Composition: Al-4%Cu.

Processing: Sand casting. Annealing (525 °C, 16 h), Water quenching and Aging (260 °C, 70 h).

Etching: 10-15 s inmersion in mixture of acids (HCl+HNO₃+HF).

Aging at 260°C corresponds to a point in the biphasic θ+κ field (see Al-Cu phase diagram). The high temperature assists the diffusion of Cu atoms that were supersaturing the κ solid solution in the X14 specimen, resulting in precipitation of several metastable phases (GP zones, θ'', θ') and, finally, the equilibrium θ (CuAl₂) phase, which is formed in the bulk of the material with a Widmanstätten-type structure that decreases the brightness of the equiaxial grains (see the macrograph).

These aging conditions do not follow the industrial practice, where shorter times and lower temperatures are typically used. The main difference is that the precipitate that forms in this specimen is large enough to be visible by optical micrograph. Commercial alloys show much smaller precipitates that can only be resolved by using an electronic microscope.

Al-Cu diagram

This work is licensed under a Creative Commons Attribution-NonCommercpial-ShareAlike 4.0 International