| Abstract |
This study was undertaken to better model the aging response of U-Nb alloys, particularly to predict properties and their scatter bands, from which lifetimes and their uncertainties can be evaluated. Predictive models of the aging time- and temperaturedependencies of seven age-sensitive properties were developed for nonbanded U-5.6 wt% Nb and U-7.7 wt% Nb alloys. These properties were total and uniform plastic tensile elongation to failure; first-yield, second-yield, and ultimate tensile strengths; first-yield elastic modulus; and Vickers microhardness. A more systematic and statistically aware kinetics modeling approach than employed previously gave reasonable models fits to accelerated aging property data in nonbanded U-5.6Nb and U-7.7Nb, and useful predictions for most of the properties studied. With minor modifications, the U-5.6Nb model was extended to banded U-6Nb. This modeling approach shared many of the key assumptions of the previous approach, including the assumption of Arrhenius behavior and the use of three adjustable parameters. Initial data returns from long-term aging experiments were used to validate the fitted models, a new feature to this study. The apparent activation energies of aging for the property of greatest interest, total elongation, were 32 kcal/mol for U-5.6Nb and 39 kcal/mol for U-7.7Nb, respectively; those for the other properties spanned 1451 kcal/mol. Comparing the goodness of the model fits for the seven properties, the best fits were obtained for second-yield strength and hardness, the first-yield modulus fit the least well, and the other properties fits were in between. The U-5.6Nb models are more robust and therefore are expected to have better predictive power than those of U-7.7Nb, especially at the lower aging temperatures of interest. Model extrapolations to longer times (up to 5 years) and lower temperatures (as low as 40oC) than those used for the model fitting agreed well with most of the validation data gathered for both nonbanded alloys, as well as banded U-6Nb, giving provisional validation of the fitted models. Property predictions for planned or already pending validation experiments are also provided. With a view towards enabling future modeling efforts, this report tabulates all replicate tensile properties and complete hardness scan data used for both model fitting and validation. For surveillance purposes, the properties most practically amenable to detecting the onset of aging at the earliest times are first-yield strength and second-yield strength. Even at aging temperatures as high as 60oC, the minimum lifetimes from this present study are beyond 100 years, giving no cause for concern, if the previously developed failure criterion based on uniaxial tensile elongation (with its caveats) is accepted. |
