PSI - Issue 23

M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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According to TEM data, the twist of 100 nm blocks by 2° degrees leads to the displacement of atoms by more than 1.7 nm and that significantly exceeds the interatomic distance. Such disorder causes the formation of the amorphous layer, which promotes the crack spread. The proposed model explains the change in the mechanism of crack growth from inter-nanocrystalline to quasi-faceted at the beginning of the air access into the crack space (Fig. 2). After that, the crack surfaces oxidized and the oxide film prevents the "cold welding" process.

Fig. 5. (a) - scheme formation of dislocations in the material under the condition of air access to the crack; (b, c) and in the condition of vacuum.

5. Conclusion It was found that a layer of highly misoriented nanocrystals with a size of 50 – 200 nm is formed in the first stage of crack formation. It has been established that fatigue cracks propagate along the boundaries of nanocrystals. A model is proposed to explain the mechanism for the formation of the misorientation of nanocrystals during the crack development without air access. The difference in the mechanisms of crack growth with and without air access is found and explained. 1. J.E. King, 1982. Surface damage and near-threshold fatigue crack growth in a ni-base superalloy in vacuum. Fatigue of Engineering Materials and Structures Vol. 5, No. 2, pp. 177-188,. 2. R.J. Kashinga, L.G. Zhao, V. V.Silberschmidt,1 R. Jiang, and P.A.S. Reed, 2018. A diffusion-based approach for modelling crack tip behaviour under fatigue-oxidation conditions. Int J Fract. 213(2), pp. 157 – 170 3. J. Radavich, D. Furrer, T. Carneiro, J. Lemsky, 2008. The Microstructure and Mechanical Properties of EP741NP Powder Metallurgy Disc Material. Superalloys 2008, TMS, pp 63-72. 4. Trunkin I.N. Artamonov M.A., Ovcharov A.V., Vasilyev A.L. 2009. Structural study of defects in granulated nickel alloy EP741NP. Crystallography Reports. T. 64. № 4 . pp. 539-543 (Rus) 5. Yousuf M., Sahu P.C., Jajoo H.K., Rajagopalan S., Govinda Rajan K., 1986. Effect of magnetic transition on the lattice expansion of nickel. Journal of Physics F., V. 16., №. 5 , pp. 373-380. 6. Taylor A., 1950. Lattice parameters of binary nickel-cobalt alloys. Journal of the Institute of Metals, V. 77, pp. 585-594. 7. Nagakura S., 1958. Study of metallic carbides by electron diffraction Part II. Crystal structure analysis of nickel carbide. Journal of the Physical Society of Japan, V. 13, №. 9 , pp. 1005-1014. 8. Taylor D., 1984. Thermal expansion data: I. Binary oxides with the sodium chloride and wurtzite structure, MO. Transactions and Journal of the British Ceramic Society, V. 83, №. 1 , pp. 5 – 9. 9. Pertlik F., 1986. Structures of hydrothermally synthesized cobalt (II) carbonate and nickel (II) carbonate. Acta Crystallographica. V. 42, №. 1 , pp. 4-5. 10. Broek, D., 1982. Elementary engineering fracture mechanics. Springer Science, pp 469. 11 V.I. Vladimirov, 1984. Physical nature of destruction. M .: Metallurgy, pp. 280. (Rus) 12 Oguma, H.; Nakamura, T., 2013. Fatigue crack propagation properties of Ti-6Al-4V in vacuum environments. International Journal of Fatigue, 50, pp.89-93. References