Effects and mechanism of Ca on refinement of AZ91D alloy

Ca addition into magnesium alloy is one of the most effective ways to improve high temperature and mechanical properties of magnesium alloys. The influence of calcium on refinement of AZ91D alloy was investigated by optical microscopy, scanning electron microscopy and thermal analysis. It was shown that increasing the amount of added Ca (≤1.0 wt.%Ca), the grain refinement tendency of AZ91D alloy intensified. The related refinement mechanism is as follows: Ca possesses a strong segregation ability in melt alloy and forms an intensive constitutional undercooling in a diffusion layer ahead of the advancing solid/liquid interface which restricts grain growth and promotes nucleation in the melt. The higher the amount of added Ca is, the higher the formation temperature of Al2Ca phase on grain boundary is, which increases the effect of constitutional undercooling, consequently intensifies the refinement tendency. Besides, with the amount of added Ca increases, the eutectic reaction extent becomes depressed lowered which results in the refinement of eutectic structure.