Influence of Phase Stability on Radiation Damage Properties: Plutonium-Gallium Alloys
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1043-T04-03
Influence of Phase Stability on Radiation Damage Properties: Plutonium-Gallium Alloys Steven M. Valone1, Michael I. Baskes1, Blas P. Uberuaga1, Richard L. Martin2, Alison Kubota3, and Wilhelm G. Wolfer3 1 Materials Science and Technology Division, Los Alamos National Laboratory, Group MST-8, Mail Stop G755, Los Alamos, NM, NM, 87545 2 Theoretical Division, Los Alamos National Laboratory, Group T-12, Mail Stop B268, Los Alamos, NM, NM, 87545 3 Lawrence Livermore National Laboratory, Livermore, CA, CA, 94550 ABSTRACT Modeling cascade and fission damage evolution of actinide materials of all kinds is essential for understanding their aging characteristics. As an example of how exotic some of the damage evolution behavior can be, plutonium-gallium (Pu-Ga) alloys in the δ-phase (fcc lattices) are explored. Aging emanates from the wide variety of spontaneous decay and fission products that, in the case of the Pu, are such species as helium (He) and uranium, among others, as well as interstitials, and vacancies. To aid in our understanding, the modified embedded atom method (MEAM) formalism is applied to the Pu-Ga-He system. The behavior of defects in the fcc (δ) phase of Pu-based materials is strongly influenced by the metastability of this phase. The influence of this metastability on minimum displacement threshold energy, point defect characteristics and He bubbles is delineated. The roles of short-range ordering and transformations of voids into stacking fault tetrahedra in the aging process are also examined. INTRODUCTION Modeling the evolution of damage due to self-irradiation in plutonium-gallium (Pu-Ga) alloys is extremely important for scientifically understanding the aging of these unusual materials [1]. At its foundations, Pu aging is a process that depends upon atomistic mechanisms. The self-irradiation begins with the spontaneous fission of a Pu atom that results in production of uranium and helium (He) atoms. The ensuing cascade damage produces point defects in the form of Frenkel pairs that quickly evolve into larger structures consisting separately of interstitials and vacancies. All types of defects after the cascade and rapid quench are believed to evolve into yet larger structures, and, in particular, the He atoms gradually accumulate into bubbles. A reliable atomic level model of the Pu-Ga-He system is key to successfully modeling the damage evolution at this level, as well as being able to inform longer length-scale models [2]. What makes this such a difficult proposition in Pu is that the metal itself exhibits six different phases between ambient temperature and its melting point (913 K). Among these, the hightemperature fcc δ-phase is of interest because it is very ductile, which makes it easy to machine and form. The δ-phase is only stable between 595K and 736 K. It can be stabilized to room temperature by addition of Ga, Al, etc. [3,4], but the stabilizing mechanisms induced by such additions are still not well understood. Theoreticians have been frustrated for the past few years in th
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