13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- image, ECCI. The purposes are to get a better understanding of the mechanisms for sub-surface fatigue crack initiation in the matrix or formation of SNDFCO in the VHCF regime and the formation of FGA. 2. Materials and Experimental 2.1 Materials Four metal materials: one nickel base alloy (Alloy 690), one martenistic–austenitic steel (MAS), one martensitic-ferritic (or bainite) steel (MFS), and one titanium alloy (Ti6Al4V), were used in this investigation. Table 1 and 2 show their nominal chemical compositions and mechanical properties. Table 1: Chemical compositions (wt%) and conventional tensile properties of the materials used Alloy C Si Mn Cr Ni Ti Fe N Mo σYT (MPa) σUT (MPa) Alloy 690 0.018 0.30 0.27 29.6 58.85 0.26 10.51 0.026 305 582 MAS 0.38 0.4 0.6 13.5 1.0 1468 1968 MFS 0.23 0.25 0.65 1.3 2.7 0.25 986 1200 Table 2: Chemical composition (wt%) and conventional tensile property of titanium alloy Alloy C Al V N O H σYT (MPa) σUT (MPa) Ti 6Al4V 0,008 5.6 4.1 130ppm 940ppm 6ppm 876 952 Fig. 1 shows the microstructures of these four materials. Alloy 690 is a single phase material. The others are two or multi-phase materials. For MAS and MFS materials, the microstructures are very fine as shown in the TEM images. (a) (b)
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