Effects of Repeated Alternating Temperatures Measurements on Lattice Behavior of δ-Phase ²³⁹Pu-Ga Alloys

Elemental plutonium is an atypical element in the periodic table, highly unstable and presenting six allotropes from room temperature to its melting point: monoclinic α, body-centered monoclinic β, face-centered orthorhombic γ, face-centered δ, body-centered δ’, and body-centered cubic ε. These phases are unstable with temperature, pressure, and time, affecting the crystallographic structure, thermodynamic and mechanical properties in ways that are still inconclusive. The high temperature face-centered cubic (fcc) δ-phase is stable in elemental Pu between 320 and 450°C, and it can be stabilized to room temperature and below by alloying it with small amounts of trivalent elements (i.e., Ga, Al, Ce, Am, or Sc). The resulting alloys are in a metastable state, of great interest for several applications and more easily worked metallurgically than the other phases, exhibiting intriguing behaviors and unusual properties, such as negative thermal expansion, large low-temperature electronic specific heat, high electrical resistivity.<br/><br/>Self-irradiation, “the inside out” aging in plutonium and its compounds¹, results in radiation damage which will affect the microstructure of the materials, with significant changes of their structure and physical and mechanical properties. X-ray diffraction and dilatometry studies have shown that the room temperature self-irradiation of δ-phase Pu-Ga alloys results in swelling of the lattice that saturates after 0.1-0.2 dpa of accumulated lattice damage. At low temperatures, self-irradiation results in significant lattice damage, but the mechanisms associated with the evolution of defects at sub-ambient temperatures are still not fully understood.<br/><br/>For a more complete picture of the poorly understood “aging” processes of defect accumulation and damage evolution in δ-phase ²³⁹Pu-Ga alloys, we need to consider the radiation dose and rate, thermal history, and composition of the materials. This talk will present experimental results on the evolution of the average structure of δ-phase ²³⁹Pu- Ga alloys when exposed to repeatedly alternating-temperatures between ambient and cryogenic values, investigated by time-of-flight neutron diffraction.