The effect of low-temperature aging on the microstructure and deformation of uranium- 6 wt% niobium: An in-situ neutron diffraction study

Abstract

The mechanical properties of uranium-niobium alloys evolve with aging at relatively low temperatures due to subtle microstructural changes. In-situ neutron diffraction measurements during aging of a monoclinic U-6Nb alloy at temperatures to 573 K were performed to monitor these changes. Further, in-situ neutron diffraction studies during deformation of U-6Nb in the as-quenched state and after aging for two and eight hours at 473 K were completed to assess the influence of microstructural evolution on mechanical properties. With heating, large anisotropic changes in lattice parameter were observed followed by relaxation with time at the aging temperature. The lattice parameters return to nearly their initial values with cooling. The active plastic deformation mechanisms including, in order of occurrence, shape-memory de-twinning, mechanical twinning, and slip-mediated deformation do not change with prior aging. However, the resistance to motion of the as-quenched martensitic twin boundaries increases following aging, resulting in the observed increase in initial yield strength.

Phase Stability in U-6Nb Alloy Doped with Ti from the First Principles Theory

“…The reactions producing α" and γ 0 are generally regarded as martensitic in nature, occurring by shear and/or displacive mechanisms requiring only cooperative, short-range motions of the atoms [5,7,27]. The highly supersaturated metastable α-phase solid solution exhibits far better corrosion resistance than unalloyed uranium and it is amenable to subsequent age hardening, permitting a wide range of mechanical properties through the selection of aging temperature and time [16,28].…”

“…Nbeq (at.%) = 13.77 at.% corresponds to the stability region of the metastable α″monoclinic structure, but Nbeq (at.%) = 16.44 at.% corresponds to the stability region of the γ 0tetragonal structure, as has been mentioned above. These arguments explain why the addition of some small amount of titanium metal to the U-6Nb alloy promotes the stabilization of the γ 0tetragonal phase at the expense of the α″-monoclinic phase, resulting in better mechanical properties of U-6Nb alloys [28].…”

“…The reactions producing α″ and γ 0 are generally regarded as martensitic in nature, occurring by shear and/or displacive mechanisms requiring only cooperative, short-range motions of the atoms [5,7,27]. The highly supersaturated metastable α-phase solid solution exhibits far better corrosion resistance than unalloyed uranium and it is amenable to subsequent age hardening, permitting a wide range of mechanical properties through the selection of aging temperature and time [16,28]. In several investigations [5,7,15,[29][30][31][32] evidence was presented suggesting that the reaction path for these diffusion-less transitions proceeded progressively as: γ(BCC) → γ 0 (tetragonal) → α″(monoclinic) → α′ (orthorhombic) (2) where the notations, α′, α″, and γ 0 has been suggested by Lehmann and Hills [33].…”

Accounting for lattice coherency in a two-phase elastic-plastic self-consistent model for nickel-based superalloys

“…w 1 = 0.55 and w 2 = 0.45 are used in Eqs. (15) and (23) for all the superalloys studied in this work, which are fixed parameters in the calculation. In this section, cases utilized in Section 3.1 were selected to perform a systematic study on the effects of f γ-γ on the calculated lattice strains, including Alloy RR1000 with fine, medium, and coarse γ microstructures at 20 ℃ from Grant et al [12] and 750 ℃ from Francis et al [13].…”

“…In situ neutron diffraction is used to quantify the evolution of lattice structure within the bulk of many polycrystalline metals. It has provided the foundation of numerous research on polycrystals with various crystalline structures [1][2][3][4][5][6][7][8] and phase components [9][10][11][12][13][14][15]. These in situ deformation studies have revealed the heterogeneous stress and strain accommodation within polycrystalline aggregates.…”