Theoretical modeling of phase boundary mediated extra tensile strength and plasticity in high entropy alloys

Abstract

High-entropy alloys (HEAs) have garnered increasing attention for their remarkable mechanical properties. However, due to the highly complex deformation mechanisms of HEAs, the current understanding of the underlying strengthening mechanisms is not yet fully developed, thereby limiting further microstructure optimization and processing. In this study, we have focused on the mechanics and theoretical modeling of Al_0.7CoCrFeNi eutectic HEAs processed by high pressure high temperature (HPHT) treatment. Our experimental results show the HPHT treatment can lead to the formation of hexagonal-like dual-phase microstructures and coherent phase boundaries with approximately doubled tensile strength and ductility. The deformation process of HPHT-treated HEAs encompasses multiple strengthening effects, including dislocation-based Taylor hardening (resulting from both statistically stored dislocations and geometrically necessary dislocations), Hall-Petch hardening, back-stress strengthening, and twinning effect. To comprehensively understand the origins of enhanced strength and ductility in HPHT-treated HEAs, we have established a constitutive theoretical model considering the multiple deformation mechanisms in the HEA samples, especially those mediated by phase boundaries, followed by crystal plasticity finite element simulations of their contributions to the mechanical properties. To ensure compatibility at large deformation, we have also proposed an algorithm for the calculation of geometrically necessary dislocations within reduced integration elements. The simulation results reveal that the enhanced coherence of phase boundaries induced by HPHT treatment is key to the activation of Taylor hardening, back-stress strengthening, and deformation twinning in the face-centered cubic (FCC) phase. Besides, due to the effective suppression of localized interface cracking, the plastic deformability of B2 phases is also enhanced, thus enabling the synergy of strength and ductility in the dual-phase system. These findings promote the understanding of strengthening mechanisms in HPHT-treated eutectic HEAs. More importantly, the constitutive model provided in this study can help theoretical investigation and quantitative analysis of metallic materials with notable interfacial effects on deformation mechanisms and mechanical properties.