HRTEM Observation of Age Hardening Precipitates in Mg-12.0%Gd-1.9%Y-0.7%Zr

The precipitation sequence in Mg-12.0 mass%Gd-1.9 mass%Y-0.69 mass%Zr alloy was investigated using high resolution transmission electron microscopy (HRTEM). The globular shaped contrast, which were probably composed of Gd and Y atoms were observed in a specimen quenched after solution heat treatment. Aging treatment was performed up to 64 hours at 473 K. The monoplanar precipitates were formed on plane (1100) of magnesium matrix of the specimen early in the aging stage at 473 K for 4 hours. In addition to the mono planar precipitates, a part of β″ phase with D019 structure and β′ phase with base centered orthorhombic structure were also observed in the same specimen. It was found that these precipitates coexisted in the specimen early at the aging stage. Based on the analysis of selected area diffraction patterns, it was revealed that β″ phase preferentially formed early in the aging stage. However, the β′ phase preferentially observed comparing to β″ phase in the specimen which had maximum hardness by aging at 473 K for 64 hours. Accordingly, the nucleation and growth of β′ phase dominates comparing to β″ in the stage of peak aging. These results suggest that coexistence of β′ phase and β″ phases, especially β′ phase attributes to the maximum hardness of aged alloy.