Thermodynamic Database Development of the Mg-Ce-Mn-Y System for Mg Alloy Design
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INTRODUCTION
ALTHOUGH magnesium-based materials have a long history of important commercial applications, including automotive, there remains much to be learned about the basic properties of the metal and its alloys. With the recent renewed interest in lightweight wrought materials, including both sheet and tube applications, there has been an increased focus on developing a better understanding of novel magnesium-alloy systems, including those that incorporate additions of such elements as Ce, Y, and Mn. These alloy systems, along with other potential candidates, are being actively pursued as possible routes to develop magnesium materials with improved ductility, or even practical room-temperature formability. The properties of cast or wrought materials derived from a specific alloy system depend first and foremost upon the phases and microstructural constituents (eutectics, precipitates, solid solutions, etc.) that are present. In an alloy with several alloying elements, the phase relationships are very complex. In order to investigate and understand these complex phase relationships effectively, it will be very useful to develop a database containing model parameters, giving the thermodynamic properties of all phases as functions of temperature and composition. Using Gibbs free energy minimization software such as FactSage,[1,2] the automotive and aeronautical industries and their suppliers will be able to access the database to calculate the amounts and compositions of all phases at equilibrium at any temperYOUN-BAE KANG, Postdoctoral Fellow, ARTHUR D. PELTON, Professor, and PATRICE CHARTRAND, Assistant Professor, are with the CRCT (Centre de Recherche en Calcul Thermochimique), De´partement de ge´nie chimique, E´cole Polytechnique, Montre´al, PQ, Canada H3C 3A7. Contact e-mail: [email protected] PHILIP SPENCER, President, is with the The Spencer Group, Trumansburg, NY 14886. CARLTON D. FUERST, Chief Scientist, is with General Motors, Warren, MI 48090-9055. Manuscript submitted October 4, 2006. Article published online June 2, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
ature and composition in multicomponent alloys, to follow the course of equilibrium or nonequilibrium cooling, to calculate corresponding heat effects, etc. Such thermodynamic databases are prepared by critical evaluation, modeling, and optimization. In a thermodynamic ‘‘optimization,’’ adjustable model parameters are calculated using, simultaneously, all available thermodynamic and phase-equilibrium data in order to obtain one set of model equations as functions of temperature, pressure, and composition. Thermodynamic data, such as activities, can aid in the evaluation of the phase diagrams, and information on phase equilibria can be used to deduce thermodynamic properties. Thus, it is frequently possible to resolve discrepancies in the available data. From the model equations, all of the thermodynamic properties and phase diagrams can be back-calculated, and interpolations and extrapolations can be made in a thermodynamically corre
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