Stress gradient versus strain gradient in polycrystalline high entropy alloy revealed by crystal plasticity finite element simulation

Abstract

Gradient structures (GS) play a crucial role in achieving a balance between strength and plasticity in metals and alloys. However, there is still a lack of understanding of the mechanisms that maintain a plasticity gradient to prevent the premature failure of fine grains in GS materials. In this work, by incorporating experimental data and the Hall-Petch relationship, we develop a size-dependent crystal plasticity model to investigate the deformation mechanisms for enhancing the strength and plasticity in polycrystalline high entropy alloys. The simulations of the GS model align well with the experimental results, exhibiting strong strain and stress gradients to improve the mechanical properties. Under the conditions of significant deformation incompatibility, the strain gradient predominantly drives the enhancement of plasticity mechanisms. As the deformation incompatibility decreases, the stress gradient begins to play a significant role in comparison with the strain gradient. This shift is attributed to the regular variations in dislocation density within different domains. As the grain size gradients and loads decrease, the dislocation density becomes more uniform across the domains, hindering the formation of strong domain boundaries. While this may impede the activation of strain gradients, it facilitates the activation of stress gradients as a supplementary measure. By designing multilayered GS structures to alter the distribution of dislocation density, we can control the activation levels of stress and strain gradients, thereby influencing the plasticity mechanisms and mechanical properties of the material.