Transient liquid-phase bonding in the NiAl/Cu/Ni system-A microstructural investigation
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I.
INTRODUCTION
THE B2 type (b phase) intermetallic compound NiAl (e.g., References 1 through 5) offers potential as a highservice-temperature material, for example, in aero gas turbine engines.[6] However, widespread application of NiAl would require access to suitable joining technologies.[6] Among possible joining technologies for NiAl, transient liquid phase (TLP) bonding offers the advantages of tolerance of the strong alumina forming tendency of NiAl and compatibility with the poor low-temperature ductility of NiAl. Techniques have been developed for the TLP bonding of NiAl-NiAl.[7,8] However, relatively high bonding temperatures are required for these procedures (ranging from around 1200 7C[7] to above 1460 7C).[8] These high bonding temperatures impede application to the joining of NiAl–Nibase superalloys (which typically have melting temperatures of around 1300 7C).[9] Thus, alternative interlayer materials are required if TLP bonding is to be applied successfully to the joining of NiAl to Ni-base superalloys. One of the authors[10] has previously investigated the application of conventional Ni-Si-B interlayers (which are widely used in Ni-base alloy–Ni-base alloy joining[11]) to NiAl–Ni-base alloy TLP bonding. In an NiAl/Ni-3.2 wt pct B-4.5 wt pct Si/Ni model system, complete isothermal solidification was achieved after only 2 hours holding at a temperature of 1065 7C.[10] However, depending upon the bonding temperature employed, problems were encountered with either excessive liquation of the Ni substrate or formation of stable borides in the Ni substrate during holding at the bonding temperature. Given the difficulties experienced with the use of Ni-SiB interlayers, the present investigation considers the use of a different interlayer material, namely, pure copper. Pure copper was chosen for investigation as an interlayer for NiAl–Ni-base alloy joining for the following reasons. W.F. GALE, Assistant Professor, and Y. GUAN, Graduate Student, are with the Materials Research and Education Center, Auburn University, Auburn, AL 36849. Manuscript submitted September 25, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
(1) Copper shows extensive solid solubility in both NiAl and nickel (the solubility of copper in NiAl exceeds 10 at pct at room temperature,[12] while the Ni-Cu system is fully isomorphous).[13] Hence, the use of pure copper as an interlayer material seemingly offers the possibility of avoiding, or at least reducing, the occurrence of undesirable second phases. (2) The melting point of pure copper (1083 7C) is sufficiently low to permit bonding at temperatures (e.g., 1150 7C) well below the solidus temperatures typical of common nickel-base superalloys. Given the rapid changes in the design of NiAl-based materials, the present article concentrates on the mechanisms governing microstructural development in an NiAl/Cu/Ni model system, rather than on detailed process data for an individual NiAl alloy–Ni-base superalloy combination. II.
EXPERIMENTAL TECHNIQUES
NiAl/Cu/Ni TLP bonds investigated in
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