13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- The methodology developed in this work is, to some extent, similar to the “local approach to fracture” (LAF) which is now widely used for predicting the variation of fracture toughness of ferritic steels with temperature (see e.g. [16]). However the LAF approach has been much less developed for high temperature applications and for Ni base superalloys. This is the reason why the present study should be considered as a preliminary attempt in this field. 2. Material and experimental procedures The chemical composition of the material is, in weight%: Ni54,18Cr17,97Fe17,31(Nb+Ta)5,4Mo2,97Ti1Al0,56Co0,14Mn0,08Cu0,03C0,023B0,0041. This heat of Inconel 718 alloy was received as a forged disk. It was given a conventional heat-treatment (720 °C – 8 h + 620 °C – 8 h) directly after forging (DA). This resulted in a small grain size (5 - 15 µm) with particles of the stable phase δ (Ni3Nb), precipitated along the grain boundaries (Fig. 1a) with small strengthening γ’ (Ni3TiAl) and γ” (Ni3Nb) precipitates (Fig. 1b). Figure 1. a) Optical micrograph showing the crystalline structure of the material and δ phase particles located at the grain boundaries (in white); (b) SEM observations showing γ’ and γ’’ precipitates within a grain. FCGR tests were performed between 500°C and 650°C under an air environment using KBr specimens shown in Figure 2. A small EDM semi-circular notch (depth 0.3mm) was introduced and the specimens were fatigue-precracked at room temperature (RT) and at a frequency of 2 Hz in order to extend the notch over a distance by about 0.3mm (Fig. 2b). Figure 2. KBr specimens used for fatigue crack growth rate measurements. a) Schematic diagram showing the dimensions (mm) and the position of the EDM notch (red dot); b) Fracture surface of a specimen where the EDM notch (0.3mm), the fatigue pre-crack (0.6mm) and the propagation area can be distinguished.
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