Elastic constants of pure body-centered cubic Mg in nanolaminates

Abstract

At ambient temperature, pressure, and sufficiently fine layer thicknesses, pure body-centered cubic (bcc) Mg can exist as a pseudo-morphic phase when coherently bonded with a substantially stiffer bcc metal, such as Nb. Compared to the hexagonal close-packed Mg/bcc Nb nanocomposite that exists in the larger layer thicknesses, the bcc Mg/bcc Nb nanocomposite was recently shown to exhibit significantly higher yield stresses and strains to failure. However, because of the morphological, spatial, and crystallographic constraints imposed by nanolayered architecture, the elastic constants of the individual bcc Mg phase cannot be directly measured experimentally. Lack of this fundamental property stands in the way of theoretical and computational modeling of the mechanical properties of the pseudo-morphic bcc phase of Mg. In this work, we employ density functional theory calculations and a strain-energy-based elasticity method to calculate the lattice and elastic constants of pure bcc Mg. For validation of these constants, we combine a set of micropillar compression experiments and microstructurally explicit finite element simulations for the fully bcc Mg/bcc Nb nanolaminate system. We conclude that (i) for the stress-free bcc Mg, the lattice parameter a0 is 3.581 Å, and the three independent elastic constants C11, C12, and C44 are 39.64 GPa, 34.14 GPa, and 31.38 GPa, respectively, and (ii) for the laminated bcc Mg (i.e., a0 = 3.347 Å), the three elastic constants are 84.68 GPa, 56.68 GPa, and 61.4 GPa, respectively.