High throughput investigation of diffusion and solid solution hardening of hcp mg alloys

Abstract

Magnesium, the lightest structural metal, is a potential alternative as weight-saving material for many industrial applications due to high specific-stiffness, superior damping capacity and biocompatibility together with excellent electrical and thermal conductivity and recyclability. However, some key issues limit its widespread application as a structural material, such as the reduced ductility and low strength at room temperature, resulting from the mechanical anisotropy and low critical resolved shear stress for basal slip and twinning. These limitations may be overcome by the addition of alloying elements and this thesis was aimed at providing a deeper understanding of the composition-microstructure-mechanical properties link in Mg alloys for structural applications. To this end, a complete assessment of the kinetic database of the hcp Mg-Al-Zn system was performed. In particular, the atomic mobility parameters were optimized based on the evaluation of the experimental diffusion data available in the literature for the Mg-Al, Mg-Zn and Mg-Al-Zn systems, in combination with the available thermodynamic databases. The optimized set of atomic mobility parameters could successfully predict the diffusion behavior for binary and ternary diffusion couple. In addition, a novel, high-throughput experimental approach was developed to investigate the anisotropic interdiffusion behavior in Mg-Al alloys by means of diffusion couples and an explicit comprehensive expression of the interdiffusion coefficients of the hcp Mg-Al alloys as a function of Al content, grain orientation and temperature was determined. Moreover, the diffusion couples in combination with micropillar compression tests, were used to estimate the influence of solute content and temperature on the critical resolved shear stress for the basal slip, twinning and pyramidal slip in Mg-Al alloys. These results were new and compared with recent theoretical estimations based on first-principle calculations. It was found that the plastic anisotropy of Mg alloys increased with the Al content. Finally, the novel highthroughput experimental strategies developed in this thesis can be readily applied to other hcp metallic alloys.