The morphology of precipitates in an AlGe Alloy-I. experimental observations

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//(111)_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 AlGe system.