Designing strong and stiff HCP and BCC Mg alloy structures by templating hard faceted phase using Sn and RE elements

Abstract

Compared to alternative materials (aluminum alloys and steel), magnesium alloys have low strength and modulus which restricts further applications in critical structural components. To overcome this limitation, we have successfully designed a strong and stiff magnesium-lithium alloy (LAZWMGT) by using Sn and RE (Gd, Y) nuclei to refine HCP α-Mg grains and forming secondary Mg2Sn inside primary Mg grains. Although the bulk Gibbs free energy favors the dissolution of HCP α-Mg and growth of BCC β-Li grains, the dispersion of thermally stable Mg2Sn, Al2Gd, and Al2Y particles inside the BCC β-Li grains can actually template the nucleation of HCP α-Mg heterogeneously. Because the Mg2Sn, Al2Gd, and Al2Y phases are hard (E > 76 GPa) and have an entropy of melting greater than 16.6 J/mol·K, they are faceted crystals and semi-coherency exists between the HCP α-Mg and Mg2Sn, Al2Gd, and Al2Y particles, leading to limited growth orientations at slow rate. As a result, new HCP α-Mg grains inside BCC β-Li do not coarsen. Therefore, both strength and stiffness are increased significantly up to YS: 319 MPa, UTS: 365 MPa, and E: 64.5 GPa. To quantify each phase's contribution to Young's modulus, nanoindentation has been used to measure the stiff particles as well as the soft BCC β-Li. It has been found by XPS experiments and DFT calculations that strong covalent Mg-Mg and Mg-X bonds in BCC β-Li enhances when Sn or RE presents either in solution heat treatment or as Mg2Sn precipitates, justifying the rigidity and directionality bonds as a result of Sn and RE. In addition, the effects of Sn and RE on grain boundary, solid solution, and precipitation strengthening have been predicted.