On the plastic anisotropy of gradient nanostructured nickel

Abstract

The plastic anisotropy of gradient nanostructured (GNS) metals profoundly influences their mechanical behaviors and performances, a complete understanding of which is crucial to the real-world industrial applications of this novel class of materials. To this end, a dislocation density-based crystal plasticity finite element model is used in this work to investigate the plastically anisotropic behaviors of GNS Ni, with a focus on their origin as well as the structure–property relationship of GNS metals in general. It is found that GNS Ni demonstrates significant plastic anisotropy, which can be attributed to both the grain size gradient and the texture. Under 90° loading, the presence of the grain size gradient leads to the plastic strain being mostly accommodated by the plastically softer coarse grains at the bottom of the specimen, resulting in diminished yield and flow stresses and consequently significant plastic anisotropy. The texture, on the other hand, induces plastic anisotropy via the Schmid effect, where the loading direction with the lowest average Schmid factor demonstrates the highest yield stress and vice versa. Increasing the grain size gradient gives rise to an increase in the plastic anisotropy measured by the average Lankford coefficient $\overline{\text{R} }$, as a result of the increasing texture gradient. In addition, a new plastic anisotropy measure is proposed for GNS metals, which demonstrates an approximately linear relationship with $\overline{\text{R} }$. The conclusions made in this work are instrumental to optimizing GNS metals via microstructural design.