Vianco-10/03 \CORRECTED\ zaida

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Specifications	Vianco-10/03 \CORRECTED\ zaida
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WELDING RESEARCH OCTOBER 2003-S268 ABSTRACT. Poor hermeticity was ob- served for Al2O3-Al2O3braze joints using a Fe-29Ni-17Co

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Content	WELDING RESEARCH OCTOBER 2003-S268 ABSTRACT. Poor hermeticity was ob- served for Al2O3-Al2O3braze joints using a Fe-29Ni-17Co alloy spacer and Ag-Cu- Ti active filler metal. Titanium was scav- enged from the filler metal by formation of a (Fe, Ni, Co)-Ti “lacework” phase. The scavenging mechanism was further stud- ied using braze joints made with elemen- tal Fe, Ni, or Co spacers. The Fe spacer caused development of FexTiyphases at its interface but allowed a significant TixOylayer to form at the Ag-Cu-Ti/Al2O3inter- face, resulting in 100% hermetic joints. The Co spacer caused a lacework phase and only intermittent TixOyreaction layer at the Ag-Cu-Ti/Al2O3interface; 75% of the buttons were hermetic. The Ni spacer caused extensive scavenging of Ti, result- ing in an absence of a TixOylayer at the Ag-Cu-Ti/Al2O3interface and no her- metic joints. Braze joint mechanical strength correlated primarily with the presence or absence of the TixOylayer. The Fe, Ni, and Co spacer experiments suggested that lacework phase formation in Fe-29Ni-17Co/Ag-Cu-Ti joints was pri- marily a result of scavenging by the Ni and Co components of the spacer material. Mechanical strength behavior and poor hermeticity were commensurate with the absence of a TixOyreaction layer at the Ag-Cu-Ti/Al2O3interface. An Fe or inac- tive metal barrier layer would provide a means to curtail the scavenging reaction. Introduction The properties of ceramic materials allow them to be suitable for applications from heat engines to electro-optical de- vices (Refs. 1, 2). Filler metal techniques (soldering and brazing) have been used to successfully join metals and ceramics. Metallization layers or active filler metals are required to promote the necessary wetting and spreading by the braze alloy over the ceramic surface (Ref. 3). Thermal expansion mismatch between metals and ceramics can generate damag- ing residual stresses within the joint (Ref. 4). Those stresses can be reduced by se- lecting a metal alloy having a thermal ex- pansion coefficient that closely matches that of the ceramic material. For example, the thermal expansion coefficient of the Fe-Ni-Co alloy, Kovar™ (Fe-29Ni-17Co, wt-%), is 6.2 ppm/°C (25–500°C) and is well matched to that of alumina (Al2O3), 7–9 ppm/°C (Refs. 4–6). An active filler metal for metal-ceramic joining is the 63.3Ag-35.1Cu-1.6Ti composition (Cusil ABA™, Ref. 7). It is based upon the eu- tectic Ag-Cu alloy (72Ag-28Cu, Teut.= 780°C) with Ti serving as the active ele- ment to promote wetting of the bare Al2O3surface. A particular application required that two pieces of Al2O3be joined together with an Fe-29Ni-17Co alloy interlayer or spacer between them. Test specimens, based upon the ASTM F-19 “tensile but- ton” configuration, were brazed with the Ag-Cu-Ti active filler metal. The proto- type braze joints exhibited satisfactory strength, but did not satisfy hermeticity re- quirements. Optical micrographs of the braze joint made at 850°C (1562°F) for 5 min are shown in Fig. 1. A lacework struc- ture developed within the filler metal (Fig. 1A). The layer ranged from 1–3 µm thick. No Ti-based reaction layer, designated TixOy, formed at the Ag-Cu-Ti/Al2O3in- terface — Fig. 1B. By comparison, A2O3/Al2O3braze joints made under the same process conditions, but in the ab- sence of a Fe-29Ni-17Co spacer, exhibited no lacework phase in the filler metal (Fig. 2A) and developed a 1.92±0.38-µm-thick TixOyreaction layer at the Ag-Cu- Ti/Al2O3interfaces — Fig. 2B. For this lat- ter case, a theoretical TixOythickness was calculated to be 1.7 µm at each interface, assuming a filler metal thickness of 51 µm and a TiO stoichiometry (Ref. 6). Electron microprobe analysis (EMPA) identified Fe, Co, Ni, Cu, and Ti in the lacework phase. Two compositional varia- tions were measured, one richer in Fe and the second richer in Ni. The Fe-rich com- position occurred when the lacework phase was located near the Fe-29Ni-17Co spacer. This case is represented by loca- tion A in the scanning electron micro- scope (SEM) photograph in Fig. 3 (back- scattered electron or BSE mode). When the phase was located further into the filler metal (location B), a Ni-rich com- position was observed. Electron micro- probe analysis determined the composi- tion of the Fe-rich phase to be (at-%) Ti, 30%; Fe, 46%; Ni, 10.1%; Co, 12.2%; Cu, 1.2%; and Ag, 0.5%. The Fe, Co, Ni, and Cu elements were combined and com- pared against the Ti content. (Silver was assumed to have had little role in the phase composition.) The Ni, Co, and Cu concentrations were normalized to the Fe content (that is, the Fe concentration equaled unity). The resulting stoichiome- try was (Fe1.0Ni0.23Co0.27 Cu0.03)2.3Ti. The composition of the Ni-rich sublayer was Ti, 24%; Fe, 6.7%; Ni, 48.4%; Co, 8.4%; Cu, 12.3%; and Ag, 0.2%. Normalized to the Ni content, this composition was rep- resented as (Fe0.14Ni1.0Co0.17Cu0.25)3Ti. The EMPA evaluation detected only a slight, intermittent Ti signal at the Ag-Cu- Ti/Al2O3interface, confirming the ab- sence of a significant TixOyreaction layer. Also, there was no appreciable Ti remain- ing within the filler metal. The above analyses suggest the follow- ing scenario. The scavenging process began by the Ti component of the filler metal reacting at the Fe-29Ni-17Co spacer KEY WORDS Brazing Active Filler Metals Titanium Scavenging Ceramic Materials Titanium Scavenging in Ag-Cu-Ti Active Braze Joints The scavenging mechanism was studied by brazing joints with elemental Fe, Ni, and Co spacers BY P. T. VIANCO, J. J. STEPHENS, P. F. HLAVA, AND C. A. WALKER P. T. VIANCO, J. J. STEPHENS, P. F. HLAVA, and C. A. WALKER are with Sandia National Laboratories, Albuquerque, N.Mex.
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