M 3 B 2 and M 5 B 3 Formation in Diffusion-Affected Zone During Transient Liquid Phase Bonding Single-Crystal Superalloy
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HIGH-TEMPERATURE brazing involves the interlayer liquid during the bonding process.[1,2] It has been used to bond or repair the single-crystal superalloys,[3] titanium alloys,[4–6] and other crack sensitive structural alloys[7,8] owing to its high reliability. However, in order to improve the high-temperature properties of the bonding joint, transient liquid phase (TLP) bonding is developed with a longer bonding stage during which the interlayer liquid will isothermally solidify completely.[9] Therefore, there are no brittle eutectics which are generated by the cooling of the residual liquid after brazing, and, after post-heat treatment, it increases the high-temperature properties of the bonding joint. TLP bonding now received a lot of investigations due to its potential to bond high-alloyed materials, for example, superalloys,[10,11] duplex stainless steels,[12] and metal composite materials.[13,14] There are usually four stages during TLP bonding,[15] which are melting of the interlayer, dissolution of the substrate and homogenization, isothermal solidification, and post-heat treatment. Several models[9,15,16] have already been established to illustrate the TLP bonding
NAICHENG SHENG, PhD Student, JIDE LIU, Associate Professor, TAO JIN and XIAOFENG SUN, Professors, ZHUANGQI HU, Professor, Academian, are with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, No. 72 Wenhua Road, Shenyang 110016, P.R. China. Contact e-mail: [email protected] XIAOBING HU, PhD Student, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China Manuscript submitted April 23, 2014. Article published online 6 January 2015
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process. However, deviations from the traditional bonding models were found during the isothermal solidification stage when TLP bonding these real world materials.[17–19] For example, lots of precipitates formed in the diffusion-affected zone (DAZ),[20] which is different from the predictions of the currently available TLP models that predict a precipitate-free substrate.[15,16,21] Idowu[22] has investigated the microstructures when TLP bonding IN738 cast superalloys. It has been found that a significant volume fraction of complex fcc Cr-MoW rich carbo-borides precipitated in the joint/base alloy interface region. However, there were no more detailed investigations on the precipitation behaviors between these borides and the substrate. Also, it was found that the kinetic of the isothermal solidification stage deviated from the standard parabolic models.[18] The recent study on the liquid/solid interface migration during the isothermal solidification stage proposed that after the borides formed, the solidification kinetics slows down.[23] Simultaneously, another study of the B distribution proved the existence of the B peak at the DAZ.[20] It has also been found that the precipitates in the DAZ may have different morphologies.[23] However, there are still no systematic and de
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