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

Highlights: • Using Cs-corrected HAADF-STEM investigated the precipitation behavior from there perpendicular directions. • The β_L′ and β_S′ precipitates were been found to co-exist in the peak-aged Mg-Gd alloy. • Four types of β_L′/β_S′ interfaces were been found. • The two precipitates have the same composition of Mg₇Gd. • The orientation relationship is determined to be [001]_βS′//[001]_βL′//[0001]_α; (100)_βS′//(100)_βL′//${\left(11\overline{2}0\right)}_{\text{α}}$. 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 β_L′) and short-structure β′ (denoted as β_S′) 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 β_S′ or β_L′ 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, ${\left[11\overline{2}0\right]}_{\text{α}}$ and ${\left[10\overline{1}0\right]}_{\text{α}}$, both the β_L′ and β_S′ precipitates are confirmed to exhibit an ordered orthorhombic structure and have a same composition of Mg₇Gd. The β_L′ phase exhibits the truncated lenticular shape, while the β_S′ phase are found to exist in the bridging structure between the β_L′ main bodies. Moreover, four different types of β_L′/β_S′ interface structures can also be observed.