Gradient nanotwinned CrCoNi medium-entropy alloy with strength-ductility synergy

In this study, a high-strain rate ultra-precision machining technology named single point cubic boron nitride turning is developed to fabricate a gradient nanotwinned CrCoNi medium entropy alloy layer. The grain size of the ~ 150 µm-thick gradient layer is gradually refined from the original ~ 17 µm to ~ 25 nm in the topmost surface, exhibiting a significantly enhanced yield strength (from ~ 450 MPa to ~ 1100 MPa) and well-retained ductility of ~ 27%. High-resolution transmission electron microscope and atomistic simulations were mainly performed to unveil the size-dependent twinning mechanisms governing the gradient refinement process from the core to the topmost surface, i.e. transiting from the parallel twins segmenting ultrafine grains, twin-twin intersections refining rhombic blocks and rotating the intersected nanograins, and finally to the zero-macrostrain deformation nanotwinning in the refined nanograins. The machining process provides sufficient equivalent stress to activate the twinning partials for forming the gradient nanotwinned structure.