13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- were compared at 600°C, and strong oxidation effects are observed in particular on the NbC particles. This oxidation mechanism increases the volume of the particles and allows the initiation and propagation of local cracks because of stress concentration [9]. In the present study, the two materials which were investigated have a mean grain size smaller than the particles size because of the forging parameters used and the DA heat treatment. Two grain sizes were selected. In the case of small grain size the scatter of fatigue life is always present [10]. So it is necessary to understand which one of these parameters really affects the fatigue life. In the present study the microstructural features are assessed via experimental observation methods. Mechanical fatigue tests are carried out and fatigue crack initiation mechanisms assessed via fractography observations. The various modes of crack initiation encountered and subsequent propagation are discussed using data from the literature to gather the ingredients for a quantitative assessment of the contributions of each crack progression step on the fatigue lifetime. 2. Materials and experimental methods The experimental pancakes used in this study were forged by Snecma. The pancakes, which were water quenched after forging, have been directly aged with the standard aging treatment, and will be designated as DA718 in the following. The standard aging treatment is: 720°C /8h/ Furnace cooling (50°C/h) down to 620°C/8h and subsequent air cooling. In the present study, two kinds of forging parameters have been investigated (such as temperature, strain amplitude or cooling speed). Indeed material 1 presents a cooling speed slower, as well as a hold time at temperature higher than the delta phase solvus temperature longer than the material 2. Inconel 718 is a nickel based superalloy hardened by γ’’ (Ni3Nb) and γ’ (Ni3[Ti,Al]) precipitates. This alloy also contains δ (Ni3Nb) phase. The δ phase particles are located both along the grain boundaries and within the grains, depending on the heat treatment applied to the alloy. Another category of coarse second phase particles present in the alloy are nitrides (TiN) and carbides (NbC) with a typical size ranging from 5 to 20 µm. These particles are preferentially located at grain boundaries [14]. Table 1 : Chemical composition of Inconel 718 in weigth % Al B C Cr Co Cu Fe MnMo Ni Nb+Ta Ti O N 0.56 0.0041 0.023 17.97 0.14 0.03 17.31 0.08 2.97 54.18 5.40 1.00 0.001 0.0062 Microstructures were assessed in term of grain size, particle (carbides and nitrides) and δ phase sizes and their distributions. Samples were polished up to 1µm diamond paste. Micrographs of these samples were taken using a Scanning Electron Microscope (SEM), LEO VP 1450 SEM. To measure precipitates and particles size and their distributions, image analysis software, ImageJ®, was used. Electron back scatter diffraction (EBSD) scans were also carried out on polished samples. EBSD patterns were collected using a Hikari digital camera, installed on a LEO SEM system. LEO VP 1450 SEM is equipped with an EDAX TSL OIM EBSD data acquisition and processing software and a Hikari digital camera. Operating conditions: high voltage 30 kV, probe current 2000 pA, working distance 22.5 mm, tilt angle 70 deg, step size 0.5 micrometers (hexagonal grid). Four areas of 500µm² were scanned on each material. Data cleanup by "Grain dilation" with threshold values of grain size and misorientation angle of 2 pixels and 5 degrees, respectively. All the mechanical tests were performed on specimens taken from the two forged material which were submitted to DA heat treatment. The specimens were oriented along the tangential direction of the pancakes. Fatigue tests were performed using a servo-hydraulic MTS testing machine with a maximum loading of 250kN. This machine was equipped with a resistance heating furnace allowing a
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