Hardness and Microstructure for Ge-Containing Al-Mg-Si-Cu alloys

Abstract

Four different Al-Mg-Si-Cu-Ge alloys with 0.002--0.037 at.\% germanium were heat treated to the T6 condition and over-aged at 250$^{\circ}$C for up to 100 hours. The hardness was tested during the over-ageing and there was a quick decrease in hardness from the start of the over-ageing. There was little difference in the hardness of the four alloys, however, there seemed to be a negative correlation between hardness and Ge-content. The alloy with 0.007 at.\% Ge-content showed the greatest hardness, but there could be an error in the alloy composition. The two alloys with the most and least Ge-content were studied using a transmission electron microscope (TEM) after 24 and 100 hours of over-ageing, and the microstructure was analysed from the TEM images. No large difference in their microstructure was observed. %After 100 hours the extreme amount of over-ageing made it difficult to get clear TEM images. The precipitates in the alloy with the least Ge-content were studied after 100 hours at 250$^{\circ}$C in a scanning transmission electron microscope (STEM). The primary precipitates found were Q$'$ (and possibly Q), and some disordered Q$'$ precipitates. Energy-dispersive X-ray spectroscopy (EDX) showed no germanium in any of the precipitates.