The effect of 0.5 wt.% Ca addition on the hot compressive characteristics and processing maps of the cast and extruded Mg–3Al–1Zn alloys

Abstract

The hot compressive characteristics and processing maps of the extruded 0.5 wt.% Ca–AZ31 (Ca–AZ31) alloy and the extruded AZ31 alloy were examined and compared in the temperature range between 473 and 673 K and in the strain rate range between 10−3 and 10 s−1. The extruded Ca–AZ31 alloy had a smaller grain size (10.64 vs. 23.44 μm) and a more homogeneous microstructure than the extruded AZ31 alloy due to the presence of thermally stable Al2Ca particles in the former. The addition of 0.5 wt.% Ca increased the ignition temperature of AZ31 by 270.5 K. While the added Ca did not affect the activation energy for plastic flow, it slightly decreased the stress exponent and increased the flow stress level. Dislocation climb creep associated with stress exponents of 5–7 dominated the plastic flow of the extruded AZ31 and extruded Ca–AZ31 alloys. The power-law breakdown did not appear in either alloy under the given testing conditions. In comparison with the extruded AZ31 alloy, the extruded Ca–AZ31 alloy exhibited a poor hot workability at low temperatures below 523 K. However, there was an improved hot workability at higher temperatures due to the presence of the Al2Ca particles that promoted dynamic recrystallization (DRX) and retarded coarsening of the DRXed grains. Compared with the extruded Ca–AZ31 alloy, the cast Ca–AZ31 alloy with a much larger grain size (86.2 μm) and a coarser secondary phase structure exhibited a poor hot workability at high strain rates (10−1–100 s−1) over the entire testing temperature range due to the occurrence of unstable flow.