Ultra-low temperature mechanical response of VCoNi medium-entropy alloy with unprecedented high strength and ductility

This work reports on the unprecedented high tensile strength and ductility of single-phase VCoNi medium-entropy alloys (MEAs) at liquid helium temperature (4.2 K) and reveals the effects of grain size on strength, strain-hardening capability, and discontinuous plastic flow (DPF) behavior at 4.2 K. The fine-grained VCoNi MEA with an average grain size of 2.2 μm exhibits exceptional yield strength of 1386 MPa and tensile stress of 1845 MPa at a high elongation of 43%. The Hall–Petch (H–P) relationship at 4.2 K for the VCoNi MEA was established for the first time, demonstrating that the yield strength enhancement with decreasing temperature primarily originates from a reduction in dislocation width, leading to an exceptionally high solid-solution strengthening contribution of 782 MPa. In addition to planar slip, nano-twinning and stacking faulting were activated at 4.2 K upon plastic deformation, contributing to a sustained strain-hardening capability. The restricted mobility of screw dislocations at 4.2 K suppresses dynamic recovery, facilitating dislocation proliferation and further enhancing strain hardening. In terms of DPF behavior, characterized by abrupt serrations in the stress-strain curves, fine-grained specimens exhibit more pronounced DPF due to the rapid accumulation of geometrically necessary dislocations. However, the maximum stress drop prior to plastic instability remains similar across all grain sizes, suggesting that dislocation density is the primary factor governing DPF behavior. These findings provide important insights into the development and mechanistic understanding of ultrastrong and ductile alloys for applications at extremely low temperatures.