Evaluating plasticity modulation quantitatively by ultra-heavy rare earth elements during annealing

This study presents a systematic investigation of the effect of the ultra-heavy rare earth element Tm on grain growth and microstructural evolution during the annealing of magnesium alloys. Mg–Zr and Mg–Tm–Zr alloys with distinct initial grain structures were fabricated by adjusting the extrusion speed. The results reveal that Tm addition promotes the formation of a stable solute segregation layer at grain boundaries, thereby suppressing rapid directional grain growth and facilitating the development of equiaxed grains; the resulting serrated rough structure extends atomic diffusion pathways and provides effective channels for slip transmission. After annealing for 60 min, the alloy achieved an elongation of 58.7%, corresponding to optimal plasticity. In situ tensile tests combined with VPSC simulations confirmed that Tm reduces the critical resolved shear stress (CRSS) for non-basal slip, which is activated in a coordinated manner during the later stages of deformation. First-principles calculations and molecular dynamics simulations revealed that the diffusion activation energy of Tm in magnesium (0.54 eV) exceeds that of Gd (0.38 eV), and the diffusion coefficient of Tm is lower; consequently, grain-boundary migration must overcome a higher energy barrier, effectively retarding recrystallisation and stabilising the segregation layer. A plasticity model comprehensively incorporating intrinsic plasticity, slip system activity, and grain orientation was established, and a multi-factor coupling expression was derived to clarify the influence of grain size and texture on the plastic response during annealing.