Effect of Ho content on the microstructural stability and grain growth kinetics of wrought Mg-Y-Ho-Zn alloys

Abstract

The low microstructural thermal stability of magnesium alloys often leads to a decrease in strength, severely limiting their elevated-temperature applications. In the present work, a microstructure with high thermal stability is developed by rare-earth Ho alloying. The influence of Ho on microstructure evolution and mechanical properties of Mg-Y-Zn alloy during annealing was investigated. The results show that Ho suppresses the generation of W phase (Mg3Zn3Y2) and promotes the precipitation of long-period stacking ordered (LPSO) structure in Mg-Y-Zn-Ho alloys. During the high-temperature applications, the W phase is prone to coarsen, which weakens the pinning effect of grain boundaries, leading to a large grain size. However, after the addition of Ho, the LPSO phase converts to lamella LPSO phases within grains and at grain boundaries. The grain sizes of Mg-Y-Zn-Ho alloys remain stable, which slows down grain growth and stabilizes microstructure. As the content of Ho increases to 2 %, the activation energies of grain boundary migration (GBM) during the grain growth increases from 55 kJ/mol to 103 kJ/mol, indicating that the addition of Ho element hinders grain growth through the formation of lamella LPSO phases. The correlation between the yield strength and grain size of all annealed alloys can be effectively explained by the Hall-Petch (H-P) relationship and LPSO phase refinement.