PSI - Issue 19

Yasuhiro Yamazaki et al. / Procedia Structural Integrity 19 (2019) 538–547 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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et al. (2015)) , since the turbine inlet temperature exceeds 1700℃ in advanced gas turbines. Therefore, they are subjected to rather complex stress and temperature cycles due to the temperature gradients that occur during start-up and shut-down or from temperature gradients within the component during steady-state operation (Bermstein et al. (1993)). The accumulation of such stress and temperature cycles leads to the possibility of failure by thermo mechanical fatigue (TMF) loading (Ito and Kobayashi (2015)). Many efforts have been made to estimate the TMF lives of Ni-base superalloys. Zhou et al revealed that the TMF life of single crystal Ni-base superalloy remarkably decreased due to the compression hold at the TMF loading cycle and depended on the volume fraction of γ’ precipitate (Zhou et al . (2003)). The controlling and deformation mechanisms were intrinsically different between the thermomechanical fatigue and isothermal low-cycle fatigue loadings in the Ni-base superalloys (Okazaki et al. (2003)). Hough et al also investigated the cyclic deformation and lifetime behavior of a cast Ni-based superalloy under both thermomechanical fatigue and isothermal low-cycle fatigue (LCF) conditions. They compared some life prediction models to evaluate the possibility of predicting the TMF lifetime by using LCF data. In addition, they also showed that the life prediction model based on the micro crack propagation proposed by Miller et al (Miller et al. (1993)) can give satisfactory TMF life prediction by using LCF data. Early growth behaviors of naturally initiated small cracks in order of sub-millimeter in size can provide some essential information for the life and the remaining life prediction because most part of fatigue life under TMF and LCF conditions is generally dominated by the propagation process. Especially, the wall thickness of the superalloy blade with the cooling flow path is thin (some millimeters), the understanding of the small crack propagation mechanisms becomes important. The small cracks may grow with the propagation rate significantly higher than that of physically long cracks (Okazaki et al. (1996)). Moreover, the growth of small cracks is notably affected by microstructures, such as grain boundary and strengthening precipitates, and also by the environment. In this study, physically short fatigue crack growth behavior in a single crystal Ni-base superalloy is investigated under the out-of-phase type TMF conditions focusing on the effect of temperature on the crack propagation behavior. In order to investigate the crack opening behavior, the numerical calculation is also conducted by means of finite element analysis taking into consideration the temperature dependences in <001> elastic modulus and yield stress together with the creep deformation. The material tested in this study is one of the typical single crystal nickel base superalloy, ICMSX-4. The chemical compositions of the materials are 6.49Cr, 0.6Mo, 1.0Ti, 5.69Al, 9.65Co, 6.37W, 6.57Ta, 0.09Hf, 2.97Re, Ni balance (mass %). The multistep solution heat treatments were given under the conditions at 1277℃ for 2h → at 1296°C for 3h → at 1304°C for 3h → at 1313°C for 3h → at 1316°C for 3h → at 1321°C for 2h in Ar. After the solution heat treatments, the aging treatments were applied at 1080°C for 4h → at 880°C for 20h in Ar. The microstructure of the tested materials is shown in Fig. 1. The material comprises the γ matrix strengthened by the cuboidal γ’ precipitates of size ranging from 0.3 μm to 0.5 μm (approximately 60% – 65% by volume). Solid cylindrical smooth specimens with a diameter of 6 mm and gauge section with a length of 16mm were machined from the materials. The specimen geometry is shown in Fig. 2. The specimen axis is parallel to the solidification direction and within 5 degrees from <001> crystallographic orientation. In this study, the flat surfaces were machined on the specimen surface of the gage section within 5 degrees of the {100} crystallographic plane. The mechanical small notch with a semi-circular shape of approximately 0.1 mm length was made by the electric discharge machining at the center of the flat surface. The surfaces of all specimens were mechanically polished to mirror surface using alumina powders before and after the tests to remove machining marks and the oxide film. The observations and measurements of crack growth behavior were conducted periodically by using a laser microscope after cooling to room temperature. 2.2. Crack propagation test 2. Experimental procedures 2.1. Material