Phase stability and mechanical property of gallium-stabilized <x content-type="symbol">δ</x>-plutonium

Abstract

Plutonium (Pu) is a very important nuclear material, acting as a key fissile component in nuclear weapons as well as in the uranium Pu mixed oxide fuel. The existence of six equilibrium solid phases at ambient pressure in its phase diagram makes Pu one of the most complex elements in the Periodic Table. The phase stability and mechanical properties play key roles in its application in nuclear industry. As a radioactive element, Pu is vulnerable to aging, decaying to uranium by emitting an α particle. This process eventually results in abundant crystal defects and helium bubbles. To maintain the effectiveness of Pu-based materials in practical applications, it is necessary to study the mechanical and structural properties of Pu. However, it is challenging to obtain the atomistic mechanism of the evolution of Pu-based materials in experimental studies due to the limitations of spatial and temporal resolution ratio. Recently, numerical simulation especially classical molecular dynamics simulation has become an important tool to study the basic properties of materials at nanoscale. The two phases of greatest interest are the monoclinic α -phase, which is the basic form of unalloyed Pu at room temperature, and the face-centered-cubic (FCC) δ -phase, which can be stabilized to room temperature by alloying with a few atomic percent (at.%) of aluminum (Al) or gallium (Ga). In this work, a molecular dynamics simulation model is set up to investigate the phase stability of Ga-stabilized FCC phase Pu ( δ -Pu). A widely accepted modified embedded atom model (MEAM) potentials are implemented to describe the interactions of the binary system using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The results indicate that unalloyed Pu deviates substantially from a typical FCC structure with lattice distortion and domain decomposition at room temperature and ambient pressure, while it is much more stable at 700 K. Furthermore, it is found that doping Ga can remarkably improve the phase stability of Pu at room temperature, and the stability increases with the atomic percent of Ga in Pu-Ga alloy. Energetically, the average potential energy per atom and the total potential energy decreases obviously with increasing the atomic percentage of Ga in the alloy. Based on the stability results above, we study the elastic property of Ga-stabilized δ -Pu alloy. According to our simulation results, δ -Pu is highly anisotropic with a Zener Index of 6.89, a much higher value than those of other metals with FCC structure such as copper, aluminum and gold. In addition, we perform uniaxial tension on single crystal nanowires with different crystallographic orientations and transverse directions. The elastic moduli of the typical low index crystal orientations [100], [110] and [111] are calculated to be 12.86, 33.47 and 74.37 GPa, respectively. Our simulation results reveal that increasing the concentration of Ga can decrease the anisotropy but increase the elastic moduli conversely, which is in good agreement with available experimental data for non-aged Ga stabilized polycrystalline δ -Pu. It is expected that our findings could lead to an improvement of fundamental understanding of the phase stability and anisotropy in Pu-Ga alloy, which is of great significance to understand the self-radiation effects and aging mechanisms of Pu-based nuclear materials.