Grain Boundary Oxidation Research Articles

Formation mechanism and microstructural and mechanical properties of in-situ Ti–Ni-based composite coatings by laser metal deposition

Abstract The in-situ TiO 2 reinforced Ti–Ni composite coating on carbon steel was successfully prepared by laser metal deposition (LMD) using Ti–Ni as-mixed powder with an atomic ratio of 60:40. With the aim of in-situ reaction design during LMD processing, a trace of oxygen mixed with the shielding gas was introduced. Different “laser energy input per unit length” (E) by changing the laser power was set to investigate the influence on the deposition quality and attendant microstructure and mechanical property of the LMD-processed layer. TiO 2 particles with unique flower-like structure formed when the applied laser energy E ≤ 96 kJ/m, while the apparent oxidation of grain boundaries was observed as E increased to 120 kJ/m. The formation mechanism of in-situ TiO 2 particles with a flower-like structure was present. At the optimized process parameter of 96 kJ/m, the LMD-processed layer showed the highest densification degree free of any pores and cracks. The corresponding mechanical properties were measured, showing the relatively high average microhardness of HV 0.2 790 and significantly improved tribological property containing lower coefficient of friction of 0.4 and more smooth worn surface.

Oxidation of iridium coatings on rhenium substrates at ultrahigh temperature in stagnant air: Its failure mechanism and life model

Abstract Electrodeposited iridium (Ir) coatings on rhenium (Re) substrate were oxidized in stagnant air at 2223 K, 2073 K and 1973 K and 100 μm-thick Ir coatings had the oxidation lifetime of about 175 min, 235 min and 310 min respectively. Both uniform thickness loss of Ir coating and preferential oxidation of Ir grain boundaries co-resulted in the oxidation failure of Ir coating. Based on uniform oxidation rate of Ir coating and diffusion rate of Re element along Ir grain boundaries, a lifetime model for the Ir coating on Re substrates was proposed. Oxidation of pure Ir and diffusion model of Re in Ir coating was studied and described respectively.

On determining the height of the potential barrier at grain boundaries in ion-conducting oxides.

The validity and limitations of two quantitative approaches for estimating the height of the potential barrier at grain boundaries, Ψgb, in polycrystalline ionic conductors are examined both theoretically and experimentally. The linear diffusion model recently proposed by Kim and Lubomirsky determines Ψgb from the value of the power exponent of the current (Igb)-voltage (Ugb) relationship at the grain boundary, dln(Igb)/dln(Ugb), while the conventional approach calculates Ψgb from the ratio of the grain boundary resistivity to the grain core resistivity. The results of our theoretical analysis demonstrate that both approaches should yield consistent values for Ψgb if the ionic current through the grain boundary is limited exclusively by space charge. While the value of Ψgb obtained by the power law procedure is relatively insensitive to other causes of current obstruction, e.g. current constriction and/or local structural disorder, the resistivity ratio method, if not explicitly corrected for these additional limitations, results in a considerable overestimate of the grain boundary potential barrier. Hence, it is possible to distinguish between grain boundary resistance due to the presence of space charge and that due to additional sources by comparing the values of Ψgb determined using each of the two methods. Our theoretical analysis is confirmed experimentally with 3 mol% Gd-doped ceria with and without an additional source of current constriction across the grain boundary.

B11-O-01Atomic-Scale STEM Characterization of Grain Boundaries in Oxides

B11-O-01Atomic-Scale STEM Characterization of Grain Boundaries in Oxides

Analysis of stress corrosion cracking in alloy 718 following commercial reactor exposure

Alloy 718 is generally considered a highly corrosion-resistant material but can still be susceptible to stress corrosion cracking (SCC). The combination of factors leading to SCC susceptibility in the alloy is not always clear enough. In the present work, alloy 718 leaf spring (LS) materials that suffered stress corrosion damage during two 24-month cycles in pressurized water reactor service, operated to >45 MWd/mtU burn-up, was investigated. Compared to archival samples fabricated through the same processing conditions, little microstructural and property changes occurred in the material with in-service irradiation, contrary to high dose rate laboratory-based experiments reported in literature. Though the lack of delta phase formation along grain boundaries would suggest a more SCC resistant microstructure, grain boundary cracking in the material was extensive. Crack propagation routes were explored through focused ion beam milling of specimens near the crack tip for transmission electron microscopy as well as in polished plan view and cross-sectional samples for electron backscatter diffraction analysis. It has been shown in this study that cracks propagated mainly along random high-angle grain boundaries, with the material around cracks displaying a high local density of dislocations. The slip lines were produced through the local deformation of the leaf spring material above their yield strength. The cause for local SCC appears to be related to oxidation of both slip lines and grain boundaries, which under the high in-service stresses resulted in crack development in the material.

Tailoring physical properties of graphene: Effects of hydrogenation, oxidation, and grain boundaries by atomistic simulations

Graphene, a two-dimensional monolayer of carbon atoms in honeycomb structure, is a research hotspot in multidisciplinary due to its excellent physical properties. To further extend the applications of graphene, various strategies have been proposed to tailor its physical properties. Recently, our group has carried out systematically computational studies on modifying graphene, including hydrogenation, oxidation, and introduction of grain boundaries. Both the hydrogenation and oxidation will convert sp2 hybridized carbons into sp3 configurations, while formation of grain boundaries only makes the sp2 carbon bonds distorted. Employing density functional theory calculations, structures, physical properties and applications of these modified graphene were explored, such as structural phase diagram, mechanical and electronic properties, and photocatalytic applications. It turns out that many physical properties of graphene are tunable, endowing graphene promising applications in various fields. In this review article, we will generally summarize our recent works on the hydrogenated graphene, graphene oxide, and graphene grain boundaries.

Uranium vacancy mobility at the Σ5 symmetric tilt and Σ5 twist grain boundaries in UO2

Ionic transport at grain boundaries in oxides dictates a number of important phenomena, from ionic conductivity to sintering to creep. For nuclear fuels, it also influences fission gas bubble nucleation and growth. Here, using a combination of atomistic calculations and object kinetic Monte Carlo (okMC) simulations, we examine the kinetic pathways associated with uranium vacancies at two model grain boundaries in UO2. The barriers for vacancy motion were calculated using the nudged elastic band method at all uranium sites at each grain boundary and were used as the basis of the okMC simulations. For both boundaries considered – a simple tilt and a simple twist boundary – the mobility of uranium vacancies is significantly higher than in the bulk. For the tilt boundary, there is clearly preferred migration along the tilt axis as opposed to in the perpendicular direction while, for the twist boundary, migration is essentially isotropic within the boundary plane. These results show that cation defect mobility in fluorite-structured materials is enhanced at certain types of grain boundaries and is dependent on the boundary structure with the tilt boundary exhibiting higher rates of migration than the twist boundary.

Controlled preferential oxidation of grain boundaries in monolayer tungsten disulfide for direct optical imaging.

Synthetic 2D crystal films grown by chemical vapor deposition are typically polycrystalline, and determining grain size within domains and continuous films is crucial for determining their structure. Here we show that grain boundaries in the 2D transition metal dichalcogenide WS2, grown by CVD, can be preferentially oxidized by controlled heating in air. Under our developed conditions, preferential degradation at the grain boundaries causes an increase in their physical size due to oxidation. This increase in size enables their clear and rapid identification using a standard optical microscope. We demonstrate that similar treatments in an Ar environment do no show this effect, confirming that oxidation is the main role in the structural change. Statistical analysis of grain boundary (GB) angles shows dominant mirror formation. Electrical biasing across the GB is shown to lead to changes at the GB and their observation under an optical microscope. Our approach enables high-throughput screening of as-synthesized WS2 domains and continuous films to determine their crystallinity and should enable improvements in future CVD growth of these materials.

Influence of orientation-dependent grain boundary oxidation on fatigue cracking behaviour in an advanced Ni-based superalloy

Fatigue tests have been conducted on an advanced disc Ni-based superalloy [low solvus, high refractory (LSHR) alloy] at 650 °C in air under three-point bend loading to investigate the role of orientation-dependent grain boundary (GB) oxidation in crack initiation and early propagation. It is found that crack initiation occurs mainly from bulged GB oxides, and cracks then predominantly propagate along the oxidised grain boundaries. These bulged oxides are extremely enriched in Co and preferentially form at the boundaries between high and low Schmid factor grains which are inclined normal to the applied tensile stress direction. Meanwhile, relatively flat/thin Ni/Ti/Al-rich oxide complexes also form at other grain boundaries, but they appear to be much less detrimental in fatigue crack initiation and propagation compared with the bulged GB Co-rich oxide complexes.

Thermal stability of AlN films prepared by ion beam assisted deposition

Abstract The thermal stability of AlN films deposited by ion beam assisted deposition was performed at 600 °C for 192 h under air ambient. The composition, morphology and optical properties were studied by X-ray photoelectron spectrometer, transmission electron microscopy, scanning electron microscopy, spectroscopic ellipsometry and UV–vis spectroscopy. The results show that the deposited film is polycrystalline, smooth, dense and homogenous. The oxidation of grain boundary takes place due to the element diffusion in the polycrystalline material. Oxidation produces amorphous oxide layers on the surface of film. As annealing time increases, surface roughness and diffuse reflection increase. Annealing has little influence on refractive index and extinction coefficient.

Isothermal high temperature low cycle fatigue behavior of Nimonic-263: Influence of type I and type II hot corrosion

Accelerated oxidation and corrosion of aero engine components due to the presence of hot salt mixture in the operating condition is known as hot corrosion. Hot corrosion degrades life of aero engine components and is broadly divided as type I and type II based on the prevailing temperature and type of salt present in the salt mixture and accordingly the mechanisms of degradation are different. Though type I and type II hot corrosion mechanisms are established but simultaneous interaction of cyclic load and hot corrosive media on aero engine material is relatively unknown. The different micro-mechanisms of interaction of types I and II hot-corrosion and low-cycle fatigue is studied in the present investigation by conducting fatigue tests at 700, 800 °C on bare and salt-coated specimens; studying specimen surface degradation and fracture surface through extensive microscopy. Type-I hot corrosion is induced at 800 °C by depositing 90% Na2SO4+10% NaCl and type-II HC is induced at 700 °C by depositing 88% Na2SO4+7% NaCl+5% NaVO3 on fatigue test specimens. Fatigue life is significantly reduced in both the hot corrosive atmospheres; however, the operative mechanisms are significantly different. Type-I atmosphere leads to sporadic fluxing of protective Cr2O3 layer exposing the substrate to hot corrosive media leading to grain-boundary oxidation and inter-granular failure whereas; type-II atmosphere leads to surface cracking and trans-granular failure.

Influence of oxygen concentration in the CdCl 2 treatment process on the photovoltaic properties of CdTe/CdS solar cells

Abstract The effect of oxygen during the CdCl 2 treatment on the optoelectronic properties of CdTe/CdS devices was studied. Higher efficiencies and greater uniformity in device performance were obtained for samples processed with higher amount of oxygen. Oxygen in the activation ambient caused an increase in the net carrier concentration near the back contact of CdTe, which ultimately seems to saturate around 5.5 × 10 15 cm − 3 when the O 2 concentration is 50%. However, the carrier concentration reduces by two orders of magnitude from the back contact to bulk CdTe. The important role of oxygen in CdCl 2 treatment is that it controls the diffusion of Cu. The disappearance of dark and light current–voltage crossover for devices annealed at higher concentration of O 2 is due to the oxidation of grain boundaries thereby blocking the diffusion of Cu towards the CdS buffer. The capacitance–voltage profiles suggest that the CdTe/CdS cell behaves more like a p–i–n than p–n device.

Electron-beam damage and point defects near grain boundaries in cerium oxide

Abstract Cerium oxide is a technologically important ceramic with applications in catalysis and potentially as an electrolyte for solid-oxide fuel cells (SOFCs). The technological interest is largely due to the behavior of oxygen vacancies in this material. Grain boundaries play an important role in oxygen vacancy diffusion and, although not completely understood, the influence of grain boundaries has been attributed to both a space-charge effect and the segregation of impurities. In this paper, results from spatially resolved electron-energy-loss spectroscopy (EELS) near grain boundaries in doped CeO 2 in a transmission electron microscope (TEM) are reported. The data are interpreted as the result of beam damage that varies as the electron beam is scanned across grain boundaries and suggest a spatially varying concentration of oxygen vacancies near the grain boundaries.

Long-term high-temperature oxidation of iridium coated rhenium by electrical resistance heating method

A continuous and compact iridium (Ir) coating with a thickness of ~ 100 μm was electrodeposited on a rhenium (Re) rod in molten salt at 580 °C for 4 h. The oxidation resistance and failure mechanism of the Ir coated Re (Ir/Re) material were investigated by resistance heating method at 2000 °C in air till the Ir coating failed. The results showed that the lifetime of the Ir/Re rod oxidized at 2000 °C in air was 183 min. After high-temperature oxidation, except for the failure position, the Ir coating in most of the heated regions kept dense and exhibited excellent adhesion on the substrate, with smooth surface and large grain size. The preferred orientation of the Ir coating changed from < 220 > to < 111 > after oxidation test. From the end to the center of the as-oxidized Ir/Re sample, the Ir coating became thinner, and the diffusion layer between Ir and Re got thicker. Meanwhile, the preferential oxidation of grain boundaries of Ir coating was more and more severe. It was found that the lifetime of the Ir/Re material in high-temperature oxidizing environment is closely related to the consumption rates of Ir coating by both the direct oxidation of Ir and the diffusion of Re into Ir coating. Based on the diffusion and oxidation kinetics of Re and Ir, the lifetime of the Ir/Re sample in the present study was calculated to be 242 min. The difference between the calculated and real lifetimes can be attributed to the ignored fact that Re diffuses rapidly along the grain boundaries of Ir coating in the calculation.

Microstructures and Electrical Properties of BaTiO&lt;sub&gt;3&lt;/sub&gt; PTC Ceramics

The microstructure, especially porosity, of PTC (positive temperature coefficient) thermistor based on BaTiO3 was controlled with a forming pressure. The relationship between theirPTCR properties and microstructureswas investigated with an optical and SEM (Scanning Electron Microscope) images and digital multimeter. Disk samples were fabricated by pressinguniaxially at various pressures of 100~15000kg/cm2 and sintering at 1265°C in reducing atmosphere and finally re-oxidizing at 700°C in air. The porosity of the samples decreased rapidly from 45% to 8% with increasing the forming pressure from 100 to 1000kg/cm2andbecame 4% at 15000kg/cm2with slowdecreasing of porosity in the pressure range of 1000~15000kg/cm2.With increasing the forming pressure, the resistivity jump of samplesdecreased rapidlyfrom 0.5 to 2.9 at about1000kg/cm2that corresponds tothe porosity of 15% and was saturated above this pressure. It is considered that there is a critical amount of porosity for having PTCR effect, which was about 15% in our samples. In addition, the porosity of the sample has a greater influence on the resistivity jump than on theresistivity at room temperature, which is due to the oxidation of grain boundary through a favorable channel of oxygen such as a pore.

Point defect–grain boundary interactions in MgO: an atomistic study

The way in which point defects interact with grain boundaries in oxides is important for understanding radiation damage evolution, sintering, and many other technologically important applications. Here, we examine how vacancies interact with three different grain boundaries in MgO, chosen as a model oxide ceramic. Further, we compare the vacancy interaction with both pristine (as constructed) and ‘damaged’ boundaries, in which excess interstitials are placed in the boundary plane to mimic the structure after a damage event. We find that the excess interstitials significantly change the interaction of the vacancies with the boundaries and that this change is sensitive to the atomic structure of the boundary. We further observe that complex electrostatic effects arise that can dominate the interaction. These results show that, as boundaries absorb defects, their interaction with other defects will change dramatically.

Examinations of Oxidation and Sulfidation of Grain Boundaries in Alloy 600 Exposed to Simulated Pressurized Water Reactor Primary Water

High-resolution characterizations of intergranular attack in alloy 600 (Ni-17Cr-9Fe) exposed to 325°C simulated pressurized water reactor primary water have been conducted using a combination of scanning electron microscopy, NanoSIMS, analytical transmission electron microscopy, and atom probe tomography. The intergranular attack exhibited a two-stage microstructure that consisted of continuous corrosion/oxidation to a depth of ~200 nm from the surface followed by discrete Cr-rich sulfides to a further depth of ~500 nm. The continuous oxidation region contained primarily nanocrystalline MO-structure oxide particles and ended at Ni-rich, Cr-depleted grain boundaries with spaced CrS precipitates. Three-dimensional characterization of the sulfidized region using site-specific atom probe tomography revealed extraordinary grain boundary composition changes, including total depletion of Cr across a several nm wide dealloyed zone as a result of grain boundary migration.

Random-walk Monte Carlo simulation of intergranular gas bubble nucleation in UO2 fuel

Using a random-walk particle algorithm, we investigate the clustering of fission gas atoms on grain boundaries in oxide fuels. The computational algorithm implemented in this work considers a planar surface representing a grain boundary on which particles appear at a rate dictated by the Booth flux, migrate two dimensionally according to their grain boundary diffusivity , and coalesce by random encounters. Specifically, the intergranular bubble nucleation density is the key variable we investigate using a parametric study in which the temperature, grain boundary gas diffusivity, and grain boundary segregation energy are varied. The results reveal that the grain boundary bubble nucleation density can vary widely due to these three parameters, which may be an important factor in the observed variability in intergranular bubble percolation among grain boundaries in oxide fuel during fission gas release.

Sintering Behavior and Properties of Si&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; Ceramics with Yttrium Chloride Additive

The big gap between the thermal conductivity of Si3N4 ceramics and the theoretical value of beta-Si3N4 single crystal implies that the low thermal conductive oxide grain-boundary phases in the ceramics possibly have a great influence on the thermal conductivity of the ceramics. In this work, yttrium chloride (YOCl) is introduced as sintering additive instead of the commonly-used Y2O3, in order to decrease the amount of the grain-boundary oxide; and the influence of YOCl on the sintering and the properties of Si3N4 ceramics was studied. Results show that YOCl additive can react with Si3N4 powders to form similar Y-Si-O-N grain-boundary phases just like Y2O3 while the Cl disappears during sintering. The thermal conductivity of the ceramics sintered with YOCl additive (85 W/m•K) increases about 15% as comparing with the ceramics sintered with Y2O3 additive, even though the relative density of the former reaches merely 97%. The microstructure of the sample sintered at different temperatures was investigated in the work, for elucidating the sintering process and the relationship between the properties and the microstructure of the YOCl added ceramics.

Multi-timescale investigation of radiation damage near TiO2 rutile grain boundaries

To understand the interactions between defects and grain boundaries (GBs) in oxides, two atomistic modeling methods were used to examine the role of GBs in a model system, rutile TiO2, in modifying radiation-induced defect production and annealing. Molecular dynamics was used to investigate defect production near a symmetric tilt GB at both 300 K and 1000 K. The damage production is found to be sensitive to the initial distance of the primary knock-on atom from the GB. We find three distinct regimes in which GBs have different effects. Similar to GBs in metals, the GB absorbs more interstitials than vacancies at certain distances while this behavior of biased loading of interstitials diminishes at other distances. Further, we obtain the statistics of both interstitial and vacancy clusters produced in collision cascades in terms of their compositions at two temperatures. Perfectly stoichiometric defect clusters represent a small fraction of the total clusters produced. Moreover, a significant reduction in the number of interstitial clusters at 1000 K compared to 300 K is thought to be a consequence of enhanced migration of interstitials towards the GB. Finally, the kinetic properties of certain defect clusters were investigated with temperature accelerated dynamics, without any a priori assumptions of migration mechanisms. Small interstitial clusters become mobile at high temperatures while small vacancy clusters do not. Multiple migration pathways exist and are typically complex and non-intuitive. We use this kinetic information to explain experimental observations and predict their long-time migration behavior near GBs.