*Corresponding Author Vol. 14 (No. 3) / 97 International Journal of Thermodynamics (IJoT) Vol. 14 (No. 3), pp. 97-105, 2011 ISSN 1301-9724 / e-ISSN 2146-1511 doi: 10.5541/ijot.296 www.ijoticat.com Published online: May 25, 2011 Thermodynamic Justification for the Ni/Al/Ni Joint Formation by Diffusion Brazing W. S. Wołczyński* 1 , T. Okane 2 , C. Senderowski 3 , D. Zasada 3 , B. Kania 4 , J. Janczak-Rusch 5 1 Institute of Metallurgy & Materials Science, Polish Academy of Sciences, Reymonta 25, 30 059 Kraków, Poland 2 AIST – National Institute of Advanced Industrial Science & Technology, Umezono 1-1-1, 305 8568 Tsukuba, Japan 3 Military University of Technology, Kaliskiego 2, 00-908 Warszawa, Poland 4 AGH – University of Science & Technology, Reymonta 19, 30-059 Kraków, Poland 5 EMPA–Swiss Federal Laboratories for Materials Science & Technology, Ueberland 129, 86 000 Duebendorf, Switzerland *nmwolczy@imim-pan.krakow.pl Abstract A theoretical model for the joint formation was developed for diffusion brazing. The phenomena of dissolution and solidification were included into the model. A thermodynamic justification for the isothermal brazing occurrence in the meta-stable conditions was developed. It involved the application of the criterion of higher temperature of the solid / liquid (s/l) interface. The dissolution of the filler metal in the substrate was described by the  ଴ െ solute concentration within the dissolution zone (liquid film) situated at the substrate surface. The selection of the  ଴ െ parameter was justified by the Thermocalc calculation of the Ni-Al phase diagram for meta-stable equilibrium. According to the model assumptions, the solidification was accompanied by undercooled peritectic reactions resulting in formation of intermetallic phases. The average Al – solute concentration measured across a given Al 3 Ni 2 /Al 3 Ni/Al 3 Ni 2 joint confirmed that the  ଴ െ solute concentration was conserved within the joint sub-layers. The Ni-Al phase diagram for meta-stable equilibrium referred to for the solidification was also calculated by means of the Thermocalc Software. It allowed to locate the solidification path, s/l interface path and redistribution path onto the mentioned diagram. Superposition of both calculated phase diagrams was also given to show that the joint formation occurred cyclically under the meta-stable conditions. Isothermal formation of the Ni/Al/Ni joints has been performed at different temperatures. The following  ோ െ temperatures have been applied: 700, 750, 800, 850, 900, 950, and 1000 0 C. The solidification was arrested and the actual morphologies frozen. It allowed to make a measurement of the Al - solute concentration across each joint by means of the EDX micro-analyzer to estimate average solute concentration,  ഥ ଴ . Regardless of the  ோ - temperature, the solidification path was always the same. Keywords: : meta-stable equilibrium, joint formation model, dissolution, solidification 1. Introduction A solidification, a dissolution and some solid/solid transformations are predicted by Tuah-Poku et al., (1988) as phenomena which are to be observed during diffusion brazing. The solidification and dissolution are subjected to consideration in the current model. However, exceptionally the first solid / solid transformation is strictly connected with solidification and dissolution through the mass balance (Kloch et al., 2005). A dissolution of substrate by the liquid filler metal occurs continuously within a zone ݔ, Figure 1. The dissolution prepares the liquid film within the zone ݔ. Once the zone becomes liquid the mass of this liquid film diffuses towards the joint symmetry axis and solidification occurs at the s/l interfaces, (Wołczyński et al., 2006a). Solidification is accompanied by the undercooled peritectic reactions, Figure 2. According to the concept of Chuang et al. (1975), undercooled peritectic reactions take place at the solid/liquid (s/l) interface. The occurrence of peritectic reactions at the s/l interface of cells formed within a given sub-layer are marked schematically with some arrows in Figure 2. Figure 1. Ni-substrate dissolution within its infinitesimally small amount,  ݔ(the so-called zone  ݔsituated just at the substrate surface) due to diffusion of liquid solution of the filler metal,  ி , through the channels situated among some cells of Al 3 Ni 2 or Al 3 Ni phases formed in a given sub-layer, respectively. A reaction within the zone is as follows: ݑݍሺ ி ሻ ൅  ՜ ݑݎ݋݋ ݑݍ ሺ ଴ ሻ.