Crystallography of the δ → α martensitic transformation in plutonium alloys

A new stress-accommodating crystallographic mechanism of the delta -> alpha martensitic transformation in plutonium alloys is proposed. According to this mechanism, an orientation variant of the alpha phase is produced by a combination of a homogeneous strain and shuffling of the alternating close-packed (111)delta planes. It is shown that the formation of stable transformation-induced twins whose twin-plane orientations and twin-shear directions do not depend on the small variations of the crystal-lattice parameters is the preferred stress-accommodating mode. Only these stable twins have dislocation-free twin boundaries, while the twin boundaries of all others are decorated by an ultradense distribution of partial dislocations. The theory predicts a crystal-lattice rearrangement mechanism involving the (205)(alpha) ((011)(delta)) stable twins. The corresponding invariant plane-strain (IPS) solutions, with special emphasis on the two simplest shuffling modes (the single and double elementary modes), are presented and compared with the existing experimental observations. It is shown that the habit-plane orientation is highly sensitive to the input values of the crystal-lattice parameters and, especially, to the accuracy of the measured volume change in the delta -> alpha transformation. An analysis of these effects on the habit-plane orientation and orientation relations is also presented.