Key Issues in a Thermodynamic Mg Alloy Database
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INTRODUCTION
COMPUTATIONAL thermodynamics (CT) is developing as a strong and wide field in materials science and engineering. When it comes to a better metallurgical understanding of truly multicomponent and multiphase alloys, these calculations require a quantitative thermodynamic description of all possible phases in the alloy system, including the liquid phase and other complex solution phases. Such thermodynamic descriptions are developed using the Calphad approach.[1] They may be compiled for large systems with many components in a thermodynamic database. In addition to the database, a thermodynamic software package based on the principle of minimizing the Gibbs energy of the multiphase system is required to perform the actual calculations, such as Pandat (CompuTherm LLC, Madison WI),[2] Thermo-Calc (Thermo-Calc Software, Stockholm, Sweden),[3] or FactSage (ThermFact Inc., Montre´al, Canada).[4] Specifically for magnesium alloys, this approach has been demonstrated as a powerful tool in focused alloy design and process optimization.[5–9] The calculated phase diagrams, phase fractions and compositions, and the solidification paths were used to design the alloy composition with balanced mechanical properties for cast Mg-Al-Sn[5] and Mg-Al-Ca[6] alloys for automotive applications. Similar calculations were applied in the design strategy for microalloyed ultraductile Mg alloys and the limiting temperatures for the hot extrusion process.[8] The hot-tearing susceptibility of the cast ternary Mg-Al-Ca[10] and Mg-Al-Sr[11] alloys was derived from calculated solidification paths, freezing ranges, and phase fractions at the end of primary solidification. It is evident that the quality of the underlying thermodynamic database is decisive for the success of such applications of CT. JOACHIM GRO¨BNER, Research Fellow, and RAINER SCHMID-FETZER, Professor, are with the Institute of Metallurgy, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany. Contact e-mail: [email protected] Manuscript submitted June 8, 2012. Article published online November 8, 2012 2918—VOLUME 44A, JULY 2013
The development of a thermodynamic Mg alloy database by experiments combined with Calphad modeling has been an ongoing effort in the authors’ group since the mid-1990s.[12] The Calphad modeling is occasionally supported by first-principles calculations.[13] Traditional and widely used Mg alloy systems such as Mg-Al-Zn (AZ) or Mg-Al-Mn (AM) are dealt with in earlier publications, e.g., Reference 14. The focus of the current work will be on more advanced Mg alloys with additions of rare earths (RE: Ce, Gd), Y, Ca, Sr, and Sn in specific combinations with or without Zn and Al. In Al-free alloys, small additions of Zr as a powerful grain refiner have to be considered. Small Mn contents, added to absorb detrimental Fe-contamination from melting crucibles, may also be taken into account. The purpose of the current study is twofold, as follows: Key issues concerning the concept, consistency,
coherency, and quality assurance in
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