Early stages of precipitation and microstructure control in Mg-Rare Earth alloys

• Published in: Philosophical magazine (Abingdon, England) England), 2006-09, Vol.86 (19), p.2797-2810
• Main Authors: Antion, Caroline, Donnadieu, Patricia, Tassin, Catherine, Pisch, Alexander
• Format: Article
• Language: English
• ISSN: 1478-6435; 1478-6433
• DOI: 10.1080/14786430600557193

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.