The effect of grain boundary misorientation on the intergranular cracking behavior of nickel-16chromium-9iron in 360 degrees C argon and high purity water.

The effect of grain boundary misorientation on the intergranular cracking behavior of pure Ni-16Cr-9Fe was assessed by determining if low angle boundaries (LABs) or coincident boundaries (CSLBs) are more crack resistant than general high angle boundaries (GHABs) in argon and high purity water. Using selected area channeling patterns imaged from grains on electropolished surfaces of coarse-grained Ni-16Cr-9Fe tensile bars, grain boundaries were classified as either LABs, CSLBs, or GHABs in accordance with the coincident site lattice model of grain boundary structure. Cracking susceptibility of boundary types was determined using constant extension rate tensile tests (CERTs) in 360$\sp\circ$C argon atmosphere and in deaerated, high purity water. Proportions of coincident boundaries were increased from 12-20% in the annealed samples to 27-44% in samples given sequential tensile deformation and heat treatments; GHAB proportions decreased accordingly. Observtions indicate that boundary densities in CSLB-enhanced samples ranged from 0.01 to 0.08, while those for annealed samples ranged from 0.07-0.10, indicating that samples with increased proportions of CSLBs are more crack resistant. No LABs cracked in either environment, although CSLBs cracked in both high purity water and argon. In annealed samples, 6.7% of characterized CSLBs cracked in water, while 1.5% of CSLBs cracked in argon; 9.3% of characterized GHABs cracked in water and 6.6% GHABs cracked in argon. Thus, CSLBs are more crack resistant than GHABs in either water or argon, and both are more crack resistant in argon than in water. The higher amounts of cracking and the higher CSLB cracking susceptibility in high purity water indicates the presence of an environmental effect on cracking behavior. The beneficial effect of LABs and CSLBs is likely due to the ability of these boundaries to induce slip in neighboring grains by either transmitting or absorbing and re-emitting lattice dislocations, thereby reducing grain boundary stresses and propensity for intergranular crack initiation. However, lower proportions of GHABs may be necessary to reduce crack propagation. The results indicate that control of grain boundary proportions can improve the intergranular stress corrosion cracking susceptibility of pure Ni-16Cr-9Fe.PhDNuclear EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105364/1/9124003.pdfDescription of 9124003.pdf : Restricted to UM users only