Growth kinetics of dispersed thoria in Ni and Ni-Cr alloys

The growth rates of dispersed thoria particles in nickel have been measured as a function of time at 1350°C in gases of two separate oxygen partialpressures. Samples were prepared for electron microscopic study after these treatments for particle size distribution measurements. The diffusion coefficient and solubility of thorium in nickel placed in contact with thoria were also determined under the same conditions. The results of the measurements then provide the necessary information for the elucidation of the kinetics of Ostwald ripening in the ThO₂-Ni system. A linear relationship was found between the cube of the mean radius of the dispersed particles and the time at temperature. Substitution of the experimental results into the Lifshitz-Wagner equation for particle growth under diffusion control yielded values of the ThO₂-Ni solid-solid interfacial energy, which are in good agreement with what is expected by comparison with the corresponding solid oxide-liquid metal interface. Further studies were made of the coarsening of thoria in 20 pct Cr-Ni alloys. It was found that the enhanced rate of particle growth could be accounted for by the increased value of the diffusion coefficient of thorium in the alloy over the value in pure nickel.