The semi-solid microstructural evolution and coarsening kinetics of AZ80-0.2Y-0.15Ca magnesium alloy

Highlights: • The coarsening rate increases first and then decreases with the reduction of solid fraction. • The critical solid fraction is 0.5 for the SIMA processed AZ80M alloy. • The modified liquid film migration model can be applied when solid fraction >0.5. • A simple liquid film migration model can be applied when solid fraction ≤ 0.5. -- Abstract: The semi-solid billets of the AZ80-0.2Y-0.15Ca magnesium alloy (AZ80M) were fabricated through the strain-induced melt activation (SIMA) process by hot extrusion and then partial remelting. The effects of isothermal temperature and soaking time on the microstructural evolution of the SIMA processed AZ80M in the semi-solid state were analyzed. The results showed that the average grain size (Deq) and the shape factor (SF) of globular grains increased with the increase of the isothermal temperature. However, the value of Deq has visibly declined trend with the isothermal temperature rising to 600 °C. The results of the coarsening kinetics of AZ80M alloy indicated that the coarsening rate (K) increases first and then decreases with the reduction of solid volume fraction (f_S) and the critical solid volume fraction (f_S0) is 0.5. The coarsening mechanism plays a dominant role in the growth behavior of the solid-grains when f_S ≥ 0.5, which can be explained by the modified liquid film migration (MLFM) model. However, the melting mechanism plays a more critical role than coarsening mechanism when f_S ≤ 0.5, which can be explained by the liquid film migration (LFM) model. In addition, the SIMA processed AZ80M alloy has a lower coarsening rate than other magnesium alloys, which most likely due to the addition of rare earth elements.