Recent experiments show that metallic nanocrystalline materials can be rejuvenated under the stimuli of femtosecond laser and the system‚Äôs hardness can be effectively tuned. However, a mechanistic understanding on such phenomenon remains unclear. Here we investigate the responses of a group of <100> symmetric tilt grain boundaries (STGBs) in a Cu modeling system under the driving of fast thermal cycling. A universal hysteresis behavior is observed during such process, and the GB is driven to a higher energy state after the pulsive stimulus. By employing an enhanced data-mining algorithm to analyze the annealing behavior of GBs at various thermal conditions, we are able to construct a high-resolution pixel map for the GB‚Äôs energetic evolution and show that it can be divided into an ageing regime and a rejuvenating regime over the broad energy‚Äîtemperature parameter space. We further demonstrate that the origin of ageing/rejuvenating is attributed to the energy imbalance along with the elementary hopping processes in the GB‚Äôs underlying potential energy landscape (PEL). The hereby established picture then allows us to develop a self-consistent kinetic equation to describe the GB‚Äôs energetic and mechanical responses to external stimuli, which can qualitatively explain the recent femtosecond laser experiment. The present study thus enables a deeper mechanistic understanding towards the non-equilibrium evolution of nanocrystalline materials.

Evolution of Metastable Grain Boundaries and Its Implications on Nanocrystals?€? Hardness Variation

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