Influence of Layer Thickness on Deformation Twinning in Mg/Nb Laminates

Abstract

Mg-based nanolayered composites can achieve remarkably high specific-strengths due to a combination of a high density of bimetal interfaces and the light-weight constituent Mg phase. However, their applicability to load-bearing applications is hindered by limited formability and the anisotropic effects of deformation twinning. Understanding the microstructural and interface properties that control twinning is crucial for the design of multilayered composites. In this study, we employ an elasto-viscoplastic fast-Fourier-transform (EVP-FFT) crystal plasticity micromechanics model to examine the effect of Mg layer thickness on the growth propensity of \( \left\{ {10\bar{1}2} \right\} \)-tensile twins that span the entire Mg layer. The analysis shows that a critical Mg layer thickness exists, below which, twin growth becomes substantially harder. This critical thickness is related to the backstresses that develop along the twin boundary in the anti-twinning direction. These backstresses result from the plastic reaction of the adjacent Nb layers to the twin shear where the Mg twin lamella intersects the Mg/Nb interfaces. Below the critical layer thickness, the strong backstresses from both ends of the twin strongly interact. They increase in intensity as the layer thickness reduces. Concomitantly, increasing amounts of external loading are required to overcome the backstress and make twin growth feasible.