Grain size dependence of microscopic strain distribution in a high entropy alloy at the onset of plastic deformation

The development and distribution of microscopic shear strain have been investigated using high-resolution digital image correlation at the onset of plastic deformation in face centred cubic CoCrFeNi high entropy alloys with different grain sizes. Microscopic strain localisation increased with grain size as shown by the presence of planar slip traces at the grain level and transgranular bands at the meso-scale. Slip traces were also found within grains larger than a critical size in the fine-grained high entropy alloy, while most of smaller grains were free of slip traces, leading to a bi-modal deformation pattern. Slip traces within coarse grains were associated with the activation of multiple slip systems with lower Schmid factor values, indicating that the local stress state is different from the global one due to the grain-to-grain interactions within the polycrystalline aggregate. In addition, the CoCrFeNi high entropy alloy, -especially the fine-grained one-, exhibits substantially lower microscopic strain heterogeneity as compared to a range of conventional engineering alloys at the onset of plastic deformation.