On the lattice correspondence of the δ → α displacive transformation in plutonium alloys

Although the low temperature delta ..-->.. ..cap alpha.. transformation which occurs on cooling or deformation of Pu alloys exhibits many of the characteristics of a martensitic transformation, classification of the transformation according to recent taxonomy) and elucidation of its crystallographic characteristics have been hampered by the lack of a clear correspondence and lattice deformation relating the FCC delta and monoclinic ..cap alpha.. structures. The transformation represents an interesting test case of crystallographic concepts due to its unusually large volume contraction of app. 20%. A correspondence was proposed by Lomer (and the associated lattice deformation described by Spriet, but these are found to be inconsistent with experimental evidence that the delta ..-->.. ..cap alpha.. lattice deformation involves a contraction normal to the (020) /SUB la/ basal plane, based on the crystallographic texture of ..cap alpha.. produced from delta by uniaxial compressive deformation. The authors explore possible lattice correspondences consistent with this contraction, and examine the classification of this transformation based on characteristics of associated lattice deformations.