Precipitation Hardening

Question 1

Simply and concisely:

Describe what precipitation hardening (age hardening).

Answer 1

Question 2

Simply:

What is the three stages of precipitation hardening?

Answer 2

1. Solution treat (at the highest temp) to form a single phase solid.
2. Quench (using water or oil) leaving a supersaturated single phase solid. The solute atoms cannot diffuse out to the grain boundaries and form percipitates. This is because the temp drops to much.
3. Precipitation harden at a lower temp growing the precipitates.

Question 3

What is required for an alloy to undergo precipitation hardening?

Answer 3

The alloy must be able to be a solid single phase at a certain temperature and have a solvus line at a lower temperature. This is seen in the shaded area.

Question 4

In vivid detail:

how does precipiration hardening work to strengthen a material?

Answer 4

• First, look for a region with a single phase, where you can cool down to cross a solvus line. Ideally the line before hitting the eutectic or eutectoid point is further to the right for more ranges of alloys.
• If the temperature is allowed to cool under equilibrium conditions over time, the second phase will form on the grain boundary. Instead, you often quench the material to take out all the energy of the material, cooling it really rapidly, this stops diffusion practically. This locks the solute atoms in the material.
• Then we heat it up back to a lower temp than before but higher. This then allows the precipitates to grow under diffusion. This is the precipitation heat treatment temp. The amount of time and temp will affect diffusion and thus the ageing and coherent or incoherent microstructure formation.
• The reason it doesn’t just continue to grow the precipitates on the grain boundary as before is that when locking those solute atoms in the lattice structure, the solute atoms just want to get as close to the other solute atoms as possible, not caring if it’s along the grain boundaries, just to form the second phase. This takes two phases to fulfil its desired phase.
• The formation of these precipitates are called GP zones and they start coherent and as time goes on and more diffusion and energy occurs they make a more suitable microstructure closer to how they want, inchorent.

Question 5

What factors affect precipiration hardening (or ageing)?

Answer 5

As it’s a diffusion based process, the arhenious equation is a big factor. Therefore the biggest two factors are:

• Temperature
• Time

Question 6

How does ageing a material change it’s strength?

This is to do with precipitation hardening

Answer 6

• Initally for an alloy, it will have its main strength from being supersaturated, where there is just the alloying affect improving its strength. By heating it we drop some strength as the atoms continue to re-arrange to be more uniformed and de-stressed.
• Then Coherency stress takes over, as the precipirates start to grow larger into another phase within the middle of the grain, best place to prevent dislocations from moving, unlike the grain boundary. The precipitates start by clumping together and alligning to a microstructure that is coherent to the rest of the structure but different in phase. This improves strength. These precipitates can be sheared by dislocations though, but this still inhibits dislocation motion.
• Then Bowing stress takes over as we get to the peak strength of the material, where the precipitates are larger and incoherent to the rest of the microstructure. These precipitates can’t be sheared and so the dislocations have to bow round the precipitates reducing their motion.
• However, overaging means they grow so large there becomes fewer of them, thus bowing stress reduces. Overaging is useful as it means that no more changes in the materials strength will occur if its required in a high temp environment.

Question 7

What are some pros and cons of precipitation hardening?

Answer 7