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At least 847 records · Page 47

Annealing texture of nanostructured IF steel

In the present work, the evolution of annealing texture in nanostructured interstitial free steel fabricated via accumulative roll bonding (ARB) process was investigated. Textural evolution after post-annealing of ARB-processed samples was evaluated using X-ray diffraction. There were several texture transitions in the γ-fiber and ζ-fiber during ARB and post-annealing treatment. It was found that with increasing the number of ARB cycles, the volume fraction of the low angle grain boundary decreased and the high angle grain boundary fraction increased. Also, the shear texture was dominant after the first cycle, while for other samples, the rolling texture was dominant. The one-cycle sample clearly indicated a weak α-fiber and γ-fiber and a relatively strong ζ-fiber. In addition, during the recrystallization and before the grain growth, the intensity of α-fiber and γ-fiber decreased, the intensity of ζ-fiber increased, and the intensity of (011)〈100〉 orientation in the ε-fiber and η-fiber increased. Moreover, it was concluded that the transition from the rolling texture to the shear one was a sign of occurrence of the recrystallization (before the grain growth). Finally, with increasing the number of ARB cycles, the intensity of rolling and shear textures saturated and a stable texture formed. - Highlights: • There were texture transitions in the γ-fiber and ζ-fiber. • When the number of cycles increased, the low angle grain boundaries decreased. • The shear texture was dominant after the first cycle. • Transition from rolling texture to shear one was a sign of recrystallization. • With increasing the number of ARB cycles, a stable texture formed.

36 MATERIALS SCIENCE↗

Microstructural characterization of sulfur-doped Bi{sub 2}Te{sub 3} crystals

In this study, the detailed microstructure of sulfur-doped Bi{sub 2}Te{sub 3} and the distribution of sulfur dopants were investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD result indicates that the interplanar distances of Bi{sub 2}Te{sub 3} are shortened after introducing sulfur dopants. HRTEM reveals that the unit cell lengths along [001, 100] are decreased as a result of the substitution of Te by smaller S atoms on the anion lattice. XRD and HRTEM analysis suggest that the distortion of the crystal lattice in Bi{sub 2}Te{sub 3} is induced by doping sulfur. High annular angler dark field scanning electron microscopy (HAADF-STEM) image shows obvious contrast variations in the Te atomic columns, indicating that Te sites were unevenly substituted by S dopants. Te columns with least contrast intensity correspond to the preferential occupation sites of S atoms. The replacement of Te atoms by S can be attributed to the evaporated sulfur powder in the gas reaction procedure. The present work is not only contributed to obtain a better understanding of the distribution of sulfur dopants introduced by gas reaction, but also can help explore the structural-property relationship of sulfur-doped Bi{sub 2}Te{sub 3}. - Highlights: • Sulfur-doped Bi{sub 2}Te{sub 3} was synthesized through modified Bridgeman method. • The introduction of S dopants results in shortening the lattice spacing of Bi{sub 2}Te{sub 3}. • HRTEM analysis indicates that S was doped through substituting Te atoms instead of incorporating interstitially. • HAADF-STEM analysis shows that Te columns with least intensity correspond to the preferential occupation sites of S.

36 MATERIALS SCIENCE↗

Microstructural evolution and mechanical properties on an ARB processed IF steel studied by X-ray diffraction and EBSD

Accumulative Roll Bonding (ARB) is one of the so-called severe plastic deformation (SPD) processes, allowing the production of metals and alloys with ultrafine (micro-nano) structures. Materials with ultrafine grains present attractive properties like the simultaneous increase in strength and ductility. Our interest in these materials is focused on their microstructural evolution during ARB processing, eventually responsible for the enhancement of those mechanical properties. In the current work we follow the evolution of the microstructure in an interstitial-free (IF) steel deformed by ARB after consecutive processing cycles, by means of Electron BackScatter Diffraction (EBSD) and X-ray diffraction (XRD). Particularly, we present results related to texture, grain (GS) and domain sizes, grain boundary character, density of Geometrically Necessary Dislocations (GND), Grain Orientation Spread (GOS), lattice parameters, microstrain, dislocation density and their spatial arrangement. After 5 ARB cycles the system shows a microstructure constituted mainly by submicrometric grains with high angle boundaries and low presence of dislocations inside the grains. - Highlights: •The evolution of microstructure is followed simultaneously by using GAM, GOS and GND (EBSD) and XRD. •LAGBs and subgrains disappear after few cycles SSDs and HAGBs persist at the end. •Dynamic recrystallization counterbalances dislocation arrays and diminishes hardening rate. •Grain size stabilization is revealed as a mechanism for increasing ductility after few ARB cycles.

36 MATERIALS SCIENCE↗

Radiation-induced extreme elastic and inelastic interactions in concentrated solid solutions

One of the biggest challenges in the radiation induced defect science is to understand the complex nature of ion-atom interactions under highly extreme conditions. Here, we report the irradiation induced non-equilibrium defect formation in NiCoCr single phase concentrated solid solution alloy due to (i) the extreme inelastic and (ii) the coupled inelastic and elastic ion-atom interactions. These two conditions are achieved at 5 and 30 μm penetration depths along the paths of swift heavy ions (1.542 GeV Bi). In general, the irradiation induced damage consists of interstitial-type dislocation loops and vacancy-type stacking fault tetrahedra (SFT). Near the surface (~5 μm) where electronic energy loss is dominating (~62.5 keV nm –1 ), the atomic motion primarily results in the formation of SFT. A noticeable increase of dislocation loop formation is observed at 30 μm near the maximum energy deposition from elastic interactions (~4.9 keV nm –1 ), as compared to the near surface region (~0.06 keV nm –1 ). As a result, insights on the complex electronic and atomic correlations of extreme energy deposition and dissipation on defect dynamics and structural stability may pave the way for new design principles of radiation–tolerant structural alloys.

36 MATERIALS SCIENCE↗

Effects of Fe atoms on hardening of a nickel matrix: Nanoindentation experiments and atom-scale numerical modeling

Significant hardening effect due to Fe concentrations in Ni-based alloys with face-centered-cubic structure has been studied by using a combined experimental and atomistic-based computational approach via nanoindentation tests. The obtained experimental load–displacement data for the [0 0 1] crystal orientation reached a qualitative good agreement with molecular dynamics simulations results, leading to strong evidence that the main strengthening factors are associated to sluggish dislocation diffusion, reduced defect sizes and the nucleation of tetrahedral stacking faults. Here, interstitial type prismatic dislocation loops mainly formed by $\frac{1}{6}$$\langle$1 1 2$\rangle$ Shockley dislocations are nucleated during the loading process, where their interaction leads to the formation of pyramidal shaped stacking fault which are mainly created by $\frac{1}{3}$$\langle$1 0 0$\rangle$ Hirth dislocations lines. Observing both types of defects coexisting in the same plastic deformation zone by both approaches. Reported mechanical data, measured experimentally and interpreted numerically, are also in accordance with microstructural SEM and TEM investigations.

36 MATERIALS SCIENCE↗

Analysis of position-dependent cavity parameters in irradiated metals to obtain insight on fundamental defect migration phenomena

Motion of point defects is a fundamental process that governs microstructure and properties of materials. Here, we examine the near-surface and grain boundary cavity swelling depth profiles in neutron- and ion-irradiated simple metals (Cu, Ni, and Fe-Cr) and investigate diffusional broadening of implanted Ni ions in Fe-Cr alloys. Vacancy migration energies and radiation-enhanced diffusion were experimentally estimated. Cavity denuded zone widths near planar sinks are shown to be dependent on temperature, damage rate, balance of point defects and sinks, and vacancy migration energies. An enhanced cavity swelling zone adjacent to the void-denuded zone was observed in specimens with low to moderate sink strength and interpreted as evidence of 1D gliding interstitial clusters. Radiation-enhanced diffusional broadening of implanted Ni ions (at 400–550 °C) in Fe and Fe-Cr was up to 250 nm toward the surface, which leads to a much broader near-peak-damage region where cavity swelling is suppressed. Diffusional broadening of implanted ions is calculated to be similarly pronounced for self-ion irradiations, particularly near the peak and higher swelling temperature regimes. Adequately high ion energies (~8–15 MeV) are recommended for ion-irradiation studies to provide a sufficiently broad midrange safe analysis region with minimized surface and injected ion effects.

36 MATERIALS SCIENCE↗

Solidification cracking of refractory alloys: a computational and machine learning study to investigate composition-dependence for improved weldability and additive manufacturability

Large-batch numerical, CALculation of PHAse Diagrams (CALPHAD)-based solidification cracking calculations are performed and then analyzed with machine learning methods to generate models that relate chemistry of refractory alloys to cracking susceptibility. Kou’s solidification cracking index is used to study the refractory alloys including O, N, C binary mixtures with Mo, Ta, Nb, and W, the molybdenum-based TZM, Niobium-based C103, and Tantalum-based T111 and Ta-10 W, as well as hypothetical refractory ternary alloys. Findings strongly validate Kou’s Crack Susceptibility Index (CSI) against Varestraint test data for Nb- and Ta-based alloys, establishing CSI thresholds where refractory alloys with CSI < 15,000 K are likely weldable, CSI > 15,000 K are prone to cracking, and CSI > 25,000 K are likely unweldable (or unprintable). Furthermore, interstitial elements C, N, and O significantly increase crack susceptibility, with some existing material specifications coinciding with peak cracking susceptibility concentrations. Finally, machine learning-derived elemental potency factors enable rapid prediction of CSI from alloy chemistry for C103, TZM, Ta-10 W, and T-111 alloys. These results provide practical guidance for feedstock selection, powder reuse limits, and alloy specification amendments for welding and additive manufacturing applications.

36 MATERIALS SCIENCE↗

Effect of rare earth Ce on the far infrared radiation property of iron ore tailings ceramics

Highlights: • Detailed process proposed for preparation of iron ore tailings ceramics. • Replace natural minerals with iron ore tailings as raw materials for preparing functional ceramics. • Impact mechanism of Ce on far infrared ceramics, as well as its optimum addition amounts can be obtained. • Propose a new perspective on considering the mechanism of far infrared radiation. - Abstract: A kind of far infrared radiation ceramics was prepared by using iron ore tailings, CaCO{sub 3} and SiO{sub 2} as main raw materials, and Ce as additive. The result of Fourier transform infrared spectroscopy showed that the sample exhibits excellent radiation value of 0.914 when doping 7 wt.% Ce. Ce{sup 4+} dissolved into iron diopside and formed interstitial solid solution with it sintered at 1150 °C. The oxidation of Fe{sup 2+} to Fe{sup 3+} caused by Ce{sup 4+} led to a decrease of crystallite sizes and enhancement of Mg–O and Fe–O vibration in iron diopside, which consequently improved the far infrared radiation properties of iron ore tailings ceramics.

36 MATERIALS SCIENCE↗

Effect of structure, size and copper doping on the luminescence properties of ZnS

Highlights: • Blue and green emission intensity form ZnS is sensitive to crystallographic form. • For ZnS nanoparticles, emission characteristics are not affected by copper doping. • Cu solubility poor in ZnS nanoparticles compared to corresponding bulk. - Abstract: Luminescence properties of wurtzite and cubic forms of bulk ZnS have been investigated in detail and compared with that of ZnS nanoparticles. Blue emission observed in both hexagonal and cubic forms of undoped bulk ZnS is explained based on electron–hole recombination involving electron in conduction band and hole trapped in Zn{sup 2+} vacancies where as green emission arises due to electron hole recombination from Zn{sup 2+} and S{sup 2−} vacancies. Conversion of wurtzite form to cubic form is associated with relative increase in intensity of green emission due to increased defect concentration brought about by high temperature heat treatment. Copper doping in ZnS, initially leads to formation of both Cu{sub Zn} and Cu{sub i} (interstitial copper) centers, and latter to mainly Cu{sub Zn} centers as revealed by variation in relative intensities of blue and green emission from the samples.

36 MATERIALS SCIENCE↗

Mo-N-co-doped mesoporous TiO{sub 2} microspheres with enhanced visible light photocatalytic activity

Highlights: • The Mo-N-co-doped mesoporous TiO{sub 2} microspheres were synthesized via a solvothermal method and further nitriding treatment. • The properties of TiO{sub 2} microspheres can be tuned by Mo-N-co-doping. • Photocatalytic rate is much higher than Pure TiO{sub 2} catalyst powder and Mo doped TiO{sub 2} microspheres. • The higher activity is attributed to the large BET surface area, narrow band gap and intense light absorbance in visible region. - Abstract: Mo-N-co-doped mesoporous TiO{sub 2} microspheres were synthesized via a solvothermal method, followed by nitriding treatment under ammonia gas flow. The efficiency of the samples was investigated by monitoring the degradation of Rhodamine-B under visible light irradiation. The experimental results revealed that Mo-N-co-doped mesoporous TiO{sub 2} microspheres showed better performances than Mo-TiO{sub 2} microspheres. It was observed that; 0.1% molar quantity of Mo doped on TiO{sub 2} at 500 °C for 2 h under nitriding conditions gave highest photocatalytic activities. The nitridation under ammonia gas of Mo-TiO{sub 2} samples created oxygen vacant sites and led to incorporation of substitutional and interstitial Nitrogen. The enhanced visible light photocatalytic activities of Mo-N-co-doped TiO{sub 2} photocatalyst was attributed to the large surface areas, narrow band gap and intense light absorbance in visible region. This study may create a promising and eco-friendly method towards synthesis of metal and non-metals co-doped on semiconductor materials to improve their photocatalytic activities and applications.

36 MATERIALS SCIENCE↗

Easily separable N-doped ZnO microspheres with high photocatalytic activity under visible light

Highlights: • N-doped ZnO microspheres are successfully prepared for environmental photocatalysis. • N doping is responsible for the highly improved photocatalytic activity. • The microspheres are easily separated from the suspension system by sedimentation. - Abstract: Here, two strategies are simultaneously applied to ZnO for easy separation of the photocatalyst particles by forming micron-sized spheres and for extension of the spectral response into the visible region by nitrogen doping. The prepared nitrogen-doped ZnO microspheres show highly improved photocatalytic activity for the degradation of humic acid (HA), a complex refractory organic compound, under visible light. Substitutional and interstitial doping of nitrogen are behind the improved visible light absorption and photocatalytic activity. Using 1 g/L of N-doped ZnO with urea/Zn molar ratio of 3, 95.4% of HA with an initial concentration of 5 mg/L can be degraded within 80 min of visible light irradiation of 122 mW/cm{sup 2}. Under identical experimental conditions, almost complete mineralization of HA is achieved within 12 h. Holes and hydroxyl radicals are mainly responsible for the degradation of HA. Significant reduction of toxicity of the photocatalytically treated HA solution is realized within 9 h.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Structural and optical analysis of Fe doped NiO nanoparticles synthesized by chemical precipitation route

Highlights: • The Fe doped NiO nanoparticles were synthesized by chemical precipitation route. • XRD analysis reveals cubic rock-salt fcc structure of the synthesized nanoparticles. • The Ni-O and Fe-O bonding were confirmed through FTIR spectroscopy. • Existence of nickel or interstitial oxygen vacancies indicates p-type nature of nanoparticles. - Abstract: The Ni{sub 1-x}Fe{sub x}O (x = 0.00, 0.02, 0.04 and 0.06) nanoparticles are synthesized by chemical precipitation route using polyvinyl-pyrrolidone (PVP) and nitrate metal salts. The results of thermogravimetric analysis (TGA) of the synthesized dried hydroxide powders showed that thermal decomposition occurs up to 500 °C so as to form pure Ni{sub 1-x}Fe{sub x}O. Therefore, all synthesized samples were calcined at 500 °C for 4 h and then used for further characterization. Energy dispersive X-ray spectroscopy (EDS) of all the samples showed the presence of Ni, O and Fe without any other impurities. From X-ray diffraction (XRD) and transmission electron microscopy (TEM) we could prove that the samples are polycrystalline in nature and are in single phase. The different vibrational modes in Ni{sub 1-x}Fe{sub x}O (x = 0.00, 0.02, 0.04 and 0.06) nanoparticles were studied by Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). Further, we analyzed the optical response from ultraviolet-visible spectra and Photoluminescence (PL) spectra.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Mapping TpPa-1 covalent organic framework (COF) molecular interactions in mixed solvents via atomistic modeling and experimental study

Complex solvent environments continue to limit the widespread adoption of organic solvent nanofiltration (OSN) in many chemical industry applications. In this paper we employ a commercially available covalent organic framework (COF), TpPa-1, and force field models to molecularly map separation performance of TpPa-1 membrane in mixed solvents. To minimize time and length scale mismatch between atomistic modeling and experiments, solvent permeance was normalized with water in modeling and experimental results to enable direct comparison. Model outputs, such as organic solvent permeance and solute rejection rate, matched well with filtration results. Since the atomistic models assume that all mass transfer is via through-pore transport, the discrepancies between modeling and experimental results provide insights on the effect of linear polymer defects, adsorption and interstitial mass transfer on polycrystalline COF membrane performance. In sum, force field models can serve as digital twins of COF membranes to simulate separation processes while capturing the effects of COF structure, chemistry, and crystallinity on membrane performance in complex organic solvent environments. Finally, this approach will provide insight into future COF design and synthesis for persisting separation challenges.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Heavy ion irradiation effects on GaN/AlGaN high electron mobility transistor failure at off-state

We investigate the effects of ion irradiation on AlGaN/GaN high electron mobility electron transistors using in-situ transmission electron microscopy. The experiments are performed inside the microscope to visualize the defects, microstructure and interfaces of ion irradiated transistors during operation and failure. Here, experimental results indicate that heavy ions such as Au 4+ can create a significant number of defects such as vacancies, interstitials and dislocations in the device layer. It is hypothesized that these defects act as charge traps in the device layer and the resulting charge accumulation lowers the breakdown voltage. Sequential energy dispersive X-ray spectroscopy mapping allows us to track individual chemical elements during the experiment, and the results suggest that the electrical degradation in the device layer may originate from oxygen and nitrogen vacancies.

36 MATERIALS SCIENCE↗

Interdiffusion and Formation of Intermetallic Compounds in High-Temperature Power Electronics Substrate Joints Fabricated by Transient Liquid Phase Bonding

Power electronics packages typically comprise a dielectric substrate bonded to a metal layer and attached to heat sinks using a low-thermal-conductivity solder. These multiple layers increase the effective thermal resistance of the package and are responsible for package failures under cyclic thermal loads. Bonding the aluminum nitride dielectric layer (AlN) directly to a low coefficient of thermal expansion (CTE) aluminum silicon carbide (AlSiC) cold plate using copper-aluminum (Cu-Al) transient liquid phase (TLP) bonding has been shown to improve the mechanical reliability of the power electronic packages while making the package more compact by bringing the cooling solutions closer to the devices. This study aims to characterize the Cu-Al bonds formed during the TLP bonding of AlSiC with three different power electronics substrates: pure aluminum nitride (AlN), aluminum (DBA), and copper (DBC). The material compositions and microstructures of the bonds were analyzed using scanning electron microscopy (SEM), x-ray spectroscopy (EDS), confocal scanning acoustic microscopy (C-SAM), and x-ray diffraction (XRD). ..alpha..-Al solid compound was identified as the dominant phase in AlN-AlSiC and Al-AlSiC, with a notable presence of SiC particles. In contrast, three intermetallic phases - ..theta..-CuAl2, ..eta..-CuAl, and ..gamma..'- Cu9Al4 - were observed in the Cu-AlSiC bond. A computational solid-state diffusion model was developed to predict the intermetallic compounds (IMCs) formed during Cu-Al TLP bonding in each system, which supported the observation that an initial Cu volume fraction of 20 % in the material layers produced a final bond composition of >95 % Al with minimal IMC growth. However, increased Cu concentrations produced higher concentration gradients, leading to increased growth of IMCs, Kirkendall voids, and interstitial cracks.

DIRECT ENERGY CONVERSION↗

Hydrogen isotope trapping in Al-Cu binary alloys

In this study, the trapping mechanisms for hydrogen isotopes in Al–X Cu (0.0 at. % < X < 3.5 at. %) alloys were investigated using thermal desorption spectroscopy (TDS), electrical conductivity, and differential scanning calorimetry. Constant heating rate TDS was used to determine microstructural trap energies and occupancies. In addition to the trapping states in pure Al reported in the literature (interstitial lattice sites, dislocations, and vacancies), a trap site due to Al–Cu intermetallic precipitates is observed. The binding energy of this precipitate trap is (18 ± 3) kJ•mol –1 (0.19 ± 0.03 eV). Typical occupancy of this trap is high; for Al–2.6 at. % Cu (a Cu composition comparable to that in AA2219) charged at 200 °C with 130 MPa D 2 for 68 days, there is ca. there is 3.15×10 –7 mol D bound to the precipitate trap per mol of Al, accounting for a third of the D in the charged sample.

36 MATERIALS SCIENCE↗

Effects of heat treatments on microstructure and properties of Ti-6Al-4V ELI alloy fabricated by electron beam melting (EBM)

Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that is used to fabricate three-dimensional near-net-shaped parts directly from computer models. Ti-6Al-4V is the most widely used and studied alloy for this technology and is the focus of this work in its ELI (Extra Low Interstitial) variation. The mechanisms of microstructure formation, evolution, and its subsequent influence on mechanical properties of the alloy in as-fabricated condition have been documented by various researchers. In the present work, the thermal history resulting in the formation of the as-fabricated microstructure was analyzed and studied by a thermal simulation. Subsequently different heat treatments were performed based on three approaches in order to study the effects of heat treatments on the singular and exclusive microstructure formed during the EBM fabrication process. In the first approach, the effect of cooling rate after the solutionizing process was studied. In the second approach, the variation of α lath thickness during annealing treatment and correlation with mechanical properties was established. In the last approach, several solutionizing and aging experiments were conducted.

36 MATERIALS SCIENCE↗