• PDF / 378,524 Bytes
• 10 Pages / 612 x 792 pts (letter) Page_size
• 19 Downloads / 186 Views

DOWNLOAD

REPORT

---

I. INTRODUCTION

COPPER alloys have a wide range of technical applications due to their good electrical and thermal conductivity and high corrosion resistance. The effect of slight alloying on the properties of copper can be derived from the physical and chemical changes in the crystal lattice (fcc) of copper, but with stronger alloying, these effects are also due to the appearance of new phases in the structure. In effective alloy development, the common trend is to apply thermodynamic models for the calculation of phase regions and equilibria in the selected alloys. For that purpose, plenty of assessed thermodynamic data are available for aluminum- and ironbased alloys, but not yet for copper-based alloys. In this study, a thermodynamic description is presented for the ternary Cu-Al-Sn system in its copper-rich corner. The unary thermodynamic data of the pure components are identical to those recommended by the Scientific Group Thermodata Europe (SGTE).[1] The binary thermodynamic data are taken from the recent assessments of the Cu-Al,[2] Al-Sn,[2] and Cu-Sn,[3] systems, although a slight reoptimization of data is made in the present work to improve the accuracy of these descriptions in the copper-rich corner. The new data are valid only up to such contents of Al and Sn at which the ordered gamma phase becomes stable. Indeed, for alloys containing more of these solutes, the original binary assessments[2,3] are still preferred. Worth noting is that the present study, focusing on an accurate description in a limited-composition region of the system (i.e., the copper-rich corner), does not observe the SGTE recommendation to make complete, easy-to-link assessments for the whole systems. Basically, there are three reasons. First, the phase equilibria in the studied system are too complex to be described accurately in any composition and temperature range. Second, due to the limited amount of experimental data, the assessor may have to face the situation in which there are too many degrees of freedom J. MIETTINEN, Senior Research Scientist, is with the Laboratory of Metallurgy, Helsinki University of Technology, 00530 Helsinki, Finland. Contact e-mail: [email protected] Manuscript submitted July 2, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A

to fix the parameters of the numerous phases. The third reason is simply to utilize the new information as soon as possible. Those interested only in copper alloys, for example, may not like to wait for years for well-prepared, complete assessments. This, in fact, is the main reason for the present “copper-corner” assessment. As it is SGTE compatible, it will be easy to extend this database later.

II. THERMODYNAMIC MODELS The classical substitutional-solution model has extensively been used to describe the thermodynamic properties of solution phases in metallic systems, although the more recent sublattice model[4] is more flexible for that purpose. For example, different ordered solution phases can easily be treated with the sublattice model, but not so easily with the