13th International Conference on Fracture June 16–21, 2013, Beijing, China smooth and notch components with and without dwells are still needed. Therefore, based on the experimental investigation of DZ125 [16-18] at high temperatures, one purpose of this study is focused on LCF behavior analysis. The other purpose seeks to develop acceptable approaches for predicting the LCF life under complicated load condition. The slip systems based critical plane method, combined with critical distance concept and Miner’s linear cumulative damage theory are adopted to evaluate their feasibility on those complicate load condition. 2 LCF behavior of DZ125 at elevated temperatures The chemical compositions (wt %) of DZ125 are 0.07-0.12C, 8.4-9.4Cr, 9.5-10.5Co, 6.5-7.5W, 1.5-2.5Mo, 4.8-5.4Al, 0.7-1.2Ti, 3.5-4.1Ta, 0.01-0.02B, 1.2-1.8Hf and balanced Ni. The alloy roughcast is melted in vacuum induction furnace, the solution and aging heat treatment procedure is as follows: 1180℃/2h +1230℃/3h/AC +1100℃/4h/AC +879℃/20h/AC (AC means air cooling). As shown in Fig. 1, the microstructure of DZ125 superalloy mainly contains γ-matrix and γ’-reinforced phase. Up to now, the author and co-worker have carried out systemically experimental investigation work on low cycle fatigue (LCF) tests of DZ125 at high temperatures. As is shown in Table 1, the experimental work is focused on exhibiting the influence of temperature, strain ratio, stress concentration, dwell types and dwell times on fatigue resistance. It is worth notice that the expression of dwell forms, such as Tt/Tc, means holding Tt seconds at tensile peak and Tc seconds at compressive peak. Smooth round bar, U-type and O-type notched plate specimens were adopted, their geometries are shown in Fig. 2. Corresponding experimental results is shown as follows. 2.0 Cyclic stress response Cyclic stress response at 850 and 980℃ at longitudinal (L) orientation is shown in Fig. 3, the cyclic stress range changes obviously at the early cycles for each strain range, and then tends to be stable soon. The strain range related cyclic hardening behavior is observed at 850℃，the larger the strain range is, the more obvious cyclic hardening phenomenon happened. However, this material also seems to be strain range independent slight cyclic softening at 980℃. Those cyclic behaviors observed above are similar to that of Ni-based superalloy CM247LC [2]. Grain boundary Eutectic Carbide Fig. 1 Microstructure of DZ125 after heat treatment Partial view：C d = 0.5±0.02 mm ρ = 0.2±0.02 mm (c) U-type notch, Kt=4.35 (b) O-type notch,Kt=3.0 (a) Smooth, Kt=1.0 other Fig. 2 Geometries and dimensions of specimens
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