CHE 333 Class 8

Precipitation Hardening

Precipitation HardeningPrecipitation 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.

Phase Diagram for Age Hardening

Need single phase region

Solution treat in zone, hold for a period

of time so phase dissolves.

Upon quench to below solvus, phase

diagram predicts + should be

present. However, quench suppresses

phase formation, so only phase

at room temperature.

Conditions for age hardening.

1. Alloy system

2. Single Phase region and a two

phase region

3. Decreasing solid solubility with

temperature

4. Slugish diffusion of solute.

Aluminum alloys, copper alloys, steels,

Processes

Solution treat in single phase regionthen quench. At room temperature,only a phase is present. The phasediagram predicts +, so no . Composition of single phase a at lowtemperature is then the average, as it cannot follow the >+ solvus.The a has considerably more of elementb in it than the phase diagram suggests.It is know as a “Super Saturated SolidSolution” It is again a metastable phasein some cases.

Time Temperature HistoryThermal 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.

Phase SequenceThe 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

Phase TransformationsAfter 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 ” phase.

For Al-Cu there is a further phase the ’ phase which is an ordered structure consisting of eight FCCunit cells to form a cube. The lattice parameters are a=b= 4.04A, c =5,8A

PropertiesThe 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 phase – called underaged.

The ” phase are ordered strutures and harden more.

The ’ phase hardens as it also an ordered stucture, but overages..

Optimum properties come from the ”and ’ phase combination prior to overaging.

StructuresGP zones in Al-4%Cu, 540C for 1hour130C for 16 hours1,000,000 mag{100} planes, 100A diam10% atomic diameter differencegrowth as plates.

GP zones Al-16%Ag520 C160C for 5 days200,000 magMinimal strain, spheres

Peak and Over Aging

” in Al-4%Cu540C130C for 24 hours800,000 magPlates on {100}

’ in Al-4%Cu540C solution treatment160C for 72 hours25,000 mag.

Homework

1. Upon quenching a 0.4wt% carbon steel from 950C to -50C, what phases will be present and what will be the composition?

1. 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?

1. What conditions need to be met for a material to be age hardenable?