The Β→α Phase Transformation in Plutonium

The β→α transformation in plutonium is discussed in terms of the crystallography of the two phases and the resulting topological modeling of the β/α interface. There has been little microscopy work on the transformation, but it is probably martensitic. β-Pu is monoclinic I2/m, while α-Pu is monoclinic P21/m. α-Pu has been described as a hexagonal close-packed pseudostructure with AB stacking of the (020)α planes with pseudo-close-packing along [100]α and two other directions. β-Pu is less obvious, but X-ray diffraction suggests that the (103)β planes, which are selected as the terrace plane, have the highest structure factor and are therefore among the closest-packed planes. Other pseudo-close-packed planes, such as {222¯}β and {321¯}β, could also act as terrace planes for the transformation. The (103)β planes have a pseudo-hexagonal grid of Pu atoms with AB stacking and pseudo-close-packing along [301¯]β and two other directions. A selection of terrace planes as (020)α//(103)β with disconnections along [100]α//[301¯]β provides the basis for topological modeling. The model predicts a habit plane that is ∼6° from the terrace plane. The extra Pu atoms in the β structure (17 for every 16 in α) are accommodated by having 16 (103)β planes transform into 17 (020)α planes at steps in the interface. Short-range interstitial diffusion of Pu atoms from β to α is required for the transformation to proceed. Possible lattice invariant deformation systems are discussed.