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The fatigue crack growth (FCG) behavior and microstructure of Ni-based alloy welded joint, fabricated by narrow gap tungsten inert gas welding (NG-TIG), were systematically investigated in this paper. The microstructure of weld metal (WM) and base metal (BM) are columnar grains and equiaxed grains respectively, plentiful irregular carbides aggregate at grain boundaries and the number of twin crystals decreases distinctly in HAZ due to the thermal input. The FCG tests results indicated that a higher FCG rate (d a /d N ) as well as a lower FCG threshold (Δ K th ) for WM were obtained compared to BM and HAZ. It was found that crack deflection and branching in the whole welded joint occurred at FCG path in near-threshold regime. The misorientation of grain boundaries is considered as the main factor to account for the deceleration and retardation behavior revealed by electron back-scattered diffraction (EBSD) analysis. EBSD results also indicate that the misorientation of 20–30° is a critical value from a tortuous path to a relative straight path. A contrasting morphology of Paris regime and near-threshold regime showed the fracture mode transition from tensile mode to intense shear mode. [ABSTRACT FROM AUTHOR]

The resolution dependence of orientation gradients was studied in a well-annealed 99.9995% pure polycrystalline copper pulled to failure in tension. Owing to the well developed neck, different regions in the sample correspond to different tensile strains. Post-mortem characterization was performed using EBSD on cross-sections containing the tensile axis. Kernel average misorientation (KAM) was calculated as a metric to establish correlation between defect accumulation and microstructural features, with a threshold of 5° to focus on intra-granular gradients. The region with the lowest strain (2%) showed high KAM values adjacent to grain boundaries compared to the grain interior, regardless of the point spacing, i.e. the spatial resolution. However, in the region with the highest strain (13%) a strong dependence on resolution was found. For point spacings of 0.5 μm or smaller, the same correlation of high KAM with locations near boundaries was found. At coarse spacings i.e. low spatial resolution, by contrast, the reverse was found in that the highest KAM values appear in the grain interiors, as previously observed in X-ray microscopy on the same sample which had a similar coarse resolution. An analysis of orientation gradients parallel to, and perpendicular to boundaries suggested that the latter tend to be the larger of the two. This helps to explain why boundary-adjacent points have low KAM values. The conclusion is that measurement of local orientation gradient requires a resolution that is comparable to the dislocation substructure. [ABSTRACT FROM AUTHOR]