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This is an author-produced, peer-reviewed version of this article. ABSTRACTGrain boundary engineering, which increases the special boundary fraction, may improve microstructural stability during annealing. Different processing routes are undertaken to establish the effectiveness of each and to better understand which microstructural features determine the resulting stability. We find that multiple cycles of grain boundary engineering result in a material that resists abnormal grain growth better than other processing routes despite similarities in special boundary fraction, grain size, and general boundary connectivity among as-processed materials.

Alloy 617, a high temperature creep-resistant, nickel-based alloy, is being considered for the primary heat exchanger for the Next Generation Nuclear Plant (NGNP) which will operate at temperatures exceeding 760 o C. Orientation imaging microscopy (OIM) is used to characterize the grain boundaries in the vicinity of creep voids that develop during high temperature creep tests (800-1000 o C at creep stresses ranging from 20-85 MPa) terminated at creep strains ranging from 5-40%. Observations using optical microscopy indicate creep rate does not significantly influence the creep void fraction at a given creep strain. Preliminary analysis of the OIM data indicates voids tend to form on grain boundaries parallel, perpendicular or 45 o to the tensile axis, while few voids are found at intermediate inclinations to the tensile axis. Random grain boundaries intersect most voids while CSL-related grain boundaries did not appear to be consistently associated with void development.

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Recently, NiTiHf-based HTSMAs have been shown to exhibit unique precipitation and mechanical behavior. In this study, a rolled plate of NiTiHf HTSMA was homogenized and heat treated at various times and temperatures and characterized using a barrage of analytical techniques including high-energy synchrotron X-ray diffraction (SR-XRD). Neither homogenization nor any of heat treatments studied significantly affect the austenitic or martensitic transformation temperatures. H-phase was observed to precipitate at heat treating times below 30 minutes and then to subsequently dissolve away for times of 30 minutes and above. The presence of H-phase dramatically increases the material strength by almost a factor of 2, Lastly, an over-aging effect occurs with increasing time due to the disappearance of the H-phase.