Critical impacts of thermodynamic instability and short-range order on deformation mechanisms of VCoNi medium-entropy alloy

Abstract

Short-range order (SRO) has made a splash in the recent research on multi-principal element alloys (MPEAs), among which the equiatomic VCoNi alloy was regarded as the most potential prototype. However, in addition to disputes over SRO, there is still a lack of agreements on the phase stability and deformation mechanism of the VCoNi alloy, which should be addressed before the SRO can be independently analyzed. To this end, microstructural evolutions with increasing long-term annealing temperatures (from 700 to 1,200 °C) were first inspected in detail to determine critical temperatures for phase transitions unambiguously. Our results revealed that the VCoNi alloy was still dominated by the face-centered-cubic (FCC) structure with a minor κ phase at 900 °C and became a single FCC solid solution at above 910 °C. Subsequently, by carefully examining dislocation configurations and stacking fault (SF) widths in the FCC phases annealed at 900 °C and 1,200 °C, significant variations in local stacking fault energy (SFE) were unveiled, and the overall SFE decreased with increasing annealing temperature. Combined with the state-of-the-art density function theory (DFT)-based lattice Monte Carlo (MC) simulation, we demonstrated how the rise/decline in SFE can be explained by the greater/lower degree of SRO. This research would not only offer new perspectives for recent controversies over phase stability, deformation mechanism, and SRO characterization technique using electron diffraction but also shed light on the intrinsic relationship between SRO and local stacking fault energy of MPEAs.