PSI - Issue 7

Ludvík Kunz et al. / Procedia Structural Integrity 7 (2017) 44–49 Ludvík Kunz / Structural Integrity Procedia 00 ( 201 7) 000–000

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concentration. These sites are often casting defects or pores. Their role in fatigue crack initiation in IN 713LC at high temperatures were investigated e.g. by Kunz et al. (2012). Under conditions of high mean stresses, small amplitudes and high temperatures another site of fatigue crack initiation may appear, namely the intergranular creep cracks. The fractographic observation of the fracture surfaces corresponding to the combination of high mean stresses and small stress amplitudes did not prove crack initiation on casting defects. All fatigue cracks were initiated on large intergranular creep cracks. The time to fracture of specimens subjected to loading at high temperatures and small high-frequency stress amplitudes is governed by the interaction of two mechanisms. For the stress amplitudes below the threshold the decisive mechanism of fracture is creep damage. Observation of fracture surfaces of specimens reveals the characteristic intergranular fracture with typical dendritic features and presence of small transcrystalline facets. They initiate on large intergranular cracks which were created by creep mechanism. Iintergranular fracture surface area, marked by ”C”, can be seen in the middle of the Fig. 4(c). This area corresponds to the intergranular crack created by creep. The arrow points to the place where an internal fatigue crack was initiated from the creep intergranular crack. It can be further seen in Fig. 4(c) that more of such cracks were initiated from this intergranular creep crack. The fatigue fracture surface is transcrystalline and non-crystallographic, which is consistent with the conclusions drawn by Šmíd et al. (2016), who found that with increasing temperature the crystallographic initiation and early propagation of fatigue cracks in MAR-M 247 is replaced (above 800 °C) by non-crystallographic one. The fatigue cracks which initiate on sufficiently large creep cracks in the final phase of the creep process are not able to shorten the lifetime effectively because their contribution to the reduction of the load carrying cross-section of the specimen is small. There is no time for their growth and formation of main fatigue crack. The time to fracture is thus primarily defined by the creep damage. The stress amplitudes above the threshold are sufficient for initiation and propagation of fatigue cracks which break the specimen earlier than it happens due to creep mechanisms. 6. Conclusions The high-frequency stress amplitude superimposed on mean stress at temperature 800 °C for IN 713LC and at 900 °C on MAR-M 247 does not influence the time to fracture until it reaches a certain threshold value. Then the time to facture rapidly decreases. This behavior is related to the gradual change of the damage mechanism leading to final fracture. Creep damage governs the time to facture for cycling with the stress amplitude below the threshold while fatigue damage by initiation and propagation of fatigue cracks on intergranular creep cracks is the main factor which controls the time to fracture above the threshold. Acknowledgements This research was supported by the project CZ.01.1.02/0.0/15 019/0004399 of the Ministry of Industry and Trade of the Czech Republic. The support is gratefully acknowledged. References Heywood, R.B., 1962. Designing Against Fatigue. London, Chapmann and Hall. Lesne, P.M., Gailetaud, G. 1987. In: Mechnical Behavior of Metals, Yan, M. G. et al., eds., Pergamon, Oxford, p. 1053. Lukáš, P., Kunz. L., Svoboda, J., 1997. Retardation of creep in <001>/oriented superalloy CMSX-4 single crystals by superimposed cyclic stress. Mat. Sci. Eng. A234-236, 459-462. Suresh, S., 1998. Fatigue of Materials. Cambridge, Cambridge University Press. Scheffler, K.D., 1972. Interaction between creep, fatigue, and strain-aging in 2 refractory-metal alloys. Metall. Trans. 3, 167-177. Sklenička V., 1999 . Quantitative assessment of creep damage and creep life prediction. In: Modelling of microstructural evolution in creep resitant materials. Strang A., McLean M, eds., IOM Communications, London. Šmíd, M., Horník, V., Hutař, P., Hrbáček K ., Kunz, L., 2016. High Cycle Fatigue Damage Mechanisms of MAR-M 247 Superalloy at High Temperatures. Trans Indian Inst. Met. 69, 393 – 397. Vitovec, F.H. 1957. On dynamic creep with special consideration of strain rate effects. Proc. Am. Soc. Test. Mater. 57, 977-986.