Low-temperature plastic deformation of microcrystalline AE21 alloy

The deformation behaviour of the Ex-ECAP AE21 alloy (Mg-1.7Al-1.5RE) was examined over a wide temperature range (0.5-400 K) to clarify the mechanisms governing its mechanical response. The experimentally investigated AE21 data in this study were directly compared with previously obtained and published results by the authors for AZ31 and AE42 alloys in order to identify and interpret the RE/Al‑related trend. The effect of deformation on microstructure was analysed in detail using a scanning electron microscope, and porosity was characterised by X-ray micro-computed tomography. The alloy exhibits distinct temperature-dependent deformation characteristics: the yield stress σ 0.2 gradually decreases from 180 MPa at 0.5 K to 150 MPa at 290 K, followed by a sharp drop to 60 MPa at 400 K. At cryogenic temperatures (0.5-210 K), deformation is characterised by a high initial strain-hardening rate and limited ductility, while at 290 K moderate hardening and greater deformability are observed. At 400 K, the alloy becomes more ductile but exhibits negligible hardening. Pronounced low-temperature serrated flow occurs at 0.5-4.2 K, with increased RE/Al ratio enhancing the serration frequency but reducing its amplitude, indicating finer barriers to dislocation avalanches. Thermal activation analysis revealed a transition in deformation mechanisms between 77 and 400 K. In the range of 77-210 K, the activation energy ( H 0 = 0.25 ± 0.05 eV) suggests dislocation motion governed by interactions with impurity atoms, while at higher temperatures, recovery and non-basal slip become dominant. The findings demonstrate that chemical composition critically influences the balance between basal slip, twinning, and recovery, defining the overall strength-ductility relationship of Ex-ECAP Mg-Al-RE alloys. • Ex-ECAP AE21 alloy studied across 0.5–400 K, including unique cryogenic conditions. • Strength and ductility vary non-monotonically with temperature due to mechanism transitions. • Serrated flow observed at 0.5 and 4.2 K; grain size and intermetallics control dislocation avalanches. • Three thermally activated processes identified in the 77–400 K temperature range. • Activation energy H₀ ≈ 0.25 eV (77–210 K) linked to solute–dislocation interactions. • Basal slip dominates near room temperature; recovery processes intensify at 400 K.