Shyvaniuk V. Physical background of design of hydrogen-resistant FCC iron-based alloys

The study deals with the development of physical background for design of FCC iron-based hydrogen-resistant alloys by means of alloying, which is based on the effect of hydrogen on the electron structure of alloys. It is shown that hydrogen dissolution and the increase in its concentration raises the density of states on the Fermi level, which suggests the increase in the concentration of free electrons and, therefore, the enhancement of the metallic component of interatomic bonds in hydrogen atmospheres around the dislocations. Such an effect of hydrogen correlates with the decrease in the shear modulus, which can be a reason for the phenomena observed by direct experimental methods, namely, the growth in the density of dislocations and the increase of their mobility, a decrease in the distance between dislocations in the pile-ups. The comparative analysis of the effect of interstitials (C, N and H) on the electron structure, dislocation mobility and mechanical properties is carried out. The ab initio theoretical calculations are performed to determine positions of hydrogen atoms in the FCC and HCP iron structures, their diffusion path and the effect of alloying elements (Ni and Mn) on these parameters. A comprehensive analysis of the alloying elements effect on the hydrogen-induced FCC HCP phase transformation, hydrogen atoms mobility, bonding between hydrogen atoms and dislocations and mechanical properties of austenitic alloys is carried out. It is established that the FCC HCP phase transformation is not a reason for hydrogen embrittlement and, on the contrary, the increase in the HCP phase fraction leads to better hydrogen resistance. A physical reason for such an effect is the shortening of the dislocation gliding path by the martensite plates and a corresponding decrease in the number of dislocations in the pile-ups. It is shown that hydrogen stabilizes the FCC phase in relation to formation of the strain-induced alpha-martensite and significantly decreases the recrystallization temperature of a metastable austenitic stainless steel. A mechanism for the hydrogen-induced localization of plastic deformation is proposed based on the hydrogen-caused increase in the concentration of thermodynamically equilibrium vacancies. Physical principles for alloying of austenitic steels aiming to increase their hydrogen resistance are formulated