A statistical model for the study of U-Nb aging (u)
- Los Alamos National Laboratory
This study was undertaken to model the aging response of U-Nb alloys in order to quantify property and lifetime predictions and uncertainties, in response to concerns that aging during long-term stockpile storage may change the microstructure and properties of U-6 wt%Nb alloy components in ways adversely affecting performance. U-6Nb has many desirable properties, but is a complex material because of its gross compositional inhomogeneity (its chemical banding spans 4-8 wt%), its metastable starting microstructure, and the fact that a variety of external factors such as temperature, stress, and gaseous species can cause aging through multiple mechanisms. The most significant aging mechanism identified in earlier studies [2007hac2] is age hardening, phenomenologically defined as increasing hardness and strength and decreasing ductility observed as a function of increasing aging time-at-temperature. The scientific fundamentals of age hardening at temperatures relevant to U-6Nb material processing ({le}200 C) and stockpile storage ({le}60 C) remain unresolved in spite of significant experimental efforts [2007hac2, 2009cla]. Equally problematic is the lack of a well-established U-6Nb component failure criterion. These limitations make the most desirable approach of property response and lifetime prediction - that based on fundamental physics - unattainable at the present time. Therefore, a semi-empirical approach was taken to model the phenomenological property evolution during aging. This enabled lifetime estimates to be made from an assumed failure criterion (derived from a manufacturing acceptance criterion) couched in terms of an age-sensitive property, namely quasi-static tensile elongation to failure. The predictions of this and other age-sensitive properties are also useful for U-6Nb component surveillance studies. Drawing upon a large body of artificial aging data obtained from nonbanded (chemically homogeneous) U-5.6Nb and U-7.7Nb material [2007hacJ ] over 100-300 C, predictive models of the aging time and temperature dependencies of seven age-sensitive properties were developed for U-5.6Nb and U-7.7Nb alloys. These two nonbanded alloys represented the average (U {approx} 5.8Nb) and upper limit (U {approx} 8Nb) of compositions present on 100-200 micron length scales in the banded U-6Nb material. Prior analyses [2002eck] gave concern that the high-Nb bands would age faster and cross the failure threshold sooner than the mean or lower-Nb bands (the lower limit of banding, U-4Nb doesn't appear to be aging fast enough to be an issue). On a conservative assessment the high-Nb band behavior would thus be the life-limiting factor for banded U-6Nb.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC52-06NA25396
- OSTI ID:
- 991249
- Report Number(s):
- LA-UR-09-04733; LA-UR-09-4733; TRN: US201021%%127
- Resource Relation:
- Conference: 28th Compatibility, Agind and Stockpile Stewardship Conference ; September 29, 2009 ; Albuquerque, NM
- Country of Publication:
- United States
- Language:
- English
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