Crystallography and Morphology of (Gd,Y)H2 Hydride in a Mg-Gd-Y-Al Alloy

Hydrogen can be easily captured by the rare-earth (RE) elements in hydrogen-rich environments, which significantly affect the phase compositions and mechanical performance of Mg-RE based alloys. However, the morphology of RE hydrides and their orientation relationships (ORs) with the Mg matrix have not been well explained. Here, a stable face-centered cubic (FCC) Gd,YH2 hydride was introduced and uniformly distributed in a Mg-15Gd-2.5Y-1Al alloy after hydrogenation treatment at 500 °C and 2 MPa for 40 h. The plate-like Gd,YH2 hydride with six variants was identified to exhibit an OR with the magnesium (Mg) matrix, which is [0001]Mg//[001]Gd,YH2, (101¯0)Mg10.5∘ from (002)Gd,YH2, (12¯10)Mg10.5∘ from (020)Gd,YH2. Further crystallographic matching calculations based on the edge-to-edge matching model suggest that such an OR is energetically favorable and provides the actual interface between the RE hydrides and the Mg matrix during precipitation. Our findings offer new insights into the microstructural regulation of Mg alloys in hydrogenation environments.