Heat
Treatment
The microstructure described so far has assumed that the processes
taking place during the phase transformations are taking place under equilibrium
conditions. In many technical situations temperatures are changed rapidly
and the alloy does not have its equilibrium structure at room temperature
because the diffusion processes required do not have enough time to be completed.
The diagram illustrates this for the Al- 4 wt% Cu system. When the
sample is slowly cooled from the a-phase
solid solution (green) the final microstructure is a polycrystalline a-matrix
with a precipitated q-phase. Quenching
(blue) causes the copper to remain in solid solution and the room temperature
microstructure is polycrystalline a-phase supersaturated with substitutional
copper atoms. This configuration is metastable and reheating in a controlled
way will permit the copper to precipitate in various second phase morphologies
(yellow and brown paths). |
