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Transient Liquid Phase bonding in the nickel base superalloy CM 247 LC Cheng, Jacky Man-Lam

Abstract

In this work, the effects of temperature and time were investigated on the microstructure and properties of Transient Liquid Phase bonds in the directionally solidified superalloy CM 247 LC with a commercial braze filler material MBF-80 (Ni-Cr-B-C). Specimens were bonded at 1070°C, 1125°C and 1170°C for durations of 15 minutes, 1 hour and 4 hours. The resultant microstructures were then examined metallographically using optical and scanning electron microscopes. The mechanical properties were determined by micro-hardness and tensile tests. The microstructure of bonded joints contained blocky and acicular precipitates in the base material. The chemical compositions of these precipitates were found to be similar and were nickel depleted and tungsten rich compared to the base metal. Mechanical tests made it evident that completion of the isothermal solidification in a TLP bond joint does not guarantee satisfactory joint properties. Extensive precipitation of hard and brittle phases and chemical inhomogeneity in the joint microstructure warrants further postbond heat treatment. Models of the TLP bonding process, available in the literature, were applied in this work. The apparent activation energy for the diffusion of boron in CM 247 LC was found to be 219 kJ/mol. Analytical models predicted isothermal solidification times in close agreement with experimental observations. Two numerical models were developed in this work included a moving boundary model based on diffusion coefficients and equilibrium compositions and a phenomenological model coupled to thermodynamic and atomic mobility databases. The moving boundary model predicted satisfactory isothermal solidification times. The phenomenological model predicted times which were generally lower than those found experimentally. From this work, it is apparent that the mobility data available for boron is insufficient for the current system.

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