Discrete slip crystal plasticity modeling of deformation in nanolayered materials

Irene J. Beyerlein¹, Tianju Chen², Rui Yuan², Caizhi Zhou^2,3

¹ Mechanical Engineering Department, Materials Department, University of California at Santa Barbara, Santa Barbara 93106, USA

² Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA

³ Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA

Nanolayered materials are a material class gaining much attention due not only to their ten-fold enhancement in strengths higher over that of its constituents, but also the tunability of this exceptional strength with layer thickness. While the positive layer size scaling with decreasing layer thickness applies to many nanolayered material systems, such as nanotwinned materials and bimetallic nanolayered composites (MNCs),its origins are not well understood, hindering exploitation of this material class in broad application.  We present and apply a novel crystal plasticity based computational method to predict the deformation response and underlying mechanisms of nanotwinned metals and MNCs without introducing any non-material, adjustable parameters. Calculations are applied to plastic anisotropy tests on these materials and agreement is achieved with measured strengths over a wide range of layer thicknesses. The analysis suggests that the origin of the layer size effect results from the limits layer thickness places on the lengths of dislocations sources lying in the grain boundaries and interfaces. We show that statistical variation in source length gives rise to strain hardening and coupling between crystal orientations and explains the observed plastic anisotropy. Notably the model predicts the emergence of Hall-Petch like behavior in yield strength in agreement with mechanical tests on bulk Cu/Nb nanolaminates.