The role of grain size distribution on the anomalous yielding of ultrafine-grained Au thin films

Ultrafine grained metals, with grain size ranging from 100 nm to 1 μm, generally exhibit increasing strength with decreasing grain size, as expected by the Hall-Petch relationship. Contrary to this trend, we observe an increase in yield strength from 310 to 520 MPa as the average grain size of Au thin films increases from 140 to 360 nm after annealing at 350 °C. Quantitative in situ TEM nanomechanical testing reveals that the grain size distribution plays a key role in determining the films’ yield strength. In the as-deposited state, a large area fraction of nanograins (<50 nm in size) adjacent to larger grains results in substantial stress-assisted grain-boundary migration and grain coalescence, leading to yielding at relatively low applied stresses. When these small grains are removed through annealing, grain boundary migration is largely suppressed, and higher stresses are required to initiate dislocation-mediated yielding, despite the coarser average grain size.