Kinetics of peritectoid transformation and interfacial reaction in the uranium - silicon - aluminium system

Abstract

The growth kinetics of interfacial reaction between metal and intermetallic compound in uranium-silicon-aluminium system have been studied as a function of annealing time and temperature. The nucleation and the growth kinetics of peritectoid transformation in that system were also investigated. Moreover, in order to understand the controlling factor in peritectoid transformation, the effect of size of parent $U_3Si_2$ precipitate in uranium matrix has been considered. In chapter III, formation of $U(Al,Si)_2$ and $U(Al,Si)_3$ and their growth kinetics have been investigated from hot dipped diffusion couples of $U_3Si$ and aluminium in the temperature range 510 to 670℃. The penetration width of aluminium atom and the chemical composition of growing phases were determined with SEM in connection with EDAX and WDX techniques. SEM-EDAX and -WDX analyses indicated aluminium atoms diffused faster into grain boundaries of $U_3Si$ than uranium or silicon did into aluminium and proved to be dominant moving species in the growing $U(Al,Si)_3$ phase. $U(Al,Si)_2$ was first formed at grain boundaries of $U_3Si$ by the diffusion of aluminium and grew finally to form $U(Al,Si)_3$ by further diffusion of aluminium. The diffusion of aluminium did not lead to any formation of $U(Al,Si)_4$. The order to disorder transformation of $U(Al,Si)_3$ phase helped recrystallization process to occur. Optical microscopy permitted one to distinguish the single phase region of recrystallized grained $U(Al, Si)_3$ from the neighbouring multi-phase region composed of $U_3Si$,$U(Al,Si)_2$ and $U(Al,Si)_3$. The course of growth of the multi-phase region as well as the $U(Al, Si)_3$ phase obeyed a respective parabolic rate law. The activation energy governing the growth of the $U(Al,Si)_3$ phase was calculated to be about 220kJ/mol. In chapter IV, the kinetics of peritectoid reaction of $3U+U_3Si_2\rightarrow2U_3Si$ as a function of size of parent $U_3Si_2$ precipitate in uranium matrix have been ...