Experimental Investigation and Thermodynamic Modeling for the Mg-Nd-Sr System

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Magnesium alloys with rare-earth (RE) elements have proved effective in enhancement of creep resistance and elevated temperature performance.[1] Among these alloys, the Mg-Al-(Ca/Sr) and Mg-Al-RE alloys surpass most of commercial die casting Mg alloys in tensile strengths at elevated temperature including AZ91D,[2] arising from solid-solution strengthening and/or formation of precipitates when the formers are alloyed with RE and/or Sr.[3] The CALPHAD (CALculation of PHAse Diagram) approach,[4] appropriately assisted with key experimentation and first principle calculations enables to exploit the phase diagram of the Mg-Al-CaSr-RE multicomponent system for designing high-performance RE-containing Mg alloys. Zhong et al.,[5,6] Janz et al.[7,8] and Aljarrah et al.[9,10] published the thermodynamic assessments for the quaternary Mg-AlCa-Sr system. Nevertheless, there is neither phase equilibrium information nor thermodynamic description for the Mg-RE-Sr systems. It is the objectives of this work to determine the phase equilibria relations of the

GUANGLONG XU, Ph.D. Student, is with the School of Materials Science and Engineering, Central South University, Changsha 410083, P.R. China, and also with Computational Alloy Design Group, IMDEA Materials Institute, 28906 Getafe, Madrid, Spain, and also with the Facultad de Informa´tica, Universidad Polite´cnica de Madrid, 28660 Boadilla del Monte, Madrid, Spain. YUWEN CUI, Staff Researcher, is with the Computational Alloy Design Group, IMDEA Materials Institute. LIJUN ZENG, Ph.D. Student, LIBIN LIU, and ZHANPENG JIN, Professors, are with the School of Materials Science and Engineering, Central South University. Contact e-mail: [email protected] XIAOMA TAO, Professor, is with the College of Physical Science and Technology, Guangxi University, Nanning 530004, P.R. China. Manuscript submitted December 27, 2012. Article published online October 19, 2013 5634—VOLUME 44A, DECEMBER 2013

Mg-Nd-Sr ternary system by studying the equilibrated Mg-rich ternary alloys and by using differential scanning calorimeter analysis, and upon the determined data to develop the thermodynamic descriptions of the Nd-Sr binary and Mg-Nd-Sr ternary systems in terms of the CALPHAD approach. The Mg-Nd-Sr ternary alloys were prepared from the binary master alloys. The weighed samples were wrapped in tantalum foils and then sealed in pure iron tubes under argon atmosphere. The tubes were heated up and held at 1173 K (900 C) for 4 hours for melting and then quenched into ice water. Each tube was turned upside down every 45 minutes to promote a homogeneous composition. The samples were kept in the tubes for the heat treatment at 673 K (400 C) for 720 hours, followed by a water quench. The actual compositions of the annealed samples were analyzed by the chemical titration technique, and the microstructure and phase present/composition were examined by powder X-ray diffraction (XRD) using a Dmax-2500 VBX diffractometer and by scanning electron microscopy/electron microprobe analysis (SEM/EPMA) using a JEOLJXA-8230. Sm

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