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INTRODUCTIONTitanium a-alloys possess a high specific strength, ductility, and corrosion resistance, a low susceptibility toward vacancy inflation, a low activation in neutron fluxes, and a low modulus of elasticity and coefficient of thermal expansion [1,2]. The unique combination of the listed properties together with the good ease of manufacture and weldability make titanium a-alloys a promising material for various structures, units, and parts of thermonuclear reactors. However, the radiation resistance of these alloys has been studied insufficiently well, which restricts their use in elements of thermonuclear reactors.The radiation resistance of titanium alloy RK-20, which was specially designed for nuclear power plants, has been studied very well. The alloy is an equilibrium binary solid solution of 20% Zr in a-titanium. This very ductile material with a medium strength virtually does not age and possesses an elevated corrosion strength in aggressive media, including hydrogen-bearing ones. The effect of neutron radiation on the mechanical .properties and corrosion resistance of alloy RK-20, including hydrogen absorption, has been studied in detail in [3,4]. The present work concerns the results of a study of the fine structure and micromechanisms of fracture in alloy RK-20 irradiated by neutrons in comparison with alloys PT-3V (Ti-AI-V system) and PT-7M (Ti-AI-Zr system).with a controlled expansion clearance. The temperature was controlled by a thermocouple. In order to attain a maximum fluence Fmax~2x1026m -2 (E>0.1MeV), we used a BOR-60 fast neutron reactor. The fine structure and the fracture surface of specimens of alloy RK-20 in the original state and after neutron irradiation with a fluence F = 4 • 1024 m -2 at about 250 and 400~ were studied by the methods of transmission and scanning electron microscopy. The special features of the evolution of defective structures in the process of plastic deformation after low-temperature (200-250~ neutron irradiation were studied for specimens deformed by uniaxial stretching at a rate of 10-3 sec-t to e = 3.3%. Figures ! and 2 show the dependences of the yield limit and the uniform and total specific elongation of a-alloys on the dose of neutron irradiation. These dependences have a characteristic incubation period in which the properties remain virtually unchanged. The length of this period depends on the temperature of the irradiation and the chemical and phase composition of the alloys. For example, alloy PT-3V in the Ti -AI -V system with 3.5% [3-phase (Table 1) has a shorter RESULTS Characteristics of radiation resistance. METHODS OF STUDY TABLE IThe chemical composition and mechanical proAlloy perties of the alloys are presented in Tables 1 and 2. The alloys were irradiated by neutrons to a dose PT-7M F = 4 x l 025 m-2 (neutron energy E > 0,5 MeV) in RK-20 the active zone of a research VVR-M water-mode-PT-3V rated reactor. At t < 100~ the irradiation was conducted directly in the flow of the heat-transfer agent (water). At a higher temperature the specimens...

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