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Hardening precipitation frequently occurs in Mg-rare earth (RE) alloys after heat treatment in the 150-200 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 0 phase. The structural relationship between DO 19 and 0 phases has been established in underaged and overaged states from the 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 0 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 0 phase, the Mg-Y-Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties.

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