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

The Effect of Grain Boundary Character Distribution on the High Temperature Deformation Behavior of Ni–16Cr–9Fe Alloys. B.Alexandreanu, B.H.Sencer, V.Thaveeprungsriporn, G.S.Was: Acta Materialia, 2003, 51[13], 3831-48

 Table 7

 Bulk Diffusivity of 51Cr in Ni-16.49Cr-7.40Fe-0.015wt%C