Grain Boundary Diffusion Of Oxygen Research Articles

Modeling the oxidation process of TiAl and Ti<sub>3</sub> Al intermetallic compounds due to grain-boundary diffusion of oxygen

A diffusion-kinetic model was proposed to analyze the oxidation process in a nanostructured material with explicit identification of grain boundaries. It was assumed that oxygen migrates faster along the boundaries than it does in the grain volume. The model takes into account the stages of decomposition and formation of intermetallic compounds, as well as the formation of oxides, both within the boundaries and in the grain volume. The problem was solved numerically, and the oxidation dynamics were compared for various materials with different grain properties.

Oxygen grain-boundary diffusion in (La,Sr)FeO3−δ perovskite-oxides probed by molecular-dynamics simulations

Faster grain-boundary diffusion of oxygen has been observed experimentally in polycrystalline samples of Fe-based perovskite oxides at low temperatures, but this behaviour is at present not well understood.

Oxidation of TiAl alloy by oxygen grain boundary diffusion

Ti–Al intermetallic alloys have a number of advantages for the manufacture of aviation and car details. The use of these alloys is limited by their oxidation at high temperatures, which is associated with structural features and the presence of micro and nanosized grains. In such materials, grain boundary diffusion of oxygen can be active, which contributes to the acceleration of oxidation upon heating. In this paper, we present a two-dimensional model for research the diffusion which is accompanied by oxidation. The structure of TiAl is specified as symmetrically alternating grains with a clear separation of boundaries and triple junctions between them. Diffusion and chemical properties of grains and boundaries are different. The problem is formulated in a two-dimensional formulation and solved numerically. The influence of changes in the composition over time on the penetration of oxygen is taken into account. For the convenience of numerical research and analysis of the results, dimensionless variables are used. To solve the diffusion equation, an implicit difference scheme of splitting in coordinates is used. The equations of chemical kinetics are solved using an algorithm similar to the implicit Euler method using an iterative procedure. The results demonstrate the influence of different variants of kinetic regimes on the concentration distribution and integral concentrations of reagents and oxides.

Rapid grain boundary diffusion in foraminifera tests biases paleotemperature records

The oxygen isotopic compositions of fossil foraminifera tests constitute a continuous proxy record of deep-ocean and sea-surface temperatures spanning the last 120 million years. Here, by incubating foraminifera tests in 18O-enriched artificial seawater analogues, we demonstrate that the oxygen isotopic composition of optically translucent, i.e., glassy, fossil foraminifera calcite tests can be measurably altered at low temperatures through rapid oxygen grain-boundary diffusion without any visible ultrastructural changes. Oxygen grain boundary diffusion occurs sufficiently fast in foraminifera tests that, under normal upper oceanic sediment conditions, their grain boundaries will be in oxygen isotopic equilibrium with the surrounding pore fluids on a time scale of <100 years, resulting in a notable but correctable bias of the paleotemperature record. When applied to paleotemperatures from 38,400 foraminifera tests used in paleoclimate reconstructions, grain boundary diffusion can be shown to bias prior paleotemperature estimates by as much as +0.86 to −0.46 °C. The process is general and grain boundary diffusion corrections can be applied to other polycrystalline biocarbonates composed of small nanocrystallites (<100 nm), such as those produced by corals, brachiopods, belemnites, and molluscs, the fossils of which are all highly susceptible to the effects of grain boundary diffusion.

Oxidation kinetics and mechanism of nickel alloys

The study covers the effect of alloying elements on the kinetics and mechanism of oxidation at 1150 °С for 30 hours of heat-resistant nickel alloys obtained using such technologies as centrifugal SHS metallurgy (SHS(M)), vacuum induction melting (VIM), elemental synthesis (ES), hot isostatic pressing (HIP). A comparative analysis was carried out for alloys based on nickel monoaluminide and standard AZhK and EP741NP alloys. It was found that kinetic dependences are described mainly by parabolic approximation. The logarithmic law of oxidation with the rapid (within 3–4 hours) formation of the primary protective layer is typical for alloys doped with molybdenum and hafnium. In the case of AZhK and EP741NP, oxidation proceeds according to a parabolic law at the initial stage (2–3 hours), and then according to a linear mechanism with the voloxidation and complete destruction of samples. Oxygen and nitrogen diffusion proceeds predominantly along the nickel aluminide grain boundaries and it is limited by the Al2O3 + Cr2O3 + XnOm protective film formation. SHS(M) alloys feature by a positive effect of zirconium and tantalum added as dopants on heat resistance. The Ta2O5 phase is formed in the intergranular space, which reduces the rate and depth of oxidation. The zirconium-containing top layer Al2O3 + Zr5Al3O0.5 blocks the external diffusion of oxygen and nitrogen, thereby improving heat resistance. Doping with hafnium also has a positive effect on oxidation resistance and leads to the formation of submicron and nanosized HfO2 inclusions that suppress the grain boundary diffusion of oxygen. MoO3, Mo3O4, CoMoO4 volatile oxides are formed in alloys with a high content of molybdenum and compromise the protective layer integrity. A comparative analysis of the oxidation kinetics and mechanism for samples consisting of the base β-alloy with Cr + Co + Hf additives showed a significant effect on the heat resistance of the sample preparation method. As the proportion of impurity nitrogen decreases and the Cr2O3 sublayer is formed, the oxidation mechanism also changes.

Characterization of internal and intergranular oxidation in Alloy 690 exposed to simulated PWR primary water and its implications with regard to stress corrosion cracking

Oxidation testing of Ni-base Alloy 690 was conducted in simulated PWR primary water, and the internal and intergranular (IG) oxidation layers were systematically characterized by analytical transmission electron microscopy to obtain insight into its IG stress corrosion cracking (IGSCC) resistance and behavior. The internal oxidation layer consisted of upper NixFe1-xCr2O4 and innermost Cr2O3. Each internal oxidation layer had a crystallographic orientation relationship with the Alloy 690 matrix, indicating that the layers formed through solid-state reactions between inward diffusing oxygen and the alloying elements. Strong evidence that the Cr2O3 layer formed through the outward grain boundary diffusion of Cr was found, leaving severe Cr depletion along the grain boundary with a steep composition gradient toward the surface. The innermost Cr2O3 layer had a continuous and compact band-shaped morphology around the surface grain boundary. This Cr2O3 layer was revealed to be protective of the inward grain boundary diffusion of oxygen from the surface and therefore responsible for the prevention of grain boundaries from IG oxidation. IG Cr carbides suppressed the internal and IG oxidation further. The key findings obtained in this work may present the main reason for the high resistance of Alloy 690 to IGSCC under PWR normal operating conditions.

Phase Transformations in the Surface Layer of Compact Zirconium Nitride-Based Ceramics

We have studied structural, morphological, and phase transformations in the surface layer of compact zirconium nitride-based ceramics during high-temperature annealing in vacuum, nitrogen, and air. The formation of oxygen-containing phases during heat treatment of ZrN is controlled by bulk and grain-boundary oxygen diffusion. At the beginning of the process, oxygen is supplied by an amorphous surface layer, which always forms during the ceramic preparation process. In air, the process is driven by oxygen indiffusion from the gas phase across an initially forming porous oxide layer. Grain-boundary oxygen diffusion leads to the formation of a crystalline zirconium oxide layer on grain boundaries and crack nucleation.

Fine-Grained Tungstates SrWO4 and NaNd(WO4)2 with the Scheelite Structure Prepared by Spark Plasma Sintering

Fine-grained SrWO4 and NaNd(WO4)2 ceramics with the scheelite structure having high relative densities (99 and 95.8%), which can appear candidate matrices for radioactive waste (RAW) management, are prepared by spark plasma sintering (SPS). The phase identity of the ceramics is determined by X-ray powder diffraction; their microstructure is studied by X-ray photoelectron spectroscopy. The tungstates under study are sintered at rather low temperatures (580–665°C). The intensity of compaction of the tungstates at the early sintering stage is determined by the degree of powder agglomeration. The activation energy of fine-grained scheelite ceramics at high temperatures corresponds with the activation energy of grain-boundary oxygen diffusion.

Oxidation behavior and incombustibility of molten Mg-Zn-Y alloys with Ca and Be addition

By addition of Ca or Be, non-flammable high strength Mg97Zn1Y2 (at%) alloys are developed. The addition of Ca results in the formation of a thin and continuous CaO and MgO outermost layer above Y2O3 inner layer, what helps to prevent the internal oxidation of Y and an increased ignition temperature can be achieved. The addition of a small amount of Be also helps to suppress the internal oxidation of Y. The most likely explanation for the enhancement of incombustibility is that the segregation of Be along the grain boundaries in the Y2O3 layer impedes the grain boundary diffusion of oxygen and cations.

Mass Transfer in Yb Silicate Environmental Barrier Coatings at High Temperatures

The oxygen permeabilities of polycrystalline Yb2Si2O7 wafers cut from sintered bodies to serve as models of environmental barrier coatings were determined at temperatures up to 1673 K under various oxygen potential gradients (dµO), produced by exposing the upper and lower surfaces of the wafer to atmospheres with different oxygen partial pressures (PO2). Oxygen permeation proceeded via grain boundary (GB) diffusion of oxygen from the higher PO2 surface to the lower PO2 surface, concurrently with GB diffusion of ytterbium in the opposite direction. Mass transfer analysis allowed for a design guideline for selectively enhancing the structural stability of the ytterbium silicate layer with applying a dµO.

Effects of an oxygen potential gradient and water vapor on mass transfer in polycrystalline alumina at high temperatures

The mass transfer mechanisms in polycrystalline alumina wafers serving as models for anti-oxidation films were investigated by evaluating oxygen permeability at 1873 K using 18O2 as a tracer. The grain boundary (GB) diffusion coefficients for oxygen in the vicinity of the higher oxygen partial pressure (PO2(hi)) surface were directly determined from the secondary ion mass spectrometry line profile for each GB, obtained from cross-sections of the exposed wafer. An especially interesting result was that, under dry conditions, the oxygen GB diffusivity obtained with an applied oxygen potential gradient (dμO) was significantly lower than that associated with oxygen self-diffusion in the absence of a dμO. Moreover, the contribution of electronic conductivity near the PO2(hi) surface was increased upon applying a dμO. The addition of H2O to the PO2(hi) environment in conjunction with a dμO accelerated oxygen GB diffusion, while the presence of D2O retarded the diffusivity compared to that observed under dry conditions. However, in the absence of a dμO, the oxygen GB diffusivity was essentially equal to that under dry conditions. The dissociative adsorption reactions of H2O or D2O over the entire PO2(hi) surface appear to greatly affect the diffusivity of oxygen and aluminum on the surface and along the GBs in the vicinity of the surface.

Effect of Grain Size on Oxidation Resistance of Unalloyed Titanium

The effect of the grain size on the high-temperature oxidation resistance of unalloyed titanium was experimentally investigated using titanium samples with two different grain sizes of 219 μm and 118 μm. The weight gain during oxidation and the penetration depth of oxygen from a metal surface were larger in the small-grain-size sample compared with the large-grain-size sample. In addition, oxygen diffusion was faster in the substrate of the small-grain-size sample. These results were attributed to the grain-boundary diffusion of oxygen. A steep change in the oxygen concentration was observed at a grain boundary. Our simulation results suggested that slower oxygen diffusion into the inner grain from the surface through the grain boundary with high diffusivity can cause the observed steep change in the oxygen concentration.

Effect of Dopant Configuration on Oxygen Shielding Properties of Polycrystalline Alumina

The oxygen permeability of polycrystalline α-alumina wafers, which served as model alumina layers, under an oxygen potential gradient ΔPO2 was evaluated at a temperature of 1873 K. When mutual grain boundary (GB) diffusion of oxygen and aluminum occurred in wafers subjected to a steep ΔPO2, the oxygen and aluminum fluxes at the inflow side of the wafer were significantly smaller than those at the outflow side. It was noteworthy that Lu and Hf segregation at the GBs selectively reduced the mobility of oxygen and aluminum, respectively. It was found that a wafer with a bilayer structure, in which a Lu-doped layer was exposed to a low partial oxygen pressure (PO2) and a Hf-doped layer was exposed to a high PO2, exhibited excellent oxygen shielding properties at high temperatures.

EFTEM and EELS analysis of the oxide layer formed on HCM12A exposed to SCW

The inner-diffusion layer interface of an HCM12A sample oxidized in 600°C supercritical water (SCW) was analyzed using EFTEM and EELS. The EFTEM analysis showed the presence of chromium-rich zones linked with the porosity within the inner layer, as well as a nanometric iron–chromium separation, which may be linked with the presence of both Fe3O4 and FeCr2O4 in this layer. The diffusion layer was characterized by large chromium-rich oxides located at the tempered martensite lath boundaries, which suggested the preferential grain boundary diffusion of oxygen and the preferential oxidation of the chromium carbides present at these boundaries. The metal grains of the diffusion layer contained nanometric chromium-rich spinel oxides. The presence of large chromium-rich oxide precipitates in the diffusion layer appears to help improve the corrosion resistance of these alloys.

Near-surface transport properties of donor doped Pb(ZrxTi1 − x)O3 (PZT) in an external electric field

18O tracer exchange experiments were performed on donor doped PZT under an applied voltage and analyzed by time-of-flight (ToF) secondary ion mass spectrometry (SIMS) measurements. The application of a voltage enhanced the oxygen diffusion coefficient perpendicular to the field, i.e. between the electrodes and evidence is provided that grain boundary diffusion of oxygen is accelerated. Additionally, changes in the surface resistivity across grain boundaries were observed by conductive mode (CM) microscopy. The field induced time-dependent resistivity changes originated from the cathode, and led to a breakdown of grain boundary barriers and to areas of homogeneous resistivity.

Diffusion-controlled processes in nuclear reactor oxide systems

Oxide films formed as corrosion products at high temperatures on metallic components of advanced nuclear systems constitute a complex environment in which a wide range of phenomena have usually an important role to play. From this wide range of phenomena, three diffusion-controlled processes are discussed in this paper. The oxidation of Ni to NiO is used as a simple model for studying growth of corrosion-resistant films in nuclear reactors. Transport processes in amorphous SiO 2 are then examined in the context of the highly protective properties of SiO 2 films. Finally, deformation processes in oxides are discussed again using NiO as an example. The observed creep rates indicate that in single crystals creep is controlled by lattice diffusion of oxygen to allow dislocations climb, whereas in polycrystalline material it is controlled by grain boundary diffusion of oxygen to allow grain boundary sliding and changes in grain shape.

Oxygen diffusion in grain boundaries: a ToF-SIMS investigation on hot-rolled steel sheets

A study of grain boundary diffusion of oxygen in hot-rolled steel sheets is carried out by means of time-of-flight secondary-ion-mass-spectrometry (ToF-SIMS). This involves polishing of the sample surface prior to the oxygen exposure. A nickel layer deposited after exposure ensures a homogeneous extraction field for ToF-SIMS measurements at the Ni-steel interface. The sample is bevelled at an angle of 11.5° to spread up the diffusion pathway by a factor of 5. The oxygen distribution is then acquired via ToF-SIMS in imaging mode from which diffusion parameters are calculated according to the Whipple-Le Claire's approach.

Effect of Dopant, Crystal Orientation, and Space Charge Layer on Oxygen Diffusion in Bi4Ti3O12 Ceramics

We studied oxygen diffusion in non- and Nb2O5-doped Bi4Ti3O12 (BIT) ceramics by two-dimensional isotope mapping. In non-doped BIT, the grain boundary acted as a barrier to oxygen diffusion, reflecting the presence of a space charge layer at the grain boundary. In Nb2O5-doped BIT, a textured morphology was found by bulk and grain-boundary diffusion of oxygen. Diffusion coefficients were determined from the individual grains and grain boundaries. The difference in diffusion paths resulted from the decrease in oxygen-defect concentration owing to the addition of Nb2O5. The bulk-diffusion coefficient along the ab-plane was about one order of magnitude larger than that in the direction of the c-axis. The grain-boundary-diffusion coefficients varied by about two orders of magnitude. The bulk and grain-boundary diffusion of oxygen depended significantly on the crystal orientation.

Oxygen grain-boundary transport in polycrystalline alumina using wedge-geometry bilayer samples: Effect of Y-doping

Novel wedge-geometry, dual-layer alumina samples, both undoped and 500 ppm Y 3+-doped, were studied in the temperature regime 1250–1400 °C to determine the effect of Y 3+ on oxygen grain-boundary transport in alumina. The samples consisted of a wedge-shaped, single-phase alumina top layer, diffusion bonded to an alumina/Ni substrate containing a fine, uniform dispersion of Ni marker particles (0.5 vol.%). The extent of the alumina spinel oxidation layer was measured as a function of the wedge thickness for a series of heat-treatment conditions. Models of the transport behavior were used to derive values for the rate constants ( k) in both the alumina top layer and the alumina/Ni substrate. It was found that the presence of yttrium slows oxygen grain-boundary diffusion in alumina by a factor of ∼5 (at 1300 °C), and increases the corresponding activation enthalpy for oxidation from 407 ± 20 to 486 ± 34 kJ mol −1. Microstructural observations suggested that yttrium also slows Ni outward diffusion. A comparison of the different k values revealed that, at 1300 °C, the presence of Ni alone enhances transport by a factor of ∼2 relative to undoped alumina.

Diffusion of Oxygen in Thermally Grown Oxide Scales

High temperature reactivity of materials under oxidizing atmospheres is based on the formation of protective oxide scales. The protectiveness is obtained when the thermally grown oxide scales are dense, continuous and adherent to the metallic substrates (even during thermal shocks); as a matter of fact, the growth of the scale has to be governed by diffusion of species across the growing scale. The diffusing species are coming from the substrate (metallic ions) and/or from the oxidizing atmosphere (oxygen ions). The understanding of growth mechanisms can be reached by making two stage oxidation experiments, using oxygen isotopes. The experiment consists in oxidizing first the metallic substrate in 16O2, evacuating the oxygen after the desired time, and replacing it by 18O2. The distribution of oxygen isotopes given by secondary ion mass spectrometry (SIMS) across the oxide scale informs about the growth mechanisms: anionic transport, cations diffusion or mixed diffusion processes. The use of marker experiment is able to determine the oxygen diffusion coefficients within the growing scales. In this case, a thick scale is grown under 16O2, followed by a shorter diffusion experiment in 18O2. The distribution of 18O isotope across the scale by plotting ln (18O intensity) versus x (depth in oxide) allows determining the oxygen effective diffusion coefficient in the scale, according to the classical Fick’s law solution. A more accurate analysis of these profiles can provide information about bulk and grain boundary diffusion of oxygen. The results can be related to kinetics according to Wagner’s theory. The proposed work consists firstly in making a state of the art review about oxygen diffusion in thermally grown oxide scales, and secondly in connecting the so-obtained outcome (effective, bulk and grain boundary diffusion) to kinetics results. The proposed oxides are chromia, alumina and zirconia.