Massive Mo alloying for enhanced strength-ductility at liquid helium temperature in single-phase complex concentrated alloy

초록

Face-centered-cubic (FCC) structured complex concentrated alloys (CCAs) have attracted considerable attention as structural materials for cryogenic applications owing to their promising intrinsic features such as substantial lattice distortion and strain-hardening capability. The incorporation of refractory elements can further enhance these advantages, yet it often leads to the formation of brittle intermetallic compounds. In this study, to fully exploit the benefits of refractory elements, we investigated the ultra-low temperature mechanical behavior of the Co42.5Ni42.5Mo15 alloy, maintaining a single-phase FCC structure despite a high concentration of Mo. The alloy exhibits excellent mechanical performance at liquid helium temperature (4.2 K), achieving a yield strength of 1172 MPa, a tensile strength of 1787 MPa, and a uniform elongation of 47.5%. This work experimentally quantifies the negligible temperature sensitivity of the stacking fault energy and establishes links among strengthening, deformation mechanisms, and macroscopic performance. Combined with limited thermal vibration at ultra-low temperature, the large atomic-size mismatch between Mo and other constituent elements introduces strong lattice distortion, which leads to substantial solid-solution strengthening. The elevated flow stress and limited cross-slip at 4.2 K cooperatively activate microband-induced plasticity and promote the formation of refined nano-twins. Atom-probe tomography reveals Mo segregation at grain boundaries, which facilitates grain-boundary-nucleated twinning. These findings highlight that the incorporation of a high fraction of Mo into multi-principal FCC solid solutions offers a promising pathway to achieve outstanding mechanical properties at ultra-low temperatures.

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