Microstructure variation effects on room temperature fatigue threshold and crack propagation in Udimet 720Li Ni-base superalloy

An assessment of the effects of microstructure on room temperature fatigue threshold and crack propagation behaviour has been carried out on microstructural variants of U720Li, i.e. as-received U720Li, U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent gamma' variant). Fatigue tests were carried out at room temperature using a 20 Hz sinusoidal cycling waveform at an R-ratio = 0.1 on 12.5 mm x 12.5 mm square cross-section SENB specimens with a 60 degrees starter notch. U720Li-LG showed the highest threshold delta K (delta K-th), whilst U720Li-LP showed the lowest delta K-th value. U720Li-LP also showed higher crack growth rates in the near-threshold regime and at high delta K (although at higher delta K levels the difference was less marked). Crack growth rates of U720Li and U720Li-LG were relatively similar both in the near-threshold regime and at high delta K. The materials showed crystallographic stage I type crack growth in the near-threshold regime, with U720Li showing distinct crystallographic facets on the fracture surface, while U720Li-LG and U720Li-LP showed mostly microfacets and a lower proportion of large facets. At high delta K, crack growth in the materials becomes flat and featureless indicative of stage II type crack growth. The observed fatigue behaviour, which is an effect of the combined contributions of intrinsic and extrinsic crack growth resistances, is rationalized in terms of the microstructural characteristics of the materials. Enhanced room temperature fatigue threshold and near-threshold long crack growth resistance are seen for materials with larger grain size and higher degree of planar slip which may be related to increased extrinsic crack growth resistance contributions from crack tip shielding and roughness-induced crack closure. Differences in the deformation behaviour, either homogeneous or heterogeneous due to microstructural variation in this set of materials may provide approximately equivalent intrinsic crack growth resistance contributions at room temperature.