Mg–Al–Ca合金における平盤状C15–Al₂Ca析出相の粗大化過程

抄録

The coarsening kinetics of the fine C15–Al₂Ca Laves phase with a plate-like morphology, which precipitated within the primary α-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, l, and aging time, t, as follows; l ∝ t ^0.23. By contrast, the α/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.

Fig. 2 HRTEM image of the coherent C15–Al₂Ca precipitate observed in the Mg–5Al–1.5Ca alloy aged at 523 K for 10 h, taken with B = [11-20]_α.