Role of misorientation in fatigue crack growth behavior for NG-TIG welded joint of Ni-based alloy.

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]