Early stages of precipitation and microstructure control in Mg-Rare Earth alloys
Résumé
Hardening precipitation frequently occurs in Mg-Rare Earth (RE) alloys after heat treatment in the 150癈-200癈 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 邟 phase. The structural relationship between DO19 and 邟 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 邟 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 邟 phase, the Mg-Y-Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties.
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