ON THE MECHANISM OF SERRATED GRAIN BOUNDARY FORMATION IN NI-BASED SUPERALLOYS WITH LOW γ' VOLUME FRACTION

The mechanism of serrated grain boundary (GB) formation and its effect on the intergranular properties in a wrought Ni-based superalloy Alloy 263 containing γ' volume fraction below 10% have been studied. It was newly discovered that GBs are considerably serrated in the absence of γ' phase or M23C6 at the GBs. The crystallographic analysis revealed that the GBs tend to serrate to have specific segments approaching the {111} low- index plane at a boundary so that interfacial free energy of GB can be decreased, which may be responsible for the driving force of GB serration. The formation of serrated GBs with lower energy leads to a change in the carbide characteristics. The creep resistance can be remarkably improved by serrated GBs, which is associated with a lower rate of cavitation and crack propagation through the modification of carbide characteristics as well as GB configuration. Furthermore, serrated GBs are highly resistant to liquation cracking in simulated weld-heat affected zone due to their lower tendency to be wetted and penetrated by the liquid phase. Electron energy-loss spectroscopy (EELS) studies suggest that serration is triggered by the discontinuous segregation of C and Cr atoms at GBs for the purpose of relieving the excessive elastic strain energy. The role of this lattice distortional energy due to the atomic size difference is confirmed on the serration behaviors through binary nickel alloy model systems. The formation of serrated GBs without GB 2 nd phase particles begins to occur in Ni-Zr binary system when the content of Zr with large atomic size is higher than 0.05%, which might lead to a sufficient development of stress exerting a force on GBs.