Orientation Relationship, Habit Plane, Twin Relationship, Interfacial Structure, and Plastic Deformation Resulting from the δ → α′ Isothermal Martensitic Transformation in Pu-Ga Alloys

The orientation relationship, habit plane, parent-product interface at the atomic level, twin relationship, and plastic deformation resulting from the δ→α′ isothermal martensitic transformation in Pu-Ga alloys are examined using optical microscopy, transmission electron microscopy (TEM), and finite element calculations. The δ → α′ transformation exhibits a ∼20 vol pct collapse when the fcc δ phase transforms to the monoclinic α′ phase, which results in unique and intriguing crystallography and morphology. Here, we show that the orientation relationship is very close to that previously reported by Zocco et al. (1990), but has small rotational misalignments between the two phases both parallel and perpendicular to the \( \left[ {110} \right]_\delta ||\left[ {100} \right]_{\alpha '} \) direction. The amount of plastic deformation is exceedingly large due to the ∼20 vol pct collapse, and TEM is used to quantify the difference in dislocation density between untransformed δ matrix and regions of δ adjacent to the transformed α′. The twins contained in α′ plates are shown to have a (205) _α orientation as the lattice invariant deformation and are found to be composed of two alternating variants that share a common <020> _α′ direction, but differ by a 60 deg rotation about <020> _α′ . A combination of electron diffraction and optical microscopy has been employed to examine the macroscopic habit plane, and the analysis suggests that a large fraction of the observed habit planes are on or near {111} _δ . Finally, high resolution TEM reveals that the interface is faceted on {111} _δ exhibiting a series of terrace and ledges.