Rostova H. Mechanisms of thermomechanical treatment effect on radiation resistance, erosion and mechanical properties of structural materials

Rostova H. Yu. Mechanisms of thermomechanical treatment effect on radiation resistance, erosion and mechanical properties of structural materials. – Qualification scientific work on the rights of a manuscript. Dissertation for the degree of Doctor of Philosophy in the specialty 104 - Physics and Astronomy (Field of Knowledge 10 - Natural Sciences) - National Science Center “Kharkiv Institute of Physics and Technology” of the National Academy of Sciences of Ukraine, Kharkiv, 2024. This thesis is dedicated to the modification of the structure of structural reactor materials using thermomechanical treatment and the establishment of a correlation between their structure and mechanical, radiation properties and resistance to cavitation erosion. Chapter 1 is concerned with the literature review on the topic of the dissertation. The types and operational characteristics of Gen-IV reactors are discussed, modern and promising structural materials for future generations of reactors, their structural characteristics, and the effect of alloying elements for each type of material on the structural and phase state and properties are presented. The topical issues of radiation materials science are reflected. Based on the studied sources, the tasks of the work are formulated. The second chapter presents the materials studied, research methods and experimental equipment used in this thesis. The materials of investigation were ferritic-martensitic steels T91 and Eurofer97, austenitic Cr-Ni-Mo alloy 42HNM and austenitic steel 08Cr18Ni10Ti. The methodology for severe plastic deformation and temperature treatment modes is presented. The chemical composition of the materials was studied by X-ray fluorescence analysis and EDX, and the structure was investigated by optical, scanning, transmission microscopy and X-ray diffraction analysis. The mechanical properties included determination of microhardness, yield strength, tensile strength, and relative elongation at break. The test methods for determining the cavitation and radiation resistance of the materials under study are also discussed. Chapter 3 presents the results of investigations of the structure and mechanical properties of T91 steel in the initial state and after thermomechanical treatment (TMT) in different temperature intervals. The use of severe plastic deformation by the method of multicycle “upsetting-extrusion” made it possible to modify the structure of T91 steel and form an ultrafine-grained state, as well as to achieve a high density of nanosized carbides of the MX type. The stability temperatures of the nanostructure for each microstructural state were determined. The optimal temperature is tempering at 550 °C for 25 hours to form the highest density of homogeneously distributed nanosized carbide precipitates. The use of thermomechanical treatment according to the developed modes made it possible to increase the strength characteristics of T91 steel by 2 times. Chapter 4 is devoted to the results of investigation of the development of the porous structure after irradiation of T91 steel in different structural states. It was found that T91 steel has high radiation resistance due to the features of the microstructure of this steel, which consists of a whole complex of effective point defect sinks - the nanostructural state of the material, high density of dislocations, and nanoscale carbide precipitates. The swelling in the ferrite structure is 0.65% compared to 0.26% for the martensitic structure and 0.12% for the ferritic-martensitic structure, which confirms the influence of the structural and phase state of T91 steel on its swelling. This difference is due to the presence of a large number of point defect sinks (lamellae, grain, and subgrain boundaries; high density of dislocations and coherent nanoscale precipitates) in the more complex ferritic-martensitic microstructure and martensite. The fifth chapter presents the results of investigation of the cavitation resistance of ferritic-martensitic steels T91 and Eurofer 97 and austenitic alloys 42HNM and 08Cr18Ni10Ti. It was found that T91 steel and 42HNM alloy have the highest resistance to cavitation wear. Their superior cavitation resistance is due to the optimal composition, microstructure features, and high hardness values. It has been determined that the erosion resistance of austenitic steel 08Cr18Ni10Ti is related to the deformation-induced phase transformation of austenite into martensite, which leads to surface hardening under cavitation and increases erosion resistance. The low wear resistance of Eurofer 97 is due to the globular structure of this steel, the presence of large carbides and the absence of alloying elements such as Mo and Nb, which increase cavitation resistance.