Strong and ductile CrCoNi medium-entropy alloy via dispersed heterostructure
Yanfei Wang, Xiaolong Ma, Fengjiao Guo, Zhifu Zhao, Chongxiang Huang, Yuntian Zhu, Yueguang Wei
Abstract
Here we advocate the strategic design of dispersed heterostructure to retain ductility and toughness in high-strength metals, using the equiatomic CrCoNi alloy as an example. Dispersed heterostructure, with nanograins and/or ultrafine grains (the hard zone) dispersed around micrometer-sized grain (the soft zone), is fabricated by cold-rolling followed by sequential flash-annealing at increasing temperatures. It displays a decent uniform elongation of ∼ 20 % and an exceptional strain energy density limit up to ∼ 240 mJ/mm3 at the strength level of ∼ 1.2 GPa, which is unattainable by its homogeneous as well as clustered heterogeneous counterparts. Grain size-dependent heterogeneous deformation evokes inter-zone interactions, which induce strain partitioning, activate additional mechanical twinning and promote developments of dislocation gradient and long-range internal stress near zone boundary sequentially, imparting a multistage work hardening with extraordinary strain hardening rate up-turn followed by slow attenuation. Dispersed heterostructure provides a higher density of zone boundary, ensuring more extensive inter-zone interaction and thus maximizing the extraordinary strain hardening to improve ductility.