Mulberry (uranium alloy)
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It is used as a non-corroding[1] or 'stainless'[2] uranium alloy.[3] It has been put forward as a structural material for the casings of the physics package in nuclear weapons, including those of North Korea.[4]
The composition is a ternary alloy,[5][6] of 7.5% niobium, 2.5% zirconium, 90% uranium.[3]
Mulberry was developed in the 1960s at UCRL.[6][7] Binary alloy compositions were first studied to avoid the mechanical problems of pure uranium: corrosion, dimensional instability, inability to improve its mechanical properties by heat treatment.[8] Uranium-molybdenum alloys were found susceptible to stress-corrosion cracking, uranium-niobium alloys to be weak, and uranium-zirconium alloys to be brittle.[8] Ternary alloys were next studied to try to avoid these drawbacks. Uranium-niobium-zirconium was found to be corrosion resistant and to permit age hardening, which could increase its hardness from 760 to 1,860 megapascals (110 to 270 ksi).[8][9]
Multiple crystal phases were observed, with a critical temperature of 650°C. Above this the body-centered cubic γ phase was stable. Water quenching to room temperature produces a γs transition phase and with aging this transforms to a tetragonal γo phase. Further aging produces a monoclinic ɑ″ phase that is observed metallographically as a Widmanstätten pattern.[10][11] The crystal structure of the alloy has been studied, particularly the γ phase.[6][7][12][13] Uranium inclusions have been observed within the alloy although, unlike the binary alloys, niobium-rich inclusions were not.[14] Early studies were uncertain as to whether these were inherent behaviours, or artifacts of their processing.
References
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