Germanium network connecting precipitates in an Mg-rich Al-Mg-Ge alloy.

Precipitates in an Al-0.87Mg-0.43Ge (at.%) alloy, heat-treated for 16 h at 200 degrees C, were investigated by transmission electron microscopy and annular dark-field scanning transmission electron microscopy (ADF-STEM). Earlier studies of Al-Mg-Si-(Cu) have shown that an Si network exists within all precipitates. Here, it was investigated whether the heavier, more easily detectable germanium atom would behave similarly. The precipitates were more similar to those found in Al-Mg-Si-Cu alloys with a high fraction of disordered phases than to ternary Al-Mg-Si. All precipitate cross-sections along [001]Al imaged by ADF-STEM showed that Ge atoms arrange in triangular columns separated by approximately 0.4 nm. Along these columns, the precipitate's 0.405-nm periodicity and coherency (along its needle axis) imply a Ge plane periodicity of 0.405 nm. A germanium network, therefore, exists in all precipitates in this alloy, with a hexagonal sub-cell (SC) a = b approximately 0.4 nm, c = 0.405 nm, which is very similar to the Si network in Al-Mg-Si-(Cu). The network always appears as ordered. Disorder in a precipitate must, therefore, be caused by the other atoms in the structure between Ge atoms. One difference between precipitates of the ternary systems Al-Mg-Ge and Al-Mg-Si is the orientation of the diamond element network (SC) base in {001}Al. In Al-Mg-Ge, a <100>SC edge falls along <100>Al. This coincides with the orientation in some precipitates in quaternary Al-Mg-Si-Cu. In ternary Al-Mg-Si, one SC base is parallel with a <510>Al direction.