Mechanism of Effects of Rare Earths on Microstructure and Properties at Elevated Temperatures of AZ91 Magnesium Alloy

By using real-space recursion method, the energetics of the undoped and A1 and/or RE atoms doped Σ(1450) <0001> symmetric tilt grain boundaries (GBs) in AZ91 alloys were investigated. Similar calculations were performed on undoped and doped bulk α Mg for comparison. The results showed that A1 atoms segregated at GBs in AZ91 alloys. When RE atoms were added, they also segregated at GBs, and their segregation is stronger than A1 atoms'. Therefore, RE atoms retard the segregation of A1 atoms. Calculations of interaction energy indicated that A1 atoms repelled each other, and could form ordered phase with host Mg atoms. On the contrary to the case of A1, RE atoms attracted each other, they could not form ordered phase with Mg, but could form clusters. Between RE and A1, there existed attractive interaction, and this attractive interaction was the origin of Al11 RE3 precipitation. Precipitation of A111 RE3 particles with high melting point and high thermal stability along GB improves high temperature properties of AZ91 alloys.