The oxidation behavior of Ni-based superalloys and Cu-Zr based amorphous ribbon

Autor: Chen, Wei-Sheng, 陳偉昇
Rok vydání: 2014
Druh dokumentu: 學位論文 ; thesis
Popis: 102
The oxidation behavior of Ni-based superalloys, containing Hastelloy X (X), Incoloy 800H (800H), and Inconel 617 (617), and their welds, containing the original substrate (M-SUB, where M could be X, 800H, or 617), the heat-affected zone (M-HAZ) and the melting zone (M-MZ) was studied at 950oC in dry/wet air. The results indicated that the oxidation kinetics of all the alloys in dry air followed a one- or two-stage parabolic-rate law, indicating that diffusion is the rate-controlling step during oxidation. In general, the steady-state rate-constants (kp values) of the M-HAZ are always faster than those of the M-SUB, while those of M-MZ may change dramatically, depending on the filler, having Inconel 82 (82) for 800H, Inconel 625 (625) for X, and Inconel 617 (617) for 617. Based on the oxidation kinetics studied, the kp values of 800H-MZ are slower than those of 800H-SUB, while those of X-MZ are faster than those of X-SUB, and nearly no difference is obtained for both 617-MZ and 617-SUB. The oxidation kinetics of the alloys in wet air initially revealed a mass-gain stage, while a significant mass-lose stage was observed for a prolong exposure; The 800H-HAZ revealed the fast oxidation rates among the alloys studied. Duplex scales always formed on all the alloys, consisting of an outer-layer of Cr2O3 and MCr2O4 (M=Fe, Mn, or Ni) mixture and an exclusive inner-layer of Cr2O3. In addition, the internal-oxidation precipitates of SiO¬2, and Al2O3 which some dissolved Ti were always observed just beneath the scale/substrate interface. The scales formed in wet-air environment are similar to those in dry air, except for forming porous structure. Furthermore, Fe2O3 and Fe3O4 were also observed for 800H-SUB and 800H-HAZ after a longer exposure of time (> 48h), indicative of an accelerated oxidation for the two zones in wet air. In addition, the oxidation behavior of a binary Cu50Zr50 bulk amorphous ribbon (CZ2-AR) was also studied over the temperature range of 350 - 425oC in dry air. The oxidation kinetics of the CZ2-AR generally followed a parabolic-rate law, indicating that diffusion is the rate-controlling step during oxidation. The oxidation rates of the CZ2-AR were strongly temperature-dependent, with its scaling-rate constants (ks values) increasing with temperature. Duplex scales formed on the CZ2-AR alloy were composed of an outer-layer of exclusive CuO and of a heterophasic inner-layer of CuO, monoclinic-ZrO2 (m-ZrO2), and tetragonal-ZrO2 (t-ZrO2). The CZ2-AR substrate started to form the crystalline Cu10Zr7 phase beneath the scales during the oxidation at T > 375oC, indicative of the crystallization of the amorphous substrate.
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