Under supervision of: V.G. Kouznetsova D. San Martin M.P.H.F.L. van Maris Eindhoven, April 2007

Martensitic Phase Transformation in Sandvik Nanoflex

Experimental research on isothermal and

strain induced transformation behavior

L.C.N. Louws

MT07.11

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Abstract Metastable steels, such as austenitic steels, have attracted a lot of interest for a number of applications due to their capability to combine high formability and high strength. The final mechanical properties of these steels depends strongly on the thermo-mechanical processing routes and are usually attained by process of martensite formation and, in some grades, followed by a final step of precipitation hardening. In this project, the steel termed Sandvik Nanoflex is investigated. This 12Cr-9Ni-4Mo-2Cu metastable austenitic stainless steel has, in the as-received state, an austenitic microstructure that can be transformed to martensite either by isothermal cooling or by straining the material. Even at room temperature it has been observed that the metastable austenite transforms isothermally to martensite. After transformation of the martensite, a final strength of 2000 MPa can be obtained in this steel by a

precipitation step at 450-550 °C. It has been observed that the martensitic transformation behavior in this steel is quite inhomogeneous from heat to heat, from sample to sample within the same heat and, also, along the thickness within the same sample. Compositional segregation of substitutional alloying elements in the microstructure has not been detected and, so far, the problem remains unsolved. It is known that grain size, grain orientation or grain boundary type can affect this transformation. For this reason, in this work, the influence of the crystallographic features of austenite on the isothermal and strain induced transformation behavior of this steel is studied. First, an approach to this problem has been designed and then put into action successfully. The combination of Orientation Imaging Microscopy (OIM) and in-situ optical microscopy with the aid of micros-tensile/cooling stages made it possible to achieve this goal. The formation of martensite comes with a volume increase that results in the martensite plates popping out of the polished surface, which makes their observation under optical microscopes much easier. The general experimental procedure carried out is described as follows: 1) samples are electropolished; 2) by using a micro-hardness tester some indentations are marked on the polished surface to reference the square area under analysis; 3) The roughness of the surface is examined by confocal microscopy; 4) the crystallographic features of austenite are analyzed by OIM; 5) the

formation of martensite is stimulated (either by isothermal cooling at -40 °C or by straining) using micro-tensile/cooling stages. The formation of martensite is studied in-situ, on the polished surface, by optical microscopy. Pictures are recorded periodically while the phase transformation is taking place; 6) the same area is analyzed again by OIM and confocal microscopy after the transformation; 7) OIM recorded data and the optical images are compared. Combining the OIM recorded data with the optical images makes possible to investigate the nucleation and growth of martensite at grain level. The analysis of the data is very time consuming and only the first stages of the transformation have been analyzed so far. Several comments can be extracted from this research: 1) Nucleation and growth of martensite induced by straining or isothermal cooling takes place in different ways; by straining, nucleation takes place at random, while isothermal martensite grows continuously as a front in very localized areas. 2) No grain orientation influence has been found during both tests. Isothermal martensite was more difficult to study, because many plates were sometimes growing at the same time. 3) Nucleation of martensite plates by straining seems to occur at grain boundaries and stop also at grain boundaries (either at twin boundary or not). Nucleation takes place first in bigger grains for the staining test while this was not the case for the isothermal cooling test where the grain size has no influence on the

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transformation behavior. 4) Comparing the total boundary length before and after for both tests showed a reduction in the relative amount of twin boundaries after the tests. This observation indicates a preferential nucleation at twin boundaries. These results are preliminary and more analysis of the data is necessary. This investigation is going on and the final results are intended to be published in International Conferences and SCI journals.

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Table of contents Abstract..................................................................................................................... 1 Table of contents....................................................................................................... 3 1 Introduction ............................................................................................................ 4

1.1 Philips .............................................................................................................. 4 1.2 Steel Sandvik Nanoflex .................................................................................... 4 1.3 Motivation and aim of this project ..................................................................... 6

2 Experimental equipment......................................................................................... 7 2.1 Automatic electropolishing machine ................................................................. 7 2.2 Vickers microhardness tester ........................................................................... 7 2.3 Confocal microscope........................................................................................ 8 2.4 Electron microscope......................................................................................... 9

2.4.1 Orientation Imaging Microscopy............................................................... 10 2.5 Optical microscope......................................................................................... 10 2.6 Cooling/heating stage..................................................................................... 11 2.7 Tensile stage.................................................................................................. 12

3 Experimental procedure ....................................................................................... 13 3.1 Polishing of the samples ................................................................................ 14 3.2 Recording of reference indentations............................................................... 14 3.3 Confocal microscopy and OIM measurements ............................................... 14 3.4 Tensile test: strain induced martensite ........................................................... 14 3.5 Cooling test: Isothermal formation of martensite............................................. 15

4 Experimental results and discussion..................................................................... 17 4.1 Electropolishing.............................................................................................. 17 4.2 Tensile test..................................................................................................... 20

4.2.1 Confocal imaging ..................................................................................... 20 4.2.2 Optical microscopy (OM).......................................................................... 20 4.2.3 OIM measurements.................................................................................. 22 4.2.4 Combination of OM and OIM measurements ........................................... 32 4.2.5 Conclusions from tensile test ................................................................... 36

4.3 Cooling test .................................................................................................... 37 4.3.1 Confocal imaging ..................................................................................... 37 4.3.2 Optical microscopy................................................................................... 38 4.3.3 OIM measurements.................................................................................. 40 4.3.4 Combination of OM and OIM measurements ........................................... 45 4.3.5 Conclusions from isothermal cooling test ................................................. 48

5 Conclusions.......................................................................................................... 49 6 Recommendations for future research.................................................................. 50 7 References........................................................................................................... 51 8 Appendices .......................................................................................................... 52

8.1 Appendix A..................................................................................................... 52 8.2 Appendix B..................................................................................................... 53 8.3 Appendix C .................................................................................................... 54

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1 Introduction

1.1 Philips

This research is part of the FOM-NIMR project entitled “Tailoring of Processable Metastable Steels” which focuses on the characterization and modeling of the phase transformation behavior of a metastable austenitic stainless steel termed Sandvik Nanoflex. The materials used by Philips to make the razor blades and other parts of the electrical shaver heads need to fulfill some special requirements. The material used for waterproof electrical shavers has to be corrosion resistant. Besides, the cutting system must be made of a hard material to minimize the wearing. The material should also be able to withstand high deformations during the production process. A material which combines these properties in a good way is the metastable austenitic stainless steel termed Sandvik Nanoflex [1]. The shaving cups and cutters in Figure 1 are made of this material. a)

b)

Figure 1 a): Philips electrical shaver head, b) shaving cup and cutter.

1.2 Steel Sandvik Nanoflex

The composition of this stainless steel contains a relative high amount of chrome, nickel and molybdenum (Table 1). Table 1: Chemical composition of Sandvik Nanoflex.

Element Cr Ni Mo Cu Ti Al Mn Si C+N Wt-% 12.0 9.0 4.0 2.0 0.9 0.4 0.3 ≤ 0.5 ≤ 0.05

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In the delivered condition the material is in the austenitic phase. It has a face centered cubic (FCC) crystal structure. This gives the material a ductile character and makes it able to undergo high deformations. The metastable character of the steel at room temperature means that the microstructure is not in equilibrium, but changing with time. Therefore austenite, which is the phase present at room temperature, is not always the most preferred one. The preferred phase at a given temperature is the one with the lowest free energy. At room temperature this is the martensite phase, which has a body centered tetragonal (BCT) crystal structure. The formation of martensite increases the yield stress and the hardness of the steel, but makes it more brittle. Figure 2 shows the temperature dependence of the chemical free energies of the martensite and austenite. At T0, the two phases are in a thermodynamic equilibrium. Ms depicts the temperature at which the transformation to martensite starts. At the temperature T1, it’s possible to induce the formation of martensite by adding a mechanical driving force U. The transformation starts when the total driving force at T1 equals the chemical driving force at Ms. At temperature Md, the chemical driving force, becomes so small that the martensitic transformation can not be induced by mechanical forces anymore [2]. In this steel martensite does not transform athermally but only isothermally [3]; therefore, the Ms could be only defined in this steel as the temperature at which isothermal martensite is thermodynamically favorable, which seems to be around room temperature (although the kinetics might be too slow to be detected). Other phases present in the microstructure of the steel, in addition to austenite and martensite, are titanium nitride (TiN) particles and an intermetallic phase called Chi-phase (Fe36Cr12Mo10). TiN particles appear in a cubic shape and have dimensions around 1-5 µm. The Chi-phase appears in the microstructure as spherical particles with diameters ranging from 0.1 to 3 µm and is present mainly at the grain boundaries. The mechanical properties of Sandvik Nanoflex are shown in Table 2. Table 2: Mechanical properties of Sandvik Nanoflex in the martensitic phase [1].

Tensile strength, Rm 1700-2000 MPa Yield strength, Rp0.2 1500-1800 MPa Elongation 6-8 %

Hardness 450-650 Hv10, about 45 to 58 HRC Toughness Impact strength (Charpy V) min. 27 J at -20 °C This steel is delivered as plates/sheets with a thickness of 440 µm. At the factory of Philips, the following procedure is used to make the shaving heads with the best mechanical properties: After the forming process of the steel plates (to make the pieces shown in Figure 1) these parts are given an austenizing heat treatment at

around 1100-1150 °C to retransform to austenite all the strain induce martensite and get rid of the stresses generated during the forming process. Subsequently, the

samples are cooled down to -40 °C for 24 hours to promote the isothermal transformation of martensite; at this temperature, the kinetics of isothermal martensite formation is the fastest. The last step is a precipitation hardening at 500

°C. By this hardening process, the tensile strength of the steel can be increased up to a maximum of 2000 MPa. Earlier research has shown that the martensitic

Figure 2: free energy change for a martensitic transformation.

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transformation starts in the middle of the cross section and forms a band wise pattern which seems to grow out toward the surface of the plate. This typical behavior is shown in Figure 3 [4]. a)

b)

Figure 3: Cross sections of two pieces of Sandvik Nanoflex clamped together. The austenite phase appears light and the martensite is the darker one. Photograph a) shows the amount of martensite after 1 day of cooling, and b) shows the growth after

8 days of cooling. The samples are isothermally cooled at a temperature of -40 °C.

1.3 Motivation and aim of this project

This inhomogeneous transformation behavior observed in Figure 3 suggests that the initial as-delivered austenitic microstructure is non-isotropic along the thickness of the steel. It has also been observed that different heats of the steel show different transformation kinetics. Several factors could be responsible for this behavior: inhomogeneous distribution of alloying elements, inhomogeneous grain size distribution or the presence of textures. The alloying elements seem to be distributed homogeneously. The differences found in grain size do not seem to explain the behavior observed but cannot be ruled out yet [4]; and it is still necessary to explore if certain textures and other crystallographic features of the austenitic microstructure could have an influence in the transformation behavior during the austenite to martensite transformation. The aim of this project is 1) to set a strategy to study the nucleation and growth of martensite and its dependence on the crystallographic features of austenite, and 2) once the strategy is set, to carry out a detailed characterization of the microstructures and the transformation. The following chapters will describe the work done during this project to fulfill the previous established goals. First of all, a description of the experimental facilities used for this research is undertaken (Chapter 2), followed by the description of the experimental procedure (Chapter 3). Then, the experimental results will be shown and discussed (Chapter 4). Finally, the work will end with the conclusions (Chapter 5) and some recommendations and suggestions for future research (Chapter 6).

440 µm

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2 Experimental equipment

2.1 Automatic electropolishing machine

The electropolishing of the samples was done with a Struers LectroPol-5 (see Figure 4). This is an automatic, microprocessor controlled electrolytic polishing and etching unit to prepare metallographic samples. The principle of electropolishing is as follows; the sample is made the anode in an electrolytic cell containing an appropriate electrolyte. The surface is smoothed and brightened by the anodic solution (electrolyte) when the correct combination of bath temperature, voltage (or current density) and time is employed. The LectroPol-5 is equipped with a scanning function. A scan of a sample generates a plot of the applied voltage versus the current density. This curve is used to define the correct voltage for polishing or etching if needed. The main parameters are the voltage and the polishing time. It is also possible to change the flow rate. Besides that, the contact area and the type of electrolyte should be defined. A maximum temperature of the electrolyte has to be set; above which the system gives a warning. This is a safety device to prevent the electrolyte from heating up. Most electrolytes are extremely flammable and some of them are even explosive. The machine consists of two separate components; the control unit and the polishing unit (see Figure 4). The control unit has a touch pad to program the settings and to start the polishing [5].

Figure 4: the Struers LectroPol-5. On the left hand side of the picture, the control unit and on the right hand side, the polishing unit.

2.2 Vickers microhardness tester

A Vickers microhardness test is a method to determine the material hardness. The method consists of indenting a test material with a diamond indenter in the form of a right pyramid. The indentation is made with a certain load, depending on the material and this load is normally applied for 10 to 15 seconds. After removing the indenter, the dimension of the squared indentation has to be measured. Afterwards, with the use of conversion tables it is possible to determine the Vickers hardness of the tested sample. For this research the hardness tester was not used to determine the hardness of the test samples but to make some indentations to be used as a reference to find back the area under analysis in the sample. The indents make life easier when different techniques are used to characterize the same region of the material.

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2.3 Confocal microscope

Confocal microscopy is used to measure the surface roughness/topography and makes possible to create a 3D image of a sample surface. The principle behind this technique (Figure 5 a)) can be described as follows: the sample is scanned vertically (z-direction) so that every point (each x,y-point) on the surface passes through the focus. The height of each pixel is found by detecting the peak of the light intensity curve created by this scan. The precision in the z-direction can be less then 10 nm. Here, the PLµ 2300 confocal microscope (Figure 5 b)) from Sensofar was used for the measurements [6]. a)

b)

Figure 5 a): Scheme of a confocal microscope, and b) PLµ 2300 Sensofar confocal microscope.

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2.4 Electron microscope

For this research, high resolution scanning electron microscope (HR-SEM) from the FEI Company, equipped with an EDAX EDS Pegasus system for chemical analysis and Orientation Imaging Microscopy (OIM) for microtexture, multiphase and grain boundary mapping was used (Figure 6) [7]. The software used for the OIM data acquisition and analysis is OIM Analysis 4.5 for windows.

Figure 6: FEI High Resolution Scanning Electron Microscope.

The principle behind a scanning electron microscope is described as follows. An electron gun, containing a filament, emits the electrons that are focused by electromagnetic lenses. The electrons hit the specimen and interact elastic or inelastic with it. Elastic scattering reflects the electrons over large angles with minimal energy loss. Amongst all possible elastic interactions (Figure 7), the electrons can leave the material as a back scattered electrons (BSE). Inelastic scattering causes the electron to loose energy in different ways, as also shown in Figure 7. All these emitted types of electrons and energy are potentially useful to create images of the sample.

Figure 7: Possible interaction ‘products’ during electron-specimen interactions.

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2.4.1 Orientation Imaging Microscopy

As pointed out in the previous introduction, a SEM-based technique for microstructure analysis is Electron Back Scatter Diffraction (EBSD), also called Orientation Imaging Microscopy (OIM). This technique makes use of the backscattered electrons to make images of the microstructure analyzed. With this technique the sample is positioned in the SEM sample chamber with a certain angle

between the electron beam and the sample surface (70° is usually the optimal angle for EBSD recording). When the reflected electrons hit the phosphor screen of the detector, they create a so called Kikuchi pattern; see Figure 8 a). This pattern gives information over the orientation and the crystal structure of the scanned point of the sample (from where the backscattered electron comes). By making a grid of sampling points whose spacing is much finer than the grain size and making a plot of the orientation at each sample point, a crystallographic map of the microstructure is obtained. This map gives information of features such as, fraction and distribution of phases, grain size, grain orientation with respect to a reference axis, grain boundary character, etc. Figure 8 b) shows a picture of an Inverse Pole Figure (orientation distribution map) obtained by OIM measurements of Sandvik Nanoflex. The black regions in this figure are the indentations made in the sample, which appear as non-indexed points, to have a reference of the area under investigation, as it was already explained in Section 2.2. a)

b)

Figure 8 a): Characteristic Kikuchi pattern, b) Inverse Pole Figure.

2.5 Optical microscope

The optical images have been taken with a Zeiss microscope Axioplan 2 with an AxioCam attached to it (Figure 9 a)) [8]. This microscope is also equipped with Differential Interference Contrast (DIC) to enhance topographic features on the sample surface analyzed.

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a)

b)

Figure 9 a): Zeiss optical light microscope in combination with a Zeiss Axiocam camera, b) the principle behind Differential Interference Contrast (DIC) microscopy for reflected light microscopy. The main components of a microscope equipped with DIC are presented in Figure 9 b). The incoming light beam passes a linear polarizer with the transmission axis oriented East-West with respect to the microscope frame. Linearly polarized light exiting the polarizer is reflected from the surface of a half-mirror placed at a 45-degree angle to the incident beam. The deflected light waves, which are now traveling along the microscope optical axis, enter a Nomarski or Wollaston prism where they are separated into polarized orthogonal components and sheared according to the geometry of the birefringent prism. The microscope objective focuses the sheared orthogonal wavefronts onto the surface of the opaque specimen. Reflected wavefronts, which experience varying optical path differences as a function of specimen surface topography, are gathered by the objective and focused on the interference plane of the Nomarski prism where they are recombined to eliminate shear. After exiting the Nomarski prism, the wavefronts pass through the half-mirror on a straight trajectory, and then encounter the analyzer (a second polarizer) positioned with the transmission axis oriented in a North-South direction. Components of the orthogonal wavefronts that are parallel to the analyzer transmission vector are able to pass through in a common azimuth, and subsequently undergo interference in the plane of the eyepiece fixed diaphragm to generate amplitude fluctuations and form the DIC image. The Zeiss microscope is equipped with a camera, which makes possible to take series of pictures with a programmed time interval. Afterwards, these pictures can be combined in a movie. The combined use of cooling/tensile stages (described below) and a light optical microscope offers the opportunity of studying, in-situ, the time evolution of phase transformations or reactions happening on the surface of a sample, like the martensitic transformation under investigation in this work.

2.6 Cooling/heating stage

To make a controlled in-situ study of the martensitic transformation during isothermal cooling, the Linkam cooling/heating stage of Figure 10 was used. It has an isolated cage and is connected to a vessel containing liquid nitrogen. The control unit pumps nitrogen through the cooling element with a constant flow. A thermocouple measures the actual temperature in the element and an electrical heater is used to keep the

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element at the desired temperature. It is also possible to use this stage for heating of

a sample. The unit has a temperature range from -196 °C to 350 °C. The stage can be mounted in a light optical microscope. This makes possible to look at the sample using light optical microscopy during the cooling test [9].

Figure 10: Linkam cooling/heating stage.

2.7 Tensile stage

A tensile stage has been used to study, in-situ, the strain induced martensitic transformation during a tensile test (see Figure 11). This stage can be mounted in a light optical microscope, which makes possible to look and record the changes induced by deformation in the sample. Tensile experiments can be carried out under different applied strain rates, varying from 0.1 to 20 µm/s. The system has a load cell with a maximum capacity of 2 kN.

Figure 11: tensile stage.

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3 Experimental procedure As pointed out in chapter 2, several experimental techniques have been used in this work. Figure 12 shows a flowchart that summarizes the experimental procedure followed in every experiment.

Figure 12: Flowchart of the experimental procedure followed in this project.

STARTING MATERIAL

Samples taken from the centre of the steel plates

CUT SAMPLES

ISOTHERMAL MARTENSITE

Cooling test

STRAIN INDUCED MATENSITE

Tensile test

SQUARED SAMPLES

TENSILE SAMPLES

ELECTROPOLISHING

Find the most proper conditions

(critical)

MICROHARDNESS

Make indentations in the selected area to be analyzed by OIM

CONFOCAL MEASUREMENTS

(Roughness)

OIM MEASUREMENTS

IN SITU OPTICAL MICROSCOPY

(Sample transformed to martensite)

ISOTHERMAL MARTENSITE

Cooling test

STRAIN INDUCED MATENSITE

Tensile test

FINAL MATERIAL

AUSTENITIC - MARTENSITIC

FULLY AUSTENITIC SAMPLE ?

Yes No END OF EXPERIMENT

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3.1 Polishing of the samples

Once the samples have been cut to the proper dimensions for the experiment, the next step is to polish the surfaces that are going to be characterized by confocal, OIM and light optical microscopy. This polishing step is critical to obtain the best results. The test samples require a very smooth, shiny and free of deformation surface for these experiments. There are two main methods of polishing the samples; 1) mechanical polishing and 2) electropolishing. The biggest disadvantage of mechanical polishing is the introduction of deformation at the surface that can already induce the formation of some martensite, which is something we have to avoid for these experiments. The transformation behavior of Sandvik Nanoflex will be influenced by this deformation. During electropolishing no mechanical force acts on the surface. The only force is the flow of the electrolyte which is minimal. Electropolishing also gives very smooth and scratch free polishing results which is crucial for the OIM measurements. It has also the advantage of short preparation times. For this reasons electropolishing is used in this research.

3.2 Recording of reference indentations

On a selected region of the polished samples, a squared area has been marked by using four indentations. The indentations have been made by using a microhardness tester. The aim of this step is to be able to come back to the same region every time an experimental technique is used. This way, it is made sure that the same area is analyzed all the time. A squared area of around 100x100 µm2 was chosen. This area is located approximately in the centre of the electropolished area of the samples.

3.3 Confocal microscopy and OIM measurements

The Confocal microscopy is used as a means of studying the evolution of the roughness in the surface, increased due to the formation of martensite and the strain applied to the sample. The OIM analysis of the reference area before and after the experiments will give us information concerning the location of the martensite formed and what austenitic crystallographic features determine or influence the formation of martensite either during isothermal cooling or during tensile testing. These measurements, in combination with the study of martensite by optical microscopy, comprise the core of this investigation. The OIM measurements are performed with a step size of 0.4 µm and this makes a scan of ±150x150 µm2 to take about 2 hours. The results will be shown in the following chapter.

3.4 Tensile test: strain induced martensite

The goal of this test is to use the tensile stage to stimulate the formation of strain induced martensite on the polished surface by performing a tensile test. This test is done at room temperature. A tensile sample is made with the use of spark erosion. This is done to minimize the formation of martensite. The bar is electropolished as shown in Figure 13 a). The

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dimensions of the tensile sample used are given in Figure 13 b). A special mask has been made to electropolish only the narrower part of the sample (see Figure 13 a)). a)

b)

Figure 13 a): a prepared tensile bar, b) dimension of a tensile bar in mm. The tensile test was performed at a speed of 0.1 µm/s with a maximal elongation of 500 µm. For this experiment, the tensile stage was mounted underneath the optical microscope. During this tensile test, in time steps of 10 s, pictures were recorded. By matching the optical images, the OIM measurements and the stress-strain curve, it is now possible to look back and study were the martensitic nucleation started in the microstructure, what level of straining was necessary to stimulate the transformation on the surface and relate the crystallographic features of austenite to the formation of martensite. Some preliminary results will be shown in the following chapter.

3.5 Cooling test: Isothermal formation of martensite

As compared to the previous tensile test, the goal of this test is to use the cooling stage to assist and accelerate the formation of martensite on the polished surface by

doing an isothermal cooling treatment of the samples at a temperature of -40 °C. At this temperature the highest martensitic transformation rate is obtained (See Figure 14).

Figure 14: TTT-diagram for Nanoflex austenized at 1050 °C for 5 minutes and subsequently quenched in water showing TTT-curves for 20 %, 40 % and 60 % by volume of martensite [3].

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10

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The test samples used for these experiments have dimensions of around 10x10x0.44 mm3. Figure 15 shows an electropolished test sample.

Figure 15: Electropolished test sample to be used in the cooling stage.

After making the indentations as explained before, the sample is mounted in the Linkam cooling stage with some conductive grease between the sample and the cooling element. The cooling stage is positioned underneath the optical light microscope. To prevent for any contact between the lens and the cooling stage, a long distance lens (50 times magnification) is used. The sample also has to be lifted by putting two aluminum plates between the cooling element and the sample. This will reduce the heat transfer between sample and cooling element but is needed to properly focus the images obtained from the polished surface. Thus, the presence of these aluminium plates causes a temperature gradient from the cooling element to

the sample. The stage has to be programmed at -60 °C to get a temperature of -40

°C at the sample surface. The temperature on top of the sample was measured with a thermocouple. A camera attached to the microscope was set to take pictures each 10 s. At the end, the pictures can be compiled in a movie which shows the growth of the martensite. This experiment makes possible to look back and see where nucleation and growth of martensite took place during the cooling test and relate them to the crystallographic features of austenite.

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4 Experimental results and discussion During this chapter, the main results will be summarized and discussed.

4.1 Electropolishing

The main problem observed during the sample preparation concerns with the formation of martensite during the electropolishing process. To avoid this problem, different electropolishing conditions were explored, looking for the one giving the best polishing results. These different settings are presented in Table 3. The voltage and the polishing time were varied, because these parameters have the most influence on the polishing results. It is known that there is hardly any martensite present in the bulk material. This can be checked with a magnet. After polishing it is easy to detect if there is any martensite induced because it will pop out at the surface. The reason for this is the increased volume as a result of the phase change. This makes the martensite easy to detect with an optical microscope. Table 3: these settings are used to find out the best polishing setting. Area (cm2) Electrolyte Flow rate Voltage (V) Time (s)

0.5 A3 [appendix A] 15 25, 30, 35, 40, 45 60, 120, 240

In addition to voltage and electropolishing time, the temperature of the electrolyte was also measured during polishing. As a result of the current going through the sample and electrolyte the temperature increases. This could affect the amount of martensite formed on the surface during polishing because the austenite is more stable at higher temperatures (see Figure 14 and 2). During the tests the temperature

increased from 25 °C to 33 °C. It is important to track the increase of the temperature because the electrolyte is extremely flammable at high temperatures (well above the ones reached in this work). Table 5 shows a picture of every polishing condition tested. These experiments have been numbered A1, A2 … A15. The voltage, the electropolishing time, the temperature and the amount of martensite formed at the surface are given in this table. These pictures show that the least martensite and therefore the best polishing results were obtained for higher voltages and longer polishing times. Increasing the voltage even more will introduce a lot of pitches which decreases the surface quality. Samples A8 and A9 show a big variation in the amount of martensite formed. Some places in these samples showed a lot of martensite while other regions in the same sample contain hardly any martensite. The images were taken with the light optical microscope described in Chapter 2, setting a magnification of 500 times and using DIC mode. Pictures with a magnification of 200 times are visible in appendix B. As a result of the polishing the sample thickness will be reduced. Figure 16 shows a confocal image and a profile of the electropolished sample used for the cooling experiments. The conditions under which the electropolishing was performed are given in Table 4 and are the same as used for the tensile test samples. The profile shows a reduction in thickness of approximately 90 µm. It can also be observed that the electropolishing does not result in a flat surface which may contribute to an inhomogeneous martensitic transformation behavior on the sample surface.

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Table 4: Electropolishing conditions used to prepare the samples for the cooling and micro-tensile tests.

Area (cm2) Electrolyte Flow rate Voltage (V) Time (s) 0.5 A3 15 40 240

a)

b)

Figure 16 a): confocal image of the electropolished cooling sample, and b) a profile of the cross section of this sample.

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Table 5: Below each picture; the number of the sample, the current temperature of the electrolyte and an estimation of the amount of martensite present at the surface.

60 s 120 s 240 s

25 V

A1 25 °C 85 % A6 26 °C 45 % A11 28 °C 30 %

30 V

A2 25 °C 60 % A7 26 °C 30 % A12 28 °C 15 %

35 V

A3 25 °C 40 % A8 27 °C 2-50 % A13 29 °C 2 %

40 V

A4 26 °C 20 % A9 29 °C 5-40 % A14 31 °C 1 %

45 V

A5 27 °C 15 % A10 29 °C 1 % A15 33 °C 1 %

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4.2 Tensile test

4.2.1 Confocal imaging

Before and after the tensile test confocal microscopy is used to create 3D images of the sample. These images are given in Figure 17. Pay attention to the difference in the scale bar for the z-direction. The 3D image has a large magnification in the z-direction. a)

b)

c)

d)

Figure 17: confocal images of the tensile bar; a) before deformation, and b) after deformation. Figure c) and d) show the 3D images.

4.2.2 Optical microscopy (OM)

Optical images were made during the tensile test. Figure 18 shows pictures of the same area before and after the test. A video of the tensile test is recorded on a CD which is in appendix C. The sample has two areas marked with indentations (both 100x100 µm2); in this report the left one will be called area 1 and the right one area 2 a) b)

Figure 18 a): tensile bar before deformation, and b) after deformation.

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It is clear that the roughness has increased a lot during the test. This is the result of two effects; on one hand the martensite is popping out of the surface and on the other hand the plastic deformation. The increased surface roughness affects the quality of the OIM measurements in a negative way. For this reason the tensile test was stopped at an elongation of 500 µm, when the roughness is important but the OIM results are still good enough as it will be shown later. The results of the tensile test are presented in Figure 19. Some images have been added to the curve to show the growth of martensite and the increasing surface roughness.

Tensile test

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600

elongation (µm)

loa

d (

N)

Figure 19: Load-Elongation curve and optical images obtained at increasing elongations of 100, 200, 300 and 400 µm. In order to analyze the transformation behavior it is important to see at which places the nucleation of martensite took place. In Figure 20 the first twelve martensite plates transformed are marked and numbered in the order as they appeared. The black arrows at both sides of the picture show the direction of straining. The sample was tested along the rolling direction of the sample. It is notable that most of the martensite plates seem to appear almost perpendicular to the tensile direction. Each martensite plate appeared at the elongations given in Table 6. The first plate of martensite was observed at an elongation of 103 µm. At this point, the deformation in the sample is mainly due to the martensitic transformation taking place; almost no load applied is needed to induce a large deformation in the sample. This deformation mechanism is the so called Transformation Induced Plasticity. Table 6: The elongations of the tensile bar when the first twelve martensitic transformations occurred.

Martensite (#) 1 2 3 4 5 6 7 8 9 10 11 12 Elongation (µm) 103 118 132 140 150 153 167 171 174 174 187 190

22

Figure 20: Optical image of the tensile test at an elongation of 190 µm. The martensite plates are numbered in the order as they appeared in the sample. The black arrows present the straining direction.

4.2.3 OIM measurements

Figure 21 shows the Image Quality (IQ) of area 1 before and after the tensile test. These images give an indication of the sharpness or quality of the Kikuchi pattern recorded for each point. A smoother surface will result in a better IQ. For OIM analysis it is always better to have a high Image Quality. Variations in the value of IQ are given in grey scale; being whites the best IQ and blacks the worst one. In Figure 21 a), grain boundaries are visible in darker color due to the fact that the crystal lattice is more distorted at grain boundaries and a poorer IQ is expected compared to the matrix. At these locations, it can also happen that the detector records a Kikuchi pattern which is a combination of the patterns of two grains and the system is not able to recognize the crystal structure. The indentations are also visible as dark squares because their shapes result in a bad reflection of electrons. There is also a notable difference in the IQ for different grains. The orientations of the grains seem to have an effect on the quality of the Kikuchi patters and, therefore, on the IQ.

1

2

3

4 5

6

7

8

9

10

11 12

23

a)

b)

Figure 21: Image Quality of area 1 of the tensile bar; a) before deformation, and b) after deformation. These pictures are made with the same grayscale from 30 (black) to 150 (white). After deformation (Figure 21 b)) the IQ has been reduced significantly, especially in certain regions. These regions have probably undergone martensitic transformation and the spike shape martensite plates have resulted in an increased surface roughness and therefore a low IQ. It’s also visible that a considerable number of grains are still unperturbed after deformation. Another parameter that gives information on the quality of the image recorded is the Confidence Index (CI), which is also low when the IQ is low. A low CI means that the software is not 100 % sure about the detected phase and orientation in these areas. The software uses a voting scheme to associate a certain measured point with a certain phase and orientation. The CI has a value between 0 and 1, and is calculated using the following formula;

idealV

VVCI

)( 21 −= (1)

Where V1 and V2 are the number of votes for the first and second solutions (in this case martensite and austenite) and Videal is the total number of votes from the detected Kikuchi bands. The charts in Figure 22 present the CI for the martensite and the austenite phase for area 1 of the tensile sample after deformation. It is clear that the values are much lower for the martensite phase which is also the case for the IQ values.

24

a) Confidence Index Austenite

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

0,2

0,22

0,02 0,12 0,21 0,31 0,40 0,50 0,59 0,68 0,78 0,87

CI

nu

mb

er

fra

cti

on

b) Confidence Index Martensite

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

0,2

0,02 0,1 0,18 0,26 0,34 0,42 0,5 0,58 0,66 0,74

CI

nu

mb

er

fra

cti

on

Figure 22: CI of area 1 of the sample after deformation; a) the austenite phase, and b) the CI of the martensite phase.

25

a) b)

c) d)

Figure 23: Phase maps of area 1 of the tensile sample; a) before deformation, and b) after deformation. Austenite and martensite are the red and green regions

respectively. Grain boundaries have also been plot in c) and d) with the Σ3 GB’s in

blue and the rest with a misorientation >10° in yellow. To quantify the volume fraction of phases before and after the deformation, a phase map was plotted. Figure 23 a) and b) show both maps before and after the

26

transformation of area 1; while c) and d) show the same phase maps in which the grain boundaries (GB’s) have also been plot with a distinction between blue and

yellow GB’s. The blue lines represent the so called Σ3 GB’s [10-11] and the rest of

the GB’s with a misorientation >10° are yellow. The Σ3 GB’s are a special type of Coincident Site Lattice (CSL) boundaries and define a misorientation between

adjacent grains of 60° ± v (v=v0*∑-n is the tolerance and is calculated using the

Brandon criterion (n=1/2)). Taking into account that for this work we have considered

v0=10 and ∑=3; thus, v=5.77. In Figure 23 a), it is evident that some martensite (green phase) has transformed induced by the deformation field around some of the indentations. In this figure, the system has detected many green dots present in the microstructure; however these dots cannot be martensite for obvious reasons. A close look into the microstructure in Figure 23 c) shows that these dots are mainly present at GB’s. Thus, and although the origin of this green dots is uncertain, it could be possible that the system recognizes these points as martensite wrongly just because they are at GB’s, where the IQ and CI are lower and the phase differentiation is more difficult. An alternative explanation could reside in the Chi-phase, which is an intermetallic phase precipitated in the austenitic matrix. This phase has a BCC crystal structure like martensite and is mainly present at GB’s. Given the step size used to analyze the sample, it could be that some of the points measured are Chi-phase particles. Since the system does not recognize this phase as present in the microstructure (because it is not included in the database); it could associate these points with the phase that has a closer crystal structure. However, if Figure 23 a) and b) are compared, we can see that the green dots present in a) are not present in b) anymore; this fact strengthens the first hypothesis postulated in this paragraph. Figure 23 b) shows in green those regions that have transformed to martensite due to the straining. If a comparison is made it is clear that the regions in Figure 21 b) with the lower IQ correspond to the places where martensite has transformed as it was expected. By looking at the whole microstructure it seems that a certain banding is observed in the transformed martensite. The regions of transformed martensite tend to align along the direction perpendicular to the straining/rolling direction. This is not obvious but worth noting. The surface fraction of martensite and austenite extracted from Figure 23 b) is 0.25 and 0.69 respectively. The remaining surface fraction (0.06) consists of non-indexed points. It is well known that grain size has an influence on the martensitic transformation in

steels [12]. It has also been observed that certain special GB’s like the Σ3 type twin boundary, if present, could provide preferential sites for the nucleation of martensite in Fe-30%Ni alloys [13-14]. For these reasons, it is important to analyze both, the type of GB’s and the distribution of grain sizes that are present in the microstructure.

Figure 24 a) shows the distribution of GB’s with an angle >10° that are present in area 1. In this figure, GB’s have been divided in two groups; the red lines represent

the so called Σ3 boundaries. The black lines represent the rest of the GB’s with a

misorientation >10° which are not Σ3.

27

a) b)

c) d)

Figure 24 a): Grain boundary map. Black lines represent misorientations >10° and

red lines are Σ3 grain boundaries; b) grain color map (unique color for each grain)

including all boundaries; c) grain color map, with all Σ3 boundaries removed; d) grain color map without twins using the trace twin criterion (only coherent GB’s removed).

There are two kinds of Σ3 GB’s; the coherent and the incoherent ones. Strictly

speaking, as twin Σ3 GB’s should be considered only those that are coherent. Both types fulfill the condition stated in the previous paragraph, but the coherent ones

28

should also meet a second condition; the boundary plane separating the two grains must be coincident (parallel) with one of the {111} planes in each of the neighboring crystals (grains) [15]. In order to perfectly characterize the GB plane, serial sectioning of the GB is needed, but this is a very time consuming technique. There is an alternative way of extracting this information with high accuracy, by following the method proposed by Wright and Larsen [16]. According to this method, a partial measurement of this plane can be obtained by measuring the angle between the boundary trace on the sample surface and the trace of the twinning plane on the same surface. This determination must be done for the crystals on both sides of the

boundary. These angles are defined as trace deviations and termed as T

Bω and T

Aω

for each grain. Figure 25 shows a schematic drawing of this measurement. In order to check if the coherent twin criteria is fulfill three factors are used by these authors;

the two angles defined before (T

Bω ,

T

Aω ), and angular deviation (

mω ) of the measured

misorientation from a perfect twin misorientation. The software used in this work is able to differentiate between coherent and non-coherent GB’s by applying this

approach. For this research the angular deviation was programmed at 8°.

Figure 25: Schematic drawing of parameters required for the characterization of coherent twin boundaries [16]. Definition of a grain as and independent feature has to be done based on the misorientation angle between two consecutive points. Generally, in OIM measurements, it is considered that two consecutive points whose misorientation is

above 10-15° belong to two different grains. In this work the threshold has been set

to 10° as mentioned before. Once this is set, the following step is to study the

presence of CSL GB’s, like the Σ3 and which of those are coherent and non-coherent. It is generally accepted that the incoherent ones play the same role in the microstructure as normal high angle GB’s and should be given the same consideration. However, coherent ones should special properties and its influence should be studied in detail. Figure 26 shows two grains extracted from Figure 24 a) where coherent boundaries are present as straight red lines. Only after every twin has been characterized as coherent or incoherent, the grain size of the microstructure can be determined and discussed. If twins are considered as individual grains, the resulting grain size distribution will be skewed toward smaller

grains. By only considering the twin misorientation relationship (60°±5.77°), grains could also incorrectly include neighboring grains that were not truly twin related (coherent). This is evident by comparing Figure 24 b) (all twins) and c) (no twins). In Figure 24 d) coherent boundaries are not taken into account by using the trace twin criterion described before. Although this microstructure is closer to the description of

29

grains done in conventional metallographic practice, still it is to be studied if these GB’s have an influence on the martensitic transformation. a)

b)

Figure 26: Two different grains extracted from Figure 24 a) in which coherent twin GB’s have been observed. These twins appear in red as straight lines within the grains.

Average grain size diameter (austenite only)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

All boundaries included Coherent twins

removed

All ∑3 boundaries

removed

(µm

)

Area 1 before deformation

Area 2 before deformation

Area 1 after deformation

Area 2 after deformation

Figure 27: Average grain size diameter before and after deformation for three cases, 1; all the grain boundaries included, 2; coherent twins removed using the trace twin

criterion and 3; all Σ3 boundaries removed. Figure 27 gives a chart with the average grain size diameter for the three cases as discussed before and shown in Figure 24 (brown bars). The average grain size has also been calculated for the final microstructure in which part of the initial austenite has been transformed to martensite (blue bars). These grain sizes are calculated using area fraction, the diameter is recalculated considering the grains circular shaped. The average grain size has reduced significantly after the transformation. There could be two reasons for this: 1) Bigger austenite grains transform first leaving the smaller ones untransformed; 2) During the growth of the martensite plates the austenite grains are split into smaller ones (By looking at the phase maps (Figure 23) we can see that some austenite remains untransformed in between some martensite regions). The most reasonable explanation is that first grains are split in two or more regions by martensite plates, making the austenite grains smaller. As the transformation proceeds, some grains will be fully transformed and others will remain partially untransformed (and smaller than their initial size). Both processes result in the reduction of the grain size.

30

To have a deeper insight, the grain size distribution was plotted in Figure 28. To make this figure, coherent grain boundaries have been removed (trace twin criterion). Both grain size distributions for area 1 and 2 are given before and after deformation. Be aware that the total area measured before and after deformation is not exactly the same. Since we are plotting the area fraction, and not absolute values, this should not modify the results. This figure shows that the number of grains smaller than approximately 5 µm has increased after the deformation. Looking at the bigger grains, it is obvious that the biggest grains disappeared during deformation; but it is still not clear which process of the two described in the previous paragraph prevails during the transformation. Therefore, it is not possible to come to any definitive conclusion until the transformation of each single grain of austenite is studied. The first observations seem to indicate that bigger grains transform first. In Chapter 4.2.4 the transformation of the first martensite plates is studied at single grain level.

Grain size distribution (coherent twins removed)

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

1 10 100

Grain diameter (µm)

Are

a f

rac

tio

n (

of

tota

l a

rea

)

Area 1 after deformation

Area 1 before deformation

Area 2 after deformation

Area 2 before deformation

Figure 28: Grain size distribution before and after deformation of area 1 and 2. The area fractions are fractions of the total measured area. Grains are constructed using austenite only.

To study whether there was any preference for the transformation to happen at Σ3

twin GB’s, the total length of all grain boundaries (misorientation >10°) and the Σ3 GB’s per unit area were compared before and after deformation, for both area 1 and 2. Since the areas analyzed before and after the deformations are not exactly equal, here we are going to compare the grain boundary length per unit area analyzed. The

results in Table 7 show that relatively more Σ3 boundaries disappeared than normal ones during phase transformation. Later on we will try to analyze whether the nucleation of martensite at twin boundaries is observed or not by comparing the OIM images with the optical micrographs.

31

Table 7: Boundary length per unit area measured, before and after deformation, for areas 1 and 2 (Austenite only). Analysis is done for high angle grain boundaries

(>10°) and Σ3 boundaries.

Before deformation After deformation Area 1 Area 2 Area 1 Area 2

Boundary length

(>10°) per unit area (mm/mm2)

(15.9/0.0217) = 733

(19.1/0.0262) = 729

(15.0/0.0259) = 579

(10.7/0.0188) = 569

Σ3 boundary length per unit area (mm/mm2)

(8.204/0.0217) = 378

(9.705/0.0262) = 370

(5.976/0.0259) = 231

(4.067/0.0188) = 216

Relative amount Σ3 (%)

51.6 50.8 39.9 38.0

The martensitic transformation could be also influenced by a preferential texture in the material. Therefore, it is also important to study the distribution of orientations in the microstructure. This information can be extracted from the Inverse Pole Figure [001] (IPF, Figure 29). The software uses a color code with a different color for each orientation; this code is given in Figure 29 b). The IPF shows that the orientation distribution is not totally homogeneous. Although grains of all possible orientation seem to be present, green and pink/purple like grains are predominant. The reason for this inhomogeneous distribution usually resides on the rolling process used for the fabrication of the steel plates. It is now one of the aims of this study to check if this inhomogeneous distribution could influence the transformation behavior; do we observe any preferential transformation at certain grain orientations? a) b)

Figure 29 a): Inverse Pole Figure of the tensile bar before deformation, and b) the color code.

32

4.2.4 Combination of OM and OIM measurements

In the previous section an analysis was done mainly on the initial austenitic microstructure, with the aim of characterizing the grain size distribution, the GB character, focusing on the presence of twin boundaries, and the distribution of grain orientations. We would like to go a step further trying to find out how all these features may be affecting the nucleation and growth of martensite under external applied strain. As described before, during the micro-tensile test, optical images were recorded each 10 s. The aim now is to follow the process of transformation, micrograph by micrograph, and compare it to the OIM figures shown in the previous section. We would like to find out if nucleation takes place in certain types of grains preferentially. This is a very tedious work that requires a considerable amount of time. Here, only information on the first eleven nucleation events will be analyzed. However, the results are still under investigation and it is planned to publish the results in International Conferences and peer review journal publications. The program Photoshop Adobe was used to match the optical and OIM images with the indentations acting as a mark. A problem for the accuracy of the combined image is caused by a shift of the electron beam during the SEM measurements. This results in some dimensional inaccuracy of the OIM images. This problem arises specially for long measurements, which was the case in here were step sizes of 0.4 µm were used and the measurements took about 2 hours. The problem is visible in Figure 30 where two OIM images are fitted around the indentations and they match less accurate in between.

For the ease of comparison, the grain color map (the one with the coherent Σ3 boundaries removed like Figure 24 d)) and the grain boundary map give information about the grain sizes and boundaries where the first transformations occurred (Figure 30 a)). The IPF is presented in Figure 30 b) to analyze the orientation of the transformed grains.

33

a)

b)

Figure 30: Combined pictures of optical image with OIM measurements; a) optical

image with grain color map (grains are constructed without the coherent Σ3

boundaries) and the grain boundaries (black lines have misorientations >10° and red

lines are Σ3 boundaries); b) optical microscopic image with the IPF (misorientations

>10° are black and Σ3 boundaries are red). Martensite plates are marked and numbers in the way they appeared and the black arrows represent the strain direction.

1

2

3

4 5

6

7

8

9

10

11 12

1

2

3

4 5

6

7

8

9

10

11 12

34

Table 8 includes important information about the first eleven nucleation events observed. There are some questions that we would like to answer by looking at the nucleation of single martensite plates: 1) do the martensite plates cross GB’s; 2) at what kind of boundary does the martensite plate nucleate; 3) what is the influence of

Σ3 GB on the transformation; or 4) how big is the grain size where the first nucleation events occurred compared to the average? In this table, the grain orientation is also denoted by using Euler angles (rotation around ND, TD and RD). The system coordinates are given in Figure 31. In our case, the rolling direction (RD) of the sample (direction along which the hot and cold rolling was done) is parallel to the transversal direction (TD) of the system shown in Figure 31 a). The IPF colors are also written down for each transformed grain. a)

b)

Figure 31 a): sample coordinates used by the software to define the Euler angles, and b); the sample coordinates for the OIM images as shown in this report.

Four martensite plates have crossed a Σ3 boundary. These boundaries are probably coherent ones as they are removed using trace twin criterion; this is the case when a

Σ3 boundary has the same grain color at each side (Figure 30 a)).

TD

RD

ND

35

36

4.2.5 Conclusions from tensile test

From the results as shown in this chapter, some conclusions can be made concerning the nucleation of martensite plates in Sandvik Nanoflex induced by straining. During the transformation, the grain size decreases which could be due to the transformation of bigger grains and/or the splitting of the grains by martensite plates in two or more regions. By looking at the first martensite nucleation events, there seems to be a preference for bigger austenite grains.

By comparing the initial and final microstructures, it seems that Σ3 GB’s are preferentially used for nucleation of martensite. Looking at single events at the initial

stages, some nucleation events seem to happen in the neighborhood of Σ3 GB’s. Martensite plates normally grow in a grain and stop growing at GB’s, while some

transformations cross a coherent Σ3 boundary; this is observed for four out of the first eleven transformations. A more detail analysis is needed to come to definitive conclusions. Looking to the orientation of the grains (considering the colors of the IPF), there were more martensite plates forming in green grains than in other grains, but these colors are also dominant in the IPF. The first observations do not show that nucleation takes place preferentially in any special orientation. By observing the total test area, the distribution of martensite looks quite homogeneous after the deformation. Some of the transformed regions seem to be aligned along the direction perpendicular to the straining direction which suggests some kind of variant selection involved. During the initial stages of the transformation, the nucleation of martensite plates didn’t induce the formation of other plates, as no special clustering of martensite was observed. However, by looking at the final microstructure it is clear that at a certain point, autocatalysis will start having an important influence on the transformation. An explanation for the observed nucleation of the first martensitic plates in different regions could be as follows; during the tensile test there will be an increasing tensile stress in the sample, which will stimulate the phase transformation. On a certain moment the first martensite plate will nucleate, which goes together with an increasing volume and this will result in a stress relaxation just around this martensite plate. This gives a lower stress level around this plate and therefore the next transformation will most likely happen somewhere else where the stress level is higher.

37

4.3 Cooling test

In this section a similar study as performed in chapter 4.2 has been undertaken. Therefore, only the final results will be shown. For the fine details on the procedure, the author refers to the previous section.

4.3.1 Confocal imaging

Confocal microscopy was used to characterize the surface roughness after the martensitic transformation took place. The places were martensite has formed are clearly differentiated from the rest of the electropolished surface, which remains unperturbed. Figure 32 a) shows a 3D image after the cooling test. A small area where martensite plates have formed has been magnified to give clearer picture of the size of martensite plates (Figure 32 b)). Although beyond the scope of this report, it is obvious from this figure, that confocal microscopy could be used to characterize the dimensions of martensite plates, which it is an interesting input for modelers dealing with this transformation. a)

b)

Figure 32: Confocal image of the sample after the isothermal cooling test; a) Final microstructure, and b) magnified area.

38

4.3.2 Optical microscopy

Figure 33 show two pictures of the cooling sample before and after the isothermal

cooling test at -40 °C. As it is shown, the transformation has been studied only to a certain degree of transformation do to experimental constrains. As it is evident in this figure, during the cooling test, the martensite was growing in both areas; however, only the red marked area is analyzed here. The green marked area is recorded in the OIM measurements; as there was hardly any transformation in the upper part of this region, it was cropped away for most images further on in this chapter. Again, pictures were recorded each 10 seconds during the cooling test, and a video was composed. This video has been recorded on a CD attached to this report. a)

b)

Figure 33 a): cooling sample before, and b) after isothermal cooling test. The red region has been used for analyzing. The green area has been used for the OIM measurements. Compared to the tensile test it is clear that the roughness also increases during this test, but now only as a result of the martensitic transformation. In this report, only the first thirteen nucleation events observed have been analyzed, starting from the moment at which the transformation entered the red marked area delimited by the indentations, this area is analyzed by OIM before the cooling test as it is situated in the green area. In Figure 34 the first thirteen transformations are marked and numbered in the order as they appeared. Table 9 shows at which time the martensitic transformations took place (note that the picture number in this table, times 10 gives the time, in seconds, needed for that particular event to happen, taking as origin the start of the transformation the microstructure shown in Figure 33 a)). It is worth noting that, compared to the case of the tensile test, several martensite plates might be transforming at the same time or very fast way one after the other. As it will be shown later, in these cooling experiments, it has been observed that the autocatalytic effect seems to be more important than for the case of strain induced martensite. The way this transformation takes place makes the analysis of the results trickier because it is difficult to follow the formation of single plates of martensite.

39

a)

b)

Figure 34 a): picture 144 with a black line to mark the martensite front, and b) picture 190 (after 1900 seconds) with the martensite numbered in the way it appeared. Table 9: The picture numbers when the first thirteen martensitic transformations occurred. The time the transformation took place is the picture number multiplied by 10 in seconds.

Martensite (#) 1 2 3 4 5 6 7 8 9 10 11 12 13 Picture (#) 145 149 152 155 156 158 172 173 179 180 183 187 190

2 3

4

5

7

8

9

6

10

11

12

13

1

40

4.3.3 OIM measurements

Figure 35 a) and b) depicts the IQ of the sample before and after cooling. As expected, the IQ of the untransformed regions has not changed that much. Small variations in the IQ could be due to some stresses accumulated in the austenitic matrix as a result of the martensitic transformation. a)

b)

c) d)

Figure 35: IQ of the cooling sample, a) before and b) after cooling test. Phase map images are presented in c) and d) where the austenite is red and the martensite has a green color. The phase map images confirm that the regions with the poorest IQ (darker areas) are those where martensite has transformed. Again, it is observed that many green points have been detected by the system, in the austenitic matrix, before the start of the transformation. As discussed in Figure 23, the origin is uncertain and more detail analysis of those areas is necessary. However, the most probable explanation relates them with poor indexed points because: 1) there was no martensite at those locations and 2) if Figure 35 c) and d) are compared, those green dots appear at

41

different locations. If the origin was physical, the green dots detected should be located at the same locations in both images. The surface fraction of martensite and austenite extracted from Figure 35 d) is 0.15 and 0.78 respectively. The remaining surface fraction (0.07) consists of non-indexed points (in black, for example the indentations). These fractions are extracted from the OIM measurements and cover a slightly bigger area than the phase maps in Figure 35, the top of the image has been cropped as there was no transformation in that region. Therefore, the fraction of martensite turns out to be lower than expected from these phase images. To study the influence of grain size and GB character, it is necessary to analyze, again, the grain boundary map and the grain color maps for all cases as carried out before in section 4.2.3, Figure 24. These are presented in Figure 36 in the same way as it was done earlier for the tensile sample (see chapter 4.2.3). a)

b)

c)

d)

Figure 36 a): Grain boundary map. Black lines represent misorientations >10° and

red lines are Σ3 grain boundaries; b) grain color map (unique color for each grain)

including all boundaries; c) grain color map, with all Σ3 boundaries removed; d) grain color map without twins using the trace twin criterion (only coherent GB’s removed).

42

Figure 37 gives the average grain size diameter of the cooling sample before and after the cooling test for the three cases as given in Figure 36. The average grain size has decreased during the cooling test, but the reduction is much smaller than for the tensile test. There are several reasons for this: 1) most austenite grains remained untransformed during the test, only the grains in contact with the martensitic areas have a reduced grain size; 2) the bigger grains are not preferentially transformed; 3) possibly, most austenite grains fully transform to martensite once a single nucleation takes place (strong autocatalytic effect).

Average grain size diameter (austenite only)

0

1

2

3

4

5

6

7

8

9

10

11

12

All boundaries included Coherent twins removed All ∑3 boundaries removed

(µm

) Before cooling

After cooling

Figure 37: Average grain size diameter before and after cooling test for three cases, 1; all the grain boundaries included, 2; coherent twins removed using the trace twin

criterion and 3; all Σ3 boundaries removed. To see if there was any preference for transformation in bigger or smaller grains, the grain size distribution (austenite only) before and after the isothermal cooling test are plotted in Figure 38. The results show for most grain diameters a reduction in area fraction. Compared to the results in section 4.2.3 (Figure 28), now there are no signs for any preference for transformation in bigger grains. Note again the increased area fraction for small grains as a result of some grains having partially transformed into martensite. The phase map image after deformation (Figure 35 d)) shows some small austenitic grains present within martensitic areas.

43

Grain size distribution (coherent twins removed)

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

1 10 100

Grain diameter (µm)

Are

a f

racti

on

(o

f to

tal are

a)

Before cooling

After cooling

Figure 38: Grain size distribution for austenitic grains before and after cooling test. The area fractions are fractions of the total measured area. Like before for the strain induced transformation, the total length of all grain

boundaries (misorientation >10°) and the Σ3 GB’s are compared before and after the cooling test. Again the total boundary length per unit area was analyzed. The results

shown in Table 10 depict a lower amount of Σ3 GB’s after the cooling test. The difference of about 5 % looks much smaller than it was for of the tensile test (reduction ±12 %), but one should take into account that the total martensitic surface fraction was much higher after that test (0.25 compared to 0.15 for the cooling test). So again, the martensitic transformation seems to have a preference for regions with

more Σ3 GB’s in it. Table 10: Boundary length per unit area measured, before and after isothermal

cooling test (Austenite only). Analysis is done for high angle grain boundaries (>10°)

and Σ3 boundaries.

Before cooling After cooling

Boundary length (>10°) per unit area (mm/mm2)

(21.3/0.0272) = 783 (19.2/0.0303) = 633

Σ3 boundary length per unit area (mm/mm2)

(10.82/0.0272) = 398 (8.694/0.0303) = 287

Relative amount Σ3 (%) 50.8 45.3

The IPF of the cooling sample (Figure 39) gives comparable results to the one for the tensile sample (Figure 29). Again, the majority of the grains have a green color. In this image there seem to be slightly less purple/pink grains compared to the tensile sample.

44

a)

b)

Figure 39 a): Inverse Pole Figure of the cooling sample before the isothermal cooling test, and b) the color code.

45

4.3.4 Combination of OM and OIM measurements

Following a similar approach as for the tensile test, the optical image is combined with the grain boundary map (the one from Figure 36 a)) to look where the first thirteen transformations took place (Figure 40). Compared to the tensile test it is notable that the transformation seems to grow only from already transformed areas and does not nucleate at random in different places. Moreover, as soon as one plate of martensite nucleates, more plates grow immediately after the first one, which makes it difficult to follow the transformations (given the fact that we are checking the microstructure each 10 seconds and not continuously in time). These features make the characterization of this transformation more difficult compared to the strain induced martensitic transformation. Looking at the different grain boundaries from Figure 40 it becomes clear that the martensite plates can cross twin GB. This happens for transformation 1, 6, 7, 9, 12 and 13.

Figure 40: Combination of optical image and the grain boundary map. Black lines

represent misorientations >10° and red lines are Σ3 grain boundaries. Martensite is again numbered in the order of its appearance. To study the influence of the grain size on the transformation behavior, the optical image has been combined with the grain color map using trace twin criterion (coherent twins removed). The combined image is presented in Figure 41 a) and shows some transformations in more grains at once. This has happened for the 1st, 2nd and 13th transformation. The growth of martensite within a single grain occurs in time steps; first, part of the grain transforms and, after some time, the rest of the grain transforms. This behavior has been observed in one grain for transformations number 1 and 12, in another grain for number 3 and 11 and once again for transformation number 7 and 9. The transformed grains are compared to the average

2 3

4

5

7

8

9

6

10

11

12

13

1

40 µm

46

grain size (9.0 µm) in Table 11. Looking at the transformation of single grains, the results show that there have been transformations both in bigger and smaller grains. The IPF in combination with the optical image is given in Figure 41 b) to study the orientation of the transformed grains. The colors of the transformed grains are shown in Table 11 and there are a lot of green grains that have been transformed. As said before, the majority of the austenitic grains have green colors; therefore it is not possible to make any conclusions toward any orientation dependent transformation. a)

b)

Figure 41 a): Combination of optical microscopy and the grain color map, and b) optical image with the IPF. The color code for the IPF is given in Figure 39 b).

2 3

4

5

7

8

9

6

10

11

12

13

1

2 3

4

5

7

8

9

6

10

11

12

13

1

20 µm

20 µm

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Table 11: Characterization of the first thirteen martensitic nucleation events.

Martensite number (see Fig. 41)

Does the martensite plate cross a boundary?

Grain size compared to the average grain size. (<</</=/>/>>)

Grain orientation, using IPF colors.

Comment.

1 Yes, Σ3 (coh) >, < Green, green/blue

Transformation in more grains.

2 No > , <, < Green, green, pink

Transformation in more grains.

3 No > Green 4 No = Green

5 No ? Purple Grain felt partly out of OIM measurement.

6 Yes, Σ3 (coh) < Green

7 Yes, Σ3 (coh) > Blue

8 No = Green

9 Yes, Σ3 (coh) > Yellow/green

10 No = Green 11 No > Green/purple 12 Yes, Σ3 (coh) > Green

13 Yes, Σ3 (coh) <, <, =, = Green, green, orange/brown

Transformation in more grains.

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4.3.5 Conclusions from isothermal cooling test

The test results in this chapter give some information about isothermal phase transformation in Sandvik Nanoflex. The experiment didn’t show any preference for the nucleation of martensite plates in special grain sizes. Transformations were observed both in small and big grains. In this way the transformation behavior varies from the strain induced test, where more transformations appeared in bigger grains. By comparing the boundary lengths before and after the cooling test, the martensite

plates showed a preference for nucleation around Σ3 GB’s compared to normal

boundaries (misorientations >10°). Again, some observations of martensite plates

crossing coherent Σ3 boundaries have been made. The orientation of the transformed grains (using the colors of the IPF) showed most transformations started in green grains. As there were many more green grains present before the test, it was not possible to confirm a preference for transformation in these special orientated grains. Looking at the total tested area, martensitic transformation seems to grow only from already transformed areas and therefore the martensite appears in big clusters while some large regions still remain totally free from martensite. This martensitic growth

can be explained as follows; at the temperature of -40 °C, when the transformation rate is highest (see Figure 14), the martensite phase will have a lower free energy than the austenite. This will result in a first martensitic transformation somewhere in the sample. The increasing volume of the martensite plate gives a pressing force to the surrounding material and will act as a mechanical driving force as shown in Chapter 1.2 (Figure 2). This makes it more likely to undergo transformation for austenite regions connected to martensite plates.

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5 Conclusions In this work the influence of the crystallographic features of austenite on the subsequent formation of martensite has been studied at the level of individual grains on a polished surface. A generic methodology has been designed and put into action for this investigation. The same method can be used in the future to study this transformation or similar transformations. The main conclusions of this investigation whose results are still under investigation are given as follows: 1) The preparation of a smooth and martensite free surface by electropolishing requires high polishing voltages and long polishing times (for the isothermal and tensile test samples 40 V and 240 s were used). These settings are critical to avoid the formation of martensite. 2) The combination of Orientation Imaging Microscopy (OIM) and in-situ Light Optical Microscopy (LOM) is a powerful tool to study the influence of austenite crystallographic features on the martensitic transformation. This is mainly possible because during the formation of martensite, spike shaped martensite plates pop up on the polished surface (as a consequence of a volume increase), which makes their observation possible. Problems arise at high deformation levels >500 µm, where grain sliding contributions to the surface roughness becomes so important that the analysis of martensite plates formed turns out to be very difficult. 3) In a 2D polished surface, different transformation behavior is observed for strain induced and isothermal transformations. Strain induced martensite nucleation takes place as individual plates at different locations in the microstructure, while for the isothermal martensite the formation occurs in clusters and in localized regions. 4) By comparing total boundary lengths before and after both tests, the material

showed a preference for transformation around Σ3 boundaries. Martensite plates also

seem to cross some coherent ∑3 boundaries during growth. 5) Looking at the orientations of the grains (using the colors of the IPF), both tests showed more transformations in green grains, but these grains were also present in higher numbers before the tests. Therefore it was not possible to detect any preference for transformation in certain oriented grains. 6) During the tensile test, the biggest grains seemed to be preferential sites for martensitic nucleation and growth. Different observations are made for the isothermal cooling test, where both big and small grains showed approximately the same amount of transformations. 7) Although it is not the purpose of this work, confocal microscopy has been shown to be very useful not only to examine the surface roughness but also to characterize the martensite formation at the level of individual martensite plates. This characterization might be useful as an input for micro-modelers. It is important to note that the experimental research is done at the surface of the samples. It is likely that the material will transform easier at a free surface where less surrounding material is present to hold up the transformation. This should be also taken into account when comparing these results to similar results measured in a bulk material. The transformation behavior could be different.

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6 Recommendations for future research The transformation behavior of the steel Sandvik Nanoflex is quite complex and hard to understand. In order to predict the phase transformation of this material, more experimental research is required. Electro polishing and optical imaging together with OIM measurements are proven as a reliable test method. The settings as presented in this report can be used for future research. Most of the data recorded during this work has to be analyzed yet. It is recommended that this task is undertaken before starting new experiments. The tests as shown in this report have been performed on a surface parallel to the steel plate surfaces and not in the transversal section. To get deeper understanding of the band wise martensite pattern as shown in Figure 3, additional and similar experiments have to be performed on the crossed section of the samples. Other experiments have shown that high temperature heat treatments can increase the kinetics of martensite formation. The current investigation has been undertaken on as-received samples. It could be interesting to perform the same analysis on samples previously heat treated at high temperatures. Questions such as: a) ‘How does the austenitic microstructure change due to annealing?’ or b) ‘Are these changes affecting the nucleation of martensite?’ are of the greatest interest. It might be interesting to do next to the isothermal tests new tests where the samples

are repeatedly cooled down to -40 °C and heated up to room temperature again to see if this will result in a different nucleation behavior compared to growing out as observed in the isothermal cooling test.

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7 References 1 www.smt.sandvik.com/nanoflex

2 V.G. Kouznetsova, Martensitic phase transformations, 2004

3 M. Holmquist, J.-O. Nilsson, A. Hultin Stigenberg, Isothermal formation of

martensite in a 12Cr-9Ni-4Mo maraging stainless steel, Scripta Materialia, 1995,

Vol., pp.

4 D. San Martin, NIMR quarterly project report, 02EMM30-3 - 2006 3rd Q San

Martin, 2006.

5 www.struers.com

6 www.sensofar.com

7 www.fei.com

8 www.zeiss.com

9 www.linkam.co.uk

10 V. Randle, Application of electron backscatter diffraction to grain boundary

characterization, International Materials Reviews, 2004, Vol. 49, pp.1-11.

11 M. Booth, V. Randle and G. Owen, Time evolution of sigma 3 annealing twins in

secondary recrystallized nickel, Journal of Microscopy, 2005, Vol. 217, pp. 162-

166.

12 HKDH Bhadeshia and RWK Honeycombe, Steels, Microstructure and Properties,

Elsevier Ltd, 2006, Third Edition, Chapter 5, pp. 95-128.

13 R. Decocker, R. Petrov, P. Governado, L. Kestens, Quatitative Evalutaion of the

Crystallographic Relation in a Martensitic Transformation in an Fe-28%Ni Alloy,

Proceedings of the 25th Riso International symposium on Materials Science:

Evolution of Deformation Microstructures In 3D, Rockilde, Denmark, 2004, pp.

275-281.

14 R. Petrov and L. Kestens, An Orientation Microscopy Study of the Martensite

Transformation in an Fe-30%Ni Alloy, published in Internet,

http://edax.ametek.com/applications/pdf/12_gent_martensite.pdf.

15 V. Randle and D. Dingley, Measurement of Boundary Plane Inclination in a

Scanning Electron Microscope, Scripta Metallurgica, 1989, Vol. 23, pp. 1565-

1570.

16 SI Wright and R.J. Larsen, Extracting twins from orientation imaging microscopy

scan data, Journal of Microscopy, 2002, Vol. 205, pp. 245-252.

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8 Appendices

8.1 Appendix A

Chemical composition of the used electrolyte. A3: 60 ml Perchloric Acid 600 ml Methanol 360 ml Butylcellosolve 2 ml Vogel’s Sparbeize

53

8.2 Appendix B

Same pictures as presented in Table 5, but now with a magnification of 200.

60 s 120 s 240 s

25 V

A1 25 °C 85 % A6 26 °C 45 % A11 28 °C 30 %

30 V

A2 25 °C 60 % A7 26 °C 30 % A12 28 °C 15 %

35 V

A3 25 °C 40 % A8 27 °C 2-50 % A13 29 °C 2 %

40 V

A4 26 °C 20 % A9 29 °C 5-40 % A14 31 °C 1 %

45 V

A5 27 °C 15 % A10 29 °C 1 % A15 33 °C 1 %

54

8.3 Appendix C

CD containing the following data:

- Confocal microscopy - OIM analysis - Presentation - Report - Tensile-stage data - Video’s