Grain size effects on stress-assisted grain boundary migration in polycrystalline Au thin films under tension

Grain boundaries play an important role in the deformation of ultrafine-grained and nanocrystalline metals and are key to understanding and improving their mechanical properties. This study combines orientation mapping with in situ transmission electron microscopy straining at low applied strain rates to investigate the grain boundary migration behavior as a function of microstructure in both as-fabricated and annealed ultrafine grained Au specimens. The main observation for the unannealed specimens is that small grains are consistently removed upon straining. This increased grain boundary mobility near small grains is attributed to grain size-dependent yielding, leading to a strain energy difference across the grain boundary. Molecular dynamics simulations reveal a local smoothing effect arising from coupled sliding-migration at curved grain boundaries, which could be an additional cause for smaller grains to shrink. Other microstructural features such as grain boundary misorientation, grain orientation, and Schmid factor are investigated in the annealed specimens (when grain removal is scarce), but no clear correlations are found.