An important scientific problem was solved in the dissertation research – the influence of thermomechanical processing on the structure and texture of hafnium was established, as well as their relationship to the main characteristics of hafnium, which affect the service life of the products in the VVER-1000 reactor core, was determined. This made it possible, in the manufacturing process, by combining deformation with annealing, to obtain hafnium products with various structural and textural states, and, therefore, with various structurally sensitive properties. The influence of various methods (forging, rolling) and types (hot, warm, cold) of deformation on the change in the structure and texture of hafnium rods is investigated. The main phenomena of structure transformation and change of properties that arise at cold, warm and hot deformation of metal nuclear grade hafnium HFE-1 rods are received and analyzed. Experimental data on the effect of annealing at temperatures in the range of 550…1100 °C on the structure of deformed rods with different hardening (produced according to different technological schemes) were obtained.
The results of studying the dependence of mechanical properties, corrosion and radiation resistances of hafnium rods on their structure are presented and reviewed in the dissertation. As observed, the rods in a fully recrystallized state with a fine-grained structure possess optimal mechanical properties (high strength and ductility). Tensile strength of these rods at room temperature is ~575 MPa, percentage elongation is 27…28%. Autoclave corrosion tests and anode polarization curves revealed that oxide films formed on hafnium samples in a fully recrystallized state are the most protective, which is due to the low surface activity at the medium–metal interface. The corrosion rate of such hafnium rods in the initial period of oxidation (before the pre-transition period) is well described by the empirical power equation with the power coefficient 0.242±0.015. After the transition point (~6000 h), the corrosion kinetics is described by a linear dependence with the oxidation rate 3.12•10-4 ± 2.07•10-5 mg/(dm2•h). The results obtained in the dissertation showed that the corrosion resistance of hafnium rods, with a recrystallized structure, is higher in comparison with other promising neutron-absorbing materials - dysprosium hafnate and iodine hafnium, as well as other structural materials that are widely used in the core of VVER-1000 reactors, such as Zr-1%Nb, 06Kh18N10Т and 08Kh18N10Т, which allows its use in the reactor core without a protective cladding. As shown, there is a correlation between the radiation growth of hafnium rods and their texture coefficient (Kearns parameter) according to the results of radiation tests carried out at the JSC “SSC RIAR” (RF), and structural-textural studies of the same samples of hafnium rods in various structural states, performed in this work. The results presented in this paper, revealed that the coefficient of radiation growth linearly decreases with an increase in the Kearns parameter to a value of 0.33. While Kearns parameters were similar, the hafnium rods with a coarse-grained structure showed a higher rate of radiation growth. The deformation of the radiation growth of rods with a recrystallized structure, the Kearns parameter of ~0.1, and an average grain size of less than 20 μm is almost equals to zero when the rods are irradiated at a temperature of 260…300 °C up to a fast neutron fluence of 7.8∙1021 cm-2. According to the results of the studies, it is shown that the combination of deformation with the processes that occur during annealing, allows to obtain hafnium products with different structural and textural state, and, accordingly, with various structurally sensitive properties. From the generalized results presented in this work, it follows that non-textured (FL = ~0.33) samples of hafnium rods with a fine-grained (up to 20 μm) recrystallized structure have the most optimal combination of mechanical and corrosion characteristics during out-of-pile testing, and are also characterized by high dimensional stability under irradiation. The obtained complex results of calculation, physical, experimental researches and tests allow to create in hafnium products, at the set parameters of thermomechanical processing, a structural-phase condition with adjustable characteristics of radiation and corrosion resistance and necessary indicators of life service of products as a part of control and protection system elements that work in the conditions of VVER type reactors.
