Beyond the “source-sink” paradigm: catalytic mechanism of low-angle tilt grain boundaries in vacancy generation and annihilation

Abstract

Grain boundary (GB)-vacancy interactions play a pivotal role in tailoring material properties. However, the prevailing paradigm that “GBs function as vacancy sources/sinks” lacks robust atomic-scale evidence, thereby failing to discriminate the intrinsic mechanisms governing vacancy dynamics and leaving critical unresolved issues—most notably the GB capacity for vacancy generation/annihilation. To fill this gap, we adopt molecular dynamics simulations to systematically observe the vacancy thermal equilibration (VTE) process, wherein low-angle tilt GBs (LATGBs) are engineered as the exclusive vacancy sources and sinks. Our results unveil a remarkable phenomenon: LATGBs maintain invariant atomic structures despite continuous massive vacancy absorption and emission throughout VTE. Accordingly, we propose that LATGBs function as catalysts for vacancy generation/annihilation, with unlimited vacancy absorption/emission capacity. Furthermore, we corroborate four critical atomic-scale findings: (1) Vacancy generation/annihilation rate at edge dislocations reaches 8.62 ns⁻¹·nm⁻¹; (2) Atomic migration rate within dislocation vacant channels is at least three orders of magnitude faster than grain interior diffusion; (3) Seven of eight sites surrounding each vacant channel are catalytically active due to ultralow vacancy formation energies; (4) Compressive-tensile stress interfaces along dislocations mediate vacancy migration between active sites and grain interiors. These atomic-scale insights establish a complete catalytic cycle governing LATGB-mediated vacancy generation/annihilation.