Thermodynamic and kinetic study of diffusion paths in the system Cu-Fe-Ni
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INTRODUCTION
ADVANCED materials are of increasing importance in modern technology. For the most part, these are complex multicomponent and multiphase materials such as composites or hybrid structures, which require various types of microjoining and bonding operations. Joinability and compatibility of dissimilar materials largely determine the properties and functionability of these multimaterial assemblies. For mass production soldering, brazing or diffusion bonding are frequently employed methods of joining. New applications of advanced materials require the development of joining materials and processes. This can be carried out effectively by first achieving a better understanding and so controlling the interracial reactions and diffusion processes. In this respect, thermodynamics and a diffusion-kinetic approach combined with detailed microanalysis techniques can be of great help. Diffusion and chemical reactions at the interfaces play an essential role in bonding as well as in the mechanical, physical, and chemical performance of the joints. It is noteworthy that these time-dependent structural changes are not only observed in the joining process but are also important during storage and in the use of bonded structures. This is of particular importance in soldering, since the operation temperatures are relatively close to the melting points of conventional solder fillers, and therefore, diffusion is still remarkably fast. For practical purposes, it is advantageous to develop a model for predicting possible diffusion paths in the joint area. However, in multicomponent systems, the existence of a liquid phase and grain boundary diffusion, as well as
K.J. RONKA, Postdoctoral Student, and J.K. KIVILAHTI, Professor, are with the Department of Materials Science and Engineering, Helsinki University of Technology, 02150 Espoo, Finland. A.A. KODENTSOV, Researcher, and F.J.J. VAN LOO, Professor, Laboratory of Solid State Chemistry and Materials Science, and P.J.J. VAN LOON, Undergraduate Student, Department of Chemical Engineering, are with Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. Manuscript submitted November 11, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
the lack of data on mobility or thermodynamic data, makes a reliable prediction of actual diffusion paths very difficult. Therefore, the relatively simple Cu-Fe-Ni system was chosen in this article to study the diffusion in two-phase systems. Results of this model system will be subsequently applied to more complex systems such as Cu-Ag-Sn, CuAg-Zn, and Cu-Sn-Zn, which are of great interest for brazing and soldering applications. Furthermore, the Cu-Fe-Ni system has the advantage of a wide range of compositions that can be chosen as starting materials in the diffusion couples, and moreover, thermodynamic and kinetic data are fairly well known. It is emphasized, however, that despite their thermodynamic simplicity, the solid solutions of the Cu-Fe-Ni system are not ideal. The recent development of thermodynamic computer softwar
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