Unusual low-temperature ductility increase mediated by dislocations alone.

Abstract

Face-centered cubic (fcc) medium- and high-entropy alloys (M/HEAs) are known to exhibit enhanced strength−ductility combination at cryogenic temperatures. These superior mechanical properties have been commonly associated with the activation of additional deformation mechanisms such as stacking faults, twinning, and/or martensitic phase transformation. Here, using in situ tensile testing with neutron diffraction, we present experimental evidence of an enhanced strain hardening in VCoNi MEA, mediated solely by dislocations instead. At 15 K, VCoNi MEA shows increased yield strength, strain hardening, and fracture strain. Analysis of the in situ neutron diffraction data demonstrates that the strain hardening in this alloy is driven by faster dislocation accumulation, without the formation of stacking/twin faults or martensite. This low-temperature behavior can be rationalized by considering the Orowan equation and challenges the conventional wisdom on strength−ductility enhancement at cryogenic temperatures in fcc M/HEAs. Our study sheds light on the influence of dislocation mobility on plastic behaviors and highlights the importance of dislocation-mediated plasticity at low temperatures.