PSI - Issue 7

M. Goto et al. / Procedia Structural Integrity 7 (2017) 248–253

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M. Goto et Al./ Structural Integrity Procedia 00 (2017) 000–000

GBs, crystallographic slip planes, and twin boundaries were commonly observed. The paths along matrix/DP phase boundaries were not often observed because of their interface stability, whereas a crack occasionally passed through the DP phases. Consequently, the occurrence of sporadic DP phases had negligible effects on the fatigue strength of the present alloy regardless of the mechanically detrimental effect of DP phases. 4. Conclusions The main findings of this study on the precipitate strengthened Cu–6Ni–1.5Si alloy (500 °C, 0.5 h aging) can be summarized as follows: 1. Microstructure was composed of Cu-matrix and sporadically distributed DP phases with cellular structure. The hardness of Cu matrix was H V = 259 which were nearly equivalent to the maximum hardness due to aging. The alloy was strengthened by nano-size intermetallic compounds ( δ -Ni 2 Si) precipitated in the matrix. Along GBs, heterogeneously precipitated δ -Ni 2 Si particles with a few tens of nanometers and PFZs were formed. 2. Fatal fatigue cracks were found to originate from the GBs. The soft PFZs and heterogeneously large particles at GB areas induced a localized high-stress/strain distribution which led to the fast crack initiation, followed by the crack growth along slip planes in grains sharing GBs. No fatal cracks were initiated from DP phases, this means that a negligible effect of sporadic DP phases on the fatigue strength, in spite of their mechanically detrimental effect. 3. Crack paths along GBs, crystallographic slip planes, and twin boundaries were commonly observed. The paths along matrix/DP phase boundaries were not often observed because of their interface stability, whereas a crack occasionally passed through the DP phases. Acknowledgements This study was supported by a Grant-in-Aid for Scientific Research (C) (No. 26420021) and for Encouragement of Scientists (No. 17H00330) from the Japan Society for the Promotion of Science, as well as a National Research Foundation of Korea (NRF) grant funded by the Global Frontier R&D Program (2013M3A6B1078874) at the Global Frontier Hybrid Interface Materials R&D Center funded by the Ministry of Science, ICT, and a Future Planning and National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (no. 2011-0030058). The authors are very grateful to the members of the Strength of Materials Laboratory of Oita University for their excellent experimental assistance. Thanks are also extended to the members of the Korea Institute of Materials Science, for performing the ECAP processing of our copper rods. References Corson, M.G., 1927. Electrical conductor alloys. Electric World 89, 137–139. Findik F. Discontinuous (cellular) precipitation. J Mater Sci Lett 1998; 17:79–83. Fujii, T., Kamio, H., Sugisawa, Y., Onaka, S., Kato, M., 2010. Cyclic softening of Cu-Ni-Si alloy single crystals under low-cycle fatigue. Materilas Science Forum 654-656, 1287–1290. Fujiwara, H., Sato, T., Kamio, A., 1998. Effect of alloy composition on precipitation behavior in Cu-Ni-Si alloys. Journal of Japan Institute of Metals 62, 301-309. Goto, M., Han, S.Z., Euh, K., Kawagoishi, N., Kim, S., Kamil, K., 2012. Statistical description of the effect of Zr addition on the behavior of microcracks in Cu–6Ni–2Mn–2Sn–2Al Alloy. Journal of Materials Science 47, 1497-1503. Goto, M., Han, S.Z., Lim, S.H., Kitamura, J., Fujimura, T., Ahn, J-H., Yamamoto, T., Kim, S., Lee, J., 2016. Role of microstructure on initiation and propagation of fatigue cracks in precipitate strengthened Cu-Ni-Si alloy. International Journal of Fatigue 87, 15–21. Han, S.Z, Lee, J., Lim, S.H., Ahn, J-H., Kim, K., Kim, S., 2016. Optimization of conductivity and strength in Cu-Ni-Si alloys by suppressing discontinuous precipitation. Metals and Materials International 22, 1049–1054. Han, S.Z., Lim, S.H., Kim, S., Lee, J., Goto, M., Kim, H.G., Han, B., Kim, K.H., 2016. Increasing strength and conductivity of Cu alloy through abnormal plastic deformation of an intermetallic compound. Scientific Reports 6, 30907. Kim, H.G., Lee, T.W., Kim, S.M., Han, S.Z., Euh, K., Kim, Y.W., Lim, S.H., 2013. Effects of Ti addition and heat treatments on mechanical and electrical properties of Cu-Ni-Si alloys. Metals and Materials International19, 61–65. Lockyer, S.A., Noble, F.W., 1994. Precipitate structure in a Cu-Ni-Si alloy. Journal pf Materials Science 29, 218–226. Lockyer, S.A., Noble, F.W., 1999. Fatigue of precipitate strengthened Cu-Ni-Si alloy. Materials and Science Technology 15, 1147–1153. Sun, Z., Laitem, C., Vincent, A., 2011. Dynamic embrittlement during fatigue of a Cu-Ni-Si alloy. Materials Science and Engineering A528, 6334– 6337.