Equation of state and strain-induced stabilization of δ-phase stabilized plutonium alloys

An in-situ high-pressure neutron diffraction experiment was conducted on a δ-phase stabilized plutonium alloy with isotope 242 Pu. Upon room-temperature compression, neither of the previously reported pressure-induced transformation paths, δ → α′ or δ → γ’ → α′, was observed up to 1.2 GPa. Instead, a drastic reduction in the diffraction intensity of the δ phase was observed when the pressure was above 0.8 GPa. At the highest pressure of the experiment (1.2 GPa), the diffraction data appear to be characteristic of an amorphous state, manifested by the diminishing intensities of all diffraction lines. In addition, no evidence was found to support the transformation to a body-centered tetragonal structure (δ’), which was previously inferred from the diffraction line broadening during initial compression to 0.1 GPa. The discrepancies between the present and previous experiments suggest a substantial strain-induced stabilization of the δ-phase and is presumably attributed to the different stress states in the high-pressure environments. From the pressure - volume measurements, the determined isothermal bulk modulus for the δ-phase is in the range of 31.9 ± 1.3–34.8 ± 1.8 GPa using different pressure scales, comparable to those obtained from the resonant ultrasound spectroscopy measurements of the alloys of similar composition. The pressure-induced elastic softening is neither convinced in the present work, nor can it be resolved from the diffraction experiments if it is intrinsically weak.