1. Precipitation Hardening
CHE 333 Class 8
Precipitation Hardening

2. 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.

3. 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 a+b 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,

4. Processes Solution treat in single phase region
then quench. At room temperature,
only a phase is present. The phase
diagram predicts a+b, so no b.
Composition of single phase a at low
temperature is then the average, as it
cannot follow the a>a+b 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.

5. 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.

6. 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 nm, Cu 0.128nm atomic radius

7. 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 a=b= 4.04A, c =5,8A

8. 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 q”and q’ phase combination prior to overaging.

9. Structures GP zones in Al-4%Cu, 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-16%Ag
520 C
160C for 5 days
200,000 mag
Minimal strain, spheres

10. Peak and Over Aging q” in Al-4%Cu 540C 130C for 24 hours 800,000 mag
Plates on {100}
q’ in Al-4%Cu
540C solution treatment
160C for 72 hours
25,000 mag.

11. Homework
Upon quenching a 0.4wt% carbon steel from 950C to -50C, what phases will be present and what will be the composition?
If an optimum age hardening treatment is 150C for 22 hours, what is the effect of raising the temperature to 180C. What is the effect of lengthening the time at 150C to 5 days or decreasing the time to 12 hours. Indicate the effects by diagrams?
What conditions need to be met for a material to be age hardenable?