The Al–2.5C master alloy is prepared to investigate the effect of the Al4C3 particle size distribution on the refining efficiency of the AZ31 alloy. The results indicate that the Al4C3 particles are potent nucleation substrates for primary α-Mg grains. With 1.0 wt% master alloy addition, the grain size is reduced from 204 to 70 μm. The grain refining efficiency of the Al4C3 particles on the AZ31 alloy is calculated to be 0.04%–0.75%. Such low refining efficiency is mainly attributed to the size distribution of the Al4C3 particles. The particle sizes are in the range from 0.18 to 7.08 μm, and their distribution is well fitted by a log-normal function. The optimum particle size range for significant grain refinement is proposed to be around 5.0–7.08 μm in the present conditions.

The Al2Y particle reinforced AZ31 alloy was prepared by adding Al and Y elements into AZ31 melt. The average dimension of these particles increased from 6.4 µm to 14.2 µm with the increase of Y and Al contents. TEM analysis proved the good interfacial bonding between the Al2Y particle and the matrix. The Al2Y particles resulted in the continuous grain refinement in the as-cast AZ31+xAl2Y (x=2, 4, 6 wt%) alloys. The as-rolled grains of Al2Y containing AZ31 alloys were also obviously refined compared with the as-rolled AZ31 alloy, which is attributed to the particle stimulated nucleation (PSN) during the hot rolling process. The as-cast/as-rolled mechanical properties were determined by the combination effects of the grain size and the Al2Y particles. Finer grains and small Al2Y particles contributed to the simultaneous improvement in strength and elongation. Furthermore, the texture of as-rolled AZ31 alloy was also weakened with Al2Y addition.

The influence of Ca addition on the as-cast microstructure, casting fluidity and mechanical properties of the Mg-4.2Zn-1.7Ce-0.5Zr (wt.%) alloy was investigated. The results showed that the as-cast alloys consisted of α-Mg matrix, Ca-contained T-phase and Mg51Zn20 phase. Addition of 0.2 wt.%–0.6 wt.% Ca led to effective grain refinement and enhanced the fluidity of the alloys. When the content of Ca was 0.2 wt.%, the alloy exhibited the finest grain size of 35.9 μm, and the filling length was increased by approximately 55.4% compared with the quaternary alloy. The improvement of the fluidity was attributed to the grain refinement, less energy dissipation and the oxidation resistance of Ce and Ca. With an increase in Ca content, the yield strength increased gradually, whereas the ultimate tensile strength and elongation showed a decreasing tendency. Moreover, the fracture surface mode was quasi-cleavage fracture.Optical images of longitudinal sections of as-cast Mg-6Zn-xSm-0.4Zr alloys (a) x=0; (b) x=2.0; (c) x=4.0; (d) x=6.0Download high-res image (119KB)Download full-size image