Tailoring nanoprecipitates to achieve ultrahigh strength (CoCrNi)94.5W3Ta2.5 medium-entropy alloys

Abstract

Metallic materials with high mechanical performance are in constant demand by various engineering applications. However, most conventional strengthening mechanisms inevitably suffer from fiercely sacrificing ductility, which severely limits service safety of metallic alloys, especially under various harsh circumstances. In this work, a novel ultra-strong (CoCrNi)94.5W3Ta2.5 medium entropy alloy (MEA) with dual nanoprecipitates is successfully developed. By conducting multiple thermomechanical processes, not only quantities of coherent γ" nanoprecipitates with D022 superlattice are introduced, but also massive semi-coherent η nanoprecipitates with hexagonal D024 superlattice are reconstructed with appropriate sizes and distributions, meanwhile ultrafine grains are achieved, together enabling superior strength-ductility synergies. This MEA exhibits not only high tensile strength of about 1.7 GPa and ultimate elongation of 23.8 % at room temperature, but also ultra-high tensile strength of about 2.2 GPa and still sufficient ultimate elongation of 13.2 % at cryogenic temperature. In-depth microstructure characterization indicates that the nanoprecipitates are effective barriers to dislocation motion at the early stages of plastic deformation. Intriguingly, the slip and deformation twinning could be transmitted across the phase boundaries with increasing applied strain, ensuring both strength enhancement and plasticity maintenance. At cryogenic temperature, much more complex deformation mechanisms are activated, e.g., multiple slip systems and high-density deformation twins, facilitating even better mechanical properties. The design concept of strengthening materials via tailoring dual coherent nanoprecipitates may afford a paradigm to develop advanced metallic materials with ultrahigh strength.