Enhancing cryogenic tensile properties of CrCoNi medium entropy alloy via heterogeneous microstructure design

Here we fabricate a heterostructure in CrCoNi medium entropy alloy consisting of deformed substructures (the hard zone) and recrystallized ultrafine/fine grains (the soft zone). Mechanical tests reveal that the heterogeneous structure displays an exceptional combination of strength and ductility, especially at cryogenic condition, with a yield strength up to 1480 MPa and a uniform elongation of 28%. Such mechanical performance is primarily attributed to the zone boundary-dictated inhomogeneous deformation. Strain incompatibility-induced inter-zone interaction produces high internal stress near zone boundary, which leads to quick piling-up of geometrically necessary dislocations and consequently extra back stress strengthening. Cryogenic deformation further improves the internal stress, which helps to activate additional mechanical twinning and promotes the multiplication and interaction of defects, thereby contributing to high strain hardening at the large plastic stage. These findings suggest that heterostructured CrCoNi medium entropy alloy is a promising candidate for structural application over a wide temperature range. •Design heterogeneous microstructure on H/MEA is aimed to understand the coupling of multiple factors including microstructure heterogeneity, intrinsic material properties and cryogenic deformation temperatures.•The heterogeneous CrCoNi MEA presents deformed substructures (the hard zone) and recrystallized ultrafine/fine grains (the soft zone).•Heterostructured CrCoNi MEA displays an exceptional combination of yield strength (1480 MPa) and tensile ductility (28%) at 77 K.•Strain incompatibility-induced inter-zone interaction produces quick accumulation of geometrically necessary dislocations and extra strengthening.•Cryogenic temperature and microstructure heterogeneity collectively promote the deformation twinning and interaction of defects that contribute to high strain hardening.