Annealing hardening effect aroused by solute segregation in gradient ultrafine-grained Mg-Gd-Zr alloy

•A remarkable annealing hardening is realized in SFTed-Mg-Gd-Zr alloy under the precipitation-free condition.•Thermal stability of UFG-structured is enhanced due to the prominent Gd segregation along grain boundaries.•Prominent Gd segregation exists around the HAGBs, LAGBs, and E-SF simultaneously.