Phase Equilibria of the Ce-Mg-Zn Ternary System at 300 °C

Ahmad Mostafa,Mamoun Medraj

doi:10.3390/met4020168

Abstract

The isothermal section of the Ce-Mg-Zn system at 300 °C was experimentally established in the full composition range via diffusion multiple/couples and key alloys. Annealed key alloys were used to confirm the phase equilibria obtained by diffusion multiple/couples and to determine the solid solubility ranges. Spot analysis was carried out, using wavelength dispersive X-ray spectroscopy (WDS), to identify the composition of the observed phases. The composition profiles were obtained using WDS line-scans across the diffusion zones. X-ray diffraction (XRD) was performed to identify the phases in the annealed alloys and to confirm the WDS results. Eight ternary compounds, in the Ce-Mg-Zn isothermal section at 300 °C, were observed from 45-80 at.% Zn. These are: τ1 (Ce6Mg3Zn19), τ2 (CeMg29Zn25), τ3 (Ce2Mg3Zn3), τ4 (CeMg3Zn5), τ5 (CeMg7Zn12), τ6 (CeMg2.3−xZn12.8+x; 0 ≤ x ≤ 1.1), τ7 (CeMgZn4) and τ8 (Ce(Mg1−yZny)11; 0.096 ≤ y ≤ 0.43). The ternary solubility of Zn in the Ce-Mg compounds was found to increase with a decrease in Mg concentration. Accordingly, the ternary solid solubility of Zn in CeMg12 and CeMg3 was measured as 5.6 and 28.4 at.% Zn, respectively. Furthermore, the CeMg and CeZn showed a complete solid solubility. The complete solubility was confirmed by a diffusion couple made from alloys containing CeMg and CeZn compounds.

Highlights

Casting magnesium alloys exhibit low mechanical strength and ductility, and Mg alloys, in general, are limited to certain applications, because of their poor plastic properties at room temperature and poor creep resistance at elevated temperatures [1]
The diffusion couple technique combined with the selected equilibrated alloys was used to achieve more reliable information about the Ce-Mg-Zn isothermal section at 300 °C
The phase equilibria were determined by means of solid-solid diffusion couples and a diffusion multiple

Summary

Introduction

Casting magnesium alloys exhibit low mechanical strength and ductility, and Mg alloys, in general, are limited to certain applications, because of their poor plastic properties at room temperature and poor creep resistance at elevated temperatures [1]. Zn is one of the potential alloying elements added to Mg to improve its mechanical properties and corrosion resistance [2]. The problem with the Mg-Zn binary alloys is the low melting point, deeming these alloys not suitable for elevated temperature applications. In order to improve the mechanical properties at elevated temperatures, rare earth (RE) elements are added to form precipitates by age hardening [3], on the one hand. The addition of rare earth elements can refine the grains and form stable compounds with a high melting point [4]. As a major component of the mischmetal, is considered one of the most important additives to modify the Mg-Zn binary alloys [5], because it forms a series of stable intermetallic compounds

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