Effect of Ca/Al Ratio on Microstructure and Mechanical Properties of Slow Extrusion High-Strength Mg-Al-Ca-Mn Alloys

The role of different Ca/Al mass ratios (ranging from 0.2 to 1.4) in modifying the microstructure, phase evolution and the mechanical properties of Mg-Al-Ca-Mn alloys was investigated. The primary second phase evolves from blocky Mg17Al12 to semi-continuous Al2Ca and then to the reticular (Mg, Al)2Ca phase and Mg2Ca phase when the Ca/Al ratio rises. Meanwhile, the average grain sizes decrease from 1.5 to 0.5 μm. Therefore, the fine grain strengthening effect of the as-extruded alloy is enhanced. The broken second phases exhibit a banding distribution along the extrusion direction, and their dimension is approximately 3 ~ 5 μm. These second phases not only refined grains but also hinder dislocation migration, resulting in dislocation accumulation and increase in dislocation density, which strengthens the alloy further. Furthermore, increasing the Ca/Al ratio causes the recrystallization degree decrease, which means more undynamically recrystallized grains with numerous residual dislocations and higher basal texture strengthening. The optimum grain orientation changes from < 11–20 > to < 10–10 > as the Ca/Al ratio increases, and the Schmid factor of the as-extruded AXM2306 alloy is smaller than that of the AXM4106 alloy. The tensile yield strength and ultimate tensile strength of as-extruded Mg-2.0Al-2.8Ca-0.6Mn alloy were ~ 401.8 and ~ 426.2 MPa, respectively.