Study of Metallic Carbide (MC) in a Ni-Co-Cr-Based Powder Metallurgy Superalloy
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INTRODUCTION
THE powder metallurgy (PM) technique, which offers segregation-less microstructures, fine grain size, and uniform distribution of precipitates, is introduced to consolidate superalloys.[1–5] As the most common consolidation method of PM techniques, hot isostatic pressing (HIP) has been applied to the consolidation of many high-performance superalloys such as Rene´ 95,[1] LSHR,[2] EP741NP,[3] Astroloy,[4] and Inconel 718.[5] However, some problems such as gas porosity and undesired precipitation of carbides along previous particle boundaries (PPBs) are also introduced into the PM superalloys.[6] Dispersed carbides at grain boundaries suppress the crack propagation and intergranular sliding during high-temperature deformation. But the presence of PPB carbides promotes interparticle fracture and leads to lower ductility and inferior stress rupture properties at elevated temperatures.[4,5,7] Previous research showed that the formation of PPB was associated with the segregation of carbon and WEN-BIN MA and PENG-HUI HU, Students, and BEN-FU HU, Professor, are with the School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. GUO-QUAN LIU, Head of Materials Science Department, is with the School of Materials Science and Engineering, and also with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing. Contact e-mail: [email protected] YI-WEN ZHANG, Senior Engineer, is with the School of Materials Science and Engineering, University of Science and Technology Beijing, and also with the High Temperature Materials Research Institute, Central Iron and Steel Research Institute, Beijing 100081, P.R. China. Manuscript submitted March 2, 2013. Article published online October 2, 2013 208—VOLUME 45A, JANUARY 2014
oxygen on the powder surface, and most of the PPB precipitates were confirmed to be carbides and oxides.[1,7] However, the formation mechanism of PPB carbides is still in dispute. Dahlen et al.[8] suggested that less stable metallic carbide (MC) in the bulk of the powder particles decomposed during HIP, and stable MC redistributed to the oxide/oxide interfaces of contacted powders for their lower interfacial energy. An alternative explanation was that the oxide particles distributed on the surface of powders provided stable sites for further growth of PPB carbides in the form of Ostwald ripening. The growth of the PPB carbides was fed by a large amount of the less stable carbides in the interior. The existence of ZrO2 nuclei in PPB MC was confirmed by Menzies et al.,[9] but it was agreed that the formation of PPB carbides included two processes: the decomposition of MC in the bulk of the powder particles and the selective precipitation of the stable MC at PPB. Both suggestions were focused on the precipitation of stable MC on the PPB, but the decomposition mechanism of less stable MC of the as-atomized powders was seldom mentioned. Much work has been done to reduce or eliminate the PPB by promoting the s
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