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 similar to 150 mu m-thick gradient layer is gradually refined from the original similar to 17 mu m to similar to 25 nm in the topmost surface, exhibiting a significantly enhanced yield strength (from similar to 450 MPa to similar to 1100 MPa) and well-retained ductility of similar to 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. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.