The theory of the electrochemical and strain-induced interactions, phase transformations and rearrangement of atoms in the hydrogen solutions in metals and alloys

Abstract

An equilibrium hydrogen atom distribution among different-type interstices in the crystal lattice of metals and their alloys is analysed. The theory leads to the conclusion that there are order-disorder phase transitions in the interstitial atom subsystem, as well as order-order ones, both with and without the hydrogen atoms preferential-coordination-change in host crystal. A model is proposed, in which the influence of both the presence of impurity atoms and a change of temperature on the volume of the elastically anisotropic host crystal of the alloy is taken into account. It is shown that the representative absolute value of energy of the indirect (“by means of the host crystal”) long-range strain-induced interaction of interstitial hydrogen atoms in the host crystal should increase considerably just below its structural instability temperature. The types of the superstructures, arising in the interstitial atom subsystem on the octahedral interstices of the hexagonal close-packed (hcp) metals, are determined. The correlation of impurity atoms with respect to a substitution of sites and the octahedral interstices in the face-centered cubic (fcc) three-component alloys is considered. In the general case of an arbitrary sort of impurity, a thermally activated replacement of the impurity atoms preferential localization both in the octahedral interstices and in the vacant sites is investigated, and its role in the formation of a mixed interstitial-substitutional mechanism of impurity self-diffusion is discussed.