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

摘要

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 S 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 S 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.