INTERNAL-FRICTION AND PHASE-TRANSFORMATION IN FCC FE-NI-MN ALLOYS SUBJECTED TO CATHODIC HYDROGEN CHARGING

In FCC Fe-Ni-Mn alloys charged cathodically with hydrogen, internal friction was measured from 170 to 370 K at about 600 Hz in the fundamental mode of free-free transverse vibration. The hydrogen peak was found in these alloys, as has been observed in austenitic stainless steel, 36%Ni Invar, Fe-Ni and Fe-Cr-Mn alloys. As a result of cathodic hydrogen charging, there occurred either of two types of hydrogen-induced phase transformation for the most composition range of Fe-Ni-Mn alloys; hydride formation or martensitic transformation. However, for some intermediate composition of the alloys such as Fe65Ni15Mn20, Fe50Ni20Mn30, Fe40Ni30Mn30 and Fe30Ni40Mn30, cathodic hydrogen charging did not induce any phase transformation and yet produced a well-defined hydrogen peak of internal friction. This means that the hydrogen peak originates from dissolved hydrogen in the FCC alloy matrix, independent of hydrogen-induced phase transformation. The activation energy of the hydrogen peak was estimated to be 37 kJ/mol for the Fe50Ni20Mn30 alloy, which is considered as that of hydrogen diffusion.