Low Temperature Age Hardening in Cast Uranium-6 wt. pct. Niobium
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
An investigation to determine whether cast U-6Nb would exhibit low temperature (<250°C) age hardening behavior, which has been well-documented in wrought U-6Nb, was performed using a 6 year old near-cylindrical U-6Nb casting, courtesy of AWE, UK. The casting was sectioned, and metallographic cubes and tensile bars were machined and subjected to a variety of heat treatments. Our results indicate that the cast U-6Nb has a larger grain size than wrought U-6Nb, ranging from ~200-500 μm, and that these grains are equiaxed. A significant quantity of inclusions are present at the <10 μm size range and appear to be carbides, based on their morphology (faceted) and color (dark gray). There does not appear to be significant porosity present and the few pores observed were <40 μm in size and spherical. Tensile bars (TBs) were lathe machined, solutionized, then water quenched prior to testing. A group of TBs were subjected to low temperature artificial aging heat treatments consisting of a hold at 200°C for 1 or 2h, followed by furnace cooling, all under a helium backfill. Mechanical testing confirms that cast U-6Nb TBs exhibit low temperature age hardening, with an increase in initial yield stress (Y1) of >425 MPa—a significant increase over wrought U-6Nb yield stress of 100-200 MPa for the same heat treatment. This increase is accompanied by a reduction in failure strain, which is indicative of a reduction in ductility. Modifying the heat treatment temperature and/or time may allow the properties to be tailored to the requirements. This study is intended as a preliminary investigation. Overall, this high impurity content casting exhibits a reasonable mechanical response which can be tailored in a consistent manner through low temperature heat treatment. As the result, a more comprehensive investigation of low temperature aging in cleaner cast U-6Nb, better representative of the state-of-the-art, shall be performed in FY19.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- Contributing Organization:
- Atomic Weapons Establishment (AWE), Berkshire (United Kingdom)
- DOE Contract Number:
- AC52-07NA27344
- OSTI ID:
- 1438735
- Report Number(s):
- LLNL-TR-748326
- Country of Publication:
- United States
- Language:
- English
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