Coexistence of 14H and 18R-type long-period stacking ordered (LPSO) phases following a novel orientation relationship in a cast Mg−Al−RE−Zn alloy

Abstract Coexistence of 14H and 18R-type long-period stacking ordered (LPSO) phases was observed in a cast Mg−Al−RE−Zn alloy, following a novel orientation relationship of (0001)14H about 70.5° from (001)18R and [1 1 ¯ 00]14H//[100]18R. Transmission electron microscopy (TEM) observations reveal that this novel structure was formed because of the concurrence of the precipitation of 18R structure in the lumpy Mg4.8Al12RE7.2 phase and the heterogeneous nucleation of 14H structure on the coherent Mg/Mg4.8Al12RE7.2 interface during isothermal heat treatment at 300 °C. This paper gives an in depth understanding of the new formation mechanisms of the coexistence of both 14H and 18R-type LPSO phases.