2012 Volume 53 Issue 8 Pages 1371-1378
The effect of 0.5 mass%Ca addition on the precipitation behavior of a Mg–6.0 mass%Zn–3.0 mass%Al (ZA63) alloy was investigated. The as-cast ZA63 and Ca-added alloys were homogenized and solid solution treated at 673 K for 12 and 1 h, respectively, followed by water-quenching. Then, both alloys were aged at 343, 373, 403 and 433 K. The cast ZA63 and Ca-added alloys were mainly composed of the primary α-Mg and Mg32(Al,Zn)49 phases. The Al2Ca phase was detected co-existing in the Ca-added alloy. The Ca addition caused the grain refinement and improved hardness after the solid solution treatment compared with the ZA63 alloy even with the trace amount of 0.5 mass% Ca. The peak hardness of the Ca-added alloy aged at each temperature was higher than that of the ZA63 alloy. The microstructures of the ZA63 and Ca-added alloys produced by peak aging at 433 K consist of rod-like, blocky, plate-like and lath-like precipitates. Moreover, the precipitates in the Ca-added alloy are more finely and densely distributed than those in the ZA63 alloy even though the volume fraction of precipitates in the Ca-added alloy estimated by the electrical conductivity during aging at 433 K for longer than 64 h is smaller than that in the ZA63 alloy. The Cliff-Lorimer plots of the EDS results for the β ’1 phase in the Ca-added alloy aged at 433 K for 1000 h were examined. It was confirmed that the β ’1 phase in the Ca-added alloy contained Ca atoms, indicating that Ca atoms inside the β ’1 phase might change the structure and/or composition of precipitates.
