In situ observation of nanocluster-constrained plasticity and diffusion in nanocrystalline Cu-Ta

Solute nanoclusters play a critical role in governing deformation and mass transport in stabilized nanocrystalline immiscible alloys at elevated temperatures. Here, we investigate the high-temperature mechanical response of nanocrystalline Cu-3 at.% Ta using in situ micropillar compression. Two distinct deformation phenomena are revealed that are absent in bulk testing of the same alloy: (i) pronounced serrated flow arising from dislocation pinning by Ta-rich nanoclusters and nanoparticles, followed by collective dislocation release events, and (ii) stress-assisted surface diffusion of Cu leading to Ta-depleted, Cu-rich surface extrusions during deformation at 400 °C. While Ta-rich nanoclusters suppress grain-boundary mobility and internal diffusion within the pillar interior, they simultaneously confine both deformation and strain-induced chemical potential gradients generated during plastic flow, thereby redirecting Cu transport toward free surfaces where diffusion barriers are lowest. These results demonstrate that Ta-rich nanoclusters and nanoparticles do not merely stabilize grain size, but reconfigure the deformation and diffusion pathways under coupled stress and temperature, giving rise to intermittent plastic flow and localized surface mass transport. Solute nanoclusters play a critical role in governing deformation and mass transport in stabilized nanocrystalline immiscible alloys at elevated temperatures. Here, we investigate the high-temperature mechanical response of nanocrystalline Cu-3 at.% Ta using in situ micropillar compression.