Uranium metal and alloys based on it are of interest as materials for fast-reactor fuel. A comprehensive investigation of the thermodynamic and diffusion properties of uranium alloys with zirconium permits drawing several fundamental conclusions that could be valid not only for this system but for other alloys as well. Thermodynamic data on the -),-phase alloys of the uranium-zirconium system are presented in [1]. These data were obtained by calculations based on data on the mutual diffusion and diffusion of the components (the calculations were performed on the basis of Darken's formula). It was shown that the computed concentration dependence of the free energy agrees with the value measured by Knudsen's method [2] (Fig. 1). It was also established that the composition dependences of the activity and integrated free energy [3] cannot be described on the basis of models of regular and subregular solutions. The cluster-variation model is an acceptable model that describes the sign-changing behavior of the activity of one of the components.An analysis of the diffusion data (both on interdiffusion and diffusion of individual components), our own data, and the published data was made in [4]. It was shown that the concentration dependences of the interdiffusion coefficients in uranium-zirconium and uranium-titanium systems do not agree with the melting point of the alloys of these systems (Fig. 2). This disagreement is due to the strong influence of thermodynamics (strong concentration dependence of the thermodynamic factor g(c) appearing in Darken's formula for the interdiffusion coefficient). The lowest values of/~(c) are characteristic for alloys with a low thermodynamic stability d2F/dc 2 --* 0, g(c) .-., O. Thermodynamic instability of the alloys results in, on the one hand, phase transformations in the system and, on the other, a decrease of the interdiffusion coefficient, activation energy of interdiffusion, and curvature of the temperature dependence of the interdiffusion coefficients. The decrease in the interdiffusion coefficients follows directly from Darken's formula/9 = (Dl*C 2 + D2*Cl)g(c), where Di*, c i (i = 1, 2) are, respectively, the diffusion coefficient of the components and the concentration; g(c) is a thermodynamic factor, whereac ! F m is the free energy of mixing, R is the universal gas constant, and T is the absolute temperature. The character of the variation of the activation energy as a function of composition and the interdiffusion coefficients as a function of the temperature require analysis. The experimental data presented in [5][6][7] attest to the fact that the interdiffusion activation energy also decreases substantially in the concentration range corresponding to low thermodynamic stability (Fig. 3). Recalling [8], the convex composition dependence of Q [9] was explained by the fact that at a lower temperature intermetallic compounds with a strong interatomic bond are formed, and the value of 0 was regarded as a heat-resistance criterion for the materials.A theoretical analysi...
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