Atomic diffusion and phase equilibria at the interfaces of the CoAl/Ir multilayer on Nb 5 Si 3 -base alloys
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I. INTRODUCTION
NICKEL-BASE superalloys have been leading the history of heat-resistant alloys for a long time, and their potential, durability, and reliability are developing even today.[1,2] Nowadays, from the viewpoint of conserving the global environment, it is important not only to improve the commercial alloys currently used but also to develop new materials that are applicable at ultra-high temperatures. It is quite beneficial to raise the applicable temperatures of gas turbines and jet engines for enhancing the efficiency of energy conversion. A possible approach from the materials science side is to design new heat-resistant alloys beyond Ni-base superalloys. “Beyond” would imply the development of new materials and many researchers are working on and struggling with materials such as Nb-silicides, Mo-silicides, and precious metal alloys such as Ir and Pt base.[3–14] Applicable temperatures must go beyond 1273 K, targeting 1473 K, and hopefully reaching 1773 K. As an example, the quaternary Co-Ni-Al-C system is of interest since E21 Co3AlC, an ordered crystal structure similar to L12 Ni3Al, can be used as an excellent strengthener. Our group reported a series of investigations to design E21 Co3AlC-based heat-resistant alloys[15,16] as well as (Nb)/Nb5Si3-based alloys.[5] Either way, we have to consider not only mechanical properties but also oxidation, interdiffusion, and phase equilibria. YOSHISATO KIMURA, Assistant Professor, TATSUYA SHIMIZU, Graduate Student, and YOSHINAO MISHIMA, Professor, are with the Materials Science and Engineering Department, Tokyo Institute of Technology, Yokohama 226-8502, Japan. Contact e-mail: [email protected] SACHIYORI SHINA, formerly Graduate Student, Materials Science and Engineering Department, Tokyo Institute of Technology, is with Canon Co., 410-1196 Shizuoka, Japan. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMDCorrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
Protection against severe oxidation damage is always an inevitable issue in designing heat-resistant alloys and it becomes much more important as the temperature increases. Functionally, multilayered thermal coatings are an attractive solution that does not substantially sacrifice mechanical properties. This idea was originally proposed by Hosoda et al. aiming at the improvement of oxidation resistance by designing smart coatings based on IrAl alloys at ultra-high temperatures.[17–20] A schematic drawing is presented in Figure 1(a) for a typical functionally multilayered thermal coating, which consists of several layers such as thermal barrier coating layer, oxidationresistant layer, diffusion-barrier layer, and stress relaxation layer. Phase equilibria informati
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