Phase stability comparison by first principle calculation and experimental observation of microstructure evolution in a Mg-6Gd-2Zn(wt%) alloy

The microstructure of a Mg-6Gd-2Zn(wt%) alloy was investigated by SEM and TEM observations; phase stability comparison was performed by first principle calculation. The long periodic structure (LPS) was found to be transformed from the Mg3Gd phase when subjected to solution treatment at 723 K for 6 h plus aging treatment at 633 K for 8 h. When aged at 503 K for 36 h after solution treatment, the LPS phase transformed to the much more stable Mg3Gd phase. The calculations of cohesive energy and formation enthalpy shown that the Mg3Gd phase is more stable in comparison with the LPS phase; the analyses on DOS (density of states) distributions proved that Zn alloying makes the LPS phase less stable due to p-d hybridization of valence electrons.