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 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 cubicγ 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.
^
abcHoge, K.G; Kuhn, B.A.; Reshenk, V.L. (1973).
Flow behavior of mulberry uranium(PDF) (Report). Vol. 4. California Univ., Livermore (USA). Lawrence Livermore Lab. UCRL-51346.
Archived(PDF) from the original on 2017-12-01. Retrieved 2017-11-18.
^Erickson, W. C.; Jaynes, G. E.; Sandstrcsn, D. J.; Seegmiller, R.; Taub, J. M. (September 1972).
Evaluation of Uranium Alloys (Report).
LASL. pp. 8–13.
Archived from the original on 2017-12-01. Retrieved 2022-07-22.
^Lopes, Denise Adorno; Restivo, Thomaz Augusto Guisard; de Lima, Nelson Batista; Padilha, Angelo Fernando (2014). "Gamma-phase homogenization and texture in U–7.5Nb–2.5Zr (Mulberry) alloy". Journal of Nuclear Materials. 449, Issues 1–3, June 2014, Pages 23-30 (1–3): 23–30.
Bibcode:
2014JNuM..449...23L.
doi:
10.1016/j.jnucmat.2014.02.030.