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

Abstract

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.

Precipitation and Hardening in Magnesium Alloys

“…[150] In the following years, several studies [180,[192][193][194][195] have been made to use modern facilities to characterize the precipitates in aged samples of WE54 and WE43. It was reported [193,194] that during the early stage of aging of WE43 alloy, monolayer precipitates with a D0 19 ordering form with two possible habit planes: 11 " 20 È É a and 1 " 100 È É a , and that the precursor 11 " 20 È É a monolayers form the b¢¢ (D0 19 ) phase whereas the precursor 1 " 100 È É a monolayers led to the formation of the b¢ phase. However, the similarity between these Mg-Y-Nd-based alloys and Mg-Gd alloys suggests that the precipitation in the early stage of the WE alloys is likely to be similar to that in Mg-Gd alloys (Table I).…”

Contribution of small-angle X-ray scattering to microstructural investigation of newly developed Mg–rare earth alloys for structural applications

“…Compression and creep tests performed up to 573 K have shown that these alloys display mechanical properties higher than the commercial alloys up to 523 K. However, above 523 K a drop of mechanical properties is observed for all alloys Antion, 2003;Antion et al, 2006). The aim of the present paper is to clarify the hardening mechanisms of these alloys by a detailed microstructural study.…”

“…The study of ageing treatments for these alloys has evidenced a large hardening potential, related to the precipitation of metastable 00 and 0 phases as revealed by transmission electron microscopy (TEM) (Antion et al, 2006). Compression and creep tests performed up to 573 K have shown that these alloys display mechanical properties higher than the commercial alloys up to 523 K. However, above 523 K a drop of mechanical properties is observed for all alloys Antion, 2003;Antion et al, 2006).…”

“…Average size of precipitates having an approximate globular section is 100 Å . Such precipitates correspond to the metastable 0 phase of the Mg-rare earth precipitation sequence having a bco crystal structure; the faceted shape of the 0 precipitates results from the way in which they are formed: plane by plane from stacking of ordered monoplanar 00 precipitates having a DO 19 structure every four magnesium planes (Antion, 2003;Antion et al, 2006). The latter corresponds to the formation of the first metastable phase for earlier time of precipitation.…”

“…In particular, varying the energy close to this value should allow detection of yttrium composition differences between magnesium solid solution and precipitates, whose composition is not clearly established (Antion, 2003;Antion et al, 2003;Antion et al, 2006).…”