Twinning of CaMgSi phase in a cast Mg-1.0Ca-0.5Si-0.3Zr alloy

Two types of twins and an orientation domain (orientation twin) were found in CaMgSi particles formed in a Mg-Ca-Si alloy by transmission electron microscopy. The two types of twins were shown to be a (1 0 2) twin with the (1 0 2) plane serving as the twinning plane and a (0 1 1) twin with the (0 1 1) plane serving as the twinning plane. In addition to the frequently observed two-fold twin, two different three-fold twins were also observed in the (1 0 2) twinned CaMgSi particles; their crystallographic features are discussed in detail. The boundaries of both the (1 0 2) and (0 1 1) twins were found to be coincident with the twinning plane. The orientation domains were formed by one segment rotating 60o about the [1 0 0] axis relative to the other. The possible formation mechanisms of the twins and orientation domain are discussed based on a crystallographic consideration