The effect of grain boundary character distribution on the high temperature deformation behavior of Ni–16Cr–9Fe alloys

The objective of this work was to test the Thaveeprungsriporn model for the dependence of creep rate on the coincident site lattice (CSL) fraction. The model attributed the large reduction in creep rate in alloys with a high population of CSL boundaries to the greater difficulty of extrinsic grain boundary dislocation (EGBD) absorption at coincident site lattice boundaries (CSLBs) vs. high angle boundaries (HABs). Ease of EGBD absorption was assessed by measuring the annihilation rates of EGBDs in both CSL-related and HABs following an anneal at 360 °C. Results showed that EGBDs are annihilated at HABs at a rate that is on average three times that at CSLBs, implying a grain boundary diffusion coefficient in CSLBs that is 12 times lower than that in HABs. The expectation that a reduction in EGBD absorption would lead to greater matrix hardening was investigated using nano-hardness measurements. Results showed that the hardness in the vicinity of CSLBs is greater than that near HABs, and the grain-averaged hardness increases with the fraction of contiguous CSLBs. Further, strain hardening is greater in CSL-enhanced samples than in reference, solution annealed samples. These results taken together substantiate the hypothesis that CSLBs impede dislocation absorption into the grain boundary, thereby increasing lattice hardening and internal stress in the sample, resulting in a reduced creep rate.