A thermodynamic description of the Al-Mg-Zn system

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I.

INTRODUCTION

BECAUSE elements Al, Cu, Mg, and Zn are basic components for many high-strength aluminum alloys such as alloy 7075, it is important to develop a thermodynamic description for this quaternary system. Developing a thermodynamic description for an alloy system means developing appropriate thermodynamic models for all the phases in the system so that we can calculate its phase equilibria and thermodynamic properties. Such a description is important not only for basic materials research in related areas, such as solidification and solid-state phase transformation, but also for alloy and processing development and improvement. However, in order to obtain a thermodynamic description for the quaternary, it is necessary to develop descriptions for its constituent ternaries first. In a previous article, we have reported a thermodynamic description for the Al-Mg-Cu system.[1] In this article, we report a description for the second ternary system Al-Mg-Zn. Developing such a description is based on the experimental phase equilibrium and thermodynamic data available in the literature and the established descriptions for the three constituent binaries Al-Mg,[2] Al-Zn,[3] and Mg-Zn.[4] In the following, we will first present a review of the experimental data, then the thermodynamic models used in this study, next optimization of the model parameters, and last a discussion of the results. II.

REVIEW OF EXPERIMENTAL DATA

Ever since the first experimental investigation of the Al-Mg-Zn system by Eger[5] in 1913, many additional studies have been reported in the literature.[6–60] The experimental results have been reviewed periodically by a number of researchers.[9,32,34,41,49,50,53,58,60] The review by Despande et al.[58] is the most detailed one considering both the

H. LIANG, Graduate Student, S.-L. CHEN, Research Associate, and Y.A. CHANG, Wisconsin Distinguished Professor, are with the Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706-1595. Manuscript submitted November 25, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A

phase equilibrium and thermodynamic data. The most recent review of the phase equilibrium data is given by Petrov.[60] Figures 1 through 3 show the assessed liquidus projection and two isothermal sections at 25 7C and 335 7C, respectively. As shown in these diagrams, there exist two ternary phases which are the T phase with a formula of (Al, Zn)49(Mg)32 and the f phase with a formula of Al20.4Mg54.9Zn24.7. The T phase was determined by Bergman et al.[33,36] and the f phase by Clark and Rhines[37] and Clark.[40] Both phases exist over wide ranges of homogeneity. Clark[40] also suggested the existence of a third ternary phase with an uncertain composition near the Mg-Zn boundary binary, but its existence was never confirmed by later investigators. Although all the binary phases in Figures 2 and 3 show appreciable solubility for the third component, there is no sufficient experimental evidence to establish their exact ternary solubility. To facil

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