Microstructural Evolution and Mechanical Properties Evaluation of IN-939 Bonds Made by Isothermal Solidification of a Liquated Ni-Cr-B Interlayer

Abstract

The IN-939 superalloy was bonded under a high vacuum condition by isothermal solidification of a liquated Ni-Cr-B amorphous foil with a 38 μm thickness as the interlayer. The progression of isothermal solidification was completed at 1120 °C in the bonding time of 45 minutes and a single-phase gamma solid solution was formed at the centerline. Insufficient bonding times led to a formation of centerline eutectic products composed of gamma solid solution, Ni-rich boride, and Cr-rich boride phases. A reverse relationship between the bonding time and the width of eutectic micro-constituents was perceived. Some precipitates with blocky and widmanstatten morphologies were observed in the diffusion-affected zone (DAZ) which seemed to be Cr-rich borides. The maximum precipitation depth, the number, and the area fraction of DAZ precipitates were investigated. It was revealed that the microhardness distribution across the joints was governed by changes in the area fraction of DAZ precipitates. The applied shear tests demonstrated an increment in shear properties of the bonds by increasing the bonding time. A remarkable joint efficiency of ~ 83 pct accompanied by a high failure energy (~ 35 J) and displacement (~ 1.8 mm) was achieved in the bond made at 1120 °C for 60 minutes. The fractographic examination of the fracture surfaces illustrated two different dimple-like and semi-cleavage morphologies for the bonds with complete isothermal solidification and partial isothermal solidification, respectively.