Intrinsic Nanoscience of δ Pu–Ga Alloys: Local Structure and Speciation, Collective Behavior, Nanoscale Heterogeneity, and Aging Mechanisms

Abstract

δ Pu−Ga alloys and their response to self-irradiation are important scientifically because of the unique complexity of Pu and technologically because of their importance in Science Based Stockpile Stewardship. The local order and structure and the role of the Ga are crucial to understanding the phase stability and the aging effects. X-ray diffraction that gives the long-range average structure of the periodic component of the materials and pair distribution functions analysis and X-ray absorption fine structure that give the overall and the element specific local structure have been used to examine a variety of new and aged materials, including a set of high purity δ Pu 1−x Ga x alloys with 1.7 ≤ x ≤ 6.4 atom % Ga that span the low [Ga] portion of the δ region of the phase diagram across the ∼3.3 atom % Ga metastability boundary, a ∼1.7 atom % Ga alloy that was enriched with Pu 238 to accelerate the aging process, and others. We find that metastable alloys contain tens of percents of a novel, "σ", Pu structure that we attribute to rearrangement of the Ga-depleted regions after the self-organization of the Ga to form quasi-intermetallic Pu 25−35 Ga. This collective and cooperative behavior involving the Ga and other defects in terms of a tendency to aggregate into domains with structures that differ from the δ host and the resulting nanoscale heterogeneity also appears to play an important role in the observation of analogous locally ordered structures in aged materials. This description of these materials and their aging is radically different from current conceptual basis derived from other experiments that are insensitive to ordering on the angstrom−nanometer length scale.

Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized δ-Pu

“…31 It is important to note, however, that this decreased damage around Ga is in contrast to the previous experiment on the same sample but with the damage recorded at T A = 30 K, where the Ga edge amplitude was similar to, but slightly higher than, that recorded from the Pu edge. 31 This variability is consistent with the changes noted by Conradson et al 53,54 leading to the conclusion that exact sample history can affect the EXAFS signal.…”

“…53,54 This impurity has been observed by multiple investigators 31 including pair-distribution function analysis of x-ray diffraction data. 68 While this peak clearly follows a similar damage dependence on T A (Fig.…”

“…Solving the role and identity of the σ-phase here is beyond the scope of this study, but will require an even more careful comparison between samples with the same histories owing to the history dependence observed here (difference relative damage fractions measured from the Ga and Pu edge between this study and the previous one 31 ) and in many other measurements. 53,54 It should be noted that the impurity is not likely to be on the surface of the sample. Considering sample geometry and the absorption length of Pu fluorescence x-rays, the Pu EXAFS signal comes from at least 5 µm of the sample.…”

Nanoscale heterogeneity, premartensitic nucleation, and a new plutonium structure in metastableδfcc Pu-Ga alloys

“…X-ray absorption fine structure measurements on these initial samples were performed at either ß80 K by mounting the holder to the cold finger of a liquid nitrogen reservoir-type cryostat or at 25-40 K using an open cycle liquid He refrigeration system. The Pu 0.973 Ga 0.027 sample used for x-ray PDF analysis (and EXAFS) was prepared from electrorefined Pu and annealed for 200 h. Another sample used for PDF is aged Pu 0.9983 Ga 0.017 material; aging processes are outside the scope of this report (although included in a companion paper [37]), but this sample was sufficiently young that its structure is almost identical to that of new samples. This study culminated in a suite of six alloys with 1.7, 2.3, 3.0, 3.7, 4.7, and 6.5 at.…”

“…% Ga [5,19]. We have previously reported on radical behavior of the local structure correlated with these transformational properties [20,21,37] that has since been corroborated in an Al alloy by x-ray pair distribution function (PDF) measurements [24] and in Pu-Ga by extended x-ray absorption fine structure (EXAFS) spectroscopy [25].…”