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I.

INTRODUCTION

THE components in the quaternary system Al-Mg-CuZn are used in many of the high-strength aluminum alloys, such as 7075. A basic understanding of the thermodynamics, phase relationships, and solidification behavior of the system is essential in the development of new materials, as well as to improve the performance of existing materials. Experimental determination of the thermodynamic and phase equilibria data for ternary and higher order alloy systems under all possible conditions is practically impossible and not normally attempted. Yet these data are absolutely essential for alloy and processing development. In order to overcome this shortcoming, we began to develop thermodynamic descriptions first for the constituent binaries and then for the ternaries for this quaternary system, employing a computational approach using data primarily available in the literature. The term ‘‘thermodynamic description’’ means that thermodynamic models are developed for all the phases in a system so that we can use them to calculate the phase diagram in question. The thermodynamic models must also yield values in accord with experimental thermochemical data. Out of the six binaries in the quaternary Al-Mg-Cu-Zn system, we have developed descriptions for Al-Mg,[1] AlZn,[2] and Mg-Cu.[3] Descriptions for the other three binaries, Al-Cu,[4] Mg-Zn,[5] and Cu-Zn,[6] are available in the literature. We have also developed thermodynamic descriptions for the four ternaries: Al-Mg-Zn,[7] Al-Mg-Cu,[8] AlCu-Zn,[9] and Mg-Cu-Zn.[10] This article presents a detailed thermodynamic description of the Al-Mg-Cu system. This description was used by Zuo and Chang[11] and Chang et

S.-L. CHEN, Research Associate, Y. ZUO and H. LIANG, Graduate Students, and Y.A. CHANG, Wisconsin Distinguished Professor, are with the Department of Materials Science and Engineering, University of Wisconsin–Madison, Madison, WI 53706. Manuscript submitted August 14, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A

al.[12] to study the solidification behavior of the Al-rich AlMg-Cu alloys. In the future, we will publish descriptions of the other three ternaries and the quaternary. We will first review the experimental data in this system and then present an overview of our approach to obtain a thermodynamic description for this system. We will then introduce the thermodynamic models used and the methodology employed to optimize the model parameters. II. REVIEW OF THE EXPERIMENTAL DATA AND AN OVERVIEW OF THE APPROACH USED TO OBTAIN A THERMODYNAMIC DESCRIPTION Phase equilibria of the Al-Mg-Cu system, as shown in Figures 1 and 2, are highly complex and not well established. To facilitate reading, the symbols used to designate the phases in the Al-Mg-Cu system are summarized in Table I. The experimental data reported in the literature have been reviewed by Phillips,[13] Mondolfo,[14] Willey,[15] Guether et al.,[16] Chang et al.,[17] Murray,[18] and Prince and Effenberg.[19] The liquidus projection shown in Figure 1 is essentially based on the data of Brommel