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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Influence of rapid heating and cooling combined with deformation at ultrahigh strain rates on the microstructure evolution of pure titanium shaped charge liner

Highlights: • A slug prepared from pure titanium SCL was recovered after detonation deformation at an ultrahigh strain rate over 10{sup 5} s{sup −1}. • The α → β → α phase transformation occurred in Ti SCL during the ultrahigh-strain -rate deformation. • The microstructures of the recovered slug included lamellar α grains, equiaxed α grains, and jagged α grains. - Abstract: A slug prepared from pure titanium shaped charge liners was recovered after detonation deformation at an ultrahigh strain rate over 10{sup 5} s{sup −1} and high strain over 500%. Optical microscopy, electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM) were employed to investigate the microstructure of the Ti shaped charged liner before and after the detonation deformation. The microstructures of the recovered slug include lamellar α grains, equiaxed α grains within localized shear bands, and jagged α grains. EBSD results indicated that the grain orientation distribution is random and no texture was observed in the slug. TEM observation revealed that the α → β → α phase transformation occurred owing to the rapid heating and cooling combined with deformation at ultrahigh strain rates in the Ti shaped charge liner. No melting phenomenon was observed during the deformation process. The deformation process consisted of the explosive detonation stage combined with rapid heating and the subsequent penetration stage combined with rapid cooling. In the explosive detonation stage, the α → β phase transformation instantaneously occurred via a martensitic mechanism. In the penetration stage, the high-temperature slug in β state was cooled rapidly and showed different deformation behaviors in three different areas.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Influence of processing on secondary phase formation and microstructural evolution at U-10Mo alloy and Zr interlayer interfaces

Thermo-mechanical processing of uranium-10 wt. % molybdenum (U-10Mo) fuel plates leads to microstructure changes at the U-10Mo/Zr interfaces. Secondary phases formed at this interface are particularly important to interfacial bond strength, process optimization, and maintaining structural integrity of the U-10Mo fuel plates during irradiation. In this work, we determined the phases and phase transformation products occurring at the interface of the U-10Mo fuel and Zr interlayer when the fuel plate is subjected to short and long hot isostatic pressure times. Interfacial morphology, structure and composition of phases formed, and relative hardness across the U-10Mo/Zr interfaces were studied using a multi-length scale, multi-modal characterization approach involving electron microscopy, atom probe tomography, and atomic force microscopy. Here, results highlight that the extent of phase transformations, secondary phase formation, and hardness variability across interfaces can be controlled by modifying processing parameters. Phase diagram construction and thermodynamic calculations were performed using the Thermocalc software to identify expected phases formed at interfaces during the maximum hold temperature of 560 °C experienced during HIP.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Modular Controllable Transformers (MCT)

Large Power Transformers (LPTs) are at the heart of the electricity infrastructure and are critical pieces in today’s grid. Aging assets, long turn-around times, transportation delays and dependence on foreign manufacturing for these components have created conditions for points of failure across the system. Moreover, there is no available dynamic control on these passive sections of the grid. For all the automation embedded on the low voltage side of the low voltage side of the system there is absolutely no flexibility on the high voltage side. Changes in power flow patterns are brought about by altering dispatch patterns for generators which requires a coordinated effort and is often time consuming. Traditional FACTS based approaches such as UPFC or HVDC light involve massive modifications to the existing infrastructure. Recent instances of physical attacks on LPTs and the slow recovery process associated with fixing or replacing these damaged units has created an increased awareness for this issue. LPT designs are highly customized owing to specific field requirements. Long lead times coupled with transportation delays make procuring and installing LPTs a lengthy process. In an attempt to minimize the recovery time following an LPT failure, transformer manufacturers have developed mobile transformer units. These units typically consist of either a single-phase or three-phase transformer mounted on a truck and characterized by compact form factors. By reducing the assembly time and transport time these units offer a fast and temporary solution to LPT failures. Several efforts led by DOE, Edison Electric Company (EEI) and North American Electric Reliability Corporation (NERC) have led to the RecX program. RecX transformers consist of single phase transformers transported on specialized assemblies to enable fast restoration and minimal installation time. NERC and EEI have instituted transformer sharing programs like Spare Equipment Database (SED) as well as Spare Transformer Equipment Database (STEP) which encourage utilities to maintain and share a fleet of LPTs. Mobile transformers are typically limited to less than 100 MVA to enable transportation on a trailer. Also, the efficiency of mobile transformer units and RecX transformers is lower since they are temporary solutions. The ideal replacement solution would be one that could utilize a set of standardized transformers while incorporating flexibility to influence metrics like power flow, apparent impedance and voltage profiles. In view of these issues the team at Georgia Tech proposed a concept called Modular Controllable Transformers (MCTs). This project was commissioned as a part of DOE’s Transformer Resiliency and Advance Component (TRAC) program. The approach consists of splitting one large unit rated upwards of 100 MVA with smaller modular, standardized units. Each of these smaller units are augmented with a converter to add power flow, voltage and impedance control. Splitting the power level into multiple units has two important implications. By making smaller units, the transportation times can be reduced drastically. Moreover, even though these units have very specific impedance and other parameters, the augmented converters can make it emulate the unit being replaced. Thus, faster replacement times and added controls are achieved making this an extremely resilient approach. In the event of one modular units failure, the other two controllable units can operate in the most resilient fashion keeping the net system overloads to a minimum. Moreover, for the same probability of failure only a fraction of the system capacity is lost. The converter is augmented on to the tertiary winding of the transformer making it retrofittable in the field without massive modifications to the system. The material proposed in this report highlights design considerations to minimize recovery time as well as results displaying overall improvement in resiliency. The cost analysis of this approach shows significant savings compared to UPFC and HVDC lite based approaches. Extensive analysis was conducted on numerous aspects of this project over the course of this project. Simulation studies were conducted to prove the 5MVA converter design. Challenging BIL constraints and transient issues were addressed over the course of this project. Simulations on EMTP platform presented in consequent sections were used to finely tune the control for the same. A final design for a back-to-back 5MVA neutral-point-clamped (NPC) converter was completed in this project. Furthermore, switching components like high power IGBTs from IXYS Westcode Press-Pack series and ABB Stackpack series were identified for this application. These devices exhibit a blocking voltage of 6.5 kV and a nominal collector current of 900 A. Potential housing options as well as existing commercial option were identified for this converter design. A 56 MVA modular transformer unit was designed with Delta Star Inc. with consultation and inputs from all project members. The transformer cooling systems were chosen to be Oil Natural Air Forced (ONAF) to optimize ease of transport. A full Product Requirement Document (PRD) for the converter and the transformer was generated. The team also investigated key aspects of resiliency and how to quantify the same. A metric was developed to analyze resiliency and numerous simulation cases were presented. Increased (N-1) resiliency and outage resistance was showcased through simulation on the Texas Large scale power system. Dynamic control capabilities were also showcased on numerous system level simulations. An IEEE 30 bus system was used to show the ability of the MCT approach to reduce line congestion. Power flow control; a part of the MCTs abilities, showcased a 29% reduction in transformer loading on the same system. Full control over tie line flows was demonstrated on a modified IEEE 13 bus system. Dynamic LTC capabilities were shown through simulation and were compared to traditional LTCs. Furthermore, the ability of the MCTs to alter apparent transformer impedance was simulated to promote standardization of modular transformer units while retaining the ability to emulate a varied range of impedance characteristics. Thus granular control over power flow, voltages, impedances and was presented. Regulatory analysis conducted by Oak Ridge National Laboratory (ORNL) further bolstered the efficacy of the MCT approach and verified the added resiliency achieved due to this approach. As a conclusion, it was proved that the resilience improvement obtained from using the MCT approach is justified by just one failure in a 30-year period. This means that the benefits obtained from using the MCT approach far outweigh the cost of setting this approach up. As a part of the future efforts, the team is proposing building a (12.47-24) kV/400 kVA scaled down unit to control 5MVA of power flows. Emphasis would be laid on using commercial components to speed up the build of these units. Further the actual transport of the modular transformer unit would be a part of this operation and would provide insight into transportation issues. The approach shows much promise and adds key dynamic control on passive sub transmission and transmission sectors.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2019

“Lattice Strain Matching”‐Enabled Nanocomposite Design to Harness the Exceptional Mechanical Properties of Nanomaterials in Bulk Forms

Abstract Nanosized materials are known to have the ability to withstand ultralarge elastic strains (4–10%) and to have ultrahigh strengths approaching their theoretical limits. However, it is a long‐standing challenge to harnessing their exceptional intrinsic mechanical properties in bulk forms. This is commonly known as “the valley of death” in nanocomposite design. In 2013, a breakthrough was made to overcome this challenge by using a martensitic phase transforming matrix to create a composite in which ultralarge elastic lattice strains up to 6.7% are achieved in Nb nanoribbons embedded in it. This breakthrough was enabled by a novel concept of phase transformation assisted lattice strain matching between the uniform ultralarge elastic strains (4–10%) of nanomaterials and the uniform crystallographic lattice distortion strains (4–10%) of the martensitic phase transformation of the matrix. This novel concept has opened new opportunities for developing materials of exceptional mechanical properties or enhanced functional properties that are not possible before. The work in progress in this research over the past six years is reported.

Zhang, Junsong↗
DOE OSTI2018

The facile synthesis, crystallization behavior and magnetic property of FeNiP amorphous nanoparticles

Highlights: • A facile chemistry method was proposed to synthesize amorphous FeNiP nanoparticle. • Phase transformation happened during sintering the amorphous nanoparticle. • Ni nanospheres and monoclinic (Ni,Fe){sub 3}[PO4]{sub 2} nanorods appeared at 800 °C. • Crystallization behavior is α-FeNiP → Ni + FeNi{sub 3} + Ni{sub 3}P → Ni + (Ni,Fe){sub 3}[PO{sub 4}]{sub 2}. • Sintered powders have varied magnetic properties due to phase proportion change. - Abstract: Amorphous FeNiP nanoparticles with homogeneous composition are synthesized by reduction of aqueous Fe{sup 2 +} and Ni{sup 2 +} with sodium borohydride (NaBH{sub 4}) and hypophosphite at room temperature. These amorphous nanoparticles can be crystallized by heating up to 300 °C to form FeNi{sub 3} and Ni{sub 3}P crystals. Ni phase appeared at the sintering temperature of 500 °C, and a special net structure was formed at 700 °C due to the joining and diffusion among different nanoparticles. At a higher temperature of 800 °C, phase transformation happened and only pure Ni nanospheres and monoclinic ((Ni, Fe){sub 3}[PO{sub 4}]{sub 2}) nanorods can be observed, which should be governed by the long-range diffusion in the net structure. The amorphous FeNiP nanoparticles have excellent soft magnetic property with Ms of 13.2 emu/g and Hc of 0.7 Oe. However, the sintered mixing nanoparticles show varied magnetic properties due to the phase transformation and proportion change among different phases involved in.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Two-step deformation-induced martensitic transformation in additively manufactured High-Si stainless steel

Deformation-induced phase transformation in the additively manufactured (AM) Fe–18Cr–8Ni-5.5Si (in wt.%) alloy was analyzed using the in-situ electron backscattered diffraction (EBSD) record during uniaxial tensile testing in a scanning electron microscope (SEM). Analysis of EBSD data was focused to investigate phase transformation behavior during deformation with consideration of mechanical interactions among phases and grains in the ultrafine AM microstructure. The initial EBSD phase map illustrated that the as-built microstructure was a duplex structure consisting of major austenite (γ) and minor ferrite phases, some of which showed non-traditional orientation relationship since the rapid solidification in the AM process has produced non-equilibrium state in phase formation. In the deformed as-built AM alloy, the two-step martensitic transformation (γ→ε→α´) was identified as the main deformation-induced transformation mechanism. Martensitic transformation steps were found to be associated with glides of Shockley partial dislocations on closed packed (111) planes under the influence of atomic disturbance in the γ and ε phases, respectively. Using the closed-packed plane pole figures and unit cell orientations, their evolutions and mechanisms accompanying with the transformation were analyzed. Finally, the results indicate that one variant of ε-martensite forms from their parent γ phase, while two variants of α´ form from their parent ε phase.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Many-Body Quantum Catalysts for Transforming Between Phases of Matter

A catalyst is a substance that enables otherwise impossible transformations between states of a system, without being consumed in the process. In this work, we apply the notion of catalysts to many-body quantum physics. In particular, we construct catalysts that enable transformations between different symmetry-protected topological (SPT) phases of matter using symmetric finite-depth quantum circuits. We discover a wide variety of catalysts, including GHZ-like states that spontaneously break the symmetry, gapless states with critical correlations, topological orders with symmetry fractionalization, and spin-glass states. These catalysts are all united under a single framework that has close connections to the theory of quantum anomalies, and we use this connection to put strong constraints on possible pure- and mixed-state catalysts. We also show how the catalyst approach leads to new insights into the structure of certain phases of matter, and to new methods to efficiently prepare SPT phases with long-range interactions or projective measurements.

quantum many-body systems↗
DOE OSTI2017

High-pressure behavior of A 2 B 2 O 7 pyrochlore (A=Eu, Dy; B=Ti, Zr)

In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A 2 B 2 O 7 pyrochlore (A=Eu, Dy; B=Ti, Zr) up to ~50GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ~41 GPa for B=Ti and ~16 GPa B=Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionic radii, i.e., A=Dy, proceeding faster than those with a larger ionic radii, i.e., A=Eu. These results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed differences in the degree of short-range order of the different compositions. Due to the large phase fraction of cotunnite at high pressure for B=Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A=Eu than A=Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu 2 Zr 2 O 7 as compared with the initially defect-fluorite structured Dy 2 Zr 2 O 7 .

36 MATERIALS SCIENCE↗
DOE OSTI2021

Developing Accelerated Test Protocols and Tuning Microstructures of the Common Materials to Improve Robustness, Reliability, and Endurance of SOFC Cells

This work is built on our prior work where we found that phase transformation in praseodymium nickelates, e.g. Pr 2 NiO 4 (PNO) and (Pr 1-x Nd x ) 2 NiO 4 (PNNO), can be electrochemically driven, and is substantially faster when compared to thermal annealing studies. The first task aims at an attempt to further accelerate the phase transformation in the oxygen electrode by alternating the current input in the cells, which lead to the development of accelerated test protocols (ATPs). ATPs showed up to 60x faster phase transformation and up to 10x faster performance degradation in (Pr 0.50 Nd 0.50 ) 2 NiO 4 electrodes, when compared to long-term operation under constant current density. Furthermore, the phase stable Nd 2 NiO 4 and (La 0.6 Sr 0.4 )(Co 0.8 Fe 0.2 )O 3 (LSCF6482) electrodes were tested in full cells under ATPs, and showed up to 10x faster performance degradation within 1,100 hours in a comparison with long-term thermal annealing studies and electrochemical operation under constant current density. The second task aims at the quantification of the contributions of cell components to the total impedance of a solid oxide fuel cell (SOFC) using electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT). Specifically, the role of gas composition at both anode and cathode was systematically studied, aiming at deconvoluting, identifying and quantifying the contributions of different electrode processes. This was achieved by first tuning the partial pressure of H 2 at the anode and subsequently varying the partial pressure of O 2 at the cathode. The results suggest that, while DRT offers a viable way of deconvoluting different times distributions, additional attention is needed before assigning a peak to a specific electrode process due to the significant overlap of the contributions from the cathode and the anode. Density function theory studies show that both Pr-vacancies and O-defects play a key role on the activity and stability for nickelates towards oxygen reduction reaction. The resident O-interstitials and oxygen ions in the PrO layer form peroxide (O 2 2- ) nearby Pr vacancies. The O 2 2- limits oxygen-ion transport due to the required additional energy to break its O-O bond. We further calculated the formation and segregation energies for different Ln ions (La, Pr, Nd, Pm, Sm, Gd, Tb, Dy, and Ho) in PNO and CeO 2 (111) surfaces. In addition to Nd, Pm and La are suggested as potential dopants in PNO to enhance it stability without decomposition due to their more negative formation energies, lower diffusion energies, and positive separation energies.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2025

Characterization of SiO 2 Thermally Grown Oxide Stress Evolution of EBCs with Al-Containing Dopants

SiC/SiC ceramic matrix composites (CMCs) are desired for use in combustion environments to achieve higher turbine operating temperatures. However, CMCs require environmental barrier coatings (EBCs) for protection from the gas environment. EBC systems are known to primarily fail through coating delamination via growth of a thermally grown oxide (TGO) at the EBC—silicon bond coating interface when exposed to steam, which accelerates the TGO growth rate. The TGO undergoes a phase transformation during thermal cycling, which results in stresses that may encourage EBC spallation. Yb-silicate EBCs with mullite and yttrium aluminum garnet (YAG) dopant additions were deposited on SiC substrates with a Si intermediate bond coating and exposed to thermal cycling in steam at 1350 °C. The impact of Al dopant additions on the TGO growth rate and the SiO 2 phase transformation was assessed. Photo-stimulated luminescence spectroscopy (PSLS) was used to characterize the Al-containing phases and to measure stress evolution in the EBC following exposure using the stress-induced peak shift of the R-lines of mullite. Raman microscopy was used to map the stresses in the Si bond coating following exposure. It was found that the TGO phase transformation upon cooling increased compressive stress in the Si bond coating within 15 µm of the TGO.

Building Materials↗
DOE OSTI2024

Finite Element Modeling of the Phase Change in Thermally-Grown SiO 2 in SiC Systems for Gas Turbines

The operating lifetimes of SiC-based components in combustion environments are directly linked to the adhesion of the protective environmental barrier coating (EBC) layer. One of the major known failure modes for EBCs is the formation of a thick SiO 2 thermally grown oxide (TGO), which decreases coating adhesion and encourages eventual coating spallation. The effect of the TGO thickness under Yb 2 Si 2 O 7 EBCs on silicon carbide was investigated using finite element models (FEMs) with various interfacial architectures and SiO 2 TGO thicknesses. Further, the FEMs incorporated a user-defined material to simulate the volume contraction of the TGO during the silica phase transformation from β-cristobalite to α-cristobalite upon cooling from the stress-free state at 1350°C to room temperature. Systems with and without a silicon bond coating intermediary layer were assessed. It was shown that the TGO phase transformation stress (1.6–1.7 GPa) dominated the increase in stress in the TGO and EBC layers. Furthermore, it was found that stress increase in the TGO was independent of TGO thickness and interface geometry. These results indicate that stabilization of the TGO to mitigate the phase transformation could dramatically improve the performance of SiC-base components with EBCs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Advanced Intra-Cycle Detection of Pre-Ignition Events through Phase-Space Transforms of Cylinder Pressure Data

The widespread adoption of boosted, downsized SI engines has brought pre-ignition phenomena into greater focus, as the knock events resulting from pre-ignitions can cause significant hardware damage. Much attention has been given to understanding the causes of pre-ignition and identify lubricant or fuel properties and engine design and calibration considerations that impact its frequency. This helps to shift the pre-ignition limit to higher specific loads and allow further downsizing but does not fundamentally eliminate the problem. Real-time detection and mitigation of pre-ignition would thus be desirable to allow safe engine operation in pre-ignition-prone conditions. This study focuses on advancing the time of detection of pre-ignition in an engine cycle where it occurs. Furthermore, phase space transforms through time-delay embedding of cylinder pressure and principal component analysis were applied to same-cycle detection of pre-ignition and shown to enable detection on the order of a crank degree earlier than deviation in cylinder pressure can be identified through direct statistical observation of the pressure data. Additionally, it appears that the deviation of the trajectory in phase space may offer the opportunity to extend this method to further extend the detection window and allow more time for mitigation actions to occur.

42 ENGINEERING↗
DOE OSTI2019

Effect of laser welding on deformation mechanisms in irradiated austenitic stainless steel

In this work, deformation mechanism of a laser weld on neutron irradiated AISI 304L stainless steel was studied by in-situ microcompression test at room temperature. The deformation-induced austenite-to-martensite phase transformation occurs in {101}-oriented grains in the irradiated base metal, while deformation twinning prevails in {101}-oriented grains in the weld heat affected zone (HAZ). A high number density of irradiation-induced voids in the base metal provide sufficient nucleation sites for the austenite-to-martensite phase transformation under compression at room temperature. A deformation map is established to predict critical twinning stress for face-centered cubic (fcc) metals and alloys. Our study suggests that irradiation can tailor the deformation-induced phase transformation in austenitic stainless steels.

36 MATERIALS SCIENCE↗
NASA NTRS2014

Assessment of Shape Memory Alloys - From Atoms To Actuators - Via In Situ Neutron Diffraction

As shape memory alloys (SMAs) become an established actuator technology, it is important to identify the fundamental mechanisms responsible for their performance by understanding microstructure performance relationships from processing to final form. Yet, microstructural examination of SMAs at stress and temperature is often a challenge since structural changes occur with stress and temperature and microstructures cannot be preserved through quenching or after stress removal, as would be the case for conventional materials. One solution to this dilemma is in situ neutron diffraction, which has been applied to the investigation of SMAs and has offered a unique approach to reveal the fundamental micromechanics and microstructural aspects of bulk SMAs in a non-destructive setting. Through this technique, it is possible to directly correlate the micromechanical responses (e.g., internal residual stresses, lattice strains), microstructural evolutions (e.g., texture, defects) and phase transformation properties (e.g., phase fractions, kinetics) to the macroscopic actuator behavior. In this work, in situ neutron diffraction was systematically employed to evaluate the deformation and transformation behavior of SMAs under typical actuator conditions. Austenite and martensite phases, yield behavior, variant selection and transformation temperatures were characterized for a polycrystalline NiTi (49.9 at. Ni). As the alloy transforms under thermomechanical loading, the measured textures and lattice plane-level variations were directly related to the cyclic actuation-strain characteristics and the dimensional instability (strain ratcheting) commonly observed in this alloy. The effect of training on the shape memory characteristics of the alloy and the development of two-way shape memory effect (TWSME) were also assessed. The final conversion from a material to a useful actuator, typically termed shape setting, was also investigated in situ during constrained heatingcooling and subsequent shape recovery experiments. Neutron diffraction techniques are also being applied to the investigation of novel high temperature SMAs with the objective of designing alloys with better stability, higher transition temperatures and ultimately superior durability.

Shape Memory Alloys↗
DOE OSTI2018

Dual Phase Synergy Enabled Large Elastic Strains of Nanoinclusions in a Dislocation Slip Matrix Composite

Freestanding nanomaterials (such as nanowires, nanoribbons and nanotubes) are known to exhibit ultra-large elastic strains and ultra-high strengths. However, harnessing their superior intrinsic mechanical properties in bulk composites has proven to be difficult. A recent breakthrough has overcome this difficulty by using a martensitic phase transforming matrix, in which ultra-large elastic strains approaching the theoretical limit is achieved in Nb nanowires embedded in the matrix. This discovery, whereas breaking a long-standing challenge, still limits our ability of harnessing the exceptional properties of nanomaterials and developing ultrahigh strength bulk materials to a narrow selection of phase transforming alloy matrices. In this study, we investigated the possibility to harness the intrinsic mechanical properties of nanoinclusions in conventional dislocation slip matrix based on a principle of synergy between the inclusion and the matrix. The small spacing between the densely populated hard and dislocation-impenetrable nanoinclusions departmentalize the plastic matrix into small domains to effectively impede dislocation motion within the matrix, inducing significant strengthening and large local elastic strains of the matrix, which in turn induced large elastic strains in the nanoinclusions. This dual phase synergy is verified in a Ti 3 Sn inclusions/B2-NiTi(Fe) plastic matrix model materials system. The maximum elastic strain of Ti 3 Sn inclusion obtained in the dislocation slip matrix is comparable to that achieved in a phase transforming matrix. As a result, this finding opens new opportunities for the development of high-strength nanocomposites.

42 ENGINEERING↗
DOE OSTI2017

Final Report, Fundamental Mechanisms of Transient States in Materials Quantified by DTEM

At the project’s inception, there was growing evidence that the time domain for in situ observations of material evolution held great promise for allowing measurements to be made in never previously contemplated regimes. Also, central to the development of the project was the knowledge that phase transformations are of central importance to the development of materials microstructure and hence properties. We addressed this opportunity by developing a transmission electron microscope that could be operated in the pulsed mode (DTEM), with exposure times down to 20 ns and interframe times down to 20 ns in the nine-frame movie mode, designed with the intent of performing in situ experiments. This unprecedented capability allowed us to investigate structural phase transformations, intermetallic formation reactions, crystallization from the amorphous phase, rapid solidification of liquid metals, transformations in phase change materials, and catalyst nanoparticles. The ability of the electron microscope to create images with high spatial resolution allows for the accurate measurement of position. Common to all of the transformations mentioned above is the presence of a distinct interface between the old phase and the growing new phase. Measuring the position of the interface as a function of time, combined with the ability to count nucleation sites as a function of time, allowed for the exceptionally accurate measure of transformation kinetics. These measurements were used to guide and constrain the development of models and simulation methods for the classes of transformations studied.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Size, structure, and luminescence of Nd 2 Zr 2 O 7 nanoparticles by molten salt synthesis

Pyrochlore materials with novel properties are in demand with multifunctional applications such as optoelectronics, scintillator materials, and theranostics. Many reports have already indicated the importance of the synthesis technique for Nd 2 Zr 2 O 7 (NZO) nanoparticles (NPs); however, no explanation has been provided for the reason behind the nature of its phase selectivity. Here, we have explored the structural and optical properties of the NZO NPs synthesized by a molten salt synthesis method. We have synthesized size-tunable NZO NPs and correlated the particle size with their structural behavior and optical performance. All NZO NPs are stabilized in defect fluorite phase. Neutron diffraction provided insight on the behavior of oxygen in the presence of heavy atoms. We have collected bright amalgam of blue and green emission on UV irradiation due to the presence of oxygen vacancies from these NPs. We have carried out in situ XRD and Raman investigations to observe the temperature-induced phase transformation in a controlled argon environment. Interestingly, we have not observed phase change for the molten salt synthesized fluorite NZO NPs; however, we observed phase transformation from a precursor stage to pyrochlore phase by in situ XRD directly. Finally, these observations provide a new strategy to synthesize nanomaterials phase-selectively for a variety of applications in materials science.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanoscale magnetic phase competition throughout the Ni 50– x Co x Mn 40 Sn 10 phase diagram: Insights from small-angle neutron scattering

The Ni 2 MnSn-derived Ni 50– x Co x Mn 25+ y Sn 25– y alloys are premier examples of a class of off-stoichiometric Heusler alloys recently discovered to exhibit attractive magnetic properties in tandem with extraordinarily reversible martensitic phase transformations. Multiferroicity, magnetic phase competition and separation, field-induced martensitic transformations, magnetic shape memory behavior, and sizable magneto-, elasto-, and barocaloric effects result, generating substantial interest and application potential. In this work we expand on a prior small-angle neutron scattering (SANS) study at a single composition (Ni 44 Co 6 Mn 40 Sn 10 ) by exploring all three main regions of the recently established Ni 50–x Co x Mn 40 Sn 10 phase diagram, i.e., at the representative y = 15 composition. Wide temperature and scattering wave-vector range (20–500K, 0.004–0.2Å –1 ) SANS data on x = 2, 6, and 14 polycrystals provide a detailed picture of the evolution in magnetic order and inhomogeneity. Consistent with recent studies with a variety of techniques, phase separation into short-range coexisting ferromagnetic and antiferromagnetic regions is deduced below the martensitic transformation at x = 2 and 6, with average ferromagnetic cluster spacing of ~13 nm. Remarkably, at x = 14, where the martensitic transformation is suppressed and ferromagnetic austenite is stabilized to low temperatures, nanoscopic magnetic inhomogeneity nevertheless persists. Further, distinct ferromagnetic clusters (~36-nm average spacing) in a ferromagnetic matrix are observed at intermediate temperatures, homogenizing into a uniform long-range ordered ferromagnet only at low temperatures. This unusual ferromagnet cluster/ferromagnet matrix inhomogeneity, as well as x -dependent subtleties of the superparamagnetic freezing of ferromagnetic clusters, are discussed in light of 55 Mn nuclear magnetic resonance data, and the recent observation of annealing-induced core/shell nanoprecipitates. The origins of nanoscalemagnetic inhomogeneity are discussed in terms of statistical variations in local composition and structure, tendency to chemical phase separation, and other forms of disorder.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗