On the Role of Grain Boundary Serration in Simulated Weld Heat-Affected Zone Liquation of a Wrought Nickel-Based Superal
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INTERGRANULAR microfissuring, which is frequently observed in a weld heat-affected zone (HAZ), is a main weldability concern in repair of nickel-based superalloys. It has been generally known that this microfissuring in HAZ is associated with constitutional liquation of precipitates at grain boundaries (GBs),[1–4] or incipient melting of GBs that are sufficiently segregated by melting point depressants, especially boron.[5,6] As HAZ liquation microfissuring is an intergranular phenomenon at elevated temperatures, the great concerns could be given to the character of GBs in the HAZ of superalloys.[5,7,8] For instance, Guo et al.[5] showed that the crystallographic character of GB influenced boron segregation, hence, GB liquation in the HAZ of Inconel 718 (Special Metals Corporation, New Hartford, NY). Therefore, grain boundary engineering (GBE) aims to increase the fraction of special boundaries that could be applied to superalloys to improve weldability as well as resistances to intergranular fracture caused by creep, fatigue, and stresscorrosion cracking. However, it has been realized that GBE processing,[9,10] which is achieved by repeated HYUN UK HONG, Senior Researcher, IN SOO KIM, BAIG GYU CHOI, YOUNG SOO YOO, and CHANG YONG JO, Principal Researchers, are with the High Temperature Materials Research Group, Korea Institute of Materials Science, 797 Changwondaero, Sungsan-gu, Changwon, Gyeongnam 641-831, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted March 18, 2011. Article published online July 28, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
cycles of deformation and annealing, finds it difficult to be practically used in a wide range of applications. It has been reported that GB serration can occur in nickel-based superalloys when controlled cooling treatments are employed.[11–15] The phenomenon of GB serration can be defined as the formation of serrated GB, which shows wavy morphology in nature. The serrated GBs are distinguishable from flat or smoothlycurved GBs because their amplitudes are larger than 500 nm.[15] Recently, the present authors have found that GB serration occurs in the absence of adjacent coarse c¢ particles or M23C6 carbides when a specimen is direct-aged with a combination of slow cooling from solution treatment temperature to aging temperature.[16,17] They have shown that this serration leads to a change in GB character as a special boundary based on the crystallographic analysis, demonstrating that the GBs tend to serrate to have specific segments approaching to one {111} low-index plane at a boundary. They also found that the creep resistance can be improved up to 40 pct by GB serration in Alloy 263. The mechanisms of the formation and growth of GB serration are not clearly understood yet. However, they suggest that the driving force of serration stems from a sufficient decrease in interfacial free energy of GB even though the GB area is increased. This process would surely be accompanied by the decrease of the system’s free energy. The present work was initiated t
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