Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “solid solution”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,369 records · Page 76
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Facet-controlled phase separation in supersaturated Au-Ni nanoparticles upon shape equilibration

Solid-state dewetting is used to fabricate supersaturated, submicron-sized Au-Ni solid solution particles out of thin Au/Ni bilayers by means of a rapid thermal annealing technique. Phase separation in such particles is studied with respect to their equilibrium crystal (or Wulff) shape by subsequent annealing at elevated temperature. It is found that (100) faceting planes of the equilibrated particles are enriched with Ni and (111) faces with Au. Both phases are considered by quantum-mechanical calculations in combination with an error-reduction scheme that was developed to compensate for a missing exchange-correlation potential that would reliably describe both Au and Ni. The observed phase configuration is then related to the minimization of strongly anisotropic elastic energies of Au- and Ni-rich phases and results in a rather unique nanoparticle composite state that is characterized by nearly uniform value of elastic response to epitaxial strains all over the faceted surface. The same conclusion is yielded also by evaluating bi-axial elastic moduli when employing interpolated experimental elastic constants. This work demonstrates a useful route for studying features of physical metallurgy at the mesoscale.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Crystal growth and characterization of high performance KSr 2 Br x I 5-x :Eu scintillators

In this work we investigated the effects of replacing some of the matrix iodine atoms in KSr 2 I 5 :Eu with bromine, forming new solid solutions of KSr 2 Br x I (5-x) :Eu (where 0.05 ≤ x ≤ 1). Transparent 15 and 22mm diameter single crystals of KSr 2 Br x I (5-x) :Eu were grown via the vertical Bridgman technique using a two-zone transparent furnace. The scintillation properties of the mixed compounds were investigated and compared to the non-mixed parent compound, KSr 2 I 5 :Eu. The addition of bromine blue-shifted the x-ray excited emission from 450 to 446 nm, increased the light yield by ~11% and decreased the scintillation decay time by suppressing its secondary component (≥3 μs), while maintaining energy resolutions comparable to the reference crystal. Thermoluminescence glow curves showed that the addition of bromine modified the shallow traps located between 25 and 55 K, suggesting that these traps are responsible for lengthening KSI’s decay time. In large sizes (10.3 cm 3 ), KSr 2 Br 0.10 I 4.90 :Eu 2% had light yields up to 79,000 ph/MeV and energy resolutions as low as 3.2% at 662 keV.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Transferring predictions of formation energy across lattices of increasing size*

In this study, we show the transferability of graph convolutional neural network (GCNN) predictions of the formation energy of the nickel-platinum solid solution alloy across atomic structures of increasing sizes. The original dataset was generated with the large-scale atomic/molecular massively parallel simulator using the second nearest-neighbor modified embedded-atom method empirical interatomic potential. Geometry optimization was performed on the initially randomly generated face centered cubic crystal structures and the formation energy has been calculated at each step of the geometry optimization, with configurations spanning the whole compositional range. Using data from various steps of the geometry optimization, we first trained our open-source, scalable implementation of GCNN called HydraGNN on a lattice of 256 atoms, which accounts well for the short-range interactions. Using this data, we predicted the formation energy for lattices of 864 atoms and 2048 atoms, which resulted in lower-than-expected accuracy due to the long-range interactions present in these larger lattices. We accounted for the long-range interactions by including a small amount of training data representative for those two larger sizes, whereupon the predictions of HydraGNN scaled linearly with the size of the lattice. Therefore, our strategy ensured scalability while reducing significantly the computational cost of training on larger lattice sizes.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Development of a novel fcc structure for an amorphous-nanocrystalline Ti-33Nb-4Mn (at.%) ternary alloy

Highlights: • An amorphized-nanocrystalline TiNbMn alloy was obtained via mechanical alloying. • The obtained TiNbMn ternary alloy evolved with the mechanical alloying time. • At lower milling time, two nanocrystalline phases (bcc and fcc) were developed. • At higher milling time, a stable amorphous-nanocrystalline fcc-alloy was obtained. • The fcc structure has not been previously published for Ti based alloys. - Abstract: In this work, a novel amorphous-nanocrystalline titanium‑niobium‑manganese solid solution ternary alloy with a Ti-33Nb-4Mn (at.%) nominal composition was developed by a High-Energy Mechanical Alloying. Nb and Mn were added to the elemental Ti as a β-phase (bcc) stabilizer and an amorphization promoter, respectively. The system evolved from the elemental Ti, Nb and Mn raw materials to a body centred cubic (bcc) TiNbMn alloy and, finally, to the formation of an original and stable face centred cubic (fcc) nanocrystalline TiNbMn alloy, not reported until now, at short milling time (20 h). This alloy remains invariant until 120 h. In turn, the partial amorphization of the system occurs and increases until at intermediate milling time (80 h). The production of both original fcc and the amorphous TiNbMn alloy may be beneficial for reducing the Young's modulus and improving the mechanical strength pursued for the Ti alloy. The optimal milling time respect to the amorphization, nanocrystalline size and Fe mount from milling media was 60 h and 80 h (TiNbMn60h and TiNbMn80h), with > 50 wt% of an amorphous phase and a crystalline domain size of approximately 5 nm.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Growth, intermixing, and surface phase formation for zinc tin oxide nanolaminates produced by atomic layer deposition

A broad and expanding range of materials can be produced by atomic layer deposition at relatively low temperatures, including both oxides and metals. For many applications of interest, however, it is desirable to grow more tailored and complex materials such as semiconductors with a certain doping, mixed oxides, and metallic alloys. How well such mixed materials can be accomplished with atomic layer deposition requires knowledge of the conditions under which the resulting films will be mixed, solid solutions, or laminated. The growth and lamination of zinc oxide and tin oxide is studied here by means of the extremely surface sensitive technique of low energy ion scattering, combined with bulk composition and thickness determination, and x-ray diffraction. At the low temperatures used for deposition (150 °C), there is little evidence for atomic scale mixing even with the smallest possible bilayer period, and instead a morphology with small ZnO inclusions in a SnO{sub x} matrix is deduced. Postannealing of such laminates above 400 °C however produces a stable surface phase with a 30% increased density. From the surface stoichiometry, this is likely the inverted spinel of zinc stannate, Zn{sub 2}SnO{sub 4}. Annealing to 800 °C results in films containing crystalline Zn{sub 2}SnO{sub 4}, or multilayered films of crystalline ZnO, Zn{sub 2}SnO{sub 4}, and SnO{sub 2} phases, depending on the bilayer period.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Quantum Corrections and Magnetotransport in 3D Dirac Semimetal Cd{sub 3 –x}Mn{sub x}As{sub 2} Films

Thin films of solid solutions based on the three-dimensional Dirac semimetal Cd{sub 3}As{sub 2} with the addition of manganese are investigated. Cd{sub 3 –x}Mn{sub x}As{sub 2} films (x = 0, 0.05, and 0.1) 50–70 nm in thickness are formed on a glassceramic substrate using vacuum-thermal deposition from cadmium arsenide ingots doped by Mn and fabricated by direct alloying elements by the vacuum-cell method. The temperature and magnetic-field dependences of the resistance are measured and the transport parameters of the films under study are determined. Positive magnetoresistance of the characteristic shape corresponding to the contribution of the weak antilocalization effect is observed for films with x = 0 and 0.05. The contribution from the weak localization effect is observed at a higher Mn content (x = 0.1). This change in the quantum correction type as applied to topological semimetals points to reconstruction of the band structure and transition from the Dirac semimetal state into a trivial semiconductor phase, which corresponds to the critical Mn content x{sub c} ~ 0.07 in this case.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Infrared spectra of small molecular ions trapped in solid neon

The infrared spectrum of a molecular ion provides a unique signature for that species, gives information on its structure, and is amenable to remote sensing. It also serves as a comparison standard for refining ab initio calculations. Experiments in this laboratory trap molecular ions in dilute solid solution in neon at 4.2 K in sufficient concentration for observation of their infrared spectra between 450 and 4000 cm{sup !1}. Discharge-excited neon atoms produce cations by photoionization and/or Penning ionization of the parent molecule. The resulting electrons are captured by other molecules, yielding anions which provide for overall charge neutrality of the deposit. Recent observations of ions produced from C{sub 2}H{sub 4} and BF{sub 3} will be discussed. Because of their relatively large possibility of having low-lying excited electronic states, small, symmetric molecular cations are especially vulnerable to breakdown of the Born-Oppenheimer approximation. Some phenomena which can result from this breakdown will be discussed. Ion-molecule reaction rates are sufficiently high that in some systems absorptions of dimer cations and anions are also observed. When H{sub 2} is introduced into the system, the initially-formed ion may react with it. Among the species resulting from such ion-molecule reactions that have recently been studied are O{sub 4}{sup +}, NH{sub 4}{sup +}, HOCO{sup +}, and HCO{sub 2}{sup !}.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Laser radiation of CdS{sub x}Se{sub 1-x} targets in a gas diode

Laser radiation of semiconductor targets of CdS{sub x}Se{sub 1-x} solid solutions excited by an electron beam in a gas-filled diode was investigated at constant and varying gas pressures. In the first case, lasing was excited by an electron beam with an energy of 170 keV and a duration of 100 ps in semiconductor targets with different x. The highest powers 125 and 96 kW were achieved at x ≈ 0.2 (λ ≈ 677 nm) and x ≈ 1 (λ ≈ 522 nm), respectively. The minimum power (26 kW) was observed in the yellow-green spectral region. The maximum slope efficiency in these experiments reached 9%. In the second case, the radiation power of CdS targets (x = 1) was studied as a function of the air pressure in the gas diode varying from 0.1 to 2.5 Torr. The experimental data well agree with the calculation results. The possibility of reducing the radiation divergence by using a conical optical fibre is demonstrated. At the lasing threshold of semiconductor targets exited by an electron beam or a streamer discharge, filamentary channels appear due to, probably, an anisotropy of the impact ionisation coefficient. (lasers)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Microstructural characterization of a new mechanically alloyed Ni-base ODS superalloy powder

The microstructure of a new Ni-base oxide dispersion strengthened superalloy powder was studied for high temperature gas turbine applications after the mechanical alloying process. In this study, an atomized powder with a composition similar to the CMSX-10 superalloy was mechanically alloyed with yttria and Hf powders. The mechanically alloyed powder included only the supersaturated solid solution γ phase without γ′ and yttria provided by severe plastic deformation, while after the 3-step aging, the γ′ phase was precipitated due to the partitioning of Al and Ta to the γ′ and Co, Cr, Re, W, and Mo to the γ phase. Mechanical alloying modified the morphology of γ′ to the new coherent γ–γ′ nanoscale lamellar structure to minimize the elastic strain energy of the precipitation, which yielded a low lattice misfit of 0.16% at high temperature. The γ′ lamellae aligned preferentially along the elastically soft [100] direction. Also, the precipitated oxide particles were refined in the γ phase by adding Hf from large incoherent YAlO{sub 3} to fine semi-coherent Y{sub 2}Hf{sub 2}O{sub 7} oxide particles with the average size of 7 nm and low interparticle spacing of 76 nm. - Highlights: • A new Ni-base ODS superalloy powder was produced by mechanical alloying. • The nanoscale γ–γ′ lamellar structure was precipitated after the aging treatment. • Fine semi-coherent Y{sub 2}Hf{sub 2}O{sub 7} oxide particles were precipitated by addition of Hf.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Study on a novel orientation domain of Mg17Ce2 in aged Mg–Y–Ce alloy by HAADF–STEM

Highlights: • A novel 90 degree orientation domain is verified in Mg{sub 17}Ce{sub 2} granules. • Orientations of domain–Ι and Π are parallel and perpendicular to α-Mg matrix. • A 〈 1-bar 010〉 direction is unchanged among domain–Ι, domain–Π and α-Mg matrix. • Domain–Ι matches the α-Mg matrix better than the domain–Π. - Abstract: After solid solution and aging treatment, Mg–3.0 wt%Y–1.0 wt%Ce ternary alloys were characterized using high angle annular dark field scanning transmission electron microscopy. The main precipitates are dominated by β{sup ′} phase and Mg{sub 17}Ce{sub 2}, a Ni{sub 17}Th{sub 2}–type intermetallic compound. An orientation domain can be discerned generally in Mg{sub 17}Ce{sub 2} granules. This domain structure of Mg{sub 17}Ce{sub 2} always consists of two types of domains (defined as domain–Ι and domain–Π) with a 90 degree orientation difference. In terms of crystallographic orientation relationships among α-Mg matrix and both domains, the domain–Ι is parallel to α-Mg matrix, whereas, the domain–Π is perpendicular to α-Mg matrix. It can be certain that a 〈 1-bar 010〉 direction is an unchanged orientation. Due to nearly equal values of lattice parameters (a and c) of Mg{sub 17}Ce{sub 2}, both domain–Ι and domain–Π can matches well with α-Mg matrix. Comparatively, the interfacial matching between domain–Ι and α-Mg matrix is better, suggesting that the domain–Ι is dominant.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Influence of Modification on Crystal Lattice Stability of Austenite in Stainless Steel

Using the methods of electron diffraction microscopy and X-ray diffraction analysis, the influence of alloying of the austenitic steel, Grade 110H13, with chromium and vanadium, as well as high-melting, ultrafine-grained TiO{sub 2}, ZrO{sub 2} powders and Na{sub 3}AlF{sub 6} cryolite on its structural-phase state and microstructure is investigated. It is shown that the matrix of non-modified steel is completely austenitic and consists of an iron-based solid solution and the interstitial (C, N, O and other) and substitutional (Cr, V and other) atoms simultaneously. Alloying with chromium and vanadium changes neither its phase composition nor defect structure, while alloy modification results in qualitatively new structural features: γ → ε-transformation, high-intensity microtwinning, defect structure changes, and a sharp increase in the scalar dislocation density. The features of the deformation-induced microtwinning and ε-martensite plates identified in the modified steel promote revealing additional microtwin systems in the matrix γ-phase, which result in structural changes making it possible to classify it as a γ′-phase. It is found out that an introduction of modifying additions gives rise to the following sequence of structural-phase transformations: γ→γ′→(γ′ +ε). The experimental data obtained demonstrate that as a result of modification the crystal lattice transits into a low-stability state. This transition is accompanied by marked structural-phase changes consisting in the formation of several microtwin systems and γ → ε-transformation. These structural-phase changes in the modified steel are due to the crystal-lattice transition into the low-stability state, followed by new structural-phase alterations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Optical properties of bimetallic Au-Cu nanocrystals embedded in glass

Highlights: • Bimetallic Au-Cu/Au-Cu sosoloid nanocrystals embedded in glass have been synthesized. • We achieved good size control of the nanocrystals. • The glasses with bimetallic nanoparticles exhibit enhanced for NLO. • The mechanisms on the nonlinear effects of the obtained glasses are discussed. - Abstract: Bimetallic gold-copper and gold-copper solid solutions nanocrystals (NCs) with orthorhombic morphologies were successfully embedded in sodium borosilicate glass; these materials were fabricated via a facile and synthetic sol-gel technique. Microstructural investigations demonstrate that the Au-Cu intermetallic nanostructure is generated in the glass host. Z-scan technique was applied to reveal the third-order nonlinear optical properties of the as-obtained glasses at the wavelength of 800 nm. The as-synthesized glasses exhibit similar nonlinear behaviors with reverse saturation absorption effects and self-focusing performances. The third-order nonlinear susceptibility χ{sup (3)} values were optimized by tuning the sizes of the Au-Cu NCs. The χ{sup (3)} of Au-Cu(1), Au-Cu(2) and Au-Cu(3) measured to be 3.1 × 10{sup −12}, 5.4 × 10{sup −12}, and 4.4 × 10{sup −12} esu for the single-pulse energy of 1 μJ. We have discussed the nonlinear optical mechanism of Au-Cu sodium borosilicate glass that may be due to the quantum confinement effects and surface plasmon resonance effects.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Domain configuration changes under electric field-induced antiferroelectric-ferroelectric phase transitions in NaNbO{sub 3}-based ceramics

We recently developed a feasible crystal chemistry strategy to stabilize the antiferroelectricity in NaNbO{sub 3} through a chemical substitution to decrease the tolerance factor and increase the average electronegativity of the system [Shimizu et al., Dalton Trans. 44, 10763 (2015) and Guo et al., J. Appl. Phys. 117, 214103 (2015)]. Two novel lead-free antiferroelectric (AFE) solid solutions, (1-x)NaNbO{sub 3}-xCaZrO{sub 3} and (1-x)NaNbO{sub 3}-xSrZrO{sub 3}, have been found to exhibit the double polarization hysteresis typical of a reversible AFE ↔ ferroelectric (FE) phase transition. In this study, as demonstrated by (1-x)NaNbO{sub 3}-xCaZrO{sub 3} system, the influence of chemical modification and electrical poling on the AFE/FE phase stability was investigated, primarily focusing on the microstructural and crystallographic evolutions. Together with the macroscopic polarization hysteresis measurements, a well-demonstrated structure-property relationship was presented. It was found that the CaZrO{sub 3} substitution into NaNbO{sub 3} can effectively destabilize the FE Q phase and correspondingly lead to a spontaneous reverting to AFE P phase. In contrast to the reversible AFE ↔ FE phase transition, the domain morphology evolution exhibits irreversible nature with a growing process of the orientational domains after applying electric field. Moreover, a multiple-zone axes electron diffraction map of P and Q phases has been summarized and is believed to be an efficient diagram to determine the AFE/FE nature of the NaNbO{sub 3}-based systems.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Magnetic properties of CeM{sub 1.5}M’{sub 0.5}Ge{sub 4}O{sub 12} (M = Mn, Co; M’ = Ni, Cu)

Highlights: • marked composition dependence of magnetic structure. • metamagnetic transition in CeCo{sub 1.5}Cu{sub 0.5}Ge{sub 4}O{sub 12}. • neutron diffraction in an applied field. - Abstract: A study of the solid solutions CeM{sub 2-x}M’{sub x}Ge{sub 4}O{sub 12} (M = Mn, Co; M’ = Ni, Cu) by X-ray diffraction and magnetometry is described. For M = Co the introduction of Ni and Cu was possible for x ≤ 0.5; for M = Mn attempts to introduce Cu were unsuccessful but CeMn{sub 1.5}Ni{sub 0.5}GeO{sub 4} was obtained as a single phase. These compositions crystallise in the space group P4/nbm with a ~9.76, c ~4.85 Å. They all order antiferromagnetically with T{sub N}1.5 Ni{sub 1.5}Ge{sub 4}O{sub 12} and CeCo{sub 1.5}Cu{sub 0.5}Ge{sub 4}O{sub 12} were shown by neutron diffraction to be the same as those adopted by CeMn{sub 2}Ge{sub 4}O{sub 12} and CeCo{sub 2}Ge{sub 4}O{sub 12}, respectively. The in-field metamagnetic transition of the latter to a weakly ferromagnetic phase was shown to persist in the Cu-doped composition. The available data suggest that the anisotropy associated with the Co{sup 2+} cation plays a dominant role in determining the magnetic behaviour of these compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Irradiation-induced damage evolution in concentrated Ni-based alloys

Understanding the effects of chemical complexity from the number, type and concentration of alloying elements in single-phase concentred solid-solution alloys (SP-CSAs) on defect dynamics and microstructure evolution is pivotal for developing next-generation radiation-tolerant structural alloys. A specially chosen set of SP-CSAs with different chemical complexity (Ni 80 Fe 20 , Ni 80 Cr 20 and Ni 40 Fe 40 Cr 20 ) are investigated using 1.5 MeV Mn ions over a wide fluence range, from 2 × 10 13 to 1 × 10 16 ions cm –2 at room temperature. Based on an integrated study of Rutherford backscattering spectroscopy in channeling geometry and molecular dynamics simulations, the results demonstrate that Ni 40 Fe 40 Cr 20 is more radiation tolerant than Ni 80 Fe 20 , Ni 80 Cr 20 and elemental Ni in the low fluence regime. While chemical complexity of this set of SP-CSAs is clearly demonstrated to affect defect evolution through suppressed defect production and enhanced recombination at early stages, the effect of the mixed ferro- and anti-ferromagnetic interactions is not the only controlling factor responsible for the improved radiation performance. As a result, the observed strong alloying effect on defect evolution is attributed to the altered defect migration mobilities of defect clusters in these alloys, an intrinsic characteristic of the complex energy landscapes in CSAs.

36 MATERIALS SCIENCE↗
DOE OSTI2013

High pressure synthesis and crystal structure of a ternary superconductor Ca{sub 2}Al{sub 3}Si{sub 4} containing layer structured calcium sub-network isomorphous with black phosphorus

The Zintl compound CaAl{sub 2}Si{sub 2} is peritectically decomposed to a mixture of Ca{sub 2}Al{sub 3}Si{sub 4} and aluminum metal at temperatures above 600 Degree-Sign C under a pressure of 5 GPa. The new ternary compound Ca{sub 2}Al{sub 3}Sl{sub 4} crystalizes with the space group Cmc2{sub 1} and the lattice parameters a=5.8846(8), b=14.973(1), and c=7.7966(5) A. The structure is composed of aluminum silicide framework [Al{sub 3}Si{sub 4}] and layer structured [Ca{sub 2}] network interpenetrating with each other. The electron probe microanalysis (EPMA) shows the formation of solid solutions Ca{sub 2}Al{sub 3-x}Si{sub 4+x} (x<0.6). The layer structured [Ca{sub 2}] sub-network is isomorphous with black phosphorus. The new ternary compound shows superconductivity with a transition temperature (T{sub c}) of 6.4 K. The band structure calculation suggests that the superconductivity should occur through the conduction bands mainly composed of 3p orbitals of the aluminum silicide framework. - Graphical abstract: A new ternary superconductor Ca{sub 2}Al{sub 3}Si{sub 4} has been prepared under high pressure and high temperature conditions, which includes layer structured calcium sub-network isomorphous with black phosphorus. Highlights: Black-Right-Pointing-Pointer A typical Zintl compound CaAl{sub 2}Si{sub 2} melts congruently at ambient pressure. Black-Right-Pointing-Pointer Under high pressure CaAl{sub 2}Si{sub 2} decomposes to Ca{sub 2}Al{sub 3}Si{sub 4} and Al at {approx}600 Degree-Sign C. Black-Right-Pointing-Pointer Ca{sub 2}Al{sub 3}Si{sub 4} contains Ca sub-network isomorphous with black phosphorus. Black-Right-Pointing-Pointer Ca{sub 2}Al{sub 3}Si{sub 4} shows superconductivity with a transition temperature of 6.4 K.

36 MATERIALS SCIENCE↗
DOE OSTI2016

A Degradation Model and Testing Protocol for Metallic Waste Forms

Metallic waste forms developed to immobilize high-level radioactive wastes generated in advanced (closed) fuel cycles are being evaluated as part of the US DOE Fuel Cycle Research and Development program. The electrometallurgical reprocessing of used metallic fuels to recover U and Pu generates two waste streams: salt waste comprised of electro-refiner salt contaminated with oxidized fission products and metallic waste comprised of activated cladding and metallic fission products that were not oxidized under the processing conditions. Metallic waste forms can be made by directly melting the cladding and fuel wastes with small amounts of trim metals to form a multiphase alloy material in which radionuclides and other waste constituents are immobilized within chemically durable intermetallic and iron-based solid solution phases. Some components of the waste streams serve as important constituents of phases comprising the waste form, such as Mo and Zr, which form Fe{sub 2}Mo and Fe{sub 2}Zr intermetallic phases. Other waste constituents are accommodated as substitutional contaminants, such as Tc within the Fe{sub 2}Mo intermetallic and U/TRU within the Fe{sub 2}Zr intermetallic. Sufficient amounts of these host phases must be available to accommodate all radionuclides in the waste stream. The compositional flexibility of the host phases to incorporate waste components is being utilized to formulate metal waste forms that will accommodate variations in waste streams compositions that will occur over long-term fuel reprocessing operations. Small amounts of trim metals can be added to tailor the phase assembly to match the waste stream composition and facilitate waste form processing. For example, additional Zr may be added during waste form production to ensure all U/TRU is sequestered in a durable Fe{sub 2}Zr host phase and trim Cr, Mo, and Ni can be added to supplement the amounts provided by the steel cladding to improve the durability of a metallic waste form. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

A new observation of strain-induced grain boundary serration and its underlying mechanism in a Ni–20Cr binary model alloy

Highlights: • Grain boundary serration was first found to occur in Ni–Cr binary alloy with no precipitate. • The mechanism of segregation-induced serration was proposed. • The serrated grain boundary was enriched with Cr atoms as high as 87 at.%. • The critical strain energy for the serration was determined to be lower than 0.97 kJ/mol. - Abstract: The formation of serrated grain boundaries and its mechanism correlated with straining exerted on grain boundaries were investigated in a Ni–20Cr binary model alloy. This model alloy is a full solid-solution which is deliberately free of carbon and aluminum to exclude precipitation of second phase. A special heat treatment involving slow cooling was used to promote grain boundary serration, which is known to enhance creep properties by decreasing interfacial free energy. No sign of serration could be observed for the Ni–20Cr model alloy, as expected. However, we found that a compressive 5% strain hold applied during slow cooling successfully induced grain boundary serration. The compressive 5% strain hold generated dislocations near the grain boundary, which promoted Cr enrichment near and at the grain boundary. To dismiss the effect of dislocations, straining was removed and the solutionizing temperature was increased in order to promote Cr enrichment near and at grain boundaries by means of vacancy-assisted diffusion of Cr atoms from the bulk to grain boundaries during slow cooling. Significant grain boundary serration was again observed. Atom probe tomography analyses showed that the serrated grain boundary with a wide width of ~ 10 nm was enriched with Cr atoms, as much as 87 at.%, while the γ matrix only contained 34 at.%. First-principles calculations showed that the strain energy necessary for the onset of grain boundary serration is lower than 0.97 kJ/mol.

36 MATERIALS SCIENCE↗