Nanoindentation Induced Deformation Near Grain Boundaries of Corrosion Resistant Nickel Alloys
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Nanoindentation Induced Deformation Near Grain Boundaries of Corrosion Resistant Nickel Alloys F. William Herbert1, Bilge Yildiz2 and Krystyn J. Van Vliet1 1 Department of Materials Science & Engineering, Massachusetts Institute of Technology, 77 Mass. Av., Cambridge, MA 02139 2 Department of Nuclear Science & Engineering, Massachusetts Institute of Technology, 77 Mass. Av., Cambridge, MA 02139 ABSTRACT The damage accumulation behavior of different grain boundary structures in Inconel 690 (Ni-29wt%Cr-9wt%Fe) was investigated in the presence of large, localized plastic strains induced by nanoindentation. Spatially-resolved hardness was measured as a function of lateral distance from ‘random’ high-angle grain boundaries and twin boundaries. The confinement of induced defects between the indenter tip and grain boundaries did not lead to significant differences in measured hardness between high angle and twin boundaries. Critical “pop-in” loads indicating the onset of incipient plasticity were lower within 1μm of grain boundaries, but were statistically equivalent for random and twin boundaries. These results suggest a comparable extent of dislocation mobility and absorption at the different grain boundary types in Inconel 690 under ambient conditions. INTRODUCTION Grain boundaries are regions of crystalline incompatibility during plastic flow where dislocations may originate [1] and/or accumulate [2] during shear. Grain boundary engineering to increase the fraction of “special”, low-energy boundaries -has been shown to improve resistance to creep [3], fracture [4], embrittlement and corrosion [5]. The response of twin (CSL Σ3) and ‘random’ high-angle grain boundaries to localized plastic strains is therefore likely to vary significantly. Understanding this nanoscopic behavior subsequently provides insight into the chemomechanical origins of complex degradation mechanisms such as inter-granular stress corrosion cracking. Previous nanoindentation experiments have variably suggested that local hardness: is raised by 10-15% within 1μm either side of grain boundaries in BCC alloys [6], is raised by ~20% on grain boundaries with a corresponding reduction in critical pop-in loads required to nucleate dislocations in Fe [7], either increases by 50% or decreases slightly within 2μm near grain boundaries in Cu [8], and remains statistically homogeneous across grain boundary regions in Ni3Al [9]. Although grain boundary types were reported in these studies, a clear picture of the misorientation effect on damage accumulation behavior has not emerged. This study focused on the boundary character influence on both nanohardness and critical loads for incipient plasticity in the vicinity of different grain boundaries in the nickel-based alloy Inconel 690.
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EXPERIMENTAL A polycrystalline sample of Inconel 690 alloy (Ni-29wt%Cr-9wt%Fe) was annealed at 1107oC for 15 minutes and water quenched to produce a grain size of roughly 60μm with a wide variety of grain orientations. The sample was mechanically polished with a final suspensi
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