The microstructure, fracture mechanism and their correlation with the mechanical properties of as-cast Mg-Nd-Zn-Zr alloy under the effect of cooling rate

The microstructure, fracture mechanism and their correlation with the mechanical properties of as-cast Mg-Nd-Zn-Zr alloy were studied, in which the effect of cooling rate on the microstructure and the mechanical properties were taken into account. The results showed that the microstructure of the alloy is composed of primary phase (α-Mg) and eutectic compounds. With the increase of the cooling rate in the range of 0.4–2.4 °C/s, the grain size of the alloy decreases from 66 μm to 44 μm, while the volume fraction of the eutectic compounds increases from 3% to 6.1%. The decrease of the grain size tends to improve the yield strength, ultimate tensile strength and elongation of the alloy. The eutectic compounds have complicated effects on the mechanical properties. It is found that the increase of the volume fraction of the eutectic compounds leads to the decrease of the yield strength while the network-like eutectic compounds formed at higher cooling rates decreases the ultimate tensile strength. The fracture pattern of the alloy changes from intergranular to quasi-cleavage fracture and then to a mixed fracture of intergranular and transgranular fracture with the increase of cooling rate.