Microstructures and mechanical properties of AZ91D alloys with Y addition

As-cast AZ91D + xY (x = 0, 0.5, 1, 2 mass%) magnesium alloys were prepared by a simple addition of Y-rich interalloy to AZ91D alloy. Influences of Y on the microstructures and tensile properties of AZ91D alloy were investigated. Influences of extrusion temperature on the microstructure and tensile properties of AZ91D + 2Y alloy were investigated. Moreover, microstructures and the resulting tensile of extruded AZ91D + 2Y alloy were also investigated under different heat-treatment conditions. The results show that the addition of Y to AZ91D alloy refines primary α-Mg matrix and β-Mg17Al12 phase. The addition of 2 mass%Y forms rod-shaped Al2Y phase. After hot extrusion, the grain sizes of AZ91D and AZ91D + 2Y alloys are greatly refined through dynamic recrystallisation. At extrusion temperature below 300 °C, the microstructure of AZ91D + 2Y alloy is characterized by fine and equiaxed grains, which leads to a high tensile strength but relatively low elongation. With increasing extrusion temperature up to 400 °C, more equiaxed grains with larger size are obtained. A low strength and large elongation are achieved compared with those extruded at low temperature. The best combination of both high strength and large elongation takes place at 325 °C. As for extruded AZ91D + 2Y alloy, solution treatment at 413 °C (T4) causes parts of broken β-Mg17Al12 phase to be dissolved into α-Mg matrix and the deterioration of tensile properties. During aging at 216 °C (T5), discontinuous precipitates appear at grain boundaries. Solid solution followed by aging (T6) treatment increases ultimate tensile strength but decreases yield strength and elongation.