Thermodynamic Assessment of the Aluminum Corner of the Al-Fe-Mn-Si System
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I.
INTRODUCTION
CHECKING for additions and impurities in cast Al-Si aluminum alloys becomes more and more important as the use of scrap materials increases. Among the most common impurities is iron, which may lead to the precipitation of various phases, either stable or metastable.[1] Figure 1(a) shows the liquidus projection in the aluminum corner of the stable ternary Al-Fe-Si system according to a recent reassessment.[2] In addition to the Al-rich (Al) solid solution and to Si, four compounds noted gamma-s2, delta-s4, alpha-s5, and beta-s6 are shown in this figure. Table I lists the crystallographic structure and the chemical formula of these phases. As will be briefly described in Section I–A, the reassessment was based on the COST-507 database[3] and limited to changes in the description of the compounds in order to describe the solubility range of each one of them. Among the other beneficial effects, manganese is added to Al-Si alloys, containing iron, in order to avoid precipitation of the beta-s6 phase that often forms as elongated plates, thus decreasing the mechanical properties of the cast parts. Such a solution has been applied for a long time to improve the quality of cast parts but should be mastered through the appropriate description of phase equilibria in the Al-Fe-Mn-Si system. Figure 1(b) presents the Al-rich corner of the liquidus projection of the Al-Mn-Si system as calculated using the COST-507 database, the features of which have been confirmed by a more recent experimental study of the JACQUES LACAZE, Senior Scientist, is with CIRIMAT, Universite´ de Toulouse, ENSIACET, BP 44362, 31432 Toulouse Cedex 4, France. Contact e-mail: [email protected] LUIZ ELENO, Postdoctoral Student, formerly with CIRIMAT, Universite´ de Toulouse, is with the Departamento de Engenharia Metalu´rgica e de Materiais, Escola Polite´cnica da Universidade de Sa˜o Paulo, Av. Prof. Mello Morais, 2463, 05508-030 Sa˜o Paulo, SP, Brasil. BO SUNDMAN, Visiting Researcher, formerly with CIRIMAT, Universite´ de Toulouse, is with INSTN, CEA Saclay, 91190 Gif sur Yvette Cedex, France. Manuscript submitted December 17, 2009. Article published online June 11, 2010 2208—VOLUME 41A, SEPTEMBER 2010
entire phase diagram.[35] Two ternary compounds show up in the Al-rich corner that are denoted alpha-AlMnSi and beta-AlMnSi (Table I for their structure). The alpha-AlMnSi phase has a very limited composition domain corresponding to the formula, Al9Mn2Si, while the beta-AlMnSi phase extends significantly around the composition (Al,Si)10Mn3.[35] Substitution of Si to Al has been studied in both alpha-AlMnSi[10] and betaAlMnSi[11] phases. For the assessment of the system limited to the Al-rich corner, a description of both phases, as line compounds with cross-substitution of Al and Si, was adopted in the COST-507 database, as shown in Table I. The most noteworthy work on the Al-rich corner of the quaternary Al-Fe-Mn-Si system is from Phillips and Varley,[4] who performed an extensive study using thermal analysis and optical microscopy. T
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