Chapter 11: Transformation in Alloys

1
Chapter 11 Transformation in Alloys

• Going from one phase to another takes time.
• Needs Transformation Rate for solid-solid
  reaction.
• For hardening, need
• A non-fully miscible A-B binary alloy.
• A compound AxBy precipitate
• (e.g. Al-Al2Cu , Fe-Fe3C, Al-Al3Mg2, )
• Required Features of the Compound
• Compound between two metals (Al2Cu)
• or a metal and non-metal (Fe3C).
• Narrow composition range.
• Strong covalent bond between the elements.
• Strong particles.

2
Transformation and Age Hardening in Alloys

• Issues to Address
• Since transformations take time, what is fraction
  of transformation vs. logarithm of t?

• Can transformation be slowed to engineer
  microstructure?
• You need to use a TTT (temperature-time-transform
  ation) diagram.
• What are microstructure for steels, i.e. fine
  pearlite, coarse pearlite, spheroidite, bainite,
  martensite, and tempered martensite?
• Are the mechanical properties of these
  non-equilibrium microstructures better?

3
Phase Transformations in Alloys

• Nucleation
• nuclei (seeds) act as templates on which crystals
  grow
• for nucleus to form rate of addition of atoms to
  nucleus must be faster than rate of loss
• once nucleated, growth proceeds until equilibrium
  is attained

• Driving force to nucleate increases as we
  increase ?T
• supercooling (eutectic, eutectoid)
• superheating (peritectic)

• Small supercooling ? slow nucleation rate - few
  nuclei - large crystals

Large supercooling ? rapid nucleation rate - many
nuclei - small crystal.
4
Solidification Nucleation Types

• Homogeneous nucleation
• nuclei form in the bulk of liquid metal
• requires considerable supercooling (typically
  80-300C)

• Heterogeneous nucleation
• easier since stable nucleating surface is
  already present e.g., mold wall, impurities in
  liquid phase
• only very slight supercooling (0.1-10ºC)

5
Homogeneous Nucleation Energy Effects
r critical nucleus for r lt r nuclei shrink
for r gtr nuclei grow (to reduce energy)
6
Solidification
Note ?Hf and ? are weakly dependent on ?T
7
Rate of Phase Transformations

• Kinetics - study of reaction rates of
  transformation.
• To determine reaction rate measure degree of
  transformation as function of time (while holding
  temp constant)

8
Rate of Phase Transformation
Fixed T
Fraction transformed, y
0.5
t0.5
Adapted from Fig. 11.10, Callister Rethwisch
3e.
log t

• Avrami equation gt y 1- exp (-kt n)
• k n are transformation specific parameters

fraction transformed
time
9
Temperature Dependence of Transformation
Transformation rate depends on T.

• For the recrystallization of Cu, sincerate
  increases with increasing temperature
• Rate often so slow that attainment of equilibrium
  state not possible!

10
Transformation and undercooling Fe - Fe3C
Eutectoid transf. (Fe-Fe3C system)
For transf. to occur, must cool to below 727C
(i.e., must undercool)
Cast irons have 3 - 4.2wtC
Pearlite Formed by Eutectoid Transformation (looks
like mother of pearl in microscope)
FCC
BCC
Dark Fe3C Light ferrite
Why solubility of C more in FCC than BCC (2.1wt
vs 0.022 wt)?
11
Fe - Fe3C Eutectoid Transformation
Coarse pearlite ? formed at higher temperatures
relatively soft Fine pearlite ? formed at
lower temperatures relatively hard
12
Nucleation and Growth
Rate results from nucleation and growth of
crystals.
Fig. 11.1
Examples
13
Isothermal TTT Diagram
Consider the Fe-Fe3C system, for Co 0.76
wt C A transformation temperature of 675C.
Transf. ends
Transf. begins
14
Austenite-to-Pearlite TTT Diagram
Eutectoid composition, Co 0.77wtC Begin
at T gt 727C Rapidly cool to 625oC and hold
isothermally.

• Curves show transformation.
• Note formation at GBs to reduce free energy.

15
Pearlite Morphology
Two cases
Ttransf just below TE --Larger T
diffusion is faster --Pearlite is coarser.
Ttransf well below TE --Smaller T
diffusion is slower --Pearlite is finer.
Adapted from Fig. 10.6 (a) and (b),Callister 6e.
16
Hypereutectoid Transformations proeutectoid
cementite
Consider C0 1.13 wt C
Adapted from Fig. 11.16, Callister Rethwisch
3e.
17
Hypo- and Hyper- Eutectoid of Fe - Fe3C
Proeutectoid (pre Eutectoid)
Proeutectoid wets grain boundaries as it has a
lower free energy by forming at surface
18
Bainite Non-equilibrium Products in Fe-Fe3C
Bainite -a lathes with long rods of Fe3C
in a-ferrite matrix. -diffusion
controlled. Isothermal TTT Diagram
Fig. 11.17, Callister Rethwisch 3e.
Adapted from Fig. 11.18, Callister Rethwisch 3e.
19
Spheroidite Non-equilibrium Products in Fe-Fe3C
Spheroidite - spherical Fe3C in an
a-ferrite matrix. - diffusion dependent.
- heat bainite or pearlite for long times
just below eutectoid. - Driving Force
reduce a-ferrite/Fe3C interfacial area.
Isothermal TTT Diagram
Fig. 11.19, Callister Rethwisch 3e.
20
Martensite Non-equilibrium Products in Fe-Fe3C
Martensite --g(FCC) to Martensite (BCT)
Fig. 11.22
Fig. 11.11
TTT Diagram
g to M transformation.. - is rapid
(dffusionless!) - transf. depends on T only.
21
Martensite Formation
? (FCC) ? (BCC)
Fe3C
slow cooling
Martensite (M) single phase has body
centered tetragonal (BCT) crystal
structure Diffusionless transformation
BCT if C0 gt 0.15 wt C BCT ? few slip planes
? hard, brittle
22
Phase Transformations of Alloys
Effect of adding other elements Change transition
temp. Cr, Ni, Mo, Si, Mn retard ? ? ? Fe3C
reaction (and formation of pearlite, bainite)
Adapted from Fig. 11.24, Callister Rethwisch
3e.
23
Continuous Cooling Transformations
Conversion of isothermal transformation diagram
to continuous cooling transformation diagram
Adapted from Fig. 11.26, Callister Rethwisch
3e.
24
Isothermal Heat Treatment Example

• On the isothermal transformation diagram for a
  0.45 wt C, Fe-C alloy, sketch and label the
  time-temperature paths to produce the following
  microstructures
• 42 proeutectoid ferrite and 58 coarse pearlite
• 50 fine pearlite and 50 bainite
• 100 martensite
• 50 martensite and 50 austenite

25
Solution to Part (a)

• 42 proeutectoid ferrite and 58 coarse pearlite

Isothermally treat at 680C -- all austenite
transforms to proeutectoid a and coarse pearlite.

26
Solution to Part (b)
b) 50 proeutectoid ferrite and 50 coarse
pearlite.
Isothermally treat at 590C 50 of austenite
transforms to fine pearlite.
Then isothermally treat at 470C all
remaining austenite transforms to bainite.
27
Solution to Part (c) and (d)
c) 100 Martensite - rapidly quench to room T.
d) 50 Martensite 50 austenite. -- rapidly
quench to 290C, hold at this T
28
Mechanical Behavior Influence of C
Pearlite (med)
ferrite (soft)
C0 lt 0.76 wt C
Adapted from Fig. 10.34, Callister Rethwisch
3e.
Hypoeutectoid
Increase C content TS and YS increase, EL
decreases
29
Mechanical Props Fine Pearlite vs. Coarse
Pearlite vs. Spheroidite
fine gt coarse gt spheroidite
Hardness
Adapted from Fig. 11.31, Callister Rethwisch
3e.
fine lt coarse lt spheroidite
RA
30
Mechanical Props Fine Pearlite vs. Martensite
Fig. 11.33, Callister Rethwisch 3e.
Hardness fine pearlite ltlt martensite.
31
Tempering of Martensite
Heat treat martensite to form tempered martensite
tempered martensite less brittle than
martensite tempering reduces internal stresses
caused by quenching
Fig. 11.34, Callister Rethwisch 3e.
9 mm
tempering produces extremely small Fe3C particles
surrounded by a.
tempering decreases TS, YS but increases RA
32
Possible Transformations
33
Needs for Processing and Age Hardening

• For hardening, need
• A non-fully miscible A-B binary alloy.
• A compound AxBy precipitate
• (e.g. Al-Al2Cu , Fe-Fe3C, Al-Al3Mg2, )
• Control the misfit dislocations.
• Required Features of the Compound
• Compound between two metals (Al2Cu)
• or a metal and non-metal (Fe3C).
• Narrow composition range.
• Strong covalent bond between the elements.
• Strong particles.

34
Precipitation Hardening
Particles impede dislocation motion. Ex
Al-Cu system
Fig. 11.41, Callister Rethwisch 3e.

• Other alloys that precipitation harden
  Cu-Be, Cu-Sn, Mg-Al

35
Influence of Precipitation Heat Treatment on TS,
EL
2014 Al Alloy
Maxima on TS curves. Increasing T
accelerates process.
36
Recall Why Precipitation Strengthens
Over-aged -avg. particle size 361b
-more widely spaced particles not as
effective.
Peak-aged -avg. particle size 64b
-closer spaced particles efficiently stop
dislocations.
Later Precipitates can relieve their lattice
misfit by creating misfit dislocations around
precipitate, thereby reducing opposing stress
field to dislocation glide. So it is not just
size. - Thus, over-aged precipitates are less
effective at stopping dislocations.
37
Basic features required for age-hardening
38
Formation of the compound
By length of annealing time
39
Fraction of the compound (Lever Rule)
40
Al-rich Al-Mg Al solid-solution ? Al3Mg2
?Al3Mg2
Note that alloy is 2/5 Mg or 40at Mg
41
Al-rich Al-Mg Al3Mg2 fraction
Anneal T250 C
Average C00.1
C?0.05 C?0.40
14 is ? phase.
42
Hardening in Al-Cu phases, microstructure, aging.

• Heat treatment at small wt Cu
• solid-solution ? precipitation
• anneal at fix T ? coarsening
• (need high enough T for diffusion)
• anneal time controls phase (? or ? in ? phase),
  size of particle and local strain.
• Ageing creates ? or ? phase in ? phase.

• Plate-like precipitates (GP zones) in Al-Cu
• initially 25 atoms wide and 2 layers thick
• coarsening creates larger zones.
• Coherent precipitate gives larger stress field.

43
Hardening in Al-Cu phases, microstructure, aging.

• Heat treatment at small wt Cu
• ?phase is disordered and coherent
• with ? phase lattice (large strain fields).

• ? phase is ordered and incoherent
• with lattice due to misfit dislocations
• that provide strain relief around particle.

• Plate-like precipitates (GP zones) in Al-Cu
• Coherent precipitate gives larger stress
  field.
• Incoherent precipitate reduces larger stress
  field.

Example of misfit dislocations
Coherent boundaries
Incoherent boundary with misfit dislocations
Solid-solution
44
Hardening in Al-Cu mechanical properties and
aging.

• Over-aging occurs when
• misfit dislocations form.
• Why does this affect TS?
• Note that temperature affects aging.
• For higher T, the faster the precipitates
• coarsen (i.e. grows) and relaxes strain.
• The higher T, the faster aging occurs.

• Boeing 767 has wing skin with Al 7150-T651
  (6.2Zn, 2.3 Cu, 2.3 Mg, 0.12 Zr) forming
• ? particles
• ? MgZn2
• GB decorated w/ ? particles.

45
Hardening in Al-alloys Boeing Airplane Skin
TEM of microstructure from 7150-T651
Al-alloy (6.2Zn, 2.3Cu, 2.3Mg, 0.12Zr,
Al-balance) Light regions Al solid-solution Dark
regions small plate-shaped (majority) ?-phase
(non-equilibrium) remainder (minority)
?-phase (equilibrium)
46
Synopsis on Hardening via Precipitates
Steels increase TS, Hardness (and cost) by
adding - C (low alloy steels) - Cr,
V, Ni, Mo, W (high alloy steels) -
ductility usually decreases w/additions.
Non-ferrous Cu, Al, Ti, Mg, Refractory, and
noble metals. Fabrication techniques
forming, casting, joining. Hardenability
increases with alloy content. Microstructure
dictates affect on mechanical behavior, and
Phase Diagram reveals possible micorstructure. e.g
., Al-Al2Cu (equilibrium case) and Fe-Fe3C
(metastable case). planar
precipitates pearlite, bainite, austenite,
ferrite, spheriodite T-T-T Diagram reveals
processing to achieve microstructure.
Solid-solutions, substitutional and interstitial
compound different. Precipitation hardening
-effective means to increase strength in Al,
Cu, and Mg alloys. - do not over age, which
introduces misfit dislocations.
47
Melting Glass Transition Temps.

• What factors affect Tm and Tg?
• Both Tm and Tg increase with increasing chain
  stiffness
• Chain stiffness increased by presence of
• Bulky sidegroups
• Polar groups or sidegroups
• Chain double bonds and aromatic chain groups
• Regularity of repeat unit arrangements affects
  Tm only

48
Summary TTT Diagram and Microstructure

• Precipitation hardening Two-phase regions
  containing metal ordered compound needed. (Al,
  Mg alloys precipitation hardenable.)
• Two-phase microstructure (with stiffer compound)
  leads to increase in TS, UTS, Hardness but to
  decrease in ductility.
• Type of microstructure dictates affect on
  mechanical behavior.
• e.g., Al-Al2Cu (equilibrium case) and Fe-Fe3C
  (metastable case).
• planar precipitates pearlite, bainite,
  austenite, ferrite, spheriodite
• Controlling composition, fractions of phases,
  microstructure via T-T-T diagram reveals
  processing route and subsequent mechanical
  behavior.
• Difference between substitutional compound or
  solid solution and interstitial compound (atom
  small impurity).