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Rawn, Claudia J.

Publications and source records attributed to Rawn, Claudia J..

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Neutron Diffraction Residual Stress Study of a AA2219-T87 Self-Reacting Friction Stir Weld

2219-T87 is a precipitation hardenable aluminum-copper alloy which sees wide use in structural aerospace components. Thick panels of this alloy are joined via self-reacting friction welding (SRFSW); however, this thermomechanical process can result in significant loss of the alloy’s strengthening precipitates and large residual stresses which are detrimental to mechanical behavior and performance. High-resolution maps of the residual strain and stress states along the normal, transverse, and longitudinal directions of 2219-T87 SRFSW were obtained using neutron diffraction measurements. Residual stress had the highest tensile value in the heat-affected zone (HAZ) and the largest compressive stress in the base metal region of the normal and transverse directions. Line profile residual stress distributions displayed an “M”-shaped distribution in each direction with the longitudinal direction being the most pronounced while 2D residual stresses displayed an hourglass-shaped pattern. Further, average maximal longitudinal residual stress values ranged from 60.6 to 85.6% of yield. Microhardness testing across the transverse weld section produced a “V”-shaped curve with slight hardness recovery in the stir zone. Maximal and minimal microhardness values were observed in the base metal (154 HV) and thermomechanically affected zone (82 HV), respectively. Microstructural evolution was recorded using optical microscopy and showed decreasing grain size from the HAZ to the stir zone.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Characterization and Rationalization of Microstructural Evolution in GRCop-84 Processed by Laser-Powder Bed Fusion (L-PBF)

In this study, prismatic geometries of GRCop-84 [Cu-8Cr-4Nb (at. pct)] were built with laser-powder bed fusion (L-PBF) process. The samples were sectioned parallel or perpendicular to the build direction and characterized in the as-built and after post-processing with a hot-isostatically pressing (HIP) treatment. The microstructure and phase evolutions were evaluated with optical microscopy, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and high-temperature X-ray diffraction (HTXRD) up to 1223 K. The samples in the as-built conditions exhibited predominantly columnar epitaxial and misoriented Cu-FCC grains. The microstructure evolutions are discussed based on locations within the overall build geometry, the dynamics of small melt pool shape and sectioning effects. The above grain structure did not change significantly during post-process HIP treatment. The stability of this FCC grain structure is attributed to the formation of primary stable Cr 2 Nb (Laves phase) during L-PBF, even before the emergence of FCC grains from liquid. The stability of Cr 2 Nb in both as-built and HIPed samples were evaluated using high-temperature X-ray diffraction measurements and compared with that of gas-atomized powder. The significance of these results is discussed with reference to aerospace applications.

36 MATERIALS SCIENCE↗
DOE OSTI2022

In situ inelastic neutron scattering of mixed CH 4 –CO 2 hydrates

An abundant source of CH 4 can be found in natural hydrate deposits. Recent demonstration of CH 4 recovery from hydrates via CO 2 exchange has revealed the potential as a fuel source that also provides a medium for carbon sequestration. It is vital to understand the structural and dynamic impacts of guest variation in CH 4 , CO 2 , and mixed hydrates and link the results to the stability of various deposits in nature, harvesting methane, and sequestering CO 2 . Molecular vibrations are examined in CH 4 , CO 2 , and mixed CH 4 -CO 2 hydrates at 5 and 190 K and Xe hydrates for comparison. Inelastic neutron scattering (INS) is an ideal spectroscopy technique to observe the dynamic modes in the hydrate structure and enclathrated CH 4 , as it is extremely sensitive to 1 H. The presence of CO 2 in hydrates tightens the lattice. It introduces more active librational modes to the host lattice, while hindering the motion of CH 4 in mixed CH 4 -CO 2 hydrate at 5 K. At 190 K, a large broadening of the CH 4 librational modes indicates disorder in the structure leading to dissociation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Local structure analysis of low-temperature neutron pair distribution function coupled with molecular dynamics simulations of CH 4 and CO 2 hydrates from 2 to 210 K

CH 4 hydrates occur naturally and are an abundant potential fuel source with a corresponding risk of potent greenhouse gas release, due to their stability conditions at low temperature and high pressure. Byproduct CO 2 can be exchanged with CH 4 in these natural deposits and can potentially stabilize the hydrates at higher temperatures. CH 4 , CO 2 , and mixed CH 4 -CO 2 hydrates are studied with in situ neutron pair distribution function experiments from 2 to 210 K to investigate the impact of varying the CH 4 -CO 2 guest composition in the gas hydrate structure. These experiments combined with Reverse Monte Carlo analysis allow for the characterization of intermolecular CH 4 and CO 2 interactions with the water molecules which form the hydrate lattice and how they impact the local structure of the lattice itself. Results indicate that when CH 4 and CO 2 co-occupy the hydrate, the host is more strongly distorted than in the pure CH 4 and CO 2 hydrates, but this becomes less defined with increasing temperature. The presence of CO 2 in mixed hydrate increases the stability range and creates a barrier for CH 4 to completely leave the structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Thermal Processing Conditions for the Synthesis of Near Theoretical Density Li 5 La 3 Ta 2 O 12 Ceramics for Ceramic Dual-Mode Detectors

A family of Li-containing garnet compounds, extensively investigated for Li-ion solid electrolyte applications, show great potential to be developed into transparent ceramic dual-mode scintillators. In particular, Li 5 La 3 Ta 2 O 12 (LLTaO) shows promise as an efficient dual-mode scintillator due to its high Li content, high density, non-hygroscopicity, and high effective-Z. Often the synthesis of LLTaO ceramics involves solid state processing techniques starting with oxide and carbonate powders, calcining, sintering for the removal of residual porosity, and post-processing. This study focuses on two objectives: (1) establishing thermal processing boundary conditions for a flexible alkoxide-based sol-gel method for synthesizing fine-grained powders; and (2), consolidating powders via hot-pressing to form high-density ceramics. In this study, near single-phase ceramics were achieved by hot-pressing sol-gel powders with varying amounts of excess Li. Rietveld refinements were used to analyze X-ray diffraction data and provide a quantitative evaluation of the phases present in the hot pressed ceramics. Some of the diffraction data showed large low angle peak asymmetry, suggesting the presence of two cubic Li 5 La 3 Ta 2 O 12 garnet phases with different lattice parameters. Depending on the synthesis and processing conditions, the refinement revealed ~ 90 wt% concentration of a phase with a ≈ 12.8 Å and ~ 10 wt% concentration of a phase with a ≈ 12.9 Å, indicating a remaining excess of Li in the hot pressed ceramics in the large lattice parameter phase. These synthesis experiments allowed for the determination of a baseline for Li loss during hot-pressing. X-ray excited radioluminescence and photoluminescence excitation and emission spectra were obtained from the hot pressed ceramics. In conclusion, the measured emission spectra featured broad tantalate emission and line emissions likely from a rare-earth impurity.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Synthesis method comparison of compositionally complex rare earth-based Ruddlesden–Popper n = 1 T'-type cuprates

The multicomponent approach has been successfully expanded to the Ruddlesden–Popper structure with the synthesis of two different high-entropy cuprate compositions: (La 0.2 Nd 0.2 Gd 0.2 Tb 0.2 Dy 0.2 ) 2 CuO 4 and (La 0.2 Pr 0.2 Nd 0.2 Sm 0.2 Eu 0.2 ) 2 CuO 4 . Additionally, the effect of synthesis method is explored using both solid-state reaction and polymeric steric entrapment (PSE) methods. It is found that PSE leads to more randomly distributed cation species, providing an advantageous method of synthesis for the growing field of high entropy oxides. In situ high-temperature x-ray diffraction tracks the amorphous to crystalline phase transformation in (La 0.2 Nd 0.2 Gd 0.2 Tb 0.2 Dy 0.2 ) 2 CuO 4 powder, synthesized using the PSE method. Using the High-Temperature XRD data, a method for gaining information on the kinetic behavior is also applied. Magnetometry of both compositions indicates ferrimagnetic behavior at low temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Synthesis and structural characterization of Ca 12 Ga 14 O 33

Ca 12 Ga 14 O 33 was successfully synthesized using a wet chemistry technique to promote the homogenous mixing of the Ca and Ga cations. Rietveld refinements on X-ray and neutron powder diffraction data confirm that the compound is isostructural to Ca 12 Al 14 O 33 , however, with a significantly larger lattice parameter allowing for the cages that result from the framework arrangement to expand. In naturally occurring Ca 12 Al 14 O 33 , the mineral mayenite, these cages are occupied by O 2- anions, however, experimental studies exchanging the O 2- anions with other anions has led to a host of applications, depending on the caged anion. The functional nature of the structure, where framework distortions coupled with cage occupants, are correlated to electronic band structure and modifications to the framework could lead to interesting physical properties. The phase evolution was tracked using thermogravimetric analysis and high temperature X-ray diffraction and showed a lower formation temperature for the Ca 12 Ga 14 O 33 analogue compared to Ca 12 Al 14 O 33 synthesized using the same wet chemistry technique. Analyzing both X-ray and neutron powder diffraction using the Rietveld method with two different starting models results in one structural model, with one Ca position and the caged O on a 24d special position, being preferred.

36 MATERIALS SCIENCE↗