| The objective of this study was to determine whether grain boundary deformation plays a direct role in intergranular stress corrosion cracking of Ni-16Cr-9Fe in high temperature water. The difference in deformation and cracking behavior between special grain boundaries, or coincident site lattice boundaries (CLSB), and high angle boundaries (HAB) is exploited to determine if a cause-and-effect exists between grain boundary deformation and IGSCC. It was hypothesized that coincident site lattice boundaries (CSLBs) are less susceptible to deformation than general (random) high angle boundaries (HABs), and are, therefore, less susceptible to cracking. The hypothesis was substantiated by showing that (1) the dislocation absorption kinetics differs substantially between CSLBs and HABs, resulting in different susceptibilities to deformation, and (2) grain boundary deformation is a precursor to intergranular cracking.; The difference in dislocation annihilation kinetics at CSLBs and HABs was determined by TEM on samples annealed at 360°C. Results showed that extrinsic grain boundary dislocations (EGBDs) are annihilated at HABs at a rate that is, on average, 3 times that at CSLBs, implying a grain boundary diffusion coefficient in CSLBs is 12 times lower than that in HABs. The expectation that a reduced EGBD absorption at CSLBs would lead to greater matrix hardening was investigated using nanohardness measurements. Results showed that the hardness in the vicinity of CSLBs is greater than near HABs, and the sample-average hardness increases with the fraction of CSLBs.; The difference in deformation behavior was investigated by SEM examination of samples strained at 360°C in an inert environment. Following 10% plastic straining in 360°C Ar, 52% of the HABs and 15% of the CSLBs were found to display deformation. The relationship between grain boundary deformation and IGSCC was demonstrated by further exposing these samples to 5% strain in 360°C primary water. Resulting analysis showed that 61% of the boundaries that deformed in argon also cracked in water while only 17% of cracked boundaries showed no prior deformation in argon. These results show that there is a cause-and-effect relationship between grain boundary deformation and IGSCC. |
