CHE 333 Class 11

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CHE 333 Class 11

• Precipitation Hardening

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EXAM Next Monday

• Martensite
• Heat treatment of steels
• Age hardening
• Defects

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Precipitation Hardening

• Precipitation Hardening is a NON EQUILIBRIUM heat
  treatment procedure.
• Process involves a solution heat treatment, that
  is transformation to a single phase, followed by
  a quench, again to suppress a phase
  transformation. The major difference between age
  hardening and martensite transformation is that
  the high temperature phase is retained to low
  temperatures.

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Phase Diagram for Age Hardening

• Need single phase region
• Solution treat in a zone, hold for a period
• of time so b phase dissolves.
• Upon quench to below solvus, phase
• diagram predicts ab should be
• present. However, quench suppresses
• b phase formation, so only a phase
• at room temperature.
• Conditions for age hardening.
• Alloy system
• Single Phase region and a two
• phase region
• 3. Decreasing solid solubility with
• temperature
• Slugish diffusion of solute.
• Aluminum alloys, copper alloys, steels,

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Processes
Solution treat in single phase region then
quench. At room temperature, only a phase is
present. The phase diagram predicts ab, so no b.
Composition of single phase a at low temperature
is then the average, as it cannot follow the
agtab solvus. The a has considerably more of
element b in it than the phase diagram
suggests. It is know as a Super Saturated
Solid Solution It is again a metastable phase in
some cases.
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Time Temperature History

• Thermal history for an aluminum alloy
• age hardening sequence is shown.
• After quench, an ageing treatment
• is applied. The ageing treatment
• involves heating the material into
• the two phase regions of the
• equilibrium phase diagram.
• The thermal energy allows
• atoms and vacancies to move
• around and form new phases.
• This is called the ageing sequence.
• The first phases formed are not
• the equilibrium ones.

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Phase Sequence

• The first phase to form is often called GP zones
  after Guinier and Preston. GP zones are clusters
• of solute atoms, with the same crystal structure
  as the single phase, FCC for aluminum alloys.
• Al 0.143 nm, Cu 0.128nm atomic radius

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Phase Transformations

• After the first GP zones are formed, for the
  Al-Cu system a second GP zone occurs which also
  has a
• FCC structure. This is the q phase.

For Al-Cu there is a further phase the q phase
which is an ordered structure consisting of eight
FCC unit cells to form a cube. The lattice
parameters are ab 4.04A, c 5,8A
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Properties

• The properties are a function of structure, which
  is controlled by the ageing time and temperature.
• GP zones are FCC and have the same slip system as
  the FCC a phase called underaged.
• The q phase are ordered strutures and harden
  more.
• The q phase hardens as it also an ordered
  stucture, but overages..
• Optimum properties come from the qand q phase
  combination prior to overaging.

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Structures
GP zones in Al-4Cu, 540C for 1hour 130C for 16
hours 1,000,000 mag 100 planes, 100A diam 10
atomic diameter difference growth as plates.
GP zones Al-16Ag 520 C 160C for 5 days 200,000
mag Minimal strain, spheres
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Peak and Over Aging
q in Al-4Cu 540C 130C for 24 hours 800,000
mag Plates on 100
q in Al-4Cu 540C solution treatment 160C for 72
hours 25,000 mag.