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)(88)Al10Ta2 (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 similar to 1.1 GPa, 1.4 GPa and 20% to similar 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 B2driven 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 132-crack tip and the crack-arresting capabilities of the FCC matrix at different temperatures. (C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study designed a duplex medium-entropy alloy with outstanding mechanical responses at low temperatures, showing that its strength and ductility vary with temperature. The alloy exhibits excellent strength-ductility synergy, especially with unique B2 cracking behavior observed at low temperatures.