Promoted room-temperature deformability and strength synergy in Nb-Mo-Ta-W-Ti refractory high-entropy alloys via carbon doping

Abstract

Carbon doping offers a novel pathway to address the strength–ductility trade-off in refractory high-entropy alloys (RHEAs); however, the underlying mechanisms responsible for the improvement in plasticity remain unclear. In this study, a comparative analysis of the plastic deformation behavior of Nb-Mo-Ta-W-Ti RHEAs with varying carbon additions revealed that introducing 0.5 at.% carbon promotes dislocation cross-slip and multiplication, thereby enhancing the strain hardening capacity compared to the carbon-free alloy. Moreover, the addition of carbon facilitates the formation of MC carbides. Under high-strain conditions, some dislocations can bypass the secondary phase through the gaps between the phases, while dislocation slip can also occur within some carbides, which effectively prevents stress concentration. This results in the alloy achieving a compressive strain exceeding 60% at room temperature. Compared to the carbon-free alloy, the strength increased by 151 MPa, while the fracture strain was enhanced by more than 70%. With a further increase in carbon content to 1.0 at.%, the volume fraction of precipitated second phases increases, and the more frequent dislocation–second phase interactions hinder sustained plastic deformation. Among these precipitates, large plate-like MC carbides tend to crack under high strain conditions, accelerating alloy failure. Nevertheless, the alloy with 1.0 at.% carbon still exhibits a compressive strain of 40%, demonstrating superior compressive plasticity compared to the carbon-free counterpart. These results demonstrate that minor carbon doping provides an effective pathway to simultaneously enhance the strength and deformability of refractory high-entropy alloys by modulating dislocation behavior and second-phase evolution.

Keywords: Refractory high entropy alloy, Mechanical properties, Strain hardening, Carbon doping, compressive plasticity