Microstructure and Texture Evolution During the Heat Treatment of 0.6Zn–0.6Ca–Mg Alloy Sheet Manufactured via Twin Roll Casting

Abstract

This study examined the microstructure evolution during the thermomechanical treatments of the twin roll cast strip of magnesium alloy. Magnesium alloy composed of 0.6 wt% Zn and 0.6 wt% Ca was manufactured by TRC and subsequently rolled at 370 °C, and the rolled sheets were heat treated at 400 °C for various periods of time. In the as-TRC plate, the dendritic cast structure inclined in the rolling direction was observed with the centerline segregation. The as-rolled sheet shows a mixture of fine recrystallized grains and the remaining cast structure. The annealing at 400 °C brings about the recrystallization and a gradual grain coarsening for 16 h. After the 16 h heat treatment, secondary recrystallization accompanying the abnormal grain growth was observed, which is attributed to the easier grain boundary motion resulting from the dissolution of Mg2Ca and Ca2Mg5Zn5 precipitates. The as-TRC plate shows a basal-type texture with a slight basal pole spread in the transverse direction, whereas the basal poles tilted by approximately 10° in the rolling direction are formed in the rolled sheet. During the 2 h heat treatment, the texture significantly weakens accompanying the formation of the basal poles inclined by approximately 30° in the transverse direction. These results suggest that a homogeneous microstructure with a weak texture can be obtained by the hot-rolling and heat treatments of the TRC plate, which is advantageous for an improved sheet formability, whereas the heat treatment longer than 16 h causes abnormal grain growth.