Chapter 11 Phase Transformation

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Chapter 11- Phase Transformation And some
Processing and Hardening
Drawing Aluminum Cans Forms and strengthens but
need right alloy

• Mechanical strength of Al-alloys enhanced by
  cold-working and alloying.
• Principal alloying elements include Cu, Mg. Si,
  Mn, and Zn.
• Both processes tend to diminish resistance to
  corrosion.

Non-heat-treatable alloys consist of
solid-solution hardened single-phase. Heat-treatab
le alloys are precipitation-hardened, where
non-Al-elements usually form an intermetallic
compound, e.g. MgZn2.
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Chapter 11- Phase Transformation
ISSUES TO ADDRESS...
Transforming one phase into another takes time.
How does the rate of transformation depend on
time and temperature ?
Is it possible to slow down transformations so
that non-equilibrium structures are formed?
Are the mechanical properties of
non-equilibrium structures more desirable
than equilibrium ones?
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Taxonomy of Metals
Adapted from Fig. 9.21,Callister 6e.
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Steels
Based on Data from Tables 11.1-4 Callister 6e.
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Non-Ferrous Alloys
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Refinement of Steel from Ore
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Metal Fabrication
FORMING
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Forming Temperature
Hot working and Cold working
-recrystallization -less energy to deform
-oxidation poor finish -lower strength
Cold worked microstructures are anisotropic!
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Metal Fabrication
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Metal Fabrication
Powder Processing (materials w/low
ductility)
Welding (when one large part is
impractical)
Heat affected zone (where microstructure
is affected).
11
Thermal Processing of Metals
Annealing Heat to Tanneal, then cool slowly.
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Hardenability of Steels
Ability to form martensite Jominy end
quench test to measure hardenability.
Fig. 14.5
Hardness versus distance from the quenched end.
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Why Hardness Changes with Position
The cooling rate varies with position.
Adapted from Fig. 11.12, Callister 6e.
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Hardenability vs. Alloy Content
Jominy end quench results, C 0.4wtC
"Alloy Steels" (4140, 4340, 5140, 8640)
- w/ Ni, Cr, Mo (0.2 to 2wt) -alloying
shifts TTT "nose". -martensite is easier to
get.
Fig. 14.8
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Quenching Medium and Geometry
Effect of quenching medium
Medium air oil water
Hardness small moderate large
Severity of Quench small moderate large
Effect of geometry When surface-to-volume
ratio increases --cooling rate
increases --hardness increases
Position center surface
Cooling rate small large
Hardness small large
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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.

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Precipitation Hardening
Procedure --Pt A solution heat treat
(get a solid solution) --Pt B quench to
room temp. --Pt C reheat to nucleate
small q crystals within a.
Particles impede dislocations. Ex Al-Cu
system
Other systems Cu-Be, Cu-Sn, Mg-Al
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Precipitation Effects on TS and EL
2014 Al Alloy
Minima on EL curves.
TS peaks with precipitation time. Increasing
T accelerates process. EL reaches minimum with
precipitation time.
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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.
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Basic features required for age-hardening
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Formation of the compound
By length of annealing time
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Fraction of the compound (Lever Rule)
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Al-rich Al-Mg Al solid-solution ? Al3Mg2
?Al3Mg2
Note that alloy is 2/5 Mg or 40at Mg
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Al-rich Al-Mg Al3Mg2 fraction
Anneal T250 C
Average C00.1
C?0.05 C?0.40
14 is ? phase.
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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.

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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
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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.

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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)
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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.