13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- than in the wrought materials, which is consistent with the fact that the carbides were “cut” during the forging process (Table 2). During forging, clusters of smaller carbides are formed that are oriented according to the fibre direction. These results were obtained from a 1mm² surface. Figure 4 : Microstructures of the different materials, comparison of the distribution of carbides and nitrides, in the radial direction. Table 2 : Comparison of the distribution and the size of the carbides in the three materials, analysis zones of 1mm². The same analysis was carried out for the nitrides which are particles with sharper angles than the carbides (Figure 4). Their density seems similar in both materials, only their size differs weakly. To carry this study further, and compare the distribution of the nitrides in the two materials, a larger area of analyses should be used to assess enough particles for a statistical study. Table 3 : Comparison of the distribution and the size of the nitrides in the three materials, analyse zones of 1mm². The last microstructural comparison concerns the proportion of δ phase in the two materials of the study (Figure 5). The fraction of δ phase is three times higher in material 2 compared to material 1; they are respectively of 3.0% and 1.1%. This δ phase precipitates preferentially on grain boundaries. Figure 5 : Comparison of the delta phase distribution between the two materials. 3.2. Mechanical test results The difference of yield stress between the material 1 and 2 is observed; they are respectively 1170 MPa and 1250MPa. Fatigue properties of these two materials are compared in relation with the grain sizes. The results are presented in Figure 6, the pseudo-stress values are normalised by the Young modulus divided by the mean yield stress of the material, and for proprietary reason this Material 1 Material 2 Material before forging 100µm 100µm 100µm
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