heat treatments phase diagram microstructure and mechanical properties for the

Heat treatments phase diagram microstructure and

This preview shows page 56 - 68 out of 68 pages.

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.
Image of page 56
Von Mises condition For 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.
Image of page 57
Requirements for high strength materials Maximise all possible strength materials Work hardening, w Solid solution hardening, s Precipitation hardening, p Dispersion hardening, D Grain size refinement, G Intrinsic lattice friction, 0 How the combined strengthening mechanisms influence - behaviour is complicated and depends alos on the testing conditions (eg, strain rate).
Image of page 58
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.
Image of page 59
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.
Image of page 60
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.
Image of page 61
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.
Image of page 62
Image of page 63
Stress dependence of dislocation velocity
Image of page 64
Image of page 65
Consider yield drop behaviour
Image of page 66
Both m and u determine 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.
Image of page 67
Properties of bcc metals Bcc metals have very high stacking energy ( >0.2J/m 2 ) and the partial dislocations are hardly separated.
Image of page 68

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture