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[en] Highlights: • Significant α′-phase is induced in Pu-1 at% Ga under compression of <300 MPa. • No α′-phase is detected in Pu-2 at% Ga alloy following compression of <300 MPa. • On thermal cycling α′-phase is detected in increasing amounts up to a plateau. • Increasing the compression to 600 MPa induces α′-phase to form in Pu-2 at% Ga alloy. • α′-phase induced in Pu-2 at% Ga follows both direct and indirect δ-phase reversion. - Abstract: The phase stability of δ-phase stabilised alloys is typically assessed as a function of time by periodically monitoring the specimens using thermal analytical methods. The specimens are subjected to repeat thermal cycling through phase transformations prior to and following conditioning, designed to test the degree of stability of the alloys. This work examines the impact of thermal cycling on the δ-phase stability of Pu-1 at% Ga and Pu-2 at% Ga alloys subjected to compressive stresses. Compressive pressure in the range of 100–300 MPa induced a significant percentage of α′-phase in the Pu-1 at% Ga alloy specimens, whereas no detectable levels of α′-phase were recorded in specimens of the Pu-2 at% Ga alloy. However, on thermally cycling the specimens, α′-phase was detected in the Pu-2 at% Ga alloy as well. Furthermore, a significant increase in α′-phase was recorded as both alloys were subjected to an increasing number of thermal cycles. The α′-phase increase reached a plateau after a given number of cycles for each of the alloys. The transformation of the α′-phase back to the δ-phase in specimens of both alloys followed an indirect path via the β-phase and γ-phase. Increasing the compressive pressure up to ∼600 MPa generated higher levels of α′-phase in the Pu-1 at% Ga alloy specimens and resulted in significant levels of α′-phase being recorded in the Pu-2 at% Ga alloy specimens. The transformation of the α′-phase back to the δ-phase appeared to follow an indirect path in the Pu-1 at% Ga alloy. This work shows new evidence that in the Pu-2 at% Ga alloy, the transformation followed both the indirect and direct (α′–δ phase) path

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[en] The topic of this study is the electronic structure of the compounds Pu₃X [X = In, Ga, Al, or Tl], reported to have room temperature L1₂ structures. The measured Pu-Pu bond length in δ-phase Pu is 3.28 Angstrom. The Pu-Pu bond lengths in Pu₃Ga and Pu₃Al are 3.18 Angstrom. The similarity in bonding in Pu₃Ga and Pu₃Al is especially notable as the equilibrium atomic volume in Ga is 18% higher than that of Al. Likewise, in Pu₃In and Pu₃Tl the bond lengths are 3.33 Angstrom and 3.32 Angstrom, respectively, although the atomic volumes of In and Tl differ by 10%. The electronic mechanism by which the 8 stabilizers Al and Ga contract the Pu-Pu bonds and whereby In and Tl stretch them may relate the stability of pure Pu to that of the Pu₃X compounds. For example, does the addition of Ga or Al shorten the bond length enough to make the close-packed Ll₂ structure more stable than does the addition of In or Tl? The incipient localization of the f electrons and the stabilization of open structures may be indicated through LDA total energies and densities of states

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[en] Temperature dependent extended x-ray absorption fine structure (EXAFS) spectra were measured for a 3.3 at. % Ga stabilized Pu alloy over the range T= 20 - 300 K. EXAFS data were acquired at both the Ga K-edge and the Pu L_III edge. Curve-fits were performed to the first shell interactions to obtain pair-distance distribution widths, σ, as a function of temperature. The temperature dependence of σ(T) was accurately modeled using a correlated-Debye model for the lattice vibrational properties, suggesting Debye-like behavior in this material. Using this formalism, we obtain pair-specific correlated-Debye temperatures, Θ_cD, of 110.7 ± 1.7 K and 202.6 ± 3.7 K, for the Pu-Pu and Ga-Pu pairs, respectively. The result for the Pu-Θ_cD value compares well with previous vibrational studies on (delta)-Pu. In addition, our results represent the first unambiguous determination of Ga-specific vibrational properties in Pu-Ga alloys, i.e, Θ_cD for the Ga-Pu pair. Because the Debye temperature can be related to a measure of the lattice stiffness, these results indicate the Ga-Pu bonds are significantly stronger than the Pu-Pu bonds. This effect has important implications for lattice stabilization mechanisms in these alloys

[en] Two plutonium-alloy hemispheres were cast, machined, and canned for use in pulsed-sphere experiments. LLL physicists will use the data from these experiments to improve physics codes. The total mass of Pu--1.0 wt percent Ga was 9.3 kg. The hemispherical shapes had a radius of 53.7 mm. Both hemispheres were cast with hollow polar cones. In one casting the cone was plugged; in the other casting the cone was left to allow fitting to the neutron generator. The hemispheres were electron beam welded into close-fitting stainless steel cans so they could be used in a non-plutonium area. This report describes the fabrication of the device, which is expected to have long-term research utility. 14 figures