Transient liquid phase diffusion bonding of an oxide dispersion strengthened nickel superalloy

Oxide dispersion strengthened (ODS) superalloys possess a specialized microstructure due to the presence of a dispersion of Y2O 3 particles in the metal matrix, which gives these advanced alloys excellent creep strength properties at elevated temperatures. In order to join ODS alloys, suitable joining techniques need to be developed with the ability to minimize damage to the unique microstructure of these alloys. Transient liquid phase (TLP) diffusion bonding has been of interest as an alternative joining method for advanced alloys when fusion welding techniques are not successful. Although the TLP bonding technique can produce joints that have a microstructure and mechanical properties similar to the parent alloy for an ideal case (i.e., pure metals), formation of intermetallic precipitates as well as some melting of the parent metal at the joint interface was reported when joining ODS alloys. Both these effects have a tendency to disrupt the microstructure of the joint region and hence the joint interface is weakened.;The formation of intermetallic precipitates adjacent to the joint interface and the parent metal dissolution were observed for bonds made using MBF 80 (Ni-Cr-B), MBF 50 (Ni-Cr-Si-B), MBF 15 (Ni-Cr-Fe-Si-B-Co) and Ni-P foil interlayers. The amount of precipitation and the degree of parent metal dissolution increased with an increase in the total volume of melting point depressant in the interlayer. The results showed that the use of a low bonding temperature with moderate applied pressure during bonding could reduce the amount of agglomeration adjacent to the joint interface.;The effects of the boron content in the coatings and the thickness of the coatings on the microstructural development across the joint region were found to be significant when bonds were made using Ni-B coatings. No visible precipitate formation and parent metal dissolution were observed for the bonds made using a 2µm thick coating with a boron content of 0.32 wt.%. The agglomeration of dispersoids normally associated with thicker interlayers was not observed when thin electrodeposited coatings were used.;In this thesis, the TLP bonding process has been employed to join a nickel based ODS superalloy, MA 758, using a variety of bonding variables, i.e., interlayer composition and thickness, bonding temperature, time and pressure to investigate the parameters affecting the TLP bonding process. The effect of post-bond heat treatments on TLP bonding behavior has also been studied. Furthermore, in order to control the extent of parent metal dissolution, the use of electrodeposited coatings based on Ni-B and Ni-P as interlayers has been investigated. A theoretical study on the effect of bonding time, temperature and pressure on the isothermal solidification process and parent metal dissolution was performed and the calculated results were compared with the experimental results.