The thesis is devoted to the study of hydrogen embrittlement phenomena in 3d transition metals-based construction materials. By means of theoretical and experimental techniques, the mechanisms of hydrogen brittleness available in the literature are analyzed. At the same time, peculiarities of hydrogen effect on the electron structure, phase transformations and dislocation properties in iron-, titanium- and nickel- based alloys were studied and the consequences for macroscopic mechanical properties of corresponding materials were derived.
Using nitrogen and carbon as indicators of correlations between electron structure and mechanical properties, the verification of electron concept of hydrogen enhanced localized plasticity has been performed. The proposed concept allows to take into account the chemical nature of interstitial elements and correctly predict their influence on dislocation properties that is the main point of hydrogen-caused brittleness model. An additional advantage of the proposed concept, in comparison with the model developed within the framework of continuum mechanics, is the possibility to propose practical recommendations for the increase in materials resistance to hydrogen brittleness.
Using mechanical spectroscopy, the Snoek-Koester relaxation was studied and it was shown that nitrogen atmospheres assist dislocations mobility, whereas, in contrast, carbon atoms retard dislocations slip. Using first principle calculations, a correlation between the carbon and nitrogen effects on the iron electron structure and dislocation properties is established. Correspondingly, similarly to hydrogen, the nitrogen-caused increase in the concentration of free electrons should assist mobility of dislocations, which is in a perfect consistency with experimental data.
Based on the theoretical results on the thermodynamics of phase transitions in the iron-based alloys and studies of crystallographic texture evolution, it was shown that during the interpretation of hydrogen induced phase transitions, along with inhomogeneous profile of hydrogen distribution on the depth, the thermodynamic factors also have to be considered because of different hydrogen effect on the free energy of phases in this system. At the same time, the plastic deformation, not only stresses, is a true reason for similarity between deformation-induced and hydrogen-induced epsilon-martensites.
The molecular dynamic calculations and autoradiography method allow to confirm that grain boundaries are the traps for interstitial atoms, particularly hydrogen and carbon in the iron. An approach is proposed to explain the enhanced migration of hydrogen atoms along the grain boundaries that is observed in hydrogen permeation measurements. According to this approach, such a behavior results from hydrogen-induced plastic deformation and corresponding transport of hydrogen atoms by dislocations, and also could be a consequence of microcrack formation in the vicinity of grain boundaries in the course of cathodic hydrogen charging which is always used during the hydrogen permeation measurements.
By means of theoretical and experimental studies, it was shown that there is similar effect of hydrogen on the electron structure and dislocation properties in titanium- and nickel-based alloys, which is an additional confirmation of correctness of the electron approach to hydrogen-enhanced localized plasticity and shows a general character of this approach for hydrogen brittleness. Particularly, on this basis, the explanation is given for different appearance of hydrogen brittleness in b-titanium alloys and austenitic steels, which allows to use hydrogen as a temporary alloying element during technological treatment of b-titanium alloys in single phase state, to provide their better deformational characteristics.
A detailed analysis of thermodynamic and kinetic processes in nickel-hydrogen system allows to conclude that the so-called “nickel hydride” is a consequence of decomposition of hydrogenated solid solution into the hydrogen-rich and hydrogen-depleted phases via spinodal mechanism.
A comparison was performed between the phenomena of hydrogen-caused brittleness and liquid metal embrittlement. Using the first principle calculations for the case of iodine in the iron it was shown that its effect on the electron structure of a material is similar to that in case of hydrogen. The theoretically predicted iodine-caused increase in the concentration of free electrons in the iron-based alloys has been confirmed by the experimental measurements using the electron spin resonance. The mechanical spectroscopy also allows to detect a similar effect of iodine and gallium atoms on the dislocation properties, which results in the earlier start of microplastic deformation and increased dislocations mobility. The obtained results confirm a similar mechanism of hydrogen and liquid metal brittlenesses.
