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• The Fe-Cr-Ni-Mn-N austenitic stainless steel exhibits an exceptional mechanical property at 4.2 K with a high yield strength of 1.5 GPa. • The deformation nanotwinning, HCP ε-martensite transformation, dislocation slip with L -C locks and stacking fault formation which provided a marked contribution to the Fe-Cr-Ni-Mn-N alloy's ultrahigh strain hardening rate and outstanding strength at liquid helium temperature 4.2 K. • The Fe-Cr-Ni-Mn-N austenitic stainless steel displays outstanding cryogenic mechanical properties through a progressive synergy of deformation mechanisms leading to exceptional strength which provides a new insight into the commercialized development of high-performance alloys for cryogenic applications. We report the mechanical performance and microstructural characteristics of a Fe-Cr-Ni-Mn-N alloy at cryogenic temperatures. The exceptionally high yield strength of 1.5 GPa combined with a high strain-hardening rate and no deterioration in ductility at 4.2 K was displayed. The evolution of deformation microstructure was examined using electron backscatter diffraction (EBSD), the transmission Kikuchi diffraction (TKD), high-resolution transmission electron microscopy (HRTEM), and aberration-corrected scanning TEM (STEM). The deformation microstructure mainly consisted of dislocation slip with L -C locks, {111} stacking fault formation, {111} deformation nanotwinning, and FCC → HCP shear transformation at 4.2 K. The occurrence of FCC-HCP shear transformation inside/near {111} twins to form γ-γ tw -ε dual-phase structure induces a dynamic Hall-Petch effect that promotes the strain-hardening rate and enhances the strength-ductility combination. We believe that this alloy displays outstanding damage tolerance through a progressive synergy of deformation mechanisms leading to exceptional strength which provides a new insight into the commercialized development of high-performance alloys for cryogenic applications. [Display omitted] [ABSTRACT FROM AUTHOR]

• Dissimilar laser welding of CrMnFeCoNi high entropy and 316N austenitic stainless steel was performed. • The ultimate strength of the welded joints is 510 MPa at room temperature and 820 MPa at 77 K, which can reach ∼90 % of the base materials at both room and cryogenic temperatures. • The dominant deformation mechanism was mainly twinning during the plastic deformation which accounting for the higher tensile strength, especially at cryogenic temperature. • The sound welded joints between CrMnFeCoNi high entropy and 316N austenitic stainless steel can be considered for cryogenic applications. The microstructure and mechanical properties of dissimilar laser beam welded joint between CrMnFeCoNi alloy and 316LN stainless steel was investigated. The results showed that the defect-free dissimilar joint was obtained by laser beam welding. The ultimate strength of the welded joints can reach ∼90% of the base materials at both room and cryogenic temperatures. The deformation substructure mainly consisted of planar dislocation, the stacking faults and the dissociation of stacking faults into nanotwins. The volume fraction of the nanotwins was increasing at cryogenic temperature. The hardness fluctuates greatly in welded joint and the lowest hardness was located at fusion zone near the fusion line. The fracture of the welded joint was located at the fusion zone in consistence with the lowest hardness area. It is mainly attributed to the coarse grain and stress concentration at this area. [Display omitted] [ABSTRACT FROM AUTHOR]