Delayed Onset of Discontinuous Precipitation-Based Phase Transformation in U-10mo Alloys Doped with Silicon

Abstract

A uranium-10 wt% molybdenum (U-10Mo) alloy is one of the primary candidates for metallic fuels that would use low-enriched uranium in place of highly enriched uranium, to support nuclear nonproliferation efforts. Optimal performance of a U-based metallic nuclear fuel can be achieved by retaining the high-temperature, body-centered cubic (bcc) allotrope (γ-U) at room temperature, which can be accomplished in the U-10Mo alloy. However, presence of minor alloying elements can influence the final constitution of room-temperature phases in the U-10Mo alloy, specifically, formation of α-U phase which results in anisotropic behavior of the fuel in reactor. Through a detailed transmission electron microscopy analysis, the present study reports the constituent phases that are present in a U-10Mo alloy containing ~0.1 wt% Si after it is subjected to homogenization heat treatment and thermomechanical processing. For comparison, results from an undoped U-10Mo alloy are also included. The experimental results reveal that γ-UMo solid solution is the major phase in a hot-rolled, Si-doped U-10Mo alloy metallic fuel foil, along with U 2 MoSi 2 C, UC, and U 2 Mo, after isothermal annealing at 460 °C for 10 h. In contrast, after the same heat treatment, the undoped U-10Mo alloy metallic fuel had formed a noticeable amount of α-U along prior γ-UMo grain boundaries through discontinuous precipitation (DP, area fraction: ~27.9%) with characteristic lamellar morphology, together with γ-UMo, UC, and U 2 Mo. This result indicates that doping with Si could mitigate the DP reaction in U-10Mo alloy and prevent formation of undesirable α-U. This work sheds light on optimizing Si-doping–dominated microstructure in U-10Mo fuels and facilitates designing and tuning of microstructures of U-10Mo alloys for tailoring the final designed performance of the fuel under irradiation.

Keywords: U-10Mo alloy, Si-doping, microstructure, Phase identification, Transmission electron microscopy