Mulberry (uranium alloy)

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]

Mulberry is a uranium alloy.

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 γ^s transition phase and with aging this transforms to a tetragonal γ^o 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.