Effect of holding time on microstructure of Mg-9Al-1Si alloy during semisolid isothermal heat treatment

The effects of isothermal holding time on the semisolid microstructure of Mg-9Al-1Si (mass fraction,%) alloy were investigated. The research results indicate that the Mg-9Al-1Si alloy with non-dendritic microstructure can be produced by the semi-solid isothermal heat treatment. With holding time varying from 5 to 30 min, the volume fraction of liquid is gradually enlarged from 29.3% to 38.6%, the morphology of α-Mg grains changes from initial dendritic shape to spherical types and their average sizes increases from 41.1 to 56.1 μm. In addition, during the isothermal heat treatment, the eutectic Mg ₂Si phase changes from the initial Chinese script shape to granule and/or polygon shape in Mg-9Al-1Si alloy. The modification of Mg₂Si phase is possibly attributed to a shift of the eutectic composition of the liquid in semi-solid slurries, towards lower silicon contents with increasing Al content due to a redistribution of Al during isothermal heat treatment.