The morphology of precipitates in an AlGe Alloy—I. experimental observations

Abstract

The morphology and crystallography of Ge precipitates formed in a quenched and aged Al-4 wt% Ge alloy have been characterized using transmission electron microscopy and electron microdiffraction. Five different precipitate forms have been distinguished in a sample aged 3 h at 200°C: (i) triangular plates on {111} Al, (ii) laths parallel 〈100〈 Al, (iii) rods parallel 〈110〉 Al, (iv) hexagonal plates on {111} Al, and (v) small regular tetrahedra with {111} Al facets. The cube-cube orientation relationship, (111) Ge//( 1 11) Al and [110] Ge//[110] Al, has been assocaited with three separate forms of precipitate: the triangular and hexagonal plates on ( 111) Al andthe tetrahedra. A second type of twinned hexagonal plate has been observed on ( 111) Al with an orientation relationships deviating from cube-cube by a rotation of 30° about the habit plane normal, [ 111] Ge . The 〈100〉 Al laths have [110] Ge//[100] Al along their long axis and ( 111) Ge //(001) Al ; the long axis is an approximately invariant line. Rods parallel to 〈110〉 Al have in common a rod axis [110] Ge//[110] Al, but several orientation relationships have been detected, differing by a rotation about this axis. A summary is presented of all precipitate forms and associated orientation relationships observed in the AlGe system.