Achieving high strength and ductility in a heterogeneous-grain-structured CrCoNi alloy processed by cryorolling and subsequent short-annealing

摘要

Despite possessing otherwise desirable properties, the single-phase CrCoNi medium entropy alloy (MEA) with coarse grain structure generally suffer from disappointingly low strength, which severely limits its practical utility. Grain refinement can make this material several times stronger, but this benefit typically comes at a drastic loss of ductility. In this work, a thermo-mechanical processing method comprised of cryorolling and subsequent short-annealing is proposed for design of CrCoNi MEA with heterogeneous grain structure (HGS) to achieve outstanding mechanical properties. Firstly, a highly non-uniform pre-deformed nanostructure containing nanotwin bundles, nanometer-sized grains and dislocation structures regions was formed by cryorolling. Subsequent short-annealing of the as-cryorolled CrCoNi MEA at 700 °C/10 min yielded a three-level HGS with grain sizes spanning from a few nanometers up to several tens of micrometers. The as-prepared heterogeneous CrCoNi MEA exhibits ultra-high yield and tensile strengths of 1123 ± 14 MPa and 1453 ± 21 MPa, respectively, together with a large tensile ductility with a uniform elongation of 31.2%, achieving an enhanced strength-ductility combination relative to those fabricated by conventional rolling and subsequent annealing. The extraordinary combination of strength and ductility came from heterogeneous grain and residual cryo-deformation structural effect.