Thermally activated migrating boundary-induced plasticity: Mechanistic and predictive framework across atomistic-to-macroscopic scales

Migrating boundary-induced plasticity (MBIP) is identified as a previously under-recognized high-temperature deformation mechanism in polycrystalline metals, driven solely by grain boundary (GB) migration under sub-yield stresses. This study combines molecular dynamics simulations with high-temperature strain measurements on Ti and Fe to directly quantify MBIP and elucidate its underlying physics. MBIP strain develops rapidly during the early stage of GB migration, then continues to accumulate at a reduced but steady rate. Higher temperatures markedly accelerate GB migration, producing larger final grain sizes and greater total strain. A newly developed constitutive model, extending the Coble creep framework to incorporate GB migration velocity, accurately captures MBIP strain rates across a range of temperatures and applied stresses. The model enables the determination of the final grain size and the activation energy for vacancy formation at GBs, thus overcoming the experimental challenge of real-time stagnation tracking. The estimated activation energies are substantially lower than bulk vacancy-formation energies, confirming that MBIP is governed by vacancy diffusion along migrating GBs rather than by lattice diffusion. These findings establish MBIP as a thermally activated, GB-mediated process distinct from dislocation creep, diffusional creep, and GB sliding. The proposed MBIP model may serve as a predictive framework for assessing the long-term stability and service life of polycrystalline structural metals, particularly under the high-temperature conditions where GB migration is active.