The B2 phase-driven microstructural heterogeneities and twinning enable ultrahigh cryogenic strength and large ductility in NiCoCr-based medium-entropy alloy

In this work, we designed heterogeneous (NiCoCr) ₈₈Al ₁₀Ta ₂ (at.%) duplex medium-entropy alloy (MEA) comprising the face-centered-cubic (FCC) matrix and the B2 phase towards outstanding cryogenically mechanical responses. The mechanical properties of this MEA are strongly temperature-dependent, i.e. , when temperatures decrease from 298 down to 77 K, the yield strength, ultimate strength and tensile ductility are increased from ∼1.1 GPa, 1.4 GPa and 20% to ∼1.5 GPa, 1.8 GPa and 25%, respectively. In the studied temperature-range, the plasticity mechanisms, involving the cooperation of planar slip, stacking fault and twinning, are temperature-independent for this MEA, whereas the low temperature (or high stress) promotes stacking fault and twinning at initial plastic deformation assisted by the significant B2-driven hardening. Multi-strengthening mechanisms coupled with the superior strain hardening capability render this alloy excellent strength-ductility synergy, especially for the 77 K stretched MEA involving temperature-dependent B2 cracking behavior. This unique B2 cracking behavior was uncovered for the first time in multi-component alloys, rationalized in terms of the competition between the intensity of the stress field ahead of the B2-crack tip and the crack-arresting capabilities of the FCC matrix at different temperatures.