Effects of Heat Treatments on Microstructures and Precipitation Behaviour of Mg94Y4Zn2 Extruded Alloy

Microstructures and precipitation behaviours of Mg94Y4Zn2 (at.%) extruded alloy during solution treatment and ageing processes were investigated. Three major phases were observed in the as-cast Mg94Zn2Y4 alloy: α-Mg, block shaped 18R long period stacking ordered (LPSO) phase and Mg24Y5 cuboid particles. After homogenization and extrusion, the block shaped LPSO phase changed into plate-like shape aligned along the direction of extrusion. During solution treatment, a small fraction of LPSO phase was transformed from 18R structure to 14H type. The nano-scale β′ phase with its close-packed planes being perpendicular to the direction of both α-Mg and LPSO structure was precipitated at ageing stage. The coexistence of β′ and LPSO phase contributes to the strengthening of the alloy, with microhardness for the matrix and LPSO structures reaching 145.8 and 155.0 HV, respectively.