Solute-controlled strain hardening in die-cast magnesium alloys: A comparative study of AE44, AM60, and AZ91

The strain-hardening behavior of die-cast magnesium alloys remains controversial, particularly regarding the existence of an athermal hardening stage analogous to that observed in face-centered cubic metals. In this work, the tensile deformation and strain-hardening behavior of three commercial high-pressure die-cast magnesium alloys—AE44 (Mg–4Al–4RE–Mn), AM60 (Mg–6Al–Mn), and AZ91 (Mg–9Al–1Zn–Mn)—are investigated over a wide temperature range (77–473 K) and at two strain rates. By analyzing both true stress–strain curves and normalized strain-hardening rate–stress relationships, the role of solute content in controlling strain hardening is isolated while the processing route and microstructural scale is broadly constant. AE44, in which aluminum is largely removed from solid solution by rare-earth additions, exhibits smooth, monotonic Voce-type hardening across all temperatures and strain rates, with the hardening rate decreasing continuously with increasing temperature. This behavior demonstrates that plastic deformation in AE44 is thermally activated from the onset of yielding and that no athermal hardening stage is present. In contrast, AM60 and AZ91 show pronounced deviations from monotonic behavior at intermediate temperatures, including clustering of hardening curves, reduced temperature sensitivity, and, in AZ91, negative strain-rate sensitivity. These features are consistent with solute-controlled dynamic strain ageing arising from aluminum (and zinc) in solid solution. A framework is proposed in which solute effects modify the balance between dislocation storage and recovery across the full temperature range. Apparent temperature-insensitive hardening in aluminum-containing alloys is shown to result from the superposition of multiple thermally activated processes rather than from a true athermal mechanism. The results clarify the physical origin of strain hardening in die-cast magnesium alloys and provide a consistent basis for constitutive modeling and alloy design.