13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- Conclusions The differences in fatigue life behavior in Inconel 718DA are studied here for two different microstructures. In both microstructures of the present study, the average grain size is smaller than the particle (nitrides) size and nevertheless crack initiation sites are different. In the larger (10µm) grain size material, stage I crystallographic cracking at or near the surface is initiated on the large grains of the material located at the free surface and having a diameter about 30 µm. In small (7µm) grain size material, crack initiation occurs preferentially on internal TiN particles followed by crack propagation with crystallographic character and subsequent Stage II crack propagation. A fish-eye crack is formed. The particles initiating the internal fatigue cracks are substantively larger than the largest particles observed on cut section from microstructural analyses. The internal crack occurs on nitrides of a mean diameter of 30µm. The different stages of the fish eye cracks are identified with: I. fracture of the large nitride during loading (or during forging), II. vacuum near threshold micropropagation with crystallographic character, III.vacuum stage II propagation (few striations), IV.crack reaches the specimen free surface, and propagation in air with marked striations and V.final ductile fracture. These crack progression stages are discussed using data from the literature to gather elements for quantitative assessment of the contribution of each crack progression step to the total specimen lifetime. Acknowledgements Snecma, Safran group, is gratefully acknowledged for financial support and material provision. References [1] C. Bathias, A. Pineau, Fatigue des Matériaux et des Structures 3, Hermes science publication, (2009). [2] D. F. Paulinis, J.J. Schirra, Alloy 718 at Pratt & Whitney – Historical perspective and future challenges, Superalloys 718, 625, 706 and derivatives, ed. by E. E. Loria, TMS, (2001). [3] R. L. Saha, K. Gopinath, K. K. Sharma and M. Srinivas, Low cycle fatigue behaviour of alloy 718 disc forging at elevated temperatures, Superalloys 718, 625, 706 and derivatives, ed. by E. E. Loria, TMS, (2001). [4] W. L. Mills and C. M. Brown, Fatigue fracture surface morphology for alloy 718, Superalloys 718, 625, 706 and derivatives, ed. by E. E. Loria, TMS, (2001). [5] J. Warren, D.Y. Wei, The cyclic fatigue behaviour of direct aged 718 at 149, 315, 454 and 538 °C, Materials Science and Engineering A 428, 106-115, (2006). [6] S. P. Lynch, T. C. Radtke, B. J. Wicks and R. T. Byrnes, Fatigue crack growth in nickel-based superalloys at 500-700°C. II: Direct aged alloy 718, Fatigue Fract. Engng Mater. Struct. Vol. 17, No. 3, pp. 313-325, (1994). [7] S. Deyber, F. Alexandre, J. Vaissaud and A. Pineau, Probabilistic life of DA718 for aircraft engine disks, Superalloys 718, 625, 706 and derivatives, ed. by E. E. Loria, TMS, (2005). [8] M. Stoschka, M. Stockinger, H.Maderbacher and M. Riedler, A closed concept to associate the hot-forging process controlled microstructure with fatigue life, Proceedings Superalloys 2012, TMS, (2012). [9] F. Alexandre, Probabilistic and microstructural aspect of fatigue crack initiation in IN718, PhD thesis, Ecole des mines de Paris, (2004). [10] V. Zerrouki, Inconel 718 et tenue en fatigue oligocyclique. Influence de la microstructure et prédiction de la durée de vie, Mémoire de DRT Génie des Matériaux, université EVRY, (2000) [11] F. Alexandre, S. Deyber, A. Pineau, Modelling the optimum grain size on the low cycle fatigue of a Ni based seperalloy in the presence of two possible crack initiation sites, Scripta Materialia, 50, pp. 25-30, (2004),. [12] Y. Desvallèes, M. Bouzidi, F. Bois, N. Beaude, Delta phase in Inconel 718 : Mechanical
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