Deformation Behavior of ReCr Sigma Phase Diffusion Barrier on Ni-based Superalloy
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0980-II08-07
Deformation Behavior of ReCr Sigma Phase Diffusion Barrier on Ni-Based Superalloy Seiji Miura, Kenji Ohkubo, Tetsuo Mohri, Takayuki Yoshioka, and Toshio Narita Mat.Sci. and Eng., Hokkaido Univ., Kita-13, Nishi-8, Sapporo, 060-8628, Japan
ABSTRACT In order to attain a better oxidation resistance of Ni-based superalloys for high performance engines, a ReCr-based σ-phase diffusion barrier coating between the MCrAlY bond coat (Al reservoir) layer and a Ni-based superalloy substrate has been proposed to suppress the diffusion of Al and other elements. The deformation behavior of TMS82+ superalloy specimens with coating layers composed of an Al-rich surface layer (corresponding to a bond coat) and a ReCr σ-phase inner layer is investigated. In-situ observation of the bending tests using a Laser scanning confocal microscopy revealed that the crack formation in the Al-rich surface layer(s) on the tension side of the bend specimen occurs at an early stage of bending. On the other hand, small cracks in the σ-phase layer under the Al-rich surface layer(s) are formed at the slip traces in the Ni-based superalloy single crystal substrate. This suggests a good adhesion between the σphase and the Ni-based superalloy. At the compression side, in both the Al-rich surface layer(s) and the σ-phase layer no cracking was observed until the Al-rich surface layer(s) shows buckling. INTRODUCTION For more than several decades large efforts have been dedicated for the development and modification of Ni-based superalloys. In order to attain a better oxidation resistance and heat resistance, base materials are coated by a thermal barrier coatings (TBC) composed of a ceramic top coat and an Al-rich bond coat [1]. The Al-rich bond coat is either vacuum plasma-splayed MCrAlY (M is Ni and/or Co) or a Pt-Al based alloy. The diffusion of oxygen through the ceramic topcoat forms Al2O3-based thermal grown oxide (TGO) layer between the topcoat and the Al-rich bond coat. To maintain the performance of the high temperature component for longterm use, the Al-rich layer is required to act as an Al-reservoir layer for providing enough amount of Al for TGO formation. However, the difference of compositions between the Alreservoir layer and the Ni-based superalloy substrate causes the interdifusion of Al and other elements, resulting in the degradation of both the Al-reservoir layer and the Ni-based superalloy substrate.
Fig.1 The phase field of σ-phase in the Cr- Re-Ni ternary phase diagram and the crystal structure of σ-phase. In order to suppress the diffusion, various kinds of diffusion barriers such as nitride-, carbide-, oxide- and refractory metal-based materials have been proposed [2, 3]. Recently Narita et al. [4] has proposed a ReCr σ-phase for the diffusion barrier, and found that the thin ReCr σphase of about 10 µm containing certain amount of Ni is effective to suppress the diffusion of not only Al but also other elements at the temperature range of about 1150 oC. The Re-Cr-Ni ternary phase diagram and the crystal structure of th
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