Phase Constitution and Oxidation Resistance of B2 (Ir, Co)Al
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Phase Constitution and Oxidation Resistance of B2 (Ir, Co)Al Hideki Hosoda, Hiroshi Noma* and Kenji Wakashima Precision and Intelligence Laboratory (P&I Lab), Tokyo Institute of Technology (Tokyo Tech), 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Phone&Fax 81-45-924-5057, email [email protected] *Graduate Student, Tokyo Institute of Technology ABSTRACT B2 iridium aluminide (IrAl) is hopeful for use as an ultrahigh temperature oxidation resistant coating above 1600K. In this study, the effect of Co substitution for Ir on phase constitution, hardness and oxidation behavior was studied for IrAl alloys. Alloys of (Ir, Co)-50mol%Al with various Co contents were fabricated by Ar-arc melting followed by hot-forging at 1773K. Oxidation behavior was evaluated using thermogravimetry (TG) in Ar-67%O2 up to 1823K. XRD and SEM were also carried out for alloy characterization. It was found that a continuous B2 solid solution (Ir,Co)Al is formed between IrAl and CoAl. Depending on the Co concentration, the oxidation products identified after heating to 1873K in Ar-67%O2 were Ir, IrO2 and A2O3 and/or Co2AlO4. Thin and continuous Al2O3 layers were observed after isothermal oxidation at 1673K when Co content is more than 20mol%Co. In this case, the weight change by isothermal oxidation at 1673K becomes higher with decreasing Co content. The (Ir,Co)Al alloys containing 20-40mol%Co exhibit higher oxidation resistance than CoAl and IrAl, and thus oxidation resistance of CoAl is improved by Ir addition. INTRODUCTION Since the oxidation resistance of ultrahigh temperature metallic and carbon-base materials is intrinsically poor, excellent coating materials are required. As the practical high temperature coatings, multifunctional layered structures are promising [1]. The requirements of a multifunctional coating are (1) good oxidation resistance, (2) low oxygen permeability, (3) self-healing, (4) similar coefficient of thermal expansion to substrate, (5) slow diffusion between coatings and substrate, (6) strong adhesion to substrate and (7) low thermal conductivity for the thermal barrier coating (TBC). Alloys of IrAl can form a self-heating multifunctional layered structure composed of an Ir layer and an Al2O3 layer [1-3]. Due to its low oxygen permeability [4], the Ir layer is the oxygen diffusion barrier of the multilayer coating. The Al2O3 layer protects the Ir layer from oxidation. It should be emphasized that the oxygen permeability of Ir is a few orders of magnitude lower than those of Al2O3 and SiO2 at ultrahigh temperature, and that 1µm thickness of Ir layer is comparable or superior to 1mm thickness of SiO2 and Al2O3 layers above 2000K. However, the oxidation resistance of the IrAl binary alloys is still insufficient [1-3]. Lee and Worrell have reported that Ir-Al alloys containing more than 55mol%Al forms continuous Al2O3 layer and oxidation resistance is much improved in this case [5]. A problem for Ir-Al alloy containing more than 52mol%Al is that the material will be at least partially melted over 1900K
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