heat treatments, phase diagram, microstructure and mechanical properties for the precipitation hardening of Al-Cu alloys. Your answer should identify the important requirements to achieve good precipitation hardening.
Von Mises conditionFor a crystal to be able to undergo any arbitary change of shape it must have at least 5 independent slip systems. – Von Mises Criterion.For a polycrystalline material to deform plastically without cracking or the formation of voids requires that all of the grains can deform in a compatible manner with the surrounding grains. They must therefore be capable of undergoing an arbitrary shape change.
Requirements for high strength materialsMaximise all possible strength materials•Work hardening, w•Solid solution hardening, s•Precipitation hardening, p•Dispersion hardening, D•Grain size refinement, G•Intrinsic lattice friction, 0How the combined strengthening mechanisms influence -behaviour is complicated and depends alos on the testing conditions (eg, strain rate).
a) A high solid solution composition for the matrix. Ordered solid solutions are strongest, but the concentration of solute remaining after any precipitation is usually too small for long range order.b) A high volume fraction of precipitates. To achieve this requires a large decrease in the solubility limit with decreasing temperature.
c) A fine grain size. This can be achieved by recrystallisation of highly worked materials. At high temperatures a large grain size may improve creep resistance.d) A fine dispersion of precipitates with spacing << 1 micrometer. Homogeneous precipitation through the materials.e) For high temperature applications the precipitate particles should have a low surface energy with the matrix. The surface/volume ratio for fine precipitates is large and there would be a large drawing force for particle coarsening.
Effect of strain rate and temperature upon mechanical properties= average dislocation velocity, and is a function of the applied stress. The higher is the faster the rate at which dislocations overcome obstacles. And the faster they move.
The thermal vibrations of the atoms around the dislocations assist them in overcoming the obstacles. So that the higher the temperature for the same applied stress, the faster the dislocations can move.
Stress dependence of dislocation velocity
Consider yield drop behaviour
Both m and udetermine the magnitude of the yield drop.If m is large the stress drop is small for a given increase in . If m is small the stress drop is very large.If the increase in is slow, the stress drop is also slow.
Properties of bcc metals•Bcc metals have very high stacking energy (>0.2J/m2) and the partialdislocations are hardly separated.