Hardness Response Surface for U-7.5Nb-2.5Zr Alloy: A Study of Recovery/Recrystallization and Phase Transformation Interactions

Abstract

Uranium alloys in monolithic form are an option for creating advanced nuclear fuels with very high uranium density, low isotopic enrichment, and high thermal conductivity. One aspect of the metallic fuel development is to demonstrate the retention of similar c-phase microstructure (c s) in the fuel alloy after fabrication and during the irradiation process. Unfortunately, there are few recent systematic studies in uranium alloys. Additionally, none of them are addressing the theme of recovery/recrystallization and phase transformations interactions that compose the basic metallurgical knowledge to manufacture nuclear fuel plates. This paper discusses this theme for U-7.5Nb-2.5Zr alloy employing a multivariate statistical technique, namely response surface methodology, to analyze the effect of the following process variables: degree of deformation, temperature, and time. Based on the results, in the range studied, the authors demonstrated that at low temperatures, the interaction between the recovery phenomenon and precipitation of metastable a¢¢ phase (isothermal martensitic shear transformation) shows a dependence with the degree of deformation, with prevalence of recovery for higher levels of this parameter. Interaction between a + c 3 phase nucleation and recrystallization is limited to a narrow temperature range [~848 K to 923 K (575°C to 650°C)] and during the first hour of annealing. Additionally, the series of phase transformations, c fi c s fi c o fi a¢¢, produced by shear was observed to be strongly dependent on crystallographic orientations and this fact could be used to enhance the retention of the metastable c s phase.