Caroline Antion

Hardening precipitation in a Mg–4Y–3RE alloy

Abstract Early stages of precipitation in a Mg–Y–Nd based alloy aged at 150 °C have been studied using TEM and small angle X-ray scattering (SAXS). The former brings information concerning nature, morphology and size of precipitates, and the latter adds qualitative and quantitative information concerning populations of precipitates in terms of size and volume fraction. Precipitation at 150 °C involves formation of DO19 monoplanar precipitates, which further develop into the β″ and β′ phases having platelet and globular morphologies, respectively. TEM observations on samples aged at 150 °C reveal the formation mechanism of the bco-β′ structure by the ordering of monoplanar DO19-β″ precipitates. Additional examinations at 250 °C revealed the DO19-β″→bco-β′ transformation, as well as β1 precipitates. Estimation of the volume fraction deduced from SAXS is discussed on the basis of the TEM results.

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°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 β′ phase. The structural relationship between DO19 and β′ 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 β′ 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 β′ phase, the Mg–Y–Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties.

Undercooling and demixing of copper-based alloys

Since the beginning of materials science research under microgravity conditions immiscible alloys have been an interesting subject. New possibilities to investigate such systems are offered by containerless processing techniques. Of particular interest is the ternary system Cu-Fe-Co, and its limiting binaries, Cu-Co and Cu-Fe. They all show a metastable miscibility gap in the regime of the undercooled melt. Within the ESA-MAP project “CoolCop”, different aspects of this alloy have been investigated; results obtained so far are reported here.

Hardening precipitation and mechanical properties in new Mg-Mn-Y-Gd alloys

Abstract Precipitation microstructures and mechanical properties of two new Mg–Mn–Y–Gd alloys have been studied. Small Angle X-ray Scattering and Transmission Electron Microscopy carried out on homogeneous and aged states (200°C and 250°C) have revealed Guinier–Preston zones formed from the homogeneous state at ambient temperature and first stages of precipitation consisting in DO19 type monolayer precipitates. These ultrafine precipitates show further organisation leading to the orthorhombic β′ phase. Peak-hardness microstructure is characterised by a high density of connected β′ precipitates. Overaged samples show a high level of mechanical strength and creep resistance. The related microstructure consists of stable β′ phase precipitates forming connected strings giving rise to a local strain within the matrix. Comparison with previous studies confirms the strong relationship between rare earth elements choice, precipitation microstructure and mechanical resistance.