MM-501 Phase Transformation in Solids

Fall Semester-2015

Engr. Muhammad Ali SiddiquiLecturer, Metallurgical Engineering Department, NED UET

BE: Mehran UET, 2007ME: NED UET, 2011

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Important Instruction• Class as per schedule Inshallah;

should maintain 75% attendance.

• LECTURES: soft copy can be taken after the class or from my Office.

• Make your OWN NOTES and read the books for in-depth understanding.

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Books for Understanding the subject & for Examination Preparation

1) Phase Transformations in Metals and Alloys, Porter and Easterling, 2nd ed

(Library call No 669 POR)Chapters, 1,2,3,5 and 6

2) Steels Microstructure and Properties, HKDH Bhadeshia(Library call No 669.142 BHA)chapters 5, 6 & 10

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Sessional Marks Distribution…..40Nos• 02 Test …………………………………………………………..20 Nos• 01 Assignment……………………………………………….. 10 Nos• Assignment Presentation/Viva……………………......05 Nos• Attendance………………………………………………………05 NosImportant Dates1. Test No:01 Sep 17,20152. Test No:02 Nov 5 or 12, 20153. Session Assignment Oct 1, 2015 (Last Date)4. Viva /presentation on Last Lecture day. (Expected

Nov 19,2015)…………..Inshallah

Type of Test:• MCQs/BCQs/

Descriptive/Problems

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Assignment Criteria • In-Time Submission• Late Submission will cost -1 marks/day and there will be seven

days

Style and Format:• No: of pages > 30 but < 45, at least 25 pages contain full text.• A-4 size paper, Typed in a single space,• 1” Margin all around the text,• Font Time New Roman #12 for text and 12Bold sub heading,

14 Main heading, • Reference Harvard University Style.• All figures and Tables must be captioned and discussed in the

text.

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Topics for Session Assignment and Presentation

Topic 1: Diffusionless Phase Transformation-IDiffusionless Character, Nucleation and Growth of Martensite, The Habit Plane, Orientation Relationships., Athermal Nature of Transformation.

Topic 2: Diffusionless Phase Transformation-IIStructure of the Interface b/w γ & α’, The Shape Deformation, Bain Distortion Model, Phenomenological Theory of Martensite, Morphology and crystallography of α’ (bcc or bct).

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• Topic 3: Reconstructive Phase TransformationAustenite to Allotriomorphic ferrite, Idomorphic ferrite, Massive ferrite and Pearlite transformation.

• Topic 4: Displacive Phase TransformationBainite (upper & lower), Acicular Ferrite and

widmanstatten ferrite transformation.

• Topic 5: Precipitation and Age Hardening Phenomena

Strengthening of Non Ferrous Alloy. E.g Al, Cu and Mg

• Topic 6: Solid Solution and Phase Diagram Solid solution, Binary and Ternary phase diagram

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Introduction Lecture

1. Iron - Phase Diagram.2. Effect of Alloying Elements on Fe-Fe3C Diagram.3. TTT Diagrams. (Construction )

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Phases Observed in Fe-C Diagram 1. Ferrite Ferrite is the interstitial solid solution of carbon in alpha iron. It has B.C.C. Structure. It has very limited solubility for carbon (maximum 0.022%/0.025 at 727°C and 0.008% at room temperature). Ferrite is soft and ductile. 2. Austenite Austenite is the interstitial solid solution of carbon in gamma (γ) iron. It has FCC structure. Austenite can have maximum 2.14% carbon at 1143°C. Austenite is normally not stable at room temperature. Austenite is non-magnetic and soft. 3. Cementite Cementite or iron carbide (Fe3C) is an intermetallic compound of iron and carbon. It contains 6.67% carbon. It is very hard and brittle. This intermetallic compound is a metastable phase and it remains as a compound indefinitely at room temperature.4. δ-ferrite It is a solid solution of carbon in δ-iron. It is stable at high temperatures. It has BCC structure.

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Phase Mixtures Observed in Fe-C Diagram

• 1. Pearlite The pearlite consists of alternate layers of ferrite and cementite. It has properties somewhere between ferrite and cementite. The average carbon content in pearlite is 0.76%

• 2. LedeburiteLedeburite is an eutetcic mixture of austenite and cementite in the form of alternate layers. The average carbon content in ledeburite is 4.3%.

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• If alloying elements are added to the iron-carbon alloy (steel), the position of the A1, A3, and Acm boundaries and the eutectoid composition are changed.

1) all important alloying elements decrease the eutectoid carbon content,

2) the austenite-stabilizing elements manganese and nickel decrease A1, and

3) the ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten increase A1.

The Effects of Alloying Elements on Iron-Carbon Alloys

Effect on the Eutectoid Point

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Changes eutectoid Temperature Changes eutectoid Composition

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Effect of alloying additions on the γ-phase field: (a) Mn;

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Effect of alloying additions on the γ-phase field: (b) Mo;

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Effect of alloying additions on the γ-phase field: (c) Cr;

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Effect of alloying additions on the γ-phase field: (d) Ti

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The Effects of Alloying Elements on Iron-Carbon Alloys

• For analyzes the effects of alloying elements on iron-carbon alloys would require analysis of a large number of ternary alloy diagrams over a wide temperature range.

• “Wever” pointed out that iron binary equilibrium systems fall into four main categories: open and closed γ-field systems, and expanded and contracted γ-field systems.

• This approach indicates that alloying elements can influence the equilibrium diagram in two ways:

by expanding the γ-field, and encouraging the formation of austenite over wider compositional limits. These elements are called γ-stabilizers.

by contracting the γ-field, and encouraging the formation of ferrite over wider compositional limits. These elements are called α-stabilizers.

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Fig. 1 Classification of iron alloy phase diagrams:

(a)open –field;

(b)expanded –field;

(c)closed –field;

(d)contracted –field

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Open γ-field • This group belong the important steel alloying elements nickel

and manganese, as well as cobalt and the inert metals ruthenium, rhodium, palladium, osmium, iridium and platinum.

• Both nickel and manganese, if added in sufficiently high concentration, completely eliminate the bcc α-iron phase and replace it, down to room temperature, with the γ-phase.

• So nickel and manganese depress the phase transformation from γ to α to lower temperatures , i.e. both Ac1 and Ac3 are lowered.

• It is also easier to obtain metastable austenite by quenching from the γ-region to room temperature, consequently nickel and manganese are useful elements in the formulation of austenitic steels.

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Fe-Mn Phase Diagram

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Fe-Ni Phase Diagram

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Closed γ-field• Many elements restrict the formation of γ-iron, causing the γ-

area of the diagram to contract to a small area referred to as the gamma loop.

• This means that the relevant elements are encouraging the formation of bcc iron (ferrite), and one result is that the δ- and γ-phase fields become continuous.

• Alloys in which this has taken place are, therefore, not agreeable to the normal heat treatments involving cooling through the γ/α-phase transformation.

• Silicon, aluminium, beryllium and phosphorus fall into this category, together with the strong carbide forming elements, titanium, vanadium, molybdenum and chromium.

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Fe-Cr Phase Diagram

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Fe-Ti Phase Diagram

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Fe-Mo Phase Diagram

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Fe-V Phase Diagram

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Expanded γ-field

• Carbon and nitrogen are the most important elements in this group.

• The γ-phase field is expanded, but its range of existence is cut short by compound formation.

• Copper, zinc and gold have a similar influence.

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Fe-Cu Phase Diagram

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contracted y-field

• Boron is the most significant element of this group, together with the carbide forming elements tantalum, niobium and zirconium.

• The γ-loop is strongly contracted, but is accompanied by compound formation.

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The Effect on the Formation and Stability of Carbides

Some alloying elements form very stable carbides when added to steel (Fig. 13.2)

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Alloy Carbides

The periodic table showing the positions of strong carbide-forming elements

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• In steel six kinds of carbides can be formed as shown in Table 1, where M denotes a sum of carbide-forming (metal) elements.

• The carbides placed in group I posses a complicated crystal structure; an examples is cementite (Fe3C), or Cr23C6.

• A specific structural feature of the carbides of group II as interstitial phases is a simple crystal lattice (e.g., TiC, WC, NbC and Mo2C).

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Isothermal Transformation (I.T) / Time Temperature Transformation (TTT)

Diagram

Today we will discuss about its Construction

What is TTT? T (Time) T (Temperature) T (Transformation) diagram is a plot

of temperature versus the logarithm of time for a steel of definite composition.

It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy.

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Step: 4 After Cooling, check sample hardness and microstructure.

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• Step 2,3 and 4 are shown in the following Figure

• If we draw a curve b/w %age of Pearlite Transformed and Time

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Step: 5 all steps are repeated at different sub-critical temperature below Ac1 in order to get sufficient points for plotting the curve.

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Composition of AISI 1080 Steel

C= 0.75-0.88, Mn= 0.60-0.90 ,

P= 0.040, S= 0.050

Fe= balance

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Example: Effect of Alloying Elements

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A cooling curve is determined experimentally by placing a thermocouple at a definite location in a steel sample and then measuring the variation of temperature with time.

Since the coordinates of the I-T diagram are the same as those for a cooling curve,

it is possible to superimpose various cooling curves on the I-T diagram. This is shown in the figure given below.

Cooling Curves and the I-T Diagram

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Thanks

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Re: Why is austenitic iron non-magnetic, but ferritic iron is?• It's not actually the Austenite phase that causes the loss of magnetic properties of Iron, it's

the temperature of the iron itself.• Magnetism in iron is believed to be caused by the alignment of the spins of the electrons

located in the third d shell of each atom. Each atom has a magnetic dipole moment, and in ferromagnetic materials, zones of atoms with similarly aligned moments tend to form throughout an object. Normally, there are enough different zones with differently oriented magnetic directions to cancel out any net magnetic field. But when you apply an external magnetic field to a piece of iron, the zones that are aligned with the field grow and the ones that are not shrink. This causes a net magnetic field in the iron, and the iron piece is attracted to the magnet. However, under certain parameters such as rubbing the iron on a magnet, applying a strong enough magnetic field, or giving a few really hard whacks (highly scientific term) on another iron object, the magnetic zones will align and cause a net magnetic field, without needing to apply an external field.

• Ok, that's cool, but how does temperature affect magnetism? All ferromagnetic materials have a Curie Temperature, above which, the material shows negligible magnetic properties. This occurs because above the Curie Temperature, the thermal energy in the material is so high that the atoms wiggle around quite a bit more, and it's harder for the zones to form. The Curie temperature for iron is 1043 K. This is close to the temperature at which the Austenite (gamma) phase of iron is stable (1183 to 1673 K) but the two properties are not related.

• In order to demonstrate this, you could heat a sample of ferritic (alpha) phase iron to about 1100 K, and check and see if it's magnetic. You could also heat a sample of iron to 1200-1600 K, quench it really quickly down to room temperaturre to keep it in the Austenitic phase, and then check to see if it's magnetic as well.