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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2.2 ε - martensitic transformation Here we discuss the effects of the ε -martensitic transformation and compare them to the negative properties of α´ -martensite. Figure 2(a) shows stress-strain curves of an Fe-32Mn (wt.%) alloy that has a deformation-induced ε martensitic transformation. Hydrogen charging at 3 A/m 2 and 10 A/m 2 did not cause hydrogen-induced mechanical degradation, and aggressive hydrogen charging at 30 A/m 2 caused a reduction in elongation. However, even at 30 A/m 2 , the degree of mechanical degradation caused by hydrogen was significantly lower than for other Fe-Mn binary alloys shown in Fig. 2(b) and for the Fe-Cr-Ni-C alloys shown in Fig. 1(a). One cause of hydrogen embrittlement in the lower Mn content alloy is the α´ -martensitic transformation, which is consistent with the discussion for the Fe-Cr-Ni-C alloys in the previous section. It is an interesting observation that the hydrogen embrittlement resistance of the Fe-32Mn alloy is higher than that of the Fe-40Mn alloy, in which austenite has a higher stability. One reason for this is microcrack arresting at the γ/ε interface , as seen in Figure 2(c). The presence of ε -martensite plates acts as barrier to small-crack propagation, realizing greater hydrogen-embrittlement resistance when the amount, ductility, and morphology are optimal (Koyama et al., 2016).

Fig. 2 (a) Nominal stress-strain curves for an Fe-32Mn (wt.%) alloy under different hydrogen charging conditions. (b) Dependence of elongation degradation on Mn content. (c) Electron channeling contrast image showing microcrack arrest. The details of the experimental conditions are given elsewhere (Koyama et al., 2016). Reproduced with permission from Metall. Mater. Trans. A , 47A , 2656 (2016), copyright 2016, The Minerals, Metals & Materials Society and ASM International. Figure 3 shows another beneficial effect of ε -martensitic transformation on fatigue behavior. First note that the Fe-30Mn-6Al (wt.%) steel, which is a stable austenitic steel, shows hydrogen-accelerated fatigue-crack growth, as exhibited in Figure 3(a). The hydrogen-accelerated fatigue crack growth was fully suppressed in the Fe-30Mn-4Si 2Al (wt.%) steel, which shows a deformation- induced ε -martensitic transformation (Figure 3(b)). Thus, deformation- induced ε -martensitic transformation at crack tips has a significant retarding effect on fatigue-crack growth in hydrogen environments. However, also note that the ductility of ε -martensite has a crucial role in the suppression of hydrogen-accelerated fatigue- crack growth, and the ductility of ε -martensite is strongly dependent on its chemical composition. For instance, the Fe-30Mn-6Si (wt.%) steel shows brittle-like cracking along the γ/ε interface during tensile and fatigue tests. Accordingly, the Fe-30Mn-6Si steel shows hydrogen-accelerated fatigue crack growth (Figure 3(c)), despite undergoing a deformation- induced ε -martensitic transformation. 3. Effects of configurational entropy Next, we discuss the hydrogen-embrittlement resistance of an equiatomic Fe-Mn-Cr-Ni-Co high entropy alloy (HEA). Zhao et al. reported that the hydrogen-embrittlement susceptibility of the HEA is lower than that of a stable austenitic stainless steel (Zhao et al., 2017), and Luo et al. claimed that hydrogen uptake improves the tensile strength/ductility (Luo et al., 2017). Furthermore, Nygren et al. clarified the importance of localized plasticity as a factor triggering hydrogen embrittlement of the HEA (Nygren et al., 2017). Accordingly, hydrogen-induced mechanical degradation of the HEA shows significant strain rate sensitivity, as shown in Figure 4; this may be due to competitive motion of the hydrogen atoms and dislocations. Hence, maximization of configurational entropy