Effect of Heat Treatment and Niobium Content on Microstructure and Phase Structure of Uranium-niobium Alloy

Microstructure and phase structure of uranium-niobium alloys are closely related to the niobium content and heat treatment process. The microstructure and phase structure evolution of uranium-niobium alloys under different heat treatment conditions were characterized and analyzed by quantitative metallography. The results show that quenched uranium-niobium alloy was single-phase supersaturated solid solution, and the phase structure transformed from the orthogonal to monoclinic and tetragonal structure with the increase of niobium content, the microstructure changed in the acicular, lath and equiaxed grains sequence. Slow-cooled uranium-niobium alloy was pearlite composed of niobium-rich (γ_1-2) and niobium-poor (α') alternating lamellae. Cellular decomposition occurred preferentially along austenite boundaries during the annealing in martensitic structure, and the decomposed structure was pearlite composed of niobium-rich and niobium-poor lamellae as well. The decomposition kinetics can be well described by the JMAK equation.