Early stages of precipitation and microstructure control in Mg-rare earth alloys

Hardening precipitation frequently occurs in Mg - rare earth ( RE) alloys after heat treatment in the 150 - 200 degrees 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 DO19 phase and the so- called orthorhombic beta' phase. The structural relationship between DO19 and beta' 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 DO19 or the beta' 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 beta' phase, the Mg - Y - Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties.