Using Fe-Mn alloys having three different crystal structures; BCC(α), HCP(ε) and FCC(γ), the hydrogen embrittlement mechanism was studied. For this study, the specimens charged with hydrogen were tensioned and compared with those of hydrogen free and also hydrogen-induced phase transformation was examined.
Tensile specimens whose Mn contents are 1.3%, 1.7%, 2.1%, 4.3%, 21.8%, 24.0%, 34.0% are electrolytically charged in 5% sulphuric acid solution for the hydrogen penetration into the specimens. These specimens were pulled by the Instron machine at a crosshead speed of 0.01cm/min. and X-ray diffraction analysis were made for the verification of the allotropic transformation due to plastic deformation. The experimental results show that BCC (α) is susceptible to hydrogen embrittlement, while HCP(ε) and FCC(γ) are relatively not. Particularly, hydrogen charged 13.2% Mn alloy which transforms from ε to α -martensite by plastic deformation was severely embrittled.
During cathodic charging, surface layer of specimen may saturate with hydrogen. One may suppose that these excess hydrogen in the surface layer accumulate in the lattice defects; microvoids, grain boundaries, matrix/inclusion interfaces, to develop high pressure gases by recombination of hydrogen atoms to molecules. This pressure developed in the defects may be added to the applied stress to decrease the load-carrying ability.
Diffusion of hydrogen atoms may be required to build up the critical concentration for the embrittlement. In BCC(α) hydrogen in FCC(γ) and HCP(ε) are about four order of magnitude slower than that in BCC(α). Therefore, one may hypothesize that FCC(γ) and HCP(ε) are relatively immune to hydrogen embrittlement due to their slow hydrogen diffusion rate.
In 13.2% Mn alloy, it is known that the structure(mainly, ε) transforms to be mainly α -martensite due to plastic deformation. The ε structure will lose hydrogen during transformation due to the solubility difference between ε and α. This evolved hydrogen will be accumulated and may form $H_2$ gas in the lattice defects to propagate them. The above facts may result in a sudden drop of ductility.
Meta stable phases on the thin-foil surface of 13.2%, 21.8% Mn alloy were transformed only with cathodic charging of hydrogen; ε to α - martensite in 13.2% Mn alloy and retained γ to ε in 21.8% Mn alloy.
Considering that these transformation also takes place by plastic deformation, it may be suggested that the high pressure, by hydrogen molecules accumulated in the lattice defects, induced actual plastic deformation and resulted in phase transformation. These experimental results may be satisfactorily explained by the planar pressure theory for the hydrogen embrittlement.
