13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- maximum temperature of 1200°C. The fatigue tests were carried out in air at 450 °C ± 2 °C. The temperature control was realized with thermocouple welded on each head of the sample surface (outside the gauge length). These specimens had a diameter of 5 mm and a gauge length of 13 mm. The specimens were tested under 1Hz triangular cycles under strain control. SEM observations of the fracture surfaces allowed us to identify the nature of the fatigue crack initiation sites using a Zeiss Gemini SEM. 3. Results and discussion 3.1. Microstructure Figure 1 shows EBSD data for the two materials. Differences in grain size are observed. In material 1, the average size of grains is about 10µm whereas it is only about 7µm in material 2. The distribution of the grain size is also different (Figure 2); the scatter of the distribution of grain size is larger in material 1. EBSD data also show that both materials do not have a preferred crystal orientation (i.e. texture in Figure 3). Figure 1 : Grain microstructures observed by EBSD scans with a step of 0.5 µm for a) material 1 and b) material 2 0 2 4 6 8 10 12 14 16 0 5 10 15 20 25 30 Grain diameter (µm) Area fraction (%) Material 2 Material 1 Figure 2 : Grain size distributions of the 2 materials calculated from the EBSD data on 1mm² zones Figure 3 : Pole figure of the texture of the two materials The size and the distribution of the particles (carbides and nitrides) were also determined by SEM observations on cut-sections first (Figure 4). Carbides are bigger in the initial, non-wrought material
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