Microstructural development in NiAl/Ni-Si-B/Ni transient liquid phase bonds
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1.
THE B2
INTRODUCTION
type (fi-phase) intermetallic compound NiAI
(e.g., References 1 through 5) has attracted considerable interest as a potential high-service-temperature material for applications such as in hot-end components of aero gas turbine engines, trj Practical application of NiA1 would require access to suitable joining technologiesY'~ Transient liquid phase (TLP) bondingC71 is capable of joining materials, such as NiA1, which are not readily suited to fusion welding or diffusion bonding. The TLP bonding process tT~relies on the initial formation, at the bonding temperature, of a liquid interlayer between the faying surfaces of the material(s) to be joined. Subsequent holding at the bonding temperature allows interdiffusion to occur between constituents of the liquid interlayer and solid substrates. Compositional changes induced at the joint line by interdiffusion result in isothermal resolidification of the interlayer. Given a suitable postisothermal-resolidification homogenization treatment, the microstructure and hence mechanical properties of the joint line can resemble those of the parent material(s). The application of TLP bonding and allied processes to NiAI-NiAI joining has been the subject of a number of recent studies (e.g., References 8 through 10). However, the joining of NiAI to Ni-base materials has not received a similar level of attention in the literature. The present article examines microstructural development in NiA1-Ni TLP bonds made using Ni-Si-B interlayers. Comparison is drawn between microstructural development in NiA1/Ni-SiB/Ni and Ni-base/Ni-Si-B/Ni-base TLP bonding. I1.
EXPERIMENTAL TECHNIQUES
NiA1/Ni-Si-B/Ni TLP bonds were fabricated using the following materials: (1) cast near-stoichiometric NiA1 (Ni-52 at. pct A1) substrates with a grain size of around 150/zm;
W.F. GALE, Assistant Professor, and S.V. OREL, Graduate Student, are with the Materials Research and Education Center, Auburn University, Auburn, AL 36849. Manuscript submitted January 4, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
(2) commercial purity nickel (99.5 wt pct Ni) substrates with a grain size of around 100 p~m; and (3) melt-spun Ni-4.5 wt pct Si-3.2 wt pct B (American Welding Society designation BNi-3) interlayers. Each substrate had a thickness of 2.5 mm, a faying surface area of 245 mm 2, and a 1000 grit SiC finish. An initial interlayer thickness of 51 /zm was employed, and 50 /zm thick tungsten spacers were inserted between the faying surfaces to maintain a constant joint gap. Joint fabrication was performed under a 10 -4 mbar vacuum atmosphere at bonding temperatures of 1065 ~ and 1150 ~ Joints were prepared using holding times at the bonding temperature ranging from zero (i.e., heating to the bonding temperature followed by immediate cooling to room temperature) to 21 hours. When short holding times were employed (0 to 5 minutes duration), it was desired to use a relatively rapid heating rate in order to minimize the influence of the heating step on joint microstructural development. F
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