The effect of co-addition of Si, Ca and RE on microstructure and tensile properties of as-extruded AZ91 alloy

The microstructure and tensile properties of as-extruded AZ91 alloy with 1.0 wt% Ca, 0.5 wt% Si, 0.1 wt% La and 0.1 wt% Ce additions (was named AZXSE91100 alloy) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The addition of multiple elements results in various compounds forming in as-cast alloys. On one hand, the massive large particles are good for grain refinement of as-extruded alloy, which can promote dynamic recrystallization (DRX) during extrusion process via the mechanism of particle stimulated nucleation (PSN). On the other hand, through hot extrusion, the coarse second phases are fragmented into finer dispersed particles including lots of heat-resistant compounds, such as CaMgSi, Mg2Si, Al2RE, which play an important role in hindering dislocation motions and grain boundary sliding at elevated temperature. Accordingly, the best tensile properties at room temperature are obtained from the peak-aged as-extruded AZXSE91100 alloy, and yield stress (YS), ultimate tensile stress (UTS) and elongation (EL) are 310 MPa, 370 MPa and 8.9%, respectively. The best tensile properties at elevated temperature are obtained from as-extruded AZXSE91100 alloy, and YS, UTS and EL at 423 K reach 222 MPa, 230 MPa and 25.9%, respectively.