Reducing deformation anisotropy to achieve ultrahigh strength and ductility in Mg at the nanoscale

Abstract

In mechanical deformation of crystalline materials, the critical resolved shear stress (CRSS; τ[subscript CRSS]) is the stress required to initiate movement of dislocations on a specific plane. In plastically anisotropic materials, such as Mg, τ[subscript CRSS] for different slip systems differs greatly, leading to relatively poor ductility and formability. However, τ[subscript CRSS] for all slip systems increases as the physical dimension of the sample decreases to approach eventually the ideal shear stresses of a material, which are much less anisotropic. Therefore, as the size of a sample gets smaller, the yield stress increases and τ[subscript CRSS] anisotropy decreases. Here, we use in situ transmission electron microscopy mechanical testing and atomistic simulations to demonstrate that τ[subscript CRSS] anisotropy can be significantly reduced in nanoscale Mg single crystals, where extremely high stresses (~2 GPa) activate multiple deformation modes, resulting in a change from basal slip-dominated plasticity to a more homogeneous plasticity. Consequently, an abrupt and dramatic size-induced “brittle-to-ductile” transition occurs around 100 nm. This nanoscale change in the CRSS anisotropy demonstrates the powerful effect of size-related deformation mechanisms and should be a general feature in plastically anisotropic materials.