Short-Range Order Clusters in the Long-Period Stacking/Order Phases With an Intrinsic-I Type Stacking Fault in Mg-Co-Y Alloys

Abstract We have investigated short-range order (SRO) solute clusters in the long-period stacking/order (LPSO) phases with an intrinsic-I (I1) type stacking faults (SFs), which have been uniquely formed in Mg-Co-Y alloys, based on atomic-resolution scanning transmission electron microscopy (STEM) combined with first-principles calculations. The Co3Y5 SRO cluster model embedded across the I1-type SFs has been successfully constructed to satisfy the observed electron diffraction, STEM images and computed energetic stabilities. The optimized Co3Y5 cluster configurations appear to be significantly deviated from the host Mg atom positions and consequently provide a significant gain for the energetic stability of the I1-type LPSO phases. This is a similar phenomenon observed for the major LPSO phases with an intrinsic-II (I2) type SFs, and therefore the solute SRO clusters play an important key role for the formation and stability of the LPSO phases in ternary Mg alloys.