Effect of pre-aging on the evolution of Ge precipitates in an Al-1.8 at.% Ge alloy

The shape distribution of Ge precipitates in a quenched and aged Al-1.8 at.% Ge alloy was characterized by transmission electron microscopy. In agreement with previous investigations, at least six different precipitate morphologies were distinguished : tetrahedra, octahedra and triangular plates all with the cube-cube orientation relationship ; as well as laths along Al directions, laths along (110) Al directions and plates on {100} Al planes, with three different orientation relationships, respectively. The distribution of precipitate shapes was found to depend strongly on the heat treatment schedule. After quenching to 0°C, samples aged at 200 °C exhibited predominantly triangular plates and tetrahedra and were insensitive to pre-aging at room temperature. By comparison, samples aged at 250 and especially at 300 °C contained mainly lath-shaped precipitates after short pre-aging times, but a sharply increasing fraction of tetrahedra, octahedra and triangular plates for longer pre-aging times. When the temperature of the quench bath was raised from 0 to 40 °C, samples aged at 200 °C remained unchanged whereas samples aged at 250 and 300 °C retained only lath shaped precipitates. Quenching directly to the three aging temperatures changed the precipitate distribution to a coarse array of large {100} plates.