Effects of cooling rate and pressure on microstructure and mechanical properties of sub-rapidly solidified Mg–Zn–Sn–Al–Ca alloy

Effects of cooling rate and pressure on microstructure and mechanical properties of sub-rapidly solidified Mg–6Zn–3Sn–2Al–0.2Ca alloy were investigated by OM, XRD, SEM and tensile testing. The alloy prepared by steel mould casting with no applied pressure is mainly composed of four phases i.e. α-Mg, Mg2Sn, MgZn and Mg32(Al, Zn)49, while those prepared by water-cooling steel mould casting under pressure and water-cooling copper mould casting under pressure consist of α-Mg, Mg2Sn, Mg51Zn20 and Mg32(Al, Zn)49. The higher cooling rate promotes the formation of the grain boundary compounds, while the applied pressure has an inhibiting effect on it. The alloy prepared by water-cooling copper mould casting under pressure is characteristics of the finest grain size, the smallest dendrite arm spacing and the much finer, well-distributed second phases among the five as-studied alloys, exhibiting the optimal mechanical properties, with the ultimate tensile strength, the yield strength and elongation to rupture of 224 MPa, 157 MPa and 16.1% respectively. In comparison with the alloy prepared by steel mould casting with no applied pressure, the ductile fracture characteristics of void coalescence is dominant for the alloy prepared by water-cooling copper mould casting under pressure.