Transformation induced softening and plasticity in high entropy alloys

Abstract

Nanostructured high-entropy alloys (HEAs) with excellent properties have recently attracted extensive attention from both academia and industry in recent years. However, as a consequence of the resolution limitations of existing experimental techniques, the effects of dynamical microstructure on the strength-and-plasticity properties in nanoscale HEAs is still not well understood. Here we report the kinetics of strain induced phase transformation from face-centered cubic to body-centered cubic (fcc→bcc) in the single-crystal and nanocrystalline HEA investigated by molecular dynamics simulations. The fcc→bcc phase transformation in HEA occurs when the critical stress induced by severe lattice distortion combined with high local strain effect exceeds the stress required for the nucleation of bcc phase in the matrix. In addition, the emergence of this phase transformation is dominated by strain rate and initial orientation, where maintaining high strength in nanocrystalline HEA is achieved without the sacrifice of ductility attributable to the strain-induced fcc→bcc transformation. Interestingly, owing to the structural dissipation and strain relaxation by the volume expansion, the phase transformation agreeing with N-W relationship could convert the stress state from tensile to compressive in single-crystal HEA. Therefore, the evolution of microstructures and fcc→bcc phase fractions is studied to provide a micromechanical understanding of phase transformation induced plasticity responsible for the combination of high strength and high ductility.