IOP PUBLISHING SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 18 (2009) 115003 (9pp) doi:10.1088/0964-1726/18/11/115003

The transformation behaviour of bulknanostructured NiTi alloysF Neves1,2,7, F M Braz Fernandes1, I Martins2, J B Correia2,M Oliveira2, E Gaffet3, T-Y Wang4, M Lattemann5,6, J Suffner5,6

and H Hahn5,6

1 CENIMAT/I3N, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa,2829-516 Caparica, Portugal2 Laboratorio Nacional de Energia e Geologia (LNEG), Estrada do Paco do Lumiar, 22,1649-038 Lisboa, Portugal3 NRG, UMR 5060 CNRS, UTBM, Site de Sevenans, F-90010 Belfort, France4 AKCMM, University of Sydney, Sydney, NSW 2006, Australia5 Joint Research Lab Nanomaterials, TUD-FZK, D-64287 Darmstadt, Germany6 INT, Forschungszentrum Karlsruhe, PO Box 3640, D-76021 Karlsruhe, Germany

E-mail: filipe.neves@ineti.pt

Received 22 April 2009, in final form 16 July 2009Published 11 September 2009Online at stacks.iop.org/SMS/18/115003

AbstractThe phase transformation behaviour of bulk nanostructured NiTi shape memory alloys,produced by an innovative approach called MARES (mechanically activated reactive extrusionsynthesis), was investigated using in situ x-ray diffraction and differential scanning calorimetrymeasurements. For the experimental conditions used, a suitable adjustment of the NiTi matrixcomposition was achieved after ageing at 500 ◦C for 7 h. The aged materials showed ahomogeneous dispersion of Ni4Ti3 precipitates embedded in a B2-NiTi matrix. Under thiscondition the B2-NiTi matrix has undergone a B2 ↔ R ↔ B19′ two-stage phasetransformation. This was attributed to the complex microstructural evolution during MARESprocessing, i.e. formation of large-scale and small-scale heterogeneities. Transmission electronmicroscopy investigations of the solution-treated materials showed the existence of equiaxednanocrystals in the nanocrystalline NiTi matrix.

1. Introduction

Among other factors, martensitic transformations havean important role in defining the properties of shapememory alloys, including the shape memory and superelasticeffects [1]. Quenched NiTi alloys with a stoichiometry ratioof 50 at.% Ni show a one-stage B2 ↔ B19′ transformation.The transformation temperature determines the temperaturerange where such an effect can be observed. Experimentally itis well known that the martensite transformation temperatureis strongly dependent on composition and on ageingtreatments [2, 3]. Moreover, when adding alloyingelements or when ageing treatments are performed, notonly the transformation temperature is changed, but alsothe transformation paths, and the transformation productcan also be changed. Under those conditions, a two-

7 Author to whom any correspondence should be addressed.

stage transformation B2 ↔ R ↔ B19′ or B2 ↔ B19 ↔ B19′is usually observed, instead of a one-stage B2 ↔ B19′transformation [3–5].

According to the Ti–Ni phase diagram, the NiTicompound, which is an intermetallic compound with B2order, shows a certain solubility of excess Ni on the Ni-richside at high temperature but cannot dissolve excess Ti (theTi-rich side is almost vertical) [4]. So, in Ni-rich alloysthe transformation temperature is strongly dependent on Niconcentration and the increase in Ni content causes a drasticdecrease in the transformation temperature. Conversely, for Ti-rich alloys the transformation temperature is almost unaffectedby composition and shows a similar behaviour to that of the Ti–50Ni alloy. Therefore, the precise control of overall chemicalcomposition and homogeneity of NiTi alloys plays a veryimportant role in the fabrication of these alloys.

In the 1980s it was found that, by changing the age-ing temperature, it was possible to adjust the transformation

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temperature of Ni-rich alloys (alloys where Ni exceeds50.5 at.%) even after the alloy has been produced [6, 7]. After-wards, it was concluded that the principle behind this methodis the (metastable) equilibrium between NiTi and Ni4Ti3 pre-cipitates [8]. The atomic structure and morphology of theseprecipitates were initially investigated by Tadaki et al [9]. Re-cently, their structure was refined by using the quantitativeelectron diffraction and the MSLS method, which is basedon least-squares (LS) optimization of diffracted beam inten-sities calculated with the multi-slice (MS) method [10, 11].Moreover, their influence on the transformation temperaturesand the occurrence of multiple-stage transformations havebeen mainly investigated by differential scanning calorime-try (DSC) measurements and conventional transmission elec-tron microscopy (TEM) [11–15].

The Ni4Ti3 precipitates are quite stable at temperaturesbelow 600 ◦C despite the fact that they are considereda metastable phase when compared with the equilibriumprecipitate Ni3Ti [4]. Thus, only Ni4Ti3 precipitationis expected for ageing treatments performed below thattemperature. It is thus possible to use the precipitation reactionof the quenched supersaturated Ti–Ni solid solution to finelyadjust the composition of the Ti–Ni matrix. Some studieshave shown that the Ni content in the matrix is dependenton the local volume fraction of Ni4Ti3 precipitates: thehigher the volume fraction of precipitates, the lower the nickelconcentration in the matrix [5, 13, 16]. The Ni needed to formthe Ni4Ti3 is obtained from a matrix region surrounding theprecipitate that ranges from tens of nm to more than 100 nm,depending on the size of the given precipitate and distancesbetween neighbouring precipitates. As a consequence, Rs

(the R-phase transformation start temperature) and Ms (themartensite transformation start temperature) will increase withageing time until a constant value is reached that correspondsto the equilibrium composition at that temperature and withoutbeing affected by the alloy composition [1]. Moreover, itis found that Rs is almost constant from very short ageingtimes (1 h) which suggest that the NiTi matrix reachesequilibrium very rapidly. Thus, Rs is affected only by theequilibrium Ni content in the NiTi matrix corresponding toeach ageing temperature and does not change when ageingtime increases [3, 5, 7]. In contrast, Ms shows a gradualincrease with ageing time, before reaching the constantvalue, due to the effect of the uniformity of distribution andthe size of Ni4Ti3 precipitates which changes with ageingtime [3, 5, 7]. This may be interpreted considering that Ni4Ti3

precipitates have a different crystal structure (rhombohedral)from that of the NiTi matrix (BCC-B2), and so its formationgives rise to the development of coherency stresses [17, 18].Hence, for R → B19′ transformation, which involves a largedeformation, the resistance to the transformation is large whenthe precipitates are small and have a high density (largecoherency stress) and becomes less when the size is large andthe density of the precipitates is low (small coherency stress).On the other hand, the B2 → R transformation, which has asmall transformation strain, is insensitive to the density andsize of the precipitates and depends mainly on the compositionof the NiTi matrix.

Recently, the authors reported two new approaches forthe fabrication of bulk NiTi alloys by powder metallurgythat were called MARES (mechanically activated reactiveextrusion synthesis) and MARFOS (mechanically activatedreactive forging synthesis) [19, 20]. With these two approachesalmost fully dense materials were obtained consisting of multi-phase nanocrystalline structures. Additional solubilization andageing heat treatments were necessary in order to promotehomogenization and to adjust the composition of the NiTimatrix. The characterization of the materials has beenmade using essentially x-ray diffraction (XRD) and scanningelectron microscopy (SEM) analysis.

In the present study we focus on the thermal characteris-tics of the transformation behaviour of the MARES NiTi alloyscombining in situ XRD and DSC measurements. In addition,TEM analysis is also used in this work in order to contributeto a better understanding of the precipitation processes in thesolution-treated materials.

2. Experimental details

The production of NiTi alloys by MARES consists of thefollowing steps:

• Mechanical activation of the elemental powder mixtureusing a planetary mill with a short milling duration.

• Densification of the mechanical activated powders by hotextrusion.

• Solution heat treatments of the extruded materials.• Rectification of the NiTi matrix composition by ageing

treatments.

Although the experimental details of the first three stepswere already shown with minutiae in [19], it is important togive again a brief description of the experimental details ofeach step.

2.1. Mechanical activation

Mixtures of elemental powders of Ti (ALFA AESAR, 99.9%,<105 μm) and Ni (ACROS ORGANICS, 99.9%, <44 μm),given a global equiatomic composition, were co-milledin a vario-planetary ball mill pulverisette 4 from Fritsch.Mechanical activation was conducted during a total time of 4 husing velocities of 350 rpm for the disc rotation and −200 rpmfor the vial rotation and a ball to powder ratio of 7/1.

2.2. Densification by extrusion

Densification experiments by extrusion were carried out at700 ◦C in a conventional tensile/compression Instron testmachine fitted with compression plates and with a constantram speed of 0.5 mm s−1. The mechanically activatedpowders were first handled in a glove box under a nitrogenatmosphere and were uniaxial pressed inside copper cans. Thedensification experiments consisted of placing the copper cans,previously coated with molykote (MoS2) lubricant, into theextrusion die. The copper cans were then heated by inductionand the temperature was controlled with a thermocouple placedthrough the bottom of the die in direct contact with the coppercan.

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Table 1. Oxygen and nitrogen contents of the MARES-processed materials.

Oxygen (wt%) Nitrogen (wt%)

ExtrusionSolution heattreatment

Ageing heattreatment Extrusion

Solution heattreatment

Ageing heattreatment

0.68 ± 0.09 0.73 ± 0.02 0.92 ± 0.04 0.59 ± 0.03 0.57 ± 0.02 0.40 ± 0.02

2.3. Solution heat treatments

Due to a lack of phase compositional homogeneity in theextruded materials [19], heat treatments were performed onfractions of those materials at 950 ◦C in an argon atmosphere.Also, to prevent oxidation, Ti powder was used as a getter.After a holding time of 24 h the materials were subsequentlywater quenched.

2.4. Ageing treatments

As is reported in [19], the solution heat treatments led to theformation of a microstructure consisting of an NiTi matrix,with a composition range of 55–56 at.% Ni, and a relativelyuniform dispersion of Ti2Ni precipitates. In order to adjustthe NiTi matrix composition and to study the transformationbehaviour of the MARES NiTi alloys, specimens of thesolution-treated materials were subsequently subjected toageing treatments at 500 ◦C in an argon atmosphere for 7 hfollowed by water quenching. This specific temperature andholding time were based on our previous experience [20].Again, Ti powder was used as a getter to prevent the oxidationof the specimens.

2.5. Characterization

X-ray diffraction (XRD) analyses were performed using aBruker diffractometer (rotating anode-XM18H, Cu Kα radi-ation, 35 kV/200 mA, D5000 goniometer) with conventionalθ/2θ scanning at room temperature. For the purpose of study-ing the phase transformations occurring in the aged materialsXRD patterns were also recorded in a 2θ range of 37◦–47◦ atvarious temperatures between a minimum and maximum of−180 ◦C and 100 ◦C, respectively, using the low temperaturechamber attachment TTK-450. XRD quantitative phase anal-yses were performed with the PowderCell 2.4 software [21].Pattern decomposition was carried out by means of the pseudo-Voigt function and the full width at half-maximum (FWHM)was taken as FWHM = f (U, V , W ). The weighted residualerror, Rwp, was used as the criterion for refinement. The fit-tings were also used to evaluate the crystallite size of the phasesusing a Williamson and Hall plot.

The transformation behaviour of the aged materials wasalso studied using a Setaram DSC 92 calorimeter. The DSCmeasurements were carried out in specimens with masses ofaround 50 mg and for a temperature range of −80 and 100 ◦Cwith heating and cooling rates of 7 ◦C min−1.

The microstructure of the aged materials was studied byscanning electron microscopy (SEM) using a Philips XL30field emission SEM, fitted with a backscattered electrondetector (BSE), and local phase composition was determinedby energy dispersive x-ray spectroscopy (EDS).

Transmission electron microscopy (TEM) was performedon specimens of the solution-treated materials using a JEOL3000F, 300 kV microscope. The specimens were prepared byelectrolytic polishing. A Tenupol 2 (Struers) twin-jet polisherwas used with a solution of methanol (75 vol%) and nitric acid(25 vol%) at −22 ◦C and 15 V.

Vickers micro-hardness measurements were carried outaccording to the ISO 6507-1 standard [22]. Oxygen andnitrogen contents were determined by using the LECO TC-436inert gas fusion oxygen and nitrogen analyzer (average of threedeterminations).

3. Results

Table 1 provides the oxygen and nitrogen contents determinedfor the MARES-processed materials. The oxygen and nitrogencontents showed different evolutions with the processing:a constant increase was determined for the oxygen whilethe nitrogen remained virtually unchanged. Although theprocessing was controlled in order to keep the impurity level aslow as possible after extrusion, a certain increase in the oxygencontent was unavoidable.

As expected from previous SEM investigations [19], TEMobservation revealed that the solution-treated material consistsof a duplex microstructure. The bright-field images show Ti2Nigrains, figure 1(b), embedded in a nanocrystalline NiTi matrix,figure 1(e). The grain size of both phases can be estimated tobe in the range of 50 to several hundred nm. However, whilethe Ti2Ni grains are single crystalline the NiTi grains exhibita nanocrystalline substructure within the grains, as shown byrings in the selected-area electron diffraction (SAED) patternin figure 1(f). These nanocrystals are equiaxed and are about30 nm in size.

The two SEM/BSE images presented in figures 2(a)and (b) are representative of the aged materials’ microstructure.The low magnification image (figure 2(a)) shows that ageingat 500 ◦C/7 h resulted in a homogeneous distribution andhigh density of Ni4Ti3 precipitates (light grey areas) in theNiTi matrix (medium grey areas). In addition, the dark greyareas corresponding to the Ti2Ni precipitates remained almostunchanged relative to what was observed in the solution-treatedmaterials [19]. As can be seen in the high magnificationimage (figure 2(b)) the Ni4Ti3 precipitates have lenticularshape, nanoscale in length and in width (roughly lower than500 nm and 50 nm, respectively). As expected, four differentorientations of those precipitates can be easily distinguished(marked as 1, 2, 3 and 4 in figure 2(b)). In fact, there are in totaleight variants of habit planes of Ni4Ti3 precipitates parallel tothe {111} planes of the NiTi matrix, but since two variantsshare the same habit plane, only four different orientationsare observed by SEM [6, 9, 10]. It should be mentioned

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Figure 1. TEM investigation of the solution-treated material. (a) SAED pattern and (b) bright-field image of the Ti2Ni grain with strongcontrast, (c) shows the structure and (d) the simulated ED pattern. (e) Bright-field image and (f) SAED pattern of the NiTi matrix.

(This figure is in colour only in the electronic version)

that, for the NiTi matrix and also for the Ni4Ti3 precipitates,no accurate EDAX measurements could be taken. This canbe explained and understood if we take into account thedimensions, the homogeneous distribution as well as the verysmall interspacing (of the order of a few tens of nanometres) ofthe Ni4Ti3 precipitates. In order to clarify some aspects of theSEM investigations, TEM investigations of the aged materialsare underway.

A Vickers micro-hardness value of 580 ± 37 HV 0.3 wasmeasured for the MARES-aged materials. When comparedwith the literature (500 HV [23]) this Vickers micro-hardness ishigher, although it represents a decrease when compared to thevalues determined for the MARES solution-treated materials(682 ± 30 HV 0.3 [19]). It is worth mentioning that the highhardness can be related to the development and presence ofcoherent Ni4Ti3 precipitates.

Figure 3 shows the XRD pattern of the aged materialsand the corresponding XRD quantitative phase analysis ispresented in table 2. As expected, the XRD peaks from B2-

NiTi, Ti2Ni and Ni4Ti3 phases were indexed. Moreover, dueto the relatively high content of oxygen measured by chemicalanalysis (table 1) it is likely that the oxygen was incorporatedin the Ti2Ni structure as interstitial atoms. In fact, Ti4Ni2Ohas basically the same structure as the equilibrium T2Ni phaseand therefore they are difficult to distinguish [20, 24]. Thus,the XRD peaks associated with the Ti2Ni phase may also beattributed to a Ti4Ni2Ox phase. However, with the XRD patterndecomposition, it was possible to discriminate the XRD peaksof these two phases, enabling the quantitative evaluation ofeach phase. Those results, presented in table 2, shows thatthe fraction of Ti2Ni was higher than the fraction of Ti4Ni2Ox .Table 2 also shows that B2-NiTi was indexed as the majorphase and the crystallite size, corresponding to each one of theindexed phases, was maintained within the nanometric range.

It is known that ageing treatment causes complextransformation behaviour of NiTi shape memory alloys.Figure 4 shows the XRD patterns obtained for the MARES-aged materials in the temperature range of 70 to −180 ◦C and

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Figure 2. (a) Low magnification and (b) high magnification of typical SEM/BSE images of the MARES-aged materials microstructure.

Figure 3. XRD pattern of the MARES-aged materials obtained at25 ◦C.

Table 2. Weight percentage (wt%) and the crystallite size (DC (nm))of the phases present in the MARES-aged materials and the relatedweighted residual error (Rwp) of the fittings.

NiTi Ti2Ni Ni4Ti3 Ti4Ni2O

wt% DC wt% DC wt% DC wt% DC Rwp

48 71 18 41 26 70 8 23 30

within a scanning range of 37◦ < 2θ < 47◦. This 2θ rangewas useful as the prominent diffraction peaks corresponding tothe B2, R and B19′ phases are found in this range. Both on thecooling (figure 4(a)) and on the heating (figure 4(b)) portionsof the thermal cycle, it is possible to detect a B2 ↔ R ↔ B19′two-stage transformation. The presence of the R-phasetransformation was expected since this transformation occursin the presence of Ni4Ti3 precipitates [5]. On cooling, the B2phase was found to be present up to 10 ◦C, the R phase wasidentified in the temperature range from −1 to −80 ◦C and theB19′ phase from −40 up to −180 ◦C. On heating, the B19′phase was found to be present up to −20 ◦C, the R phase wasidentified in the temperature range from −1 to 10 ◦C and theB2 phase from 10 up to 70 ◦C.

The transformation behaviour of the MARES-agedmaterials measured using DSC is shown in figure 5. On heatingthere are two endothermic peaks while on cooling apparentlythere is only one exothermic peak. However, the beginning ofa broad transformation at a temperature of around −30 ◦C canbe discerned on the cooling DSC curve of figure 5. This broad

Figure 4. XRD patterns obtained in the temperature range (a) 70 to−180 ◦C and (b) −180 to 70 ◦C and for a 2θ range where the majordiffraction peaks could be identified.

transformation on cooling was incomplete to the minimumtemperature (−80 ◦C) reached during the measurement and,according to the XRD results of figure 4(a), can be associatedwith the R → B19′ transformation. Thus, the first endothermicpeak on heating corresponds to its reverse transformation.From the small hysteresis between the first peak on coolingand the second peak on heating we can conclude that theycorrespond to the B2 ↔ R transformation.

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Figure 5. DSC curve obtained for the MARES-aged materials.

Summarizing, in situ XRD and DSC measurementsrevealed that MARES-aged materials exhibit a transfor-mation sequence of B2 → R → B19′ on cooling and ofB19′ → R → B2 on heating. The DSC peak temperaturesfor the most apparent transformations were TB2→R = 12 ◦C,TB19′→R = 0 ◦C and TR→B2 = 10.5 ◦C. It is clear then thatthe pair of transformations B2 ↔ R had a small hysteresis of1.4 ◦C, which is a typical occurrence in this type of transfor-mation [4]. Moreover, the transformation of the MARES-agedmaterials showed distinct behaviour on the R ↔ B19′ transfor-mation: on cooling, this transformation occurred in a gradualmanner and was shifted to very low temperatures, while onheating, the opposite was observed.

4. Discussion

The transformation behaviour of NiTi alloys is very sensitive tothe composition of the alloy, i.e. to the composition of the NiTiphase [1]. When a powder metallurgy (PM) approach, such asthe MARES process, is used, the final properties of an alloyare strongly dependent on the microstructure development,which in turn is mainly a function of the following factors:composition of the initial elemental powder blend, thermaland/or thermomechanical treatment steps and the presence ofimpurities (mainly oxygen) [19, 20, 25–31]. In addition, it iswell known that the formation of Ti2Ni and Ni3Ti precipitates,which affect the NiTi matrix composition and may cause adegradation of the functional and mechanical properties, isa common feature during the production of NiTi alloys byPM [19, 20, 25, 26, 31, 32]. The same can be asserted withregard to impurity-related precipitates (for example, Ti4Ni2Ox

if in the presence of oxygen) [20, 26, 27, 29, 30]. In general, theformation of those precipitates also results in an increase in theNi/Ti ratio of the matrix and thus in an undesirable decrease oftransformation temperatures. For those reasons, and in order tofully understand the transformation behaviour of an alloy, it isimportant to establish a model of the microstructural evolutionduring the different processing steps. For the particular case ofthe MARES process this is illustrated in figure 6.

In the first step of the MARES process a blend ofelemental Ni and Ti powders (figure 6(a)) was mechanicallyactivated (figure 6(b)). Mechanical activation (MA) wasbasically a solid-state mixing process due to chaotic processes(fracture and welding). It led to the formation of a layeredstructure, constituted by thin layers of the starting metals(figure 6(b)). In some areas, which were named dissolutionareas, some chemical combination between Ni and Ti was

Figure 6. (a)–(e) Schematic model of NiTi alloys’ microstructural evolution during MARES processing.

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already observed, with an atomic ratio of around 1. Therefore,MA was able to increase the contact surface area betweenNi and Ti producing a powder blend with a nanocrystallinestructure exhibiting high reactivity. This fact modified thephase transformation kinetics and allowed the formation ofintermetallic phases at a relatively low temperature [19].

In the second step, densification by extrusion at 700 ◦C,the layered microstructure was maintained but instead of beingcomposed only by the two constituted metals, layers fromthe three stable intermetallics phases, namely Ti2Ni, NiTiand Ni3Ti, were also developed (figure 6(c)). This typeof microstructure was representative of a diffusion-controlledreaction sintering. Several hypotheses have been proposedin the literature trying to explain why equiatomic elementalpowder blends always yield a mixture of NiTi, Ti2Ni and Ni3Tiphases after processing [28, 31–34]. The most realistic isthe one that considers that the formation and the coexistenceof those three intermetallic phases is controlled by solid-state diffusion reactions between Ni and Ti [31, 32]. Infact, irrespective of the homogeneity or the composition ofthe powder blend, the synthesis reaction always involvesinterdiffusion between elemental Ni and Ti. As figure 6(c)shows, on the Ni-rich side, the diffusion of Ti into Ni led tothe formation of Ni3Ti, while on the Ti-rich side, the diffusionof Ni into Ti resulted in the formation of Ti2Ni. Consideringthis, NiTi was formed in the middle by the interaction betweenNi3Ti and Ti2Ni. Actually, the direct formation of NiTi fromelemental Ni and Ti is thermodynamically not favoured whilethe secondary reactions for the formation of NiTi are also weakevents [32]. Thus, MA played an important rule on decreasingthe diffusion paths.

The third step, solution heat treatment at 950 ◦C/24 hfollowed by water quenching, led to the formation ofa microstructure consisting of a nanocrystalline Ni-richNiTi matrix, with a nanocrystalline substructure within thegrains, and a relatively uniform dispersion of Ti2Ni/Ti4Ni2Ox

precipitates (figure 6(d)). Two considerations, that in someway are interconnected, may explain the development of sucha microstructure. The first one relies on the higher mobilityof the Ni atoms compared to the Ti atoms and on the factthat Ti2Ni/Ti4Ni2Ox precipitates are more stable than NiTiand are difficult to remove by solid state diffusion [25, 32].The second one is related to the presence of impurities, inparticular oxygen. It is well known that the NiTi phase cannotincorporate any notable amounts of oxygen and, as referredto previously, Ti4Ni2Ox represents Ti2Ni with oxygen in solidsolution [24]. In addition, some studies have shown that aprocessing-related pickup of oxygen in NiTi alloys initiallyresults in compositional changes of Ti2Ni which incorporatethe oxygen atoms [27, 29, 30, 35]. When the saturation isreached, further pickup of oxygen leads to an increase ofthe Ti2Ni volume fraction in the microstructure. The firstmicrostructural process is potentially beneficial since excessoxygen will not affect the NiTi matrix when it is incorporatedin the Ti2Ni precipitates. In contrast, the second processcan have a negative effect on functional properties of NiTiSMAs because higher volume fractions of Ti2Ni are associatedwith an increased Ni/Ti ratio and hence with decreasing

transformation temperatures [1]. Taking into account thoseexplanations and the level of oxygen pickup that was reached(table 1) during the current experimental conditions, theformation with the solution heat treatment of an Ni-rich NiTimatrix (55–56 at.% Ni [19]) is more understandable.

The fourth step, ageing heat treatment at 500 ◦C/7 hfollowed by water quenching, resulted in the formation ofNi4Ti3 precipitates in the NiTi matrix (figure 6(e)). Moreover,the Vickers micro-hardness result (table 1) suggested that astrong coherent stress field was present in the aged materials,showing the existence of coherent Ni4Ti3 precipitates. Thepresence of these precipitates in the aged MARES NiTialloys can be explained by the fact that the development ofTi2Ni/Ti4Ni2Ox precipitates during the solution heat treatmentmade the NiTi phase so Ni-rich that the solubility of theNiTi range was exceeded and the extra nickel then formedthe Ni4Ti3 phase. Moreover, the high Ni content of theNiTi matrix may also be the genesis for the homogeneousdistribution and high density of the Ni4Ti3 precipitation.According to the literature, for a specific ageing temperaturethere are two competing factors, which control the resultantdistribution of those precipitates [2, 36]. The first factoris the presence of grain boundary or similar defects, whichfavour a preferred precipitation along grain boundaries,i.e. heterogeneous nucleation. The second factor, which hasan opposite effect, is the Ni content in an NiTi matrix, i.e. theNi supersaturation. The competition between these two factorsdetermines whether there will be a localized/heterogeneous ora uniform/homogeneous distribution of precipitates. When Nicontent is low (roughly below 50.6 at.% Ni), the nucleationrate is very small and thus precipitation of Ni4Ti3 is verysensitive to the presence of grain boundaries. In this case,nucleation rate at a grain boundary is much larger than at agrain interior. Consequently, precipitation mainly occurs at thegrain boundary and this makes the grain interior essentiallyprecipitate-free. When Ni content is high (roughly above51.5 at.% Ni), the difference in nucleation rate between grainboundary and interior is small and thus precipitation occurshomogeneously without being affected by a grain boundary.Therefore, high Ni content, i.e. high supersaturation, promotesa homogeneous distribution of Ni4Ti3 precipitates.

As illustrated in section 3, MARES-aged materialsshowed a multiple-stage phase transformation on cooling aswell as on heating, namely a B2 ↔ R ↔ B19′ two-stagetransformation. Although several explanations for this kindof transformation mechanism were presented, this behaviourcan find a unified and natural explanation by the theory ofprecipitation kinetics that was already concisely stated in theprevious paragraph [2, 36]. So, it is generally acceptedthat these multiple-stage transformations are related to themicrostructural heterogeneity which leads to localized phasetransformations [5, 12, 14, 16, 23]. In the present study,the homogeneous precipitation of Ni4Ti3, obtained across thewhole B2-NiTi matrix, and also the small inter-precipitatespacing may then explain the two-stage transformation. Theseresults are also in agreement with some studies that suggest thattransformation behaviour is also dependent on the atmosphereduring heat treatment [37]. In those studies it was found that

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two-stage rather than three-stage transformation occurs whenthe materials are protected from oxidation by a Ti getter duringsolution treatment.

Concerning the almost absence of the R → B19′transformation within the DSC temperature window, it shouldbe mentioned that this occurrence can be understood if we takeinto account the high Ni content of the NiTi matrix in thesolution-treated materials. This fact led to the formation ofa high density of coherent Ni4Ti3 precipitates in the B2-NiTimatrix, with the subsequent ageing heat treatment [2, 5, 38].Consequently, it seems likely that the presence of thosecoherent precipitates produced a strong resistance to the largelattice deformations associated with the formation of B19′ and,for that reason, the R → B19′ transformation only took placeat very low temperatures, as the in situ XRD analysis showed.

Finally, the mechanism of formation of the nanocrystallinesubstructure of the NiTi grains as well as the evaluationof the Ni4Ti3 precipitates in the MARES-aged materialsthrough TEM investigations are subjects for ongoing research.However, it should be mentioned that TEM observations ofthe MARFOS-aged materials revealed that the nanocrystallinesubstructure of the NiTi grains was maintained with ageingtreatment, and so it is expected to observe a similarsubstructure in the MARES-aged materials [39].

5. Conclusions

In this work, the transformation behaviour of bulk NiTialloys produced by MARES has been characterized byin situ XRD and DSC measurements. These analysesrevealed that aged materials exhibit a transformation sequenceof B2 → R → B19′ on cooling and B19′ → R → B2 onheating. This behaviour is attributed to the microstructuralevolution that was established during the different steps ofthe MARES process. With the solution heat treatment of theextruded material a duplex nanocrystalline microstructure wasproduced consisting in Ti2Ni grains and an Ni-rich NiTi matrixwith a substructure of equiaxed nanocrystals. Afterwards,ageing heat treatment produced a homogeneous distributionand high density of coherent Ni4Ti3 precipitates in the B2-NiTimatrix. This type of microstructure was effective in causing aB2 ↔ R ↔ B19′ two-stage phase transformation. The theoryof precipitation kinetics explains this transformation behaviourwhich is generally related to structural heterogeneity of thematrix, both in terms of composition and of internal stress field,caused by the formation of the coherent Ni4Ti3 precipitatesin the B2-NiTi matrix. The formation of martensite, i.e. theR → B19′ transformation, at a very low temperature was alsoattributed to the presence of the coherent Ni4Ti3 precipitates.

Acknowledgments

FN is supported by an FCT/MCTES grant (SFRH/BPD/38354/2007). This research was supported by project NA-MAMET (Processing of NAnostructured MAterials throughMEtastable Transformation): STREP Project—VI FrameworkProgramme-Priority 3—European Union (more information isavailable on the official NAMAMET web site: http://www2.

polito.it/ricerca/namamet/). FN and FMBF acknowledge thepluriannual funding of CENIMAT by FCT/MCTES.

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