Coarsening of Plate-Like C15-Al2Ca Precipitates in a Mg-Al-Ca Alloy

The coarsening kinetics of the fine C15-Al2Ca Laves phase with a plate like morphology, which precipitated within the primary a Mg grains, were investigated for the Mg-5Al-1.5Ca alloy aged at 523 K. The Al2Ca precipitate coarsened with remaining coherency in the aging time below 300 h, and the quantitative relationship was obtained between precipitate length. I, and aging time, t, as follows; l proportional to t(0.23). By contrast, the alpha/C15 coherent interface turned to the semi coherent one by the introduction of misfit dislocations on the planar surface of the precipitates in the aging time above 300 h, which resulted in the promotion of Al2Ca coarsening. The Al2Ca phase was deduced to precipitate through a nucleation and growth mechanism rather than spinodal decomposition, and its coarsening was explained using the terrace ledge kink mechanism. The aspect ratio of Al2Ca precipitates was predominantly decided by the aging temperature, and it decreased at higher aging temperatures.