Hardening precipitation frequently occurs in Mg-Rare Earth (RE) alloys after heat treatment in the 150°C-200°C range. Early stages of precipitation have been studied in detail by transmission electron microscopy in two Mg-RE alloys (Mg-Y-Gd and Mg-Y-Nd). Two types of structures may be involved in the precipitation sequence: a DO 19 phase and the so-called orthorhombic β' phase. The structural relationship between DO 19 and β' phases has been established in underaged and overaged states from the

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... observations at peak ageing. We show that the earliest precipitates play a key role in the selection of phases developing in overaged states. Depending on the habit plane of the precipitates present in the early states, either the DO 19 or the β' phase will grow in further ageing. The Mg-Y-Gd and Mg-Y-Nd alloys illustrate the different microstructures resulting from such selection. Due to the selective growth of the β' phase, the Mg-Y-Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties.