Local atomic ordering strategy for high strength Mg alloy design by first-principle calculations

In this study, solute X (X = Li, Al, Mn, Zn, Y, Zr, Nd, and Gd) solution strengthening in Mg alloys have been screened by ab initio density functional theory calculations to quantify not only the substitutional stacking-fault configurations but also the solute ordering sequence as a function of local segregation. Interestingly, it has been found that the strengthening effects of single atom addition to a supercell made of 64 atoms can be mostly attributed to lattice distortions (Mn>Nd>Gd>Y>Zn>Al>Zr>Li), while the local ordering arrangements of Mg-X complex actually contribute most to the strengthening when the solute concentration rises. For example, Nd can induce a large local atomic ordering and significantly increase the basal critical-resolved shear stress (CRSS). A linear relationship between solute concentrations and ideal strength, and the quasi-quadratic relationship between solute atomic radii and ideal strength have been observed. Simultaneously, the higher the solute concentration, the higher degree of the solid solution strengthening, resulting in a smaller quasi-quadratic function curve opening. Based on the screening of the chemical (including stacking fault energy and atomic bonds) and strain (lattice distortion energy) energy calculations, we have discovered that the solute strengthening follows the ordering sequence of Nd> Mn> Gd> Y> Zn> Al> Zr> Li.