Atomistic model of helium bubbles in gallium-stabilized plutonium alloys

Abstract

The varying thermodynamic stability of gallium- (Ga-) stabilized plutonium (Pu) alloys with temperature affords a unique setting for the development of self-irradiation damage. Here, fundamental characteristics of helium (He) bubbles in these alloys with respect to temperature, gallium concentration, and He-to-vacancy ratio are modeled at the atomistic level with a modified embedded atom potential that takes account of this varying stability. Aside from the bubbles themselves, the surrounding matrix material is single-crystal metal or alloy. As a function of temperature, with a 2:1 He-to-vacancy ratio in a $5--\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ Ga fcc lattice, a 1.25-nm bubble is very stable up to about $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the bubble distorts the surrounding lattice and precipitates a liquid zone, as is consistent with the phase diagram for the model material. Between 300 and $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, this same bubble relaxes slightly through interstitial emission. At $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, with a 2:1 He-to-vacancy ratio in a $2.5--\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ Ga fcc lattice, the Ga stabilization is less effective in the model to the point where the bubble distorts the local lattice and expands significantly. Similarly, at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, if the He-to-vacancy ratio is increased to 3:1, there is significant local lattice distortion, as well as ejection of some He atoms into the lattice. The formation of new bubbles is not observed, because those events take place on a longer time scale than can be simulated with the present approach.