MICROSTRUCTURE VARIATION EFFECTS ON ROOM TEMPERATURE FATIGUE THRESHOLDS 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 γ′ variant). Fatigue tests were carried out at room temperature using a 20Hz sinusoidal cycling waveform at an Rratio=0.1. U720Li-LG showed the highest threshold ΔK (ΔKth), whilst U720Li-LP showed the lowest ΔKth value. U720Li-LP also showed higher crack growth rates in the near-threshold regime and at high ΔK (although at higher ∆K levels the difference was less marked). Crack growth rates of U720Li and U720Li-LG were relatively similar both in the near-threshold and high ΔK regime. 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 ΔK, crack growth in the materials becomes flat and featureless indicative of stage II type crack growth. The observed performance of the materials is rationalised in terms of their microstructural characteristics. Enhanced room temperature fatigue threshold and long crack growth resistance are seen for larger grained materials due to increased extrinsic crack growth resistance contributions from crack closure. Differences in heterogeneity of deformation behaviour in this set of material variants appear to give approximately equivalent intrinsic crack growth resistance at room temperature due to the respective effects of each deformation behaviour on intrinsic crack growth resistance.