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

In mechanical deformation of crystalline materials, the critical resolved shear stress (CRSS; tau(CRSS)) is the stress required to initiate movement of dislocations on a specific plane. In plastically anisotropic materials, such as Mg, tau(CRSS) for different slip systems differs greatly, leading to relatively poor ductility and formability. However, tau(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 tau(CRSS) anisotropy decreases. Here, we use in situ transmission electron microscopy mechanical testing and atomistic simulations to demonstrate that tau(CRSS) anisotropy can be significantly reduced in nanoscale Mg single crystals, where extremely high stresses (similar to 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.