Experimental Investigation and Thermodynamic Calculation of the Phase Equilibria in the Mg-Gd-Mn Ternary System

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MG-BASED alloys have attracted increasingly world-wide attention due to their excellent properties such as low density, excellent specific strength, good machinability, and high recycle ability.[1] In recent years, many efforts have been devoted to the development of the magnesium alloys with the high creep resistance at elevated temperatures. It was revealed that the rare earth element Gd could improve the high-temperature behavior of Mg alloys remarkably[2–4] and the element Mn could refine the microstructure during extrusion.[2] For the advanced development of the Mg-Gd-based alloys, knowledge of the phase equilibria in the Mg-GdX systems becomes very important. Up to now, no experimental information on the MgGd-Mn ternary system is available in the literature. Among the three binary systems, the Gd-Mg phase diagram was investigated over the whole composition range by Manfrinettic and Gschneidner[5] using the differential thermal analysis (DTA) and X-ray diffrac-

FAN ZHANG, Doctoral Candidate, HONGHUI XU and ZHANPENG JIN, Professors, SHUHONG LIU, Associate Professor, and TAO ZHOU, Senior Research Fellow, are with the State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, Hunan, P.R. China. Contact e-mail: [email protected] BIAO HU, Lecturer, is with the School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, Anhui, P.R. China. Manuscript submitted October 1, 2014. Article published online July 25, 2015 4804—VOLUME 46A, OCTOBER 2015

tion (XRD) measurement. There exist four compounds in the Mg-Gd system, viz. Mg5Gd, Mg3Gd, Mg2Gd, and MgGd, which were formed by peritectic reactions at 931 K, 993 K, 1029 K, and 1141 K (658 C, 720 C, 756 C, and 868 C), respectively. And then this system was critically assessed by Nayeb-Hashemi and Clark,[6] and thermodynamically evaluated by Guo et al.[7] and Hampl et al.,[8], respectively. The establishment of the binary Gd-Mn phase diagram is due to Kirchmayr and Lugscheider.[9] There exist three compounds in the GdMn system, viz, GdMn2, Gd6Mn23, and GdMn12. Based on these experimental data and the estimated enthalpies of formation of the Gd-Mn compounds referring to the known values of the Mn-Y system, the Gd-Mn system was thermodynamically optimized by Gro¨bner et al.[10] A summary of all experimental investigations of the MgMn system was performed by Nayeb-Hashemi and Clark,[11] which suggested that there exists no intermetallic phase in the Mg-Mn system. Later on, Gro¨bner et al.[12] measured the monotectic reaction L¢¢ = L¢ + Mn (at about 1473 K (1200 C)) using DTA and thermodynamically evaluated this system. Table I summarizes all the intermetallic phases in the Mg-Gd-Mn system. The objective of the present work is to determine the phase equilibria at 773 K (500 C) of the Mg-Gd-Mn system via an equilibrated alloy method, and further to thermodynamically evaluate this system using the CALPHAD method for computer-assisted design of magnesium-based materials. The alloys were prepared by melt