Small additions of Ga can stabilize the fcc δ phase in Pu down to room temperature. Upon further cooling of Ga-doped Pu, a diffusionless martensitic partial transformation of the alloy occurs. In the transformed regions, the δ structure, in which Ga atoms are soluble and stable, changes to the monoclinic α structure, in which Ga atoms are insoluble but effectively trapped because of the very slow diffusion rates of Ga in the α matrix. This metastable phase consisting of Ga dopant atoms trapped within a monoclinic α-Pu matrix is termed the α' phase.
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