Competitive nucleation and growth of (Ge) and CuAl2phases subjected to different solidification conditions

Abstract

We adapted DSC and drop tube processing techniques to investigate the solidification behavior of (Ge) and CuAl2 phases in the Al–Cu–Ge ternary alloy comparatively. For the DSC sample only undercooled to 32 K with a low cooling rate, (Ge) phase is the leading nucleating phase and then CuAl2 phase nucleates. Primary (Ge) block, (Ge+CuAl2) and (Ge+Al) pseudobinary eutectics coexist in the microstructure. In the drop tube, the more competitive nucleation and growth of (Ge) and CuAl2 phases are caused primarily by high cooling rate and then secondly by large undercooling. On condition that undercooling and cooling rate exceed critical values, simultaneous nucleation of (Ge) and CuAl2 phases from the alloy melt and subsequently their independent growth contribute to the formation of anomalous (Ge+CuAl2) pseudobinary eutectic grain. Under such condition, a small amount of ternary eutectic grain forms due to the late exclusion of Al solute.