PSI - Issue 2_B

Takuya Murakoshi et al. / Procedia Structural Integrity 2 (2016) 1383–1390 Author name / Structural Integrity Procedia 00 (2016) 000–000

1389

7

in a FBR. In this study, therefore, the change of the micro texture was observed in detail during high cycle fatigue tests by using EBSD method in order to investigate the fracture mechanism of this alloy at high temperatures. Figure 4 shows the typical change of the KAM map of the steel at 600 o C as a function of the amplitude of the applied stress. When the amplitude of the stress was lower than 100 MPa, no change of the initial micro texture was observed. On the other hand, the micro texture started to change when the amplitude of the stress exceeded about 150 MPa. The change rate was accelerated clearly with the increase of the amplitude of the applied stress. Thus, this change was caused by the effect of superposition of temperature and mechanical stress. Similarly, IQ value of this steel started to change when the amplitude of the applied stress exceeded a certain critical value as shown in Fig. 5. The critical value at 600 o C was about 150 MPa, and this value agreed well with the critical stress at which KAM value started to change as shown in Fig. 4. The critical value increased to about 200 MPa when the ambient temperature was decreased from 600 o C to 550 o C. This result also indicates that the degradation process of this steel was observed quantitatively by using I Q value. In this steel, the change of the micro texture was mainly attributed to the change of the density of dislocations. 5. Conclusion Initial degradation process of heat-resistant materials at elevated temperatures was evaluated by electron back scatter diffraction analyses from the viewpoint of the change of the order of atom arrangement. The order of atomic arrangement was measured by analyzing the sharpness of Kikuchi pattern as IQ value obtained from the observed area. The degree of disorder of atom arrangement caused by various defects such as vacancies, dislocations, the change of local composition, and so on was evaluated quantitatively. The degradation processes of heat-resistant alloys such as nickel-base superalloy and Cr-Mo steel were successfully observed by this method. In particular, the stress-induced anisotropic diffusion of component element was validated as a dominant factor of the degradation and it was found that there was critical stress at which the accelerated diffusion started to occur. Therefore, this IQ value is effective parameter which evaluates the order of degradation of atom arrangement of heat-resistant materials at elevated temperatures. Acknowledgements This research activity has been supported partially by Japanese special coordination funds for promoting science and technology, and Tohoku University. References Humphreys, F. J., 2004, Characterisation of fine-scale microstructures by electron backscatter diffraction (EBSD), Scripta Materialia 51, 771–776. Kirka, MM., Brindley, KA., Neu, RW.,Antolovich, SD.,Shinde, SR., Gravett, PW., 2015, Influence of coarsened and rafted microstructures on the thermomechanical fatigue of a Ni-base superalloy, Int. Journal of fatigue 81, 191-201. Kobayashi, H., Todoroki, A., Oomura, T., Sano, T., Takehara, T., 2004, Elevated temperature ultra-high-cycle fatigue properties and fracture mechanism of modified Cr-Mo steels, Transactions of the Japan Society of Mechanical Engineers, A 70, 1087-1092. Komazaki, S., Shoji, T., 1997, Formation of the Al-rich phase on grain boundary and the creep damage mechanism in directionally solidified Ni base super-alloy, Metall. Mater. Trans. A 28, 1945-1949. Koblischka-Veneva, A., Koblischka, M.R., Qu, T., Han, Z., Mu¨cklich, F., 2008, Texture analysis of monofilamentary, Ag-sheathed (Pb,Bi) 2 Sr 2 Ca 2 Cu 3 O x tapes by electron back-scatter diffraction (EBSD), Physica C 468, 174–182. Komazaki, S., Shoji, T., Sato, M., 2000, Creep life prediction of Ni-base superalloy used in advanced gas turbine blades by electrochemical method, JSME International Journal Series A, Solid Mechanics and Material Engineering 43, 156-165. Miura, H., Suzuki, K., Sasaki, Y., Sano, T., and Murata, N., 2011, High temperature damage of ni-base superalloy caused by the change of microtexture due to the strain-induced anisotropic diffusion of component elements, Proc. of International Mechanical Engineering Congress & Exposition (IMECE2011), 62411. Matsumori, Y., Nemoto, J., Ichikawa, Y., Nonaka, I., Miura, H., 2012, High Cycle Fatigue Properties of Modified 9Cr-1Mo Steel at Elevated Temperatures, Proc. of the ASME 2012 International Mechanical Engineering Congress and Exposition, 87329, 1-6. Morito, S., Huang, X., Maki, T., Hamnse, N., 2006, The morphology and crystallography of lath martensite in alloy steels, Acta Materialia, 54 ， 5323-5331. Humphreys, F. J., 2001, Review: Grain and subgrain characterisation by electron backscatter diffraction, Journal of Materials Science 36, 3833 3854.