Modeling of Thermodynamic Properties and Phase Equilibria for the Al-Sm Binary System
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I.
INTRODUCTION
THE formation of crystalline phases in aluminum rare-earth (Al-RE) binary alloys has received considerable attention due to a number of interesting phenomena related to rapid solidification,[1] glass formation,[2–5] devitrification behavior,[6] nanocrystalline applications,[7,8] and metastable phase transformations.[9–11] By virtue of these features, it is clear that the binary Al-Sm system is one in which the metastable phases play a major role in the various solidification and devitrification phenomena. As such, accurate thermodynamic treatments of equilibrium and metastable phases are required to use this system as a model for the investigation of fundamental competition and selection principles in systems far from equilibrium. Considerable work has been done to quantify the various phase equilibria in the Al-Sm system,[12–20] and the reader is referred to our recent review of reported experimental and modeling efforts.[11] In addition, the reader is referred to Table I of Reference 11 for a comprehensive summary of the structure and composition of the various stable and metastable phases observed in the Al-Sm binary system. The most current Al-Sm phase diagram is shown in Figure 1,[20] and, for the sake of brevity, the inconsistencies that remain unresolved are summarized here. First, prior models[20] take no account of the metastable c phase (orthorhombic, Al4U type), which has been
S.H. ZHOU, Assistant Scientist, is with Materials and Engineering Physics, Ames Laboratory, USDOE, 204A Wilhelm Hall, Ames, IA 50011. R.E. NAPOLITANO, Associate Professor, Department of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, IA 50011, and Scientist with Materials and Engineering Physics, Ames Laboratory, USDOE, 116 Wilhelm Hall, Ames, IA, 50011. Contact e-mail: [email protected] Manuscript submitted May 23, 2007. Article published online January 25, 2008 502—VOLUME 39A, MARCH 2008
observed in various devitrification sequences.[9,10] We have previously investigated the relative stability of the c phase along with other Al4Sm and Al11Sm3 phases and incorporate our prior results into the general model here.[11] Second, while experimental evidence for the relative stability of the a phase is unclear,[12–20] it has been treated as a stable phase, and it appears in the equilibrium phase diagram proposed by Saccone,[20] as shown in Figure 1. Our prior work, however, suggests that a has no range of full stability but rather that it is metastable down to 0 K,[11] which is reflected in the current treatment. Third, reported experimental data for the region between the L fi b + r and b + r fi d reactions are not sufficient to clearly establish the associated phase equilibria, as indicated in Figure 1. Specifically, calorimetry measurements[14,15,20] suggest that there may be an invariant near 1490 K, between the L fi b + r and b + r fi d reactions. This feature would be similar to that observed in the Al-La system, which exhibits a stable Al7La3 phase in this temperature range. Moreover, incon
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