Ramiro A. Pereyra

The gamma to alpha phase transformation in cerium

At 295 K the fcc γ phase of cerium transforms at about 8 kbars pressure to the fcc α phase, the transformation volume decrease being about 18 pct. During decompression, the transformation reverses with a hysteresis of about 2 kbars. The microstructural integrity suggests a mediating phase, but none was found. Complete transformation to alpha and its reversal deformed the internal grain microstructure and drastically increased the lattice misorientation of the individual γ grains. Deformation bands were present along (111) planes. A similar transformation occurred during cooling of a pressurized specimen.

An X-ray diffraction study of the δ → α′ transformation in a Pu-2 at% Al alloy

Abstract The face-centered cubic (fcc) δ phase of plutonium is stable from 310° to 458°C. An addition of 2.0 at% aluminum retains the δ phase metastably to room temperature. This δ phase transforms to α′ during subzero thermal treatments. Under hydrostatic compression at room temperature, the δ transforms initially to β′, and this then transforms to α′ at the higher pressures. During mechanical polishing, the δ phase transforms to α′. An X-ray diffraction study of this transformed zone by sequential removal of thin surface layers by electropolishing shows that this transformation is not directly from δ to α′, but from δ to γ with the γ phase then transforming to α′. There is no evidence for the presence of either the β or β′ phases when the δ phase is transformed by mechanical polishing.

Characterization of Indentation-induced ‘Particle Crowding’ in Metal Matrix Composites

A direct experimental characterization of reinforcement configuration in an indentation-deformed metal matrix composite is undertaken. The primary objective is to quantify the possible local increase in particle concentration, which has been proposed to cause inconsistency in the indentation hardness and the overall composite strength. Quantitative metallography on the post-indented material is carried out to measure the particle volume fraction. Multiple cross sections of an indentation are investigated with the statistically significant results obtained. A distinct increase in particle concentration induced by the indentation is found. The spatial distribution of particle concentration is also examined in detail. The residual compressive stress field remained in the material upon unloading, as illustrated by the finite element analysis, is shown to be in qualitative agreement with the measurement.

Initial electron backscattered diffraction observations of a plutonium alloy

Abstract In this work, the first electron backscattered diffraction patterns (EBSPs) were captured for a plutonium–gallium (Pu–Ga) alloy. The experimental techniques used for EBSP acquisition are described in detail. This demonstrated sample preparation and characterization technique is expected to be a powerful means to further understand phase transformation behavior, orientation relationships, and texture in the complicated Pu and Pu-alloy systems.

Initial electron back-scattered diffraction observations of cerium

The first electron back-scattered diffraction Kikuchi patterns and grain orientation maps were captured for pure n-phase (fcc) Ce. The sample preparation technique used for electron back-scattered diffraction orientation mapping of this surface-reactive metal included ion sputtering the surface using a scanning Auger microprobe followed by vacuum transfer of the sample from the scanning Auger microprobe to the scanning electron microscope. The effect of ion sputtering on the microstructure as well as preliminary electron back-scattered diffraction microstructural characterization is presented. Based on the sputtering data, the room-temperature diffusivity of O in n-Ce was estimated.

Electron backscatter diffraction of a plutonium–gallium alloy

Abstract An experimental technique has recently been developed to characterize reactive metals, including plutonium (Pu) and cerium, using electron backscatter diffraction (EBSD). Microstructural characterization of Pu and its alloys by EBSD had been previously elusive primarily because of the extreme toxicity and rapid surface oxidation rate associated with Pu metal. The experimental technique, which included ion-sputtering the metal surface using a scanning Auger microprobe (SAM) followed by vacuum transfer of the sample from the SAM to the scanning electron microscope (SEM), used to obtain electron backscatter diffraction Kikuchi patterns and orientation maps for a Pu–gallium alloy is described and the initial microstructural observations based on the analysis are discussed. The phase transformation behavior between the δ (face-centered cubic) and e (body-centered-cubic) structures is explained by combining the SEM and EBSD observations.

Detection and quantification of residual α-phase in δ-stabilized plutonium

The temperature range of the δ-phase field of plutonium can be expanded by alloying with Group IIIA elements. Ga is a particularly potent δ-stabilizer and effectively stabilizes the δ-phase to room temperature. Due to a strong propensity to wards s olute redistribution during cooling through the e→δ phase field, regions of the material often do not contain enough solute to stabilize the δ-phase even after extensive homogenization annealing in the δ-phase field. The result is a small, but persistent, fraction of α-phase in the material. A technique using differential scanning calorimetry to measure the enthalpy of transformation of the plutonium α→β transformation is described which can detect and quantify α-phase in a δ-phase matrix at levels as low as ~0.1 wt. %. Complications arise due to interference from the pressure-induced α-phase, and a peak separation method was developed to accurately measure the heat signal from each phase. A set of δ-stabilized Pu-1.7 atomic % Ga alloys was examined using the technique and found to contain 0.32 ± 0.06 weight % α-phase. The onset temperature of the α→β transformation in these specimens was found to be 140.2°C, significantly higher than that for the transformation in pure plutonium, 126.2°C. This increase in onset temperature is a consequence of significant Ga content in the α-phase.

Utilization of principal component analysis on plutonium EXAFS data from the advanced photon source

Since the 1941 discovery of plutonium (Pu) by Glenn Seaborg, this enigmatic metal has been the subject of intense scientific investigation. Despite these efforts, there is still much to be learned about the unusual physical and mechanical properties of plutonium and its alloys. In particular, unalloyed Pu undergoes six allotropic phase transformations upon cooling from the melt to room temperature. Many of these phase transformations result in large volume changes and produce low-symmetry crystal structures. These unusual characteristics have made the metallurgy of Pu and Pu alloys particularly challenging.

Confined Tube Crimp Using Portable Hand Tools

The Lawrence Radiation Laboratory developed handheld tools that crimp a 1/16 inch OD tube, forming a leak tight seal1 (see Figure 1). The leak tight seal forms by confining the 1/16 inch OD tubing inside a die while applying crimp pressure. Under confined pressure, the tube walls weld at the crimp. The purpose of this study was to determine conditions for fabricating a leak tight tube weld. The equipment was used on a trial-and-error basis, changing the conditions after each attempt until successful welds were fabricated. To better confine the tube, the die faces were polished. Polishing removed a few thousandths of an inch from the die face, resulting in a tighter grip on the tubing wall. Using detergent in an ultrasonic bath, the tubing was cleaned. Also, the time under crimp pressure was increased to 30 seconds. With these modifications, acceptable cold welds were fabricated. After setting the conditions for an acceptable cold weld, the tube was TIG welded across the crimped face.

Characterization of Indentation-Induced “Particle Crowding” in Metal Matrix Composites

A direct experimental characterization of reinforcement configuration in an indentation-deformed metal matrix composite is undertaken. The primary objective is to quantify the possible local increase in particle concentration, which has been proposed to cause inconsistency in the indentation hardness and the overall composite strength. Quantitative metallography on the post-indented material is carried out to measure the particle volume fraction. Multiple cross sections of an indentation are investigated with statistically significant results obtained. A distinct increase in particle concentration induced by the indentation is found. The spatial distribution of particle concentration is also examined in detail. The residual compressive stress field remained in the material upon unloading, as illustrated by the finite element analysis, is shown to be in qualitative agreement with the measurement.Copyright