Thermodynamics of Ti in Ag-Cu alloys
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w o r k . [8,9] Moorhead e t al. t8] showed that standard free
INTRODUCTION
O N E of the challenges involved in fabricating reliable structural components from ceramics is the development of effective techniques for joining ceramics such as A1203, ZrO 2, Si3N4, and SiC to themselves and to metals. An important method of joining ceramics is brazing, and it depends on the wetting of liquid metal alloys on ceramic surfaces to produce strong bonds. The wetting of oxide ceramics by metals has been the subject of many experimental studies, tl-71 and it is well established that the wetting properties of various alloys can be dramatically improved by small additions of titanium, t2'3,5-71 The improvements in wetting behavior produced by Ti additions are often accompanied by improvements in the adhesion of metal alloys to oxide surfaces t51 or in the strength of brazed joints, tTl Therefore, understanding the behavior of Ti is an important part of developing a better understanding of the physical and chemical factors responsible for strong bonding of ceramics and for the development of new brazing alloys for ceramic joining. However, the effect Ti has on wetting is not well understood, in part, because of a lack of thermodynamic data for its behavior in various liquid metal alloys. The ability of Ti additions to promote wetting of oxides has been explained as being due to the high affinity of Ti for oxygen, which can result in a reduction reaction with the oxides such as A1203 to form a titanium oxide at the ceramic-metal interface. A possible reaction is 3Ti + x A 1 2 0 3 (s) --- 3TiOx (s) + 2xA1
[1]
where x denotes the stoichiometry of the titanium oxide phase and the underlined Ti and A1 are dissolved in the braze filler metal. The extent of this reaction and the nature of the reaction product depend on the activities of Ti and AI in the liquid metal alloy. The thermodynamics of the reaction of Ti in braze fdler metals with A1203 have been discussed in previous J.J. PAK is with the Steelmaking Research Department, Pohang Steel Company, Pohang, South Korea. M.L. SANTELLA is with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6096. R.J. FRUEHAN, Professor, is with the Metallurgical Engineering and Materials Science Department, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted February 15, 1989. METALLURGICAL TRANSACTIONS B
energy data would not predict the reduction of A1203 by Ti at brazing temperatures, 800 ~ to 1000 ~ normally used for Ag- or Cu-based alloys. Nicholas tg] discussed the stability of TiO~ phases, noted that reduction reactions such as Eq. [ 1] have been observed, and concluded that this meant that the activity coefficient of Ti must be much greater than that of A1 in the liquid braze filler metals. Nicholas e t a l J 61 also discussed the effects of elements such as indium and tin on the wetting behavior of Ti-containing melts on A1203 substrates. It was observed that In or Sn additions could decrease the concentration of Ti needed to induce
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