Co-existences of the two types of β′ precipitations in peak-aged Mg-Gd binary alloy

In this work, an atomic-scale investigation on the phase structure in the Mg-Gd alloy during isothermal ageing at 230 °C was conducted by means of Cs-corrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). Two types of precipitates including the long-structure β′ (denoted as βL′) and short-structure β′ (denoted as βS′) are found to co-exist in the peak-aged Mg-11.5 wt% Gd alloy, which is largely different from the previous reports that only βS′ or βL′ precipitate in the peak-aged Mg-Nd, Mg-Y and Mg-Gd binary alloys. According to the micrographs observed in the three directions of [0001]α, [112¯0]α and [101¯0]α, both the βL′ and βS′ precipitates are confirmed to exhibit an ordered orthorhombic structure and have a same composition of Mg7Gd. The βL′ phase exhibits the truncated lenticular shape, while the βS′ phase are found to exist in the bridging structure between the βL′ main bodies. Moreover, four different types of βL′/βS′ interface structures can also be observed.