13th International Conference on Fracture June 16–21, 2013, Beijing, China -105. Summary In the present study, the effects of the artificial second-step-aging prior to deformation as well as the effects of dynamical aging during deformation in Fe-18Ni maraging steels (300 and 350 grades) were investigated. The analysis of the increase in hardness with aging time in the second-step-aging at 473 K and 673 K showed that the activation energy of second-step-aging is 113 kJ/mol for initially under-aged 300G steel and 82.9 kJ/mol for initially peak-aged 350G steel. This activation energy corresponds to that of the pipe diffusion of solute atoms along dislocations. The solute atmosphere is considered to form around dislocations via the pipe diffusion and impede the motion of dislocations. Particularly the dynamic aging process takes place in the fatigue tests at 473 K and 673 K, of which effect is nearly the same. This also indicates that the fast diffusion of solute atoms to mobile dislocations causes the dynamic aging which improves the fatigue strength markedly. The microstructural observations do not provide clear evidence that new precipitates are formed during the second-step-aging. This fact also is indirect evidence that the evolution of solute atmosphere, very fine clusters or precipitates invisible in TEM are responsible for the hardening during the static and dynamic aging. The improvement in fatigue strength in humid air by the second-step-aging may be correlated closely with the microstructural change and hardening mechanism mentioned above. References [1] W. Sha, Z. Guo, Maraging Steel, Modelling of Microstructure, Properties and Applications, Woodhead Publishing Limited, Oxford, 2009. [2] W.C. Leslie, The Physical Metallurgy of Steels, McGraw-Hill, New York, 1981. [3] K. Nakashima, Y. Fujimura, H. Matsubayashi, T. Tsuchiyama, S. Takaki, Tetsu-to-Hagane, 93 (2007) 459−465. [4] N.E. Frost, K.J. Marsh, L.P. Pook, Metal Fatigue, Dover, New York, 1999. [5] N. Kawagoishi, T. Nagano, M. Moriyama, Y. Ohzono, T. Ura, Trans JSME A, 71 (2005) 14−20. [6] N. Kawagoishi, M. Miyazono, T. Nagano, M. Moriyama, J-JSMS, 58, (2009) 787−792. [7] N. Kawagoishi, K. Hayashi, T. Nagano, Y. Nakamura, M. Moriyama, Y. Maeda, J-JSMS, 61 (2012) 787−794. [8] N. Kawagoishi, T. Iwamoto, T. Nagano, K. Morino, J-JSMS, 58 (2009) 781−786. [9] D. Tabor, The Hardness of Metals, Oxford Press, Oxford, 1951. [10]K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, 1985. [11] J.W. Christian, The Theory of Transformations in Metals and Alloys, Part I, 2nd Edition, Pergamon Press, Oxford, 1975. [12] V. Gerold, Precipitation Hardening, in F.R.N. Nabarro (Ed.), Dislocations in Solids, Vol. 4, North-Holland, Amsterdam, 1979, pp.219−260. [13] F.C. Larché, Nucleation and Precipitation on Dislocations, in: ibid., pp. 135−153. [14]L.J. Cuddy, W.G. Leslie, Acta Met, 20 (1972) 1157−1167. [15]Y. Nakamura, N. Kawagoishi, unpublished. [16]J.R. Stephens, W.R. Witzke, J Less Common Metals, 48 (1976) 285−308. [17]A.F. Guillermet, Calphad, 6 (1982) 127−140. [18] K.C. Hari Kumar, P. Wollants, L. Delaey, Calphad, 18 (1994) 223−234. [19]H. Nitta, T. Yamamoto, R. Kanno, K. Takasawa, T. Iida, Y. Yamazaki, S. Ogu, Y. Iijima, Acta Mater, 50 (2002) 4117−4125. [20]F. Christien, M.T.F. Telling, K.S. Knight, Scripta Mater, to be published. [21]U.K. Vsiwanathan, G.K. Dey, M.K. Asundi, Met Trans A, 24 (1993) 2429−2442. [22]R. Tewari, S. Mazumder, I.S. Batrai, G.K. Dey, S. Banerjee, Acta Mater, 48 (2000) 1187−1200. [23]W. Sha, G.D.W. Smith, A. Cerezo, Surface Science, 266 (1992) 378−384. [24]N. Kawagoishi, K. Kariya, T. Nagano, M. Moriyama, Y. Nakamura, Y. Maeda, Proc 31st Symp on Fatigue, JSMS, 2012, pp.119−123.
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