PSI - Issue 13

Motomichi Koyama et al. / Procedia Structural Integrity 13 (2018) 292–297 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 5 (a) Tensile ductility (Ichii et al., 2018) and (b) fatigue crack resistance of an Fe-30Mn-10Cr-10Co metastable HEA pre-charged with hydrogen gas; these properties were measured by CT testing. The fatigue-crack growth-rate data for type 304 austenitic stainless steel, which has an α´ - martensitic transformation, were taken from a previous paper (Matsunaga et al., 2017). Reproduced with permission from Scripta Mater. , 150 , 74 (2018), copyright 2018, Elsevier. 4. Summary In this paper, we have summarized the positive effects of the metastability of the ε -phase and of increasing the configurational entropy on hydrogen-embrittlement resistance. The ε -martensitic transformation shows distinct beneficial effects on microcrack arrestability and fatigue crack growth resistance in hydrogen environments. However, the ductility of the ε -martensite is also crucial; i.e. , ε -martensite without sufficient ductility causes serious hydrogen induced mechanical degradation during both the tensile and fatigue tests. Moreover, increasing the configurational entropy enhances hydrogen-embrittlement resistance, and this is highly compatible with the metastability concept for maximizing the ductility-strength balance and fatigue-crack resistance with and without hydrogen. Acknowledgements This study was financially supported by the Japan Science and Technology Agency (JST) (grant number: 20100113) under Industry-Academia Collaborative R&D Program and JSPS KAKENHI (Grant Numbers JP16J06365; JP17H04956). M.K. greatly appreciate helpful discussion with Dr. Sawaguchi at National Institute for Materials Science. Gludovatz, B., Hohenwarter, A., Catoor, D., Chang, E.H., George, E.P., Ritchie, R.O., 2014. A fracture-resistant high-entropy alloy for cryogenic applications. Science 345, 1153-1158. He, Y., Li, Y., Chen, C., Yu, H., 2017. Diffusion coefficient of hydrogen interstitial atom in α - Fe, γ - Fe and ε -Fe crystals by first-principle calculations. International Journal of Hydrogen Energy 42, 27438-27445. Ichii, K., Koyama, M., Tasan, C.C., Tsuzaki, K., 2018. Comparative study of hydrogen embrittlement in stable and metastable high-entropy alloys. Scripta Materialia 150, 74-77. Ju, Y.-B., Koyama, M., Sawaguchi, T., Tsuzaki, K., Noguchi, H., 2016. In situ microscopic observations of low-cycle fatigue-crack propagation in high-Mn austenitic alloys with deformation- induced ε -martensitic transformation. Acta Materialia 112, 326-336. Koyama, M., Ogawa, T., Yan, D., Matsumoto, Y., Tasan, C.C., Takai, K., Tsuzaki, K., 2017. Hydrogen desorption and cracking associated with martensitic transformation in Fe-Cr-Ni-Based austenitic steels with different carbon contents. International Journal of Hydrogen Energy 42, 26423-26435. Koyama, M., Okazaki, S., Sawaguchi, T., Tsuzaki, K., 2016. Hydrogen embrittlement susceptibility of Fe-Mn binary alloys with high Mn content: effects of stable and metastable ε -martensite, and Mn concentration. Metallurgical and Materials Transactions A 47, 2656-2673. Li, Z., Pradeep, K.G., Deng, Y., Raabe, D., Tasan, C.C., 2016. Metastable high-entropy dual-phase alloys overcome the strength-ductility trade off. Nature 534, 227-230. Luo, H., Li, Z., Raabe, D., 2017. Hydrogen enhances strength and ductility of an equiatomic high-entropy alloy. Scientific Reports 7, 9892. Matsunaga, H., Takakuwa, O., Yamabe, J., Matsuoka, S., 2017. Hydrogen-enhanced fatigue crack growth in steels and its frequency dependence. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 375, 20160412. References