Medium entropy alloy-induced strong size dependence in the strengthening and shear instability of nanolayered metallic composites

Medium/high entropy alloys (M/HEAs)-based nanolayered metallic composites have attracted intensive scientific interests due to their superior mechanical properties. However, the shear instability of the M/HEAs-based composites is rarely studied and the underlying mechanism remains uncovered. A combination of theoretical analysis and nano/microindentation tests was conducted to investigate the size-dependent strengthening and shear instability of Cu/CrCoNi composites. The results show that the shear banding-induced instability of the composites exhibit a clear transition from layer interfaces-mediated kinking to grain boundaries-accommodated one at a critical layer thickness of 25 nm, which apparently differs from those in traditional M/HEAs-free composites. The sudden change in the deformation mode originates from the MEA-induced size-dependent microstructure transformation, i.e., from horizontally aligned layer interfaces at large layer thicknesses to vertically aligned grain boundaries at small ones. The size-dependent strengthening and plasticity can be respectively captured by the confined layer slip model and the proposed theoretical model.