Abstract: Grain boundaries (GB) can exhibit multiple phases with distinct atomic structures, significantly impacting the physical and mechanical properties of materials. Under external stimuli, transitions between different GB phases frequently occur, yet the underlying atomistic mechanism remains insufficiently understood. Here, using molecular dynamics simulation, we uncover an accelerated GB relaxation mechanism through metastable-to-equilibrium GB phase transition induced by thermal cycling in aluminum. In the [100] symmetrical tilt GB, thermal cycling between 200 and 400 K accelerates the split-kite to normal-kite phase transition. This phase transition is driven by the hydrostatic stress-induced formation of localized vacancies and the immediate insertion of nearby atoms at the GB, distinguishing it from the conventional transition mechanism involving long-range atomic diffusion in isothermal annealing process. The kinetics of GB phase transitions is systematically investigated, clarifying their dependence on critical thermal cycling parameters, including maximum temperature T_max, thermal amplitude ΔT, and cyclic number N. A saturated transition ratio of approximately 30% is also observed through thermal cycling, which exceeds that after isothermal annealing, leading to enhanced GB mobility.

Key words: Grain boundary, Thermal cycle, Phase transition, Grain boundary mobility