Strengthening Micromechanisms in Cold-Chamber High-Pressure Die-Cast Mg-Al Alloys

The contributions from grain boundary, solid solution, and dispersion strengthening to the yield strength of cast-to-shape specimens were calculated for seven binary alloys with compositions ranging from very dilute (0.5 mass pct Al) to concentrated (12 mass pct Al). Experimentally and theoretically determined parameters were used to explicitly account for the different microstructures at the skin and core regions of specimens’ cross sections. Microhardness maps were used to identify the specimens’ skin. The specimens’ strength was calculated as the weighted addition of the respective strengths of skin and core. The calculated strengths reproduced well the experimental values for the dilute alloys but underestimated the strength of the most concentrated alloys by as much as ~35 MPa. It is argued that the presence of the percolating network of Mg₁₇Al₁₂ eutectic intermetallic, particularly in the skin region, in conjunction with highly efficient dispersion hardening due to the convoluted shape of the intermetallics, accounts for the shortfall in the calculated strength.