On the grain size dependence of competing deformation mechanisms in a CrCoNi medium entropy alloy

Grain refinement is an effective approach to tailoring the deformation mechanism and thus the mechanical properties of a material. In this study, the deformation behaviors of the CrCoNi medium entropy alloy with different grain sizes were investigated by in-situ neutron diffraction and transmission electron microscopy observations. For the coarse-grained CrCoNi alloy, the initial plastic deformation was driven by dislocation slip, while stacking faults and twinning became activated at a later stage. The critical stress for stacking faults in coarse-grained samples exhibited a weaker grain size sensitivity than that of dislocation slip. As a result, below a grain size threshold, the critical stress needed to trigger the dislocation slip caught up with that of stacking faults. It was shown that for CrCoNi, when the grain size was below ∼1.5 μm, all the three deformation modes, namely, dislocation slip, stacking faults, and twinning, were activated simultaneously, which resulted in a grain-size dependence deviating from the Hall-Petch relationship due to the extra strengthening at yielding. The contributions from dislocations and planar faults to the work hardening were quantified, which revealed a dominant role of dislocations in the hardening behaviors of the CrCoNi alloy. However, the relative magnitude of these contributions changed as the grain size was reduced, with the contribution from planar faults, especially the stacking faults, becoming increasingly significant.