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Devaraj, Arun

Publications and source records attributed to Devaraj, Arun.

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

New frontiers in atom probe tomography: a review of research enabled by cryo and/or vacuum transfer systems

There has been a recent surge in the use of cryo and/or vacuum specimen preparation and transfer systems that are broadening the scope of research enabled by the microscopy technique of atom probe tomography. This is driven by the fact that, like for many microscopes, the application of atom probe to air and temperature sensitive materials or wet biological specimens has previously been limited by through-air-transfer and through-vacuum-transfer at room temperature. Here we provide an overview of the critical areas of research that require these new transfer and analysis protocols, as well as a review of current advances in transfer devices, environmental cells and glove boxes for controlled sample manipulation. This includes the study of catalysis and corrosion, the application of atom probe to biological samples, the study of liquid-solid interfaces, natural ageing investigations and the analysis of hydrogen in metals.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Precipitation-site competition in duplex stainless steels: Cu clusters vs spinodal decomposition interfaces as nucleation sites during thermal aging

Competing microstructural evolution mechanisms can exist simultaneously when duplex stainless steels are operating for several decades in a high temperature service environment. Such competition between different microstructural evolution pathways can be difficult to ascertain using simple model alloy systems necessitating detailed structural and compositional analysis of phase transformation mechanisms in complex alloys Thus, in this study duplex stainless steels with complex but well understood chemistries were used to investigate the relative importance of different heterogeneous nucleation sites – specifically spinodal decomposition and Cu clustering – on precipitation of Ni-Si-Mn particles during long term thermal aging. Precipitation of Ni-Si-Mn particles in duplex stainless steels and ferritic steels during thermal aging and irradiation is known to have profound effect on mechanical properties. Using duplex stainless steels with custom modified compositions along with atom probe tomography characterization and first-passage kinetic Monte Carlo (FPKMC) simulations it is revealed that while the interface between Cr and Fe formed during spinodal decomposition can be a pathway for diffusion of Ni, Si, Mn, and Cu, it is not a site that encourages precipitation of Ni-Si-Mn rich particles. Instead, the presence of a higher concentration of Cu in these steels leads to formation of small Cu clusters with high energy interfaces that act as nucleation sites for Ni-Si-Mn particles because of its strong interaction particularly with Ni increasing the chemical driving force for precipitation. These results will help inform predictive models for the use of duplex stainless steels and other precipitation-hardened alloys for extended operation at high temperatures.

Duplex stainless steel, Thermal aging, Precipitati↗
DOE OSTI2020

An image-driven machine learning approach to kinetic modeling of a discontinuous precipitation reaction

Micrograph quantification is an essential component of several materials science studies. Machine learning methods, in particular convolutional neural networks, have previously demonstrated performance in image recognition tasks across several disciplines (e.g. materials science, medical imaging, facial recognition). Here, we apply these well-established methods to develop an approach to microstructure quantification for kinetic modeling of a discontinuous precipitation reaction in a case study on the uranium-molybdenum system. Prediction of material processing history based on image data (classification), calculation of area fraction of phases present in the micrographs (segmentation), and kinetic modeling from segmentation results were performed. Results indicate that convolutional neural networks represent microstructure image data well, and segmentation using the k-means clustering algorithm yields results that agree well with manually annotated images. Classification accuracies of original and segmented images are both 94% for a 5-class classification problem. Kinetic modeling results agree well with previously reported data using manual thresholding. The image quantification and kinetic modeling approach developed and presented here aims to reduce researcher bias introduced into the characterization process, and allows for leveraging information in limited image data sets.

36 MATERIALS SCIENCE↗
DOE OSTI2020

A Mechanistic Understanding of Nonclassical Crystal Growth in Hydrothermally Synthesized Sodium Yttrium Fluoride Nanowires

Sodium yttrium fluoride (NaYF 4 ) is an important upconverting material with many potential uses in chemistry, materials science, and biology, which can be synthesized hydrothermally in both cubic (α) and hexagonal (β) crystallographic polymorphs. Understanding the mechanisms underlying the phase conversion between the cubic and hexagonal polymorphs is of great interest to help inform future synthetic efforts, for example in the design of atomically precise quantum materials with well-defined sizes and morphologies. In this work, we use a combination of analytical transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), powder X-ray diffraction (XRD), in situ liquid cell TEM, atom probe tomography (APT), and extended X-ray absorption fine structure (EXAFS) measurements to show that the hexagonal NaYF 4 nanowires form through a nonclassical crystal growth mechanism involving the formation and subsequent oriented attachment of mesocrystals consisting of cubic (α) phase units. Additionally, EXAFS measurements suggest that substitutional Yb 3+ point defects within NaYF 4 are distributed evenly throughout the crystal lattice without clustering and also that they may exhibit selective substitution into one of the two possible trivalent yttrium sites in the unit cell for hydrothermally synthesized β-NaYF 4 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Enhanced Catalyst Durability for Bio-Based Adipic Acid Production by Atomic Layer Deposition

Harsh reaction environments challenge the lifetime productivity of conventional catalysts and impede emerging biobased chemical processes. Condensed-phase processing of muconic acid, a biobased platform chemical, leads to problematic levels of catalyst leaching and fouling. Atomic layer deposition (ALD) can improve catalyst durability using protective metal oxide coatings; however, a targeted coating strategy and cost models thereof are needed for application-specific deactivation challenges that can surmount these challenges while accounting for process implications. Herein, we utilize thin (<5 wt% Al) Al2O3 ALD coatings to increase the lifetime productivity of Pd/TiO2 during muconic acid hydrogenation to adipic acid. The ALD coating decreases Pd leaching by >3x and markedly improves the thermal stability of Pd and TiO2. Economic analysis for an adipic acid biorefinery supports that increased material costs and reduced catalyst productivity can be justified through lifetime extension. Broadly, this work lays the foundation for reducing the cost of biobased chemical processes through catalyst stability.

Settle, Amy↗
DOE OSTI2019

PIE of Cycle 13 316SS Cladding Using Aberration Corrected STEM Image and Analys

Optical, SEM and TEM analysis confirmed the presence of moderate sensitization in the 20% cold worked 316SS after the iron aluminide coating was applied. The Cr depletion occurs on the grain boundaries between the closely spaced carbides, and at the interface between the carbides and matrix. Cr depletion varied from locally along the grain boundaries, in many cases showing levels down to 15 wt%, but a few isolated regions had depletion levels down to 12.5 wt%. Optical and SEM images taken from a weld heat affected zone revealed the recrystallization occurred after an end plug cap had been welded to the coated cladding. Irradiation did not affect the carbides in a noticeable manner, but radiation-induced segregation/depletion altered the Cr depletion profiles by removing the wide Cr depletion profile, and replacing it with a much deeper, more narrow profile. Cr levels were depleted to 8-10 wt%, Ni and Si were enriched to levels of 40 wt% and 12 wt% respectively. Substantial precipitation formed in the matrix, and on line dislocations in the matrix and on grain boundary dislocations. Voids were observed in the upper region of the TPBAR where the irradiation temperature exceeds 320C. The levels of Cr depletion and Ni/Si enrichment indicate a material in a state susceptible to irradiation assisted stress corrosion cracking.

316SS cladding↗
DOE OSTI2019

Discontinuous Precipitation in U-10 wt.%Mo Alloy: Reaction Kinetics, Effect of Prior gamma-UMo Microstructure, the Role of Grain-Boundary Misorientation, and the Effect of Ternary Alloying Addition

Eutectoid transformation in U-10 wt.% Mo (U10Mo) alloy initiates through discontinuous precipitation (DP) resulting in the formation of a lamellar microstructure that consists of an a-U and Mo-enriched bcc ?-UMo phase along the prior ?-UMo grain boundaries. DP is the main mode of transformation at 500°C. However, continuous precipitation (CP) of ?’ U2Mo phase together with grain-boundary DP is observed at 400°C. The reaction kinetics are noted to be very sluggish at 400°C compared to 500°C. The degree of eutectoid transformation is related to the prior ?-UMo grain size, and a smaller average grain size leads to a larger amount of transformation at a given reaction temperature. Minor addition (0.2 wt.%) of a third alloying element (Cr, Ni, Co) leads to significant changes in the nature of the DP mechanism in U10Mo alloy. In addition, high-angle grain boundaries with a misorientation of 30–45° are observed to be the major DP initiation sites. The microstructural evolution of U10Mo alloy with regard to DP-based reaction is discussed.

Jana, Saumyadeep↗
DOE OSTI2019

Exploring New Science Domains with Atom Probe Tomography Enabled by an Environmental Transfer Hub

The liquid/solid and gas/solid interface represents a rich scientific and technological frontier for discovery and innovation in many areas of science. However, the direct nano- to atomic-scale characterization of these interfaces is technically challenging within broad range of analytical tools that require high to ultrahigh vacuum conditions. This challenge has been largely addressed within the electron microscopy (EM) community through the development of unique specimen holders and differentially-pumped vacuum systems that allow researchers to probe, in situ, liquid/solid and gas/solid interfaces using electron probes [1]. However, the direct application of APT to liquid/solid and gas/solid interfaces is not possible and requires the development of unique specimen preparation and handling of cryogenically-frozen specimens and/or work with specimens under environmentally-protected conditions. At PNNL, we have developed unique suite of hardware and experimental protocols for the preparation and handling of environmentally-sensitive materials centered around the combination of a unique environmental transfer hub, specimen suitcase device, and modified cryo FIB/SEM capabilities (Fig. 1a) [2-3]. The ability to prepare, manipulate, and transfer specimens under either cryogenic or vacuum conditions thus provides a means to uniquely apply APT analysis to a wide variety of material systems that would not be possible otherwise. Here we describe some specific examples, shown in Figure 1b, of APT analysis applied to temperature sensitive material systems requiring cryo specimen preparation and transfer, as well as air sensitive materials systems requiring vacuum or inert vacuum transfer.

Cryo atom probe tomography, Atom Probe, cryogenic ↗
DOE OSTI2019

Composition-Dependent Microstructure-Property Relationships of Fe and Al Modified Ti-12Cr (wt.%)

b-Titanium (Ti) alloys have applications in several industries (e.g. aerospace, automotive, and biomedical) where material performance requirements vary widely. To tailor the microstructure and mechanical properties of b-Ti alloys for various applications, it is critical to understand the influence of individual alloying elements. Toward this goal, we investigated the effect of individual alloying additions on the microstructure and resultant mechanical properties of four model b-Ti alloys: Ti-12Cr, Ti-12Cr-3Al, Ti-12Cr-1Fe, and Ti-12Cr-1Fe3Al (wt.%). The microstructures of these alloys were studied using x-ray diffraction, electron microscopy, and atom probe tomography. The mechanical properties were analyzed via Vickers and Rockwell hardness measurements and tensile testing. The addition of 1 wt.% Fe resulted in an approximate 5% increase in elongation-to-failure (ef), while the addition of 3 wt.% Al did not appear to significantly affect ef. The addition of Fe and Al decreased the yield and ultimate tensile strengths.

Ballor, Joann P.↗
DOE OSTI2019

Crystallographic and Compositional Analysis of impurity phase: U2MoSi2C in UMo Alloys

Minor impurity elements in metallic nuclear materials can critically influence the microstructural evolution and mechanical properties, making it crucial to understand the structure, composition and distribution of the impurity phases. Using Transmission Electron Microscopy, Atom Probe Tomography and ab-initio calculations we provide the atomic scale crystallographic structural and compositional analysis of a new impurity phase: U2MoSi2C in an important nuclear fuel U-10Mo alloy. We identify the new phase to have tetragonal symmetry with lattice parameters of a=6.67Å, c=4.33Å, and space group P4/mbm (No.127). Ab-initio calculations were performed to verify structural stability and to perform atom positions refinement.

Kovarik, Libor↗
DOE OSTI2019

Injection of oxygen vacancies in the bulk lattice of layered cathodes

Surfaces, interfaces, and grain boundaries are, classically, known to be sinks at which defects generated within bulk lattice will preferentially migrate to and get annihilated, therefore rejuvenating a crystalline lattice to restore the intended functionalities.1, 2, 3 Here we report an inverse case by which the defects generated at the particle surface are continuously pumped into the bulk lattice and consequentially compromising the intended functionalities. We discovered that during the operation of a rechargeable battery, oxygen vacancies generated at cathode particle surfaces migrate toward inside of a perfect lattice in lithium-rich layered cathodes, featuring electrochemical driven surface vacancy creation, injection and condensation phenomenon and revealing an unrecognized surface process induced bulk lattice degradation featured with void formation and lattice reorganization. This process is associated with a high-cutoff voltage at which the anionic redox is activated. Coupling with calculations, we substantiate that triggering of anionic redox leads to sharply lowering of both formation energy of oxygen vacancy and migration barrier of oxidized oxide ions, therefore enabling migration of oxygen vacancy via oxidized oxide ion and leading to the injection of oxygen vacancy generated at the particle surface into the bulk lattice. This work unveils unpredicted behavior of oxidized oxide ion, leading to essential insight for designing of high capacity cathode for better batteries through coupled cationic and anionic redox dynamics in layered cathodes. In a broad term, this work provides insight on the intimate coupling between interfacial process and bulk lattice behavior for governing electrode functionality.

Yan, Pengfei↗
DOE OSTI2019

Thermally activated nucleation and growth of cobalt and nickel oxide nanoparticles on porous silica

Integrating mesoscale to the molecular level understanding of nanoparticle nucleation phenomena can drive the bottom-up synthesis approach for target applications. In this work, we studied the thermal evolution of cobalt and nickel oxide nanoparticle structural phases on porous silica host from over wide spatial scale using multi-modal analysis involving scanning transmission electron microscope (STEM), X-ray absorption (XANES) and nuclear magnetic resonance (NMR) spectroscopies along with density functional theory (DFT) based calculations. The TEM analysis reveals thermally activated nanoparticle clustering and subsequent interaction with the porous host material. The Co and Ni K-edge XANES spectra revealed the evolution from metal hydroxide to metal oxide and subsequently metal silicate composites with calcinination temperature. 29 Si NMR analysis revealed the role of surface functional groups of silica host for silicate composite formation which is corroborated by DFT studies.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Outstanding radiation resistance of tungsten-based high-entropy alloys

A body-centered cubic W-based refractory high entropy alloy with outstanding radiation resistance has been developed. The alloy was grown as thin films showing a bimodal grain size distribution in the nanocrystalline and ultrafine regimes and a unique 4-nm lamella-like structure revealed by atom probe tomography (APT). Transmission electron microscopy (TEM) and x-ray diffraction show certain black spots appearing after thermal annealing at elevated temperatures. TEM and APT analysis correlated the black spots with second-phase particles rich in Cr and V. No sign of irradiation-created dislocation loops, even after 8 dpa, was observed. Furthermore, nanomechanical testing shows a large hardness of 14 GPa in the as-deposited samples, with near negligible irradiation hardening. Theoretical modeling combining ab initio and Monte Carlo techniques predicts the formation of Cr- and V-rich second-phase particles and points at equal mobilities of point defects as the origin of the exceptional radiation tolerance.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Grain boundary segregation and intermetallic precipitation in coarsening resistant nanocrystalline aluminum alloys

In-spite of all of the unique properties of nanocrystalline materials, they are notorious when it comes to their susceptibility to thermally induced grain coarsening, thus imposing an upper limit to their application temperature. In this study, we demonstrate a coupled Monte Carlo-molecular dynamics simulation-guided experimental approach of improving the resistance to thermally induced grain coarsening in light-weight nanocrystalline Al-Mg alloys. The structure, grain boundary segregation of Mg, and extent of grain coarsening of the Al-Mg alloys were characterized using plan view and cross-sectional transmission electron microscopy and atom probe tomography. Coarsening resistance is attributed to a combination of thermodynamic stabilization of grain boundaries by controlled Mg segregation, and kinetic stabilization through pinning of the boundaries with nanoscale intermetallic precipitates. Thus, we highlight the opportunities in extending the upper limit of application temperature for nanocrystalline alloys by using a complementary thermodynamic and kinetic stabilization approach.

Nanostructured materials, Al Alloys, Thermal stabi↗
DOE OSTI2018

Co-dependent microstructural evolution pathways in metastable d-ferrite in cast austenitic stainless steels during thermal aging

Cast austenitic stainless steels (CASS) are excellent alloys because they combine high corrosion resistance with high strength and toughness. However, they are susceptible to embrittlement upon long-term thermal aging at elevated temperatures. Thus, the microstructural evolution pathways during thermal aging need to be understood to predict and potentially prevent embrittlement. Atom probe tomography was used to identify and quantify the microstructural evolution pathways of the d-ferrite in different CASS alloys aged for up to 10,000 hours at temperatures between 290 °C and 400 °C. The four steels – CF8, CF8M, CF3, and CF3M – which vary by Mo and C concentration, each experienced spinodal decomposition of the d-ferrite, and precipitation of G-phase clusters and Cu clusters attached to the G-phase. There were large differences in the extent of these features due to their Mo and C concentration. Using radial distribution function analysis, the interactions of constituent elements was found to determine the evolution of these features, with Mo and C specifically influencing the movement of Cr, Ni, Si, Mn, and Cu atoms due to their relative miscibility with these elements. The results will help inform predictive models for the use of duplex stainless steels for extended operation at high temperatures.

Duplex stainless steel, Aging, Spinodal decomposit↗