Insights into the effect of different thermomechanical processing on the microstructure, phases, texture and tensile properties in Mg-0.9Al-0.6Mn-0.2Si-0.1Ca alloy

Abstract

In this study, a Mg-0.9Al-0.6Mn-0.2Si-0.1Ca alloy (in as-cast form) has been subjected to homogenization treatment followed by different thermomechanical processing, namely hard plate hot forging (HPHF) and hot rolling (HR), at a fixed working temperature (723 K). The HPHF specimen has developed a bimodal grain size distribution comprising of fine recrystallized grains and coarse deformed (non-recrystallized) grains due to incomplete dynamic recrystallization. Most of these grains have basal orientations along the forging plane, leading to a strong basal fiber texture with low Schmid factor and low Taylor factor. The HR specimen, on the contrary, has developed a uniform grain size distribution consisting of mostly fine grains, with twins in some of the relatively coarser grains. Although a large number of grains in the HR specimen have basal orientations, some of the very fine newly recrystallized grains and the twinned regions attain non-basal orientations, leading to an overall weaker basal fiber texture. The Mg17Al12 phase from the as cast specimen has dissolved during both the thermomechanical treatments owing to its low melting point. Contrarily, the thermally stable Al8Mn5 precipitates are present in all the specimens. The finer grain size and higher dislocation density, along with the finer nano-sized Al8Mn5 precipitates, have resulted in greater strengthening in the HR specimen, as compared to the HPHF specimen. Furthermore, the weaker basal fiber texture, higher Schmid factor and higher Taylor factor have led to much better ductility as well as work hardening response in the HR specimen, than in the HPHF condition.