Movchan D. Interatomic interaction and phase transformations in alloys based on the Fe-H and Ni-H systems.

The hydrogen effect on the thermodynamic stability of iron- and nickel-based solid solutions has been experimentally and theoretically studied. The interatomic interaction in Fe-H, Ni-H and Fe-Ni systems has been studied using the ab initio calculations of the electron structure. The obtained energies of the interatomic bonds Fe-Fe, Ni-Ni, Fe-Ni, Fe-H, Ni-H and H-H allow to explain the splitting of ? refleсtions in the X-ray diffraction patterns of hydrogenated austenitic steels and iron-nickel alloys by the existence of the short-range atomic decomposition in the substitution solid solution. The role of hydrogen amounts to the visualization of this effect due to a difference in the energies of interatomic bonds between the hydrogen and iron and nickel atoms and different hydrogen solubility in the areas enriched in the iron or nickel, respectively. The results of theoretical calculations have been confirmed by the results of the X-Ray diffraction and transmission electron microscopy studies. The calculations of hydrogen effect on the cohesion energy in the fcc and hcp phases of Fe-Cr-Mn steels give the evidence that, in presence of hydrogen, manganese does not intensify the "gamma"-"epsilon" transformation, but makes transformation weaker, which, ex facte, is at variance with the effect of manganese on the stacking fault energy in the fcc iron. The comparison with similar calculations on the Fe-Cr-Ni steel suggests that the latter is more inclined to the hydrogen induced "gamma"-"epsilon" transformation than the Fe-Cr-Mn one. This result has been confirmed based on the existing experimental data on the hydrogen-charged CrNi and CrNiMn steels and gives the possibility to assume that, in case of the combined alloying of the iron and the saturation with hydrogen, the effect of the alloying elements on the stacking fault energy is non-additive. As a result of the X-Ray study of the hydrogen-charged nickel, it has been established that, in contrast to the widely spread opinion, the so-called "nickel hydride" is actually the Ni-H solid solution resulting from the miscibility gap in the nickel-hydrogen system, and not a chemical compound.