Hydrogen-Induced Hardening and Embrittlement in FCC Fe-Ni-Mn Alloys Subjected to Cathodic Charging

Hardening and embrittlement caused by cathodically charged hydrogen were measured at room temperature in Fe50Ni50−xMnx alloys, with special attention to the subsidiary effect of hydrogen-induced phase transformation. There are two types of hydrogen-induced phase transformation in most FCC iron alloys; γH and εH. After cathodic hydrogen charging, the γH phase is formed in the nickel-rich composition of Fe50Ni50−xMnx alloys and εH phase in the manganese-rich composition, but there occurs no transformation in the intermediate composition such as Fe50Ni20Mn30. The effect of dissolved hydrogen on their mechanical properties can therefore be distinguished from the influence of two types of phase transformation by testing these alloys.Hydrogen-induced hardening and embrittlement were found in all the compositions of Fe50Ni50−xMnx alloys, apparently being a complex effect of dissolved hydrogen and hydrogen-induced phase transformation. The hardening was restored almost completely to the original state after outgassing of hydrogen. In addition, the embrittlement seemed to be more remarkable in the transformation-free Fe50Ni20Mn30 alloy than in other alloys. This means that the hardening and embrittlement are mainly caused by dissolved hydrogen in the FCC matrix, where the hydrogen-induced phase transformation does not play a very important role. The conclusion may apply to any FCC iron alloys including austenitic stainless steel.