Microstructural evolution of rheo-diecast AZ91D magnesium alloy during heat treatment

Microstructures and mechanical properties of an AZ91D magnesium alloy prepared with rheo-diecasting (RDC) were characterised in as-cast and various heat treatment conditions. The RDC alloy exhibited a unique microstructure featuring primary α-Mg globules uniformly distributed in the matrix of fine secondary α-Mg grains and β-Mg17Al12 intermetallic. Solution treatment at 413 °C caused microstructural evolution, involving four processes: dissolution of the β-phase; formation of a fine-grain structure; morphological change of the α globules into traditional grain shape; and grain coarsening. The RDC alloy exhibited a faster β-phase dissolution rate than a high-pressure diecast alloy. During aging at 216 °C, discontinuous precipitates were preferentially initiated at some of the grain boundaries and then continuous precipitates appeared in the grain interiors with accelerated age-hardening kinetics for the RDC alloy. A newly developed heat treatment, Tx, carried out at a typical temperature and time of 365 °C and 2 h, produced an improved combination of mechanical properties compared to the conventional heat treatments studied in the present work.