Constructing multi-level heterogeneous structures in multi-principal element alloys for superior cryogenic tensile properties

Abstract

The multi-level heterogeneous structures, consisting of heterogeneous grain structure, B2 nanoprecipitates, and chemical short-range ordering (CSROs), were designed and fabricated in the (CoNiV)₉₅Al₅ multi-principal element alloy. The material exhibits superior tensile properties at 298 K, with simultaneous enhancement of yield strength and uniform elongation at 77 K. Specifically, it achieves ∼1.65 GPa yield strength and ∼19 % uniform elongation at room temperature, improving to ∼2.0 GPa and 22 % under cryogenic conditions (77 K). Under identical tensile strain, the average density of geometrically necessary dislocations (GNDs) increases more significantly at 77 K than at 298 K, enhancing hetero-deformation-induced hardening and improving tensile ductility. Uniformly dispersed nano-scale B2 nanoprecipitates provide strong precipitation hardening, while dense chemical short-range ordering (CSRO) imparts precipitation-like hardening through dislocation pinning. The strengthening at each level and the synergistic hardening effects among different levels are responsible for the excellent tensile properties in the multi-level heterogeneous structures, especially at 77 K.