Joining of NiAl to iron-based alloys by reactive casting
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INTRODUCTION
AN intermetallic compound of nickel monoaluminide, NiAl, has attractive combinations of low density (5.86 g/ cm3), high melting temperature (1911 K), high strength, good corrosion and oxidation resistance, high thermal conductivity, and low cost.[1–4] Therefore, it has been slated for high-temperature applications, such as in hot-end components of the turbine blades of aeronautic engines. However, NiAl is brittle at temperatures below 600 K, which is one of the most significant disadvantages of NiAl and impedes the practical use of it as a structural material.[5,6,7] Although reducing the grain size or alloying with some elements can improve the ductility of NiAl, the degree of the improvement is not remarkable.[8,9] A possible way to overcome the problem of the brittleness of NiAl and to apply it to practical use will be joining of it to ductile structural materials. Gale and Orel[10] and Gale and Guan[11] studied joining between NiAl and nickel using the transient liquid phase (TLP) bonding method.[12] In those two studies, a Ni-Si-B ternary eutectic alloy foil and copper foil were used as the interlayer material, respectively. During holding the joint couple at a temperature higher than the melting point of the interlayer material, diffusion solidification[13] of the interlayer proceeds, and it leads to the joining between the two base materials. The TLP bonding is a simple and useful technique; however, its most significant disadvantage is a long processing time, 40 hours at 1423 K, for example.[11] We have proposed a new joining technique with a very short processing time based on an exothermic reaction, demonstrating the joining between NiAl and a structural material.[14,15,16] In that technique, NiAl is synthesized from elemental powders of aluminum and nickel by the selfpropagating high-temperature synthesis (SHS or combustion synthesis), and at the same time, it is joined to the base KIYOTAKA MATSUURA, Instructor, and MASAYUKI KUDOH, Professor, Division of Materials Science and Engineering, and HIROSHI KINOSHITA, Instructor, and HEISHICHIRO TAKAHASHI, Professor, Center for Advanced Research of Energy Technology, are with the Hokkaido University, Hokkaido 060-8628, Japan. Manuscript submitted September 6, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
material. Namely, aluminum and nickel powders placed on a steel block exothermically react and produce NiAl when heated to 900 K under a pressure, and the NiAl produced is simultaneously joined to the steel. The processing time of that technique is as short as a few minutes. Although we have proved the feasibility of the joining technique based on the SHS reaction in our previous study,[16] the tensile strength of the NiAl synthesized was lower than that of hotextruded NiAl.[6] It was suggested that oxide inclusions in the NiAl, which originated in the oxide film on the surface of the elemental powders, reduced the strength of the NiAl. We have considered that if elemental liquids of aluminum and nickel can be used as the starting materials ins
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