Kinetics/Phase Transformation

Kinetics Heat Treatment

Material Sciences and Engineering MatE271

Week8

1

Alloying

Baking Temp.

L

g-Fe (FCC) austenite

g+L g+b

a a+g a-Fe (BCC) ferrite

Composition wt%

- Ingredient

- Composition (wt%)

- Baking temperature

- Equilibrium diagram

- Baking time

- Cooling time (kinetics)

Material Sciences and Engineering

Material Sciences and Engineering, MatE271

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eutectic b

eutectoid a+b (pearlite) Fe

Time-dependent phase transformation

b+L

g

b: Fe3C (cementite) b: C Week 8

C

rapid cooling slow cooling 2

1

Goals for this unit (Ch. 10) Ø Understanding how temperature and cooling can be used to alter properties (e.g. Fe-C system). - The TTT-diagram (Ch. 10.1-2) - Applications: (Ch. 10.3-5) - Hardening (Steel alloys) - Precipitate hardening (Aluminum alloys) - Annealing (recrystallization and grain growth)


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Nonequilibrium Cooling - All previous discussion has been for “slow” cooling - Many times, this is TOO slow, and unnecessary - Nonequilibrium effects - Phase changes at T other than predicted - The existence of nonequilibrium phases at room temperature



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10.1 Time, the third dimension - Phase diagrams only represent what should happen in equilibrium (e.g. slow cooling) - Most materials are not processed under such conditions -

- Time - temperature history required to generate a certain microstructure - Time - temperature - transformation (TTT) diagrams Material Sciences and Engineering

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Week 8

Time effect at 100% of A Melting Temp. (Pure A)

Melting Temp. (Pure B)

Temperature

Temperature

Liquid Liquidus A + Liquid

A+B (both solids) Time

Liquid + B

Eutectic Line

A

Invariant Point

Composition, %B

B

You have to drop Temp slightly to start solidification



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3

Transformation - Most transformations do not take place instantaneously e.g. to change crystal structures, atoms must diffuse Which takes time

Net energy change

Energy is required to form phase boundaries between parent and product phases Liquid

solid Material Sciences and Engineering

Surface energy +ve Net energy

rc

Nucleation and growth MatE271

volume energy -ve Week 8

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Transformation by Nucleation and Growth Ø Nucleation The formation of very small particles of the new phase Often begins at imperfection sites – especially grain boundaries

Ø Growth The nuclei increase in size Some or all of the parent phase disappears Complete when system reaches equilibrium



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4

10.2 The TTT Diagram at 100% of A Melting Temp. (Pure A)

Melting Temp. (Pure B)

1 50

100 % completion of reaction

Temperature

Temperature

Liquid Liquidus A + Liquid Liquid + B

Eutectic Line A+B (both solids) A

Time

Invariant Point

Composition, %B

B

Time required for reaction completion


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Week 8

at 100% of A

-The fraction of reaction that has occurred is measured as a

Temperature

Rate of Transformation 1 50

100 % completion of reaction

function of time - Usually at a constant T

Time

- Progress is usually determined by microscopy or other physical property - Data is plotted as fraction transformed vs. log time



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Phase Transformation: when? Ø Phase transformations occur when either Ø Temperature is most common method to induce phase transformations Ø Phase boundaries are crossed during heating or cooling


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Phase Diagram vs. TTT Diagram ØWhen a phase boundary is crossed, the alloy proceeds towards equilibrium according to the phase diagram Ø Most phase transformations require a finite time ØPhase diagrams cannot indicate how long it takes to achieve equilibrium Ø Many times the preferred microstructure is metastable Ø The required transformation time is obtained from the TTT-Diagram



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Phase Transformation Ø Metallic Materials are extremely versatile - They possess a wide range of mechanical properties Ø Microstructure development occurs by phase transformations - Diffusional Transformation: - Diffusionless Transformation Ø Properties can be tailored by changing microstructure


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Diffusional Transformation

13

Week 8

g-Fe (FCC) austenite

a-Fe (BCC) ferrite a+g

spheroidite

g

g+b

a

g

coarse pearlite fine pearlite

eutectoid a+b ( 0.77% C )

upper bainite

g+ a+ Fe

a+Fe3C lower bainite

3C

Fe



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Week 8

Composition wt%

C

14

7

Diffusional Transformation (Pearlite) - Consider the eutectoid reaction g (0.77 wt% C) ® a (0.22% C) + Fe3C (6.70% C) Austenite transforms to ferrite and cementite – through Carbon diffuses away from ferrite to cementite Temperature affects the rate: Construct isothermal transformation diagrams from % transformation diagrams


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Week 8

Pearlite Transformation (diffusional) Austenite grain boundary

Austenite (g)

Growth direction Of Pearlite

Austenite (g) Ferrite, a

Check Fig. 9.2 P. 306

Fe3C cementite Pearlite

g (0.77 wt% C) ® a (0.22% C) + Fe3C (6.70% C) Austenite Material Sciences and Engineering


Ferrite MatE271

Cementite Week 8

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8

Mechanical Properties of Pearlite Ø Pearlite is a mix of cementite and ferrite (

)

- Cementite is harder but more brittle than ferrite

Ø Layer thickness also has an effect - Fine pearlite is harder and stronger than coarse


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Fe3C in Pearlite and Bainite



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Isothermal Diagrams

Ø Only valid for a particular composition for a particular system - Other compositions will have different curves Ø Only valid when the temperature is constant throughout the transformation Material Sciences and Engineering

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Diffusionless Transformation: Martensitic Transformation

Ø Crystal: g (FCC) a (BCC) Ø FCC accommodates C easily than BCC ØC Fe3C ( ) - trapped in the FCC lattice Ø Form Body center tetragonal lattice, BCT Material Sciences and Engineering


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The Full Isothermal TTT spheroidite

Coarse pearlite fine pearlite upper bainite lower bainite

martensite 100% martensite Material Sciences and Engineering

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Mechanical Properties of Martensite Ø Strongest, hardest, and most brittle ØHardness is dependent on C content Ø Martensite is not as dense - therefore when it transforms it causes stress (

)

Ø Tempering (heat treatment) of martensite relieves stress - makes it tougher and more ductile Note - other alloy system experience diffusionless (or martensitic) transformation Material Sciences and Engineering


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Martensite Tempering- stress reliving

Tempering temperature

Check Fig. 1010-18 P. 370

martensite

Tempered Martensite:

a +Fe3C

M


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(isolated particles) 23

Week 8

F

10.3 Hardenability Hardness: surface resistance to indentation d

H= F/Aprojected

Ap Hardneability: relative ability of steel to hardened by quenching - Related to



and

of Martensitic transformation

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Week 8

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12

Jominy End-Quench measure hardness heat to above Teutectoid

cool

- Cylindrical specimen is cooled from the end by a spray or water - Specimen size, shape is specified - Water spray and time is specified - The hardness is measured with respect to the distance from the quenched end - Rockwell hardness measured (a hardness scale) Material Sciences and Engineering

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10.4 Precipitate Hardening Al-alloy 7150-T651 (6.2Zn, 2.3Cu, 2.3Mg, 0.12 Zr)

500nm Material Sciences and Engineering

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Precipitate Hardening Al-Cu alloy (96% Al-4%Cu) T k k

Slow cooling k

q

q +k

Time Material Sciences and Engineering


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Age Hardening k T

Fine dispersion of q particle

k

Coherent interface

quench q +k

aging

Time Material Sciences and Engineering

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Age Hardening Aging time

Super saturated k-solid solution Material Sciences and Engineering


q-phase growth

q-phase precipitate MatE271

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GP zone and service life Alloy load carrying capacity

coalescence growth

Aging Time Material Sciences and Engineering

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10.5 Annealing - Loss of hardness at high temperature - relief of residual stresses - reduction of dislocation density

Force Stress = Area

- Link between deformation and microstructure - Cold work - Recovery - Recrystallization - Grain growth ØDeformation is measured by percentage dimensional changes Material Sciences and Engineering


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Strain =

Week 8

dL 100% L 32

16

Cold-working The degree of plastic deformation is expressed as % cold worked: Ao Af

%CW =

Ao - Af x100% Ao

Why does this occur? Ü

Dislocation-dislocation strain field interactions

Ü

Dislocation density increases with cold working so the average separation between dislocations decreases


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Cold-working-cont. Ü

Strain hardening may be removed by annealing (heating to higher T to allow dislocations to move) Brass Cu-Zn CW

3 sec at 580oC

8 sec



4 sec 1 hr

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Recovery, Recrystallization ÜPlastic deformation results in changes in microstructure and properties - Grain shape - Strain hardening - Increased dislocation density ÜOriginal properties can be regained by appropriate heat treatment Recovery, recrystallization, grain growth Material Sciences and Engineering

Recovery

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Recrystallization temperature

Brass

Ü

Some of the stored strain energy is relieved by movement of dislocations at high T - Number of dislocations is reduced - Configuration of dislocation is altered



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Recrystallization - Even after recovery, grains are still in a high energy state (they have been deformed) - Recrystallization is the formation of a new set of strain-free equiaxed grains. - New grains form by nucleation and growth Short range diffusion - Requires time and temperature - Recrystallization temperature: Temperature at which recrystallization reaches completion in 1 hr. Material Sciences and Engineering

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Stages of Recrystallization • Cold Worked • Initial Stage • Intermediate Stage • Complete Recrystallization • Grain Growth • Grain Growth, higher temperature Material Sciences and Engineering


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Grain Growth - Occurs in all crystalline materials - why? - Energy is associated with grain boundaries – As grain size increases, total boundary area decreases -All grains can’t grow – Large ones grow at the expense of small ones -Fine grains

superior properties

- How to produce fine grain structure???


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Week 8

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Reading Assignment READ Class Notes & relevant portions of Shackelford, 2001(5th Ed) – Chapter 10, pp 354-389 -HW5 will be available on Friday, Oct 19 Due Friday Oct 26 Will not accept HW stashed under my door



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