Pryadko T. Mechanism and kinetics of hydrogenation of the Ti-V alloys and eutectic alloys of the Ti - Zr - Mn system.

This thesis is devoted to the study of the influence of preliminary thermal activation treatment, hydrogen pressure and temperature on the mechanism and kinetics of hydrogenation of Ti, the Ti-V alloys and eutectic alloys of the Ti-Zr-Mn system. The aim is to obtain new materials with a high hydrogenation capacity and parameters of hydrogenation acceptable to commercial production. The initial alloys were studied by means of SEM-EDS technique, X-ray diffraction and DTA. Hydrogen storage properties were evaluated using a Siverts-type technique. The phase composition and crystal structures of the hydrogenation products were determined. The work is based on the four logically related conceptions: the first consists in evaluation of the influence of the preliminary temperature activation of the Ti specimens' surface by means of annealing and laser irradiation; the second is related to the estimation of the influence of hydrogen pressure on the absorbing properties, such as the start temperature of sorption, the rate of hydrogenation and hydrogen storage of pure Ti with different microstructures; the third consists in the study of kinetic parameters of interaction between the Ti-V alloys with hydrogen in the concentration range from pure Ti to pure V with hcp and bcc structures; in the fourth part the results of study of interaction with hydrogen of two-phase eutectic alloys of the Ti-Zr-Mn system are presented. The last part also includes the results of phase equilibria study in the Ti-Mn and the Ti-Zr-Mn systems with the aim to ascertain the composition range of the ro-TiMn phase existence and the location of binary eutectic. It was shown for the first time that Ti-V alloys with V content from 0.25 to 5 mass.% have abnormally high hydrogen capacity, up to H/Me=2.18. This result was explained by phase transformation in hydrides that increases density of defects of crystal lattice, such as metal-metal vacancies and dislocations. In the present work the mechanism of realization of the conception concerned with hydrogenation of two-phase alloys consisted of bcc and Laves phases has been proposed, once every phase participates in the hydrogenation. In this case, the bcc phase absorbed hydrogen in a favorable manner, like intermetallic compound. This situation was obtained for alloys of the Ti-Zr-Mn system with eutectic structure, where conditions of low Mn content in the Laves phase alloyed with Zr and bcc Ti-based solid solution were "automatically" realized. Since the Ti-Zr-Mn system has not been constructed yet, in this part of the present work the original results concerned with the conditions of ro-TiMn phase formation under crystallization from liquid state and phase equilibria in the Ti-TiMn2-ZrMn2-Zr partial system are presented. They include the mathematical description of the solidus surface in the Ti corner of the phase diagram, the liquidus and solidus projections, as well as the TiMn2-ZrMn2, Ti50Mn50-Zr50Mn50, and Ti60Mn40 - Zr67.5Mn32 polythermal sections. It was shown that the eutectic 47.5Ti-30Zr-22.5Mn (at.%) alloy in monolithic state could react with hydrogen after 3 hours exposure at room temperature at a pressure of 0.4 MPa of hydrogen without any preliminary thermal activation. The hydrogen capacity was measured as 2.62 mass.%H. The hydrogenation product obtained under these conditions consisted of an hydride with tetragonal structure of ThH2 type and a Laves phase-based hydride with MgZn2 type structure. Upon heating in a hydrogen atmosphere with a pressure of 0.5 MPa the activation of specimen in monolithic state begins at temperature somewhat higher than the temperature of the alfa-beta transus start, which may be related to the destruction of the thin oxide layer on the surface of specimen due to the difference between atomic volumes in the and phases. The hydrogen capacity is measured as 2.58 mass.%. The hydrogenation product consisted of a hydride with a cubic crystal structure of CaF2 type and a Laves phase-based hydride with MgZn2 type structure.