Crystallography of the δ→α phase transformation in a PuGa alloy

The crystallography of the δ→α phase transformation in a Pu nominally 1.0 wt% Ga alloy has been examined through the application of Transmission Electron Microscopy (TEM), giving new insight into the martensitic nature of the transformation. This transformation is of particular interest because of its unusually large volume change of ∼20%. Direct examination by TEM has revealed (205)_α twinning as a lattice invariant deformation mode, an orientation relationship giving near parallelism of the (111)_δ and (020)_α planes and the [110]_δ and [100]_α directions, and an α phase habit plane near (132)_α. A survey of twinning in α-Pu has shown that the compound twinning system K₁ = (205)_α, K₂ = (203)_α, η₁ = [502]_α, η₂ = [302]_α requires the lowest shear and no lattice shuffling in a pseudo-structure closely related to α-Pu. Upon application of these experimentally determined parameters to established martensite crystallographic theory, it has been shown that a reasonable solution is obtained, giving a habit plane of (0.732, 0.535, 0.422)_δ with a total transformation shape strain of m_1δ = 0.331, and a lattice invariant shear of m₂ ^α = 0.014 corresponding to a twin fraction of 0.194. The results strongly suggest this transformation occurs via a martensitic mechanism despite the large volume change.