Abstract: In this study, a remarkable annealing hardening effect was detected in gradient ultrafine-grained (UFG) Mg-0.32Gd-0.11Zr (at.%) alloy sheet fabricated by sliding friction treatment (SFT). Under the precipitation-free condition, the annealed UFG structure shows an obvious hardness increment from 1.40 GPa to 1.89 GPa after 200 °C heating for 12 h, which exhibits a much higher hardening response than the annealed coarse-grained (CG) structure. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping reveal prominent segregation of solute Gd atoms along grain boundaries, which endows the UFG structure with excellent grain boundary stability. Moreover, Gd segregation is also found around the extrinsic stacking fault (E-SFs) and the low-angle grain boundaries composed of edge dislocations. The large-scale solute partitioning provides a significant segregation hardening effect, which completely resists the softening effect aroused by the grain coarsening and dislocation annihilation. This work realizes a good combination of surface mechanical processing for fabricating UFGs and subsequent heat treatment, which earns desirable segregation hardening effects.

Key words: Ultrafine grains, Segregation, Annealing hardening