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

Abstract

The orientation relationship, habit plane, parent-product interface at the atomic level, twin relationship, and plastic deformation resulting from the d fi a¢ isothermal martensitic transformation in Pu-Ga alloys are examined using optical microscopy, transmission electron microscopy (TEM), and finite element calculations. The d fi a¢ transformation exhibits ã 20 vol pct collapse when the fcc d phase transforms to the monoclinic a¢ 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 110 ½ d jj 100 ½ a 0 direction. The amount of plastic deformation is exceedingly large due to thẽ 20 vol pct collapse, and TEM is used to quantify the difference in dislocation density between untransformed d matrix and regions of d adjacent to the transformed a¢. The twins contained in a¢ plates are shown to have a (205) a orientation as the lattice invariant deformation and are found to be composed of two alternating variants that share a common <020> a¢ direction, but differ by a 60 deg rotation about <020> a¢ . 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} d . Finally, high resolution TEM reveals that the interface is faceted on {111} d exhibiting a series of terrace and ledges.

Unconventional δ-phase stabilization in Pu–Ga alloys

“…In the case of thermal cycling, d M a 0 t causes plastic deformation in d and results in a high dislocation density. Transmission electron microscopy of similar thermally cycled material has shown that the dislocation density of samples heated at 300°C for 4 h is reduced 86 times in the highly damaged regions near the a 0 t plates [5]. Previous experiments on samples a Time required to transform during isothermal segment shown in parentheses.…”

Ambient-temperature Conditioning as a Probe of Double-C Transformation Mechanisms in Pu-2.0 at. % Ga

“…By integrating the endothermic peak of the DSC curves shown in Figure 1b to yield the area under each curve, the measured heat as a function of conditioning temperature can be obtained as plotted in Figure 2; the error bars are estimated to be ±100 mJ/g from the noise in the data and uncertainty in the subtracted baseline. The martensitic δ→α' transformation in Pu-2.0 at.% Ga is strain limited, resulting in an incomplete transformation yielding a maximum transformation of approximately 25% [9]. As such, the measured heat is proportional to both the thermodynamic heat of transformation (∆Η α ' →δ ) and the amount of α' transformed and thus subsequently reverted.…”

Reproducible phase transformation in a single Pu–1.9 at.% Ga specimen

“…Like all martensitic transformations, the d ! a 0 transformation in Pu-Ga alloys is diffusionless, is compositioninvariant, and produces a product phase (a 0 ) with a specific orientation relationship to the parent phase (d) [4,5]. Furthermore, this d !…”

“…a 0 transformation yields an a 0 phase that has a volume approximately 18% smaller than the d phase (V a 0 % 0:82V d ) [1,10]. This volume difference causes significant elastic and plastic strains in the vicinity of the transforming a 0 particle [5], and these strains likely limit the volumetric amount of a 0 product phase to about 25%. Thus, after undergoing the partial d !…”

“…Heating above 100°C completes the a 0 ! d reversion, which removes the a 0 phase and its interfacial energy, but potentially leaves elastic and plastic strains as well as defects or dislocations in the d matrix [5]. The microstructural path resulting from a thermal cycle starting in the d phase, cooling through the martensitic transformation (Eq.…”