METALS AND METAL MATRIX COMPOSITES |
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Improvement of Microstructures and Segregation Behavior of the Third Generation Nickel-based Directionally Solidified Superalloys by Carbon and Boron Additions |
WANG Xiaojuan1,2, LIU Lin1, ZHAO Xinbao3, HUANG Taiwen1, YANG Wenchao1, ZHANG Jun1, FU Hengzhi1
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1 State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072 2 School of Science, Xi’an Polytechnic University, Xi’an 710048 3 School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 |
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Abstract Nickel-based superalloys have been widely used in aircraft and industrial gas turbine. However, the casting characteristics of these modern high-refractory alloys are significantly worse than those of earlier alloys, such as a wider range of crystal orientations, the formation of grain defects, hot tearing and so on. To improve these properties of the alloys, minor elements of carbon (C) and boron (B) additions have been reco-gnized one of effective methods. But unfortunately, few investigations were related with higher generation directionally solidified superalloys containing rhenium (Re) up to now. In this paper, the microstructure and dendritic segregation of Ni-based superalloy containing Re (5wt%) were investigated by directional solidification with withdrawal rates of 200 μm/s. The results showed that the addition of carbon and boron did not remarkably change primary dendrite arm apacing λ1. Carbon and boron additions varied the volume fraction of the γ/γ′ eutectic and the distribution of carbides. The volume fraction of γ/γ′ eutectic was reduced and the amount of carbides increased with increasing carbon content; when boron was added into the alloy, the volume fraction of γ/γ′ eutectic was increased and the volume fraction of carbides decreased. The morphology of carbides was mainly Chinese script-type. The average size of carbides became smaller and their distribution was sparse at lower carbon content. However, these carbides were much bigger and their distribution was denser at higher carbon content. The morphology of Chinese script-type carbides changed from a rod to a part sheet, then again to a larger sheet with the increase of boron content. The rise of segregation coefficients of Re and W was followed by the fall with the increase of carbon content, the peak value of segregation coefficient was found at 0.040wt% carbon content. The effect of boron on segregation was just opposite of carbon. For the element of Ta, there was not significant tendency of segregation variation with the increase of carbon and boron contents.
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Published: 29 August 2019
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About author:: Xiaojuan Wangreceived her Bachelor of Science degree in July 2002 and Master of Engineering degree in June 2007 from Shaanxi Normal University. From 2002 to date, she works at Xi’an Polytechnic University. Now, she is studying for her Ph.D. at Northwestern Polytechnical University, focusing on the research of microstructure and mechanical properties of directionally solidified superalloy.Lin Liureceived his Ph. D. in current inaugural university in 1988 and was a research fellow of Alexander von Humboldt Foundation in Germany from 1991 to 1992. His research interests are alloy development and solidification structure control of nickel-based superalloys, high thermal gradient directional solidification and electro-magnetic processing of metallic materials. These researches integrate artificial intelligence, computer modeling in automated design, experimental investigations of structure and properties at laboratory scale, and manufacturing applications. Dr. Liu have published about 210 research papers in internationaland leading domestic journals in metallic materials science and given 15 keynote or invited presentation in international conference in aeronautical material area. |
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