Table of Contents

Materials and Structure




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).

From: Higgins, "Engineering Metallurgy,"
Arnold (1983)