Grain boundary stability dictates the rate-limiting deformation process in nanocrystalline metals

Nanocrystalline metals can exhibit both dislocation-mediated and grain boundary (GB)-mediated plasticity, making it difficult to identify the rate-controlling unit process. Here, we vary GB structural stability in Ni- and Pt-based films through deposition route and alloying, and determine the governing deformation mechanisms by coupling transient activation-parameter measurements with simultaneous in situ transmission electron microscopy observations. Despite having comparable mean grain sizes, the films with different nonequilibrium GB states separate into two reproducible mechanical regimes. In the grain-coarsening regime, rapid stress-assisted grain growth influences the plastic flow rate, and the measured activation volume is small (∼4-5 b ³ in Ni and 3-4 b ³ in Pt). In the stable-grain regime, the plastic flow rate is controlled primarily by dislocation activity, and the measured activation volume is larger (∼7-12 b ³ in Ni and 6-10 b 3 in Pt 90 Au 10 ). The measured activation volumes for the stable-grain condition agree quantitatively with atomistic predictions for GB dislocation nucleation, whereas the smaller values measured during grain coarsening are attributed to diffusion-assisted GB motion. These results show that, in addition to grain size, GB structural stability is a key variable influencing which localized process controls plastic flow in nanocrystalline face-centered cubic metals.