PSI - Issue 13

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

295

4

improves hydrogen embrittlement resistance when hydrogen-enhanced localized plasticity can be controlled; e.g., by increasing strain rate and decreasing hydrogen diffusivity. This high-entropy concept is a breakthrough for the design of hydrogen-resistant steels.

Fig. 3 Compact tension (CT) test results of the (a) Fe-30Mn-6Al, (b) Fe-30Mn-4Si-2Al, and (c) Fe-30Mn-6Si alloys in air and hydrogen environments at ambient temperature. The frequency and stress ratio are 1 Hz and 0.1, respectively. The specimen geometry and experimental conditions are the same as those used for a previous study (Tsuzaki et al., 2016). The Fe-30Mn-4Si-2Al and Fe-30Mn-6Si alloys show ductile and brittle ε -martensite, respectively. The other microstructural information is given in previous studies (Ju et al., 2016; Nikulin et al., 2013).

Fig. 4 Hydrogen embrittlement in an equiatomic Fe-Mn-Cr-Ni-Co HEA at ambient temperature. Hydrogen was introduced through exposure to 100 MPa hydrogen gas at 543 K for 200 h. The details of this experiment are given elsewhere (Ichii et al., 2018). Reproduced with permission from Scripta Mater. , 150 , 74 (2018), copyright 2018, Elsevier. Here, note that the high-entropy concept is compatible with ε -metastability-based alloy design. For instance, Li et al. reported (Li et al., 2016) that a metastable Fe-30Mn-10Cr-10Co (at.%) HEA, which has an ε -martensitic transformation, shows a higher ductility-strength balance than the equiatomic HEA. Previously, we noted the compatibility of the ε metastability and high-entropy concepts for the design of hydrogen-resistant steels, and we, therefore, investigated the hydrogen-embrittlement susceptibility, as shown in Figure 5(a). Even in the 100 MPa hydrogen environment, the metastable HEA shows significant ductility; the strength of the metastable HEA are comparable to those of the stable HEA shown in Figure 4. In other words, the ε -martensitic transformation improves the work-hardening capacity and associated uniform elongation in air, like the α´ -martensitic transformation, but it does not cause critical mechanical degradation, unlike the α´ -martensitic transformation. Furthermore, as shown in Figure 5(b), the fatigue-crack growth rate of the metastable HEA pre-charged with 100 MPa of hydrogen gas is lower than that of type 304 stainless steel pre-charged with 10 MPa of hydrogen gas. Consequently, we expect the metastable high-entropy concept to enable maximization of the ductility-strength balance, both with and without hydrogen.