Studies of electrical discharge surface alloyed superalloy Haynes 230 and its high temperature oxidation behaviors

• Main Authors: Ching-Yuan Bai, 白清源
• Other Authors: 顧鈞豪
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/11144372990344435291

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 94 === This research is to examine the surface modification (treatment) of the Ni-based superalloy Haynes 230 through electrical discharge alloying (EDA) method. The effect of EDA parameters on the alloying surface conditions; such as the composition, the microstructure, and the surface integrity of the superalloy are all investigated. In the EDA process, the alloying elements, Al and Mo, would be blended into the surface layer of the superalloy. Therefore, an alloyed layer with rich Al and minor Mo is expected to form on the surface of the superalloy. The micro-hardness test and the isothermal oxidation examination at high temperature are performed to inspect the EDA results on the mechanical and chemical properties of the alloy surface layer. The EDA process is carried out with a conventional die-sinking electrical discharge machine. A uniform mixture of the powders with 85 at% Al and 15 at% Mo is applied for the surface alloying materials. The Al-Mo powder mixture is made to be a green-compact composite electrode by the powder metallurgy process. To reach the purpose, the surface alloying treatments may employ two different ways: (1) By using the Al-Mo composite electrode and utilizing the pure kerosene or the distilled water as the dielectric (working fluid), or (2) By using the electrolytic copper electrode and adding/suspending the Al-Mo powder mixture in the dielectric (kerosene or distilled water), during the EDA process. For employing the Al-Mo electrode with positive polarity, the alloyed layer constituted mainly of the NiAl phase is formed on the EDA specimen, in either the kerosene (P-AlMo-Kero specimen) or the distilled water (P-AlMo-Water specimen). The alloyed layer of P-AlMo-Kero contains a mixture of NiAl, Al8Mo3, Cr23C6, and Al4C3; while that of P-AlMo-Water consists of NiAl, AlCr2, Al5Cr, and Al2O3 phases. The P-AlMo-Water exhibits the highest hardness, whereas the P-AlMo-Kero owns the smallest surface roughness and with the best alloying efficiency among all the EDA specimens. Employing the Al-Mo electrode with negative polarity and alloying in kerosene (N-AlMo-Kero specimen), a lot of discontinuous piled-layers comprised mostly of Al3Mo8 and AlMo3 phases are accumulated on the surface of the specimen; while alloying in distilled water fails owing to the difficulty of discharging under such an EDA condition. For suspending Al-Mo powder mixture in kerosene and utilizing the Cu electrode with negative polarity, the Cr23C6, WC1-x, and the graphite phase may exist in the recast layer of the EDA specimen (N-Cu-Kero(AlMo)). While applying the Cu electrode with positive polarity, the EDA specimen, P-Cu-Kero(AlMo), exhibits no alloying phase. For using the Cu electrode and adding Al-Mo powder mixture in distilled water, the electrical discharge alloying (EDA) cannot be proceeded. Doesn''t matter with positive or negative electrode polarity, the discharging procedure is always disturbed by a lot of metal oxides. The superalloy Haynes 230 and the EDA specimens are subjected to isothermal oxidation at 1000 ~ 1200℃ in static air. Experimental results of the oxidation at 1000℃ and 1100℃, respectively, indicate that a dense and continuous oxide scale composed of Al2O3 forms on the surface of the P-AlMo-Kero specimen and protects the substrate of the superalloy from oxidation. The P-AlMo-Water specimen shows a superior oxidation resistance at 1000℃, but lose its protective ability at 1100℃ or 1200℃, respectively, in static air, because of the structure defects in its alloyed layer by electrical discharge process. The oxidation resistance of the N-AlMo-Kero, N-Cu-Kero(AlMo), and P-Cu-Kero(AlMo) specimens with invalid alloying results is even worse than that of unalloyed superalloy Haynes 230. In summary, the P-AlMo-Kero specimen is more resistant to the oxidation than the unalloyed superalloy and the other EDAed specimens, and its effective anti-oxidation temperature can be elevated to 1100℃.