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 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.
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 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.