Small-scale mechanical properties and deformation mechanisms of a laser welded CrCoNi medium-entropy alloy joint

Abstract

Attributed to deformation twins, laser welded CrCoNi medium-entropy alloy joints exhibit best combination of strength and ductility among the joints of Cantor alloy and its derivatives. However, the nano/microscale mechanical properties and associated deformation mechanisms of the welded CrCoNi alloy joint remain mysterious. Here, we fabricated cylindric pillars with diameter from 500 nm to 2000 nm in a single crystal of the fusion zone, and then compressed these pillars in a nano-indenter. Results showed that the yield stress and flow stress of these pillars were size-dependent, with a “smaller is stronger” trend. Via examination and analysis of the deformed microstructure using electron backscattered diffraction, transmission electron microscope, and large-scale atomistic simulations, we found that deformation mechanism in the single crystal at nano/microscales was the formation of nanoscale stacking-fault networks. Large-scale atomistic simulations further revealed that dislocation-based mechanisms, such as entanglements, and annihilation from the free surface of the pillar, dominated the compressive deformation. Finally, a dislocation-based theoretical model was successfully utilized to predict the critical resolved shear stress and the size effect, which matched well with our experimental results.