Novel structures observed in Mg-Gd-Y-Zr during isothermal ageing by atomic-scale HAADF-STEM

Mg-Gd-Y-Zr alloy has been widely studied due to its excellent properties. Its ageing precipitation sequence was identified as a four-stage precipitation including S.S.S.S.-beta ''-beta'-beta(1)-beta. In this paper, the precipitate phases of Mg-Gd-Y-Zr during isothermal ageing at 225 degrees C are investigated by atomic resolution HAADF-STEM and, as a result, novel structures are observed. Viewed from [0 0 0 1]Mg axis, the general morphology of rare earth (RE) atoms is main bodies linked by bridges. In bridge areas, some RE atoms form separated chains of hexagons, which has not been clearly reported previously. Furthermore, a defect in the precipitation process, which is the interface between two encountered beta' parts, is characterized. (C) 2015 Elsevier B.V. All rights reserved.