Strength softening mitigation in bimodal structured metals

Polycrystalline metals become stronger with a decrease in the constitute grain size, yet strength softening takes over at the nanometer regime. Here, we find that this softening rate can be tailored and mitigated by tuning structural heterogeneity and grain size uniformity. With reducing the small grain size in bimodal structures, the strength reduction is markedly alleviated due to the enhanced strengthening of large grains. This mitigation of strength softening is mediated by promoted intragranular slip, extra dislocation storage, and reduced grain boundary sliding in bimodal structures. Our findings signifying the role of grain size non-uniformity on crystal plasticity shed light on a broad class of heterogeneous structured materials with improved strength and ductility.