Chemical interactions of solute atoms during L1₂ cluster formation in Mg–Zn–Gd alloys with long-period stacking ordered structure

Magnesium-based alloys containing transition metal (TM) and rare-earth (RE) atoms form L1₂-type TM₆RE₈ clusters, which are regularly arranged into long-period stacking ordered (LPSO) structures. X-ray absorption spectroscopy (XAS) and ab initio calculations were performed on the solution-treated Mg₉₇Zn₁Gd₂ alloy before and after aging at 673 K to understand the L1₂ cluster formation mechanism and the interactions between the TM and RE atoms. Ab initio simulations of Zn K X-ray absorption near edge structure (XANES) spectra showed that the sharpness of the white line depended on the number of Gd atoms. The fine spectral structure changed with the local structure around the Zn atom. The measured XANES spectrum indicated that the Zn in the solution-treated Mg₉₇Zn₁Gd₂ alloy was initially dissolved in Mg₃Gd and then incorporated within L1₂ clusters after aging. Furthermore, an analysis of the l-projected density of states showed that the sharp white line peak in the XANES spectrum resulted from the interaction of Zn p and Gd f states, which occurred with spinodal decomposition and structural relaxation of L1₂ clusters. This spinodal decomposition and structural relaxation, caused by the electronic interaction between the RE and TM atoms, may be one of the reasons why rare-earth elements are indispensable for forming an LPSO structure.