Intergranular Oxide Research Articles

A mechanism for stress-aided grain boundary oxidation ahead of cracks

A comparison of literature data indicates that intergranular oxide intrusions formed in the nickel-based superalloy RR1000 ahead of stationary cracks grow at a rate higher than that observed on flat surfaces. This is attributed to the effect on vacancy equilibria of the tensile stress that exists at the tip of the intrusion. A stress-aided oxidation rate parameter is defined that is stress dependent and varies with the nature of the oxide. The threshold condition for crack advance is also discussed.

XAFS investigations of nitrided NbN–SiO2 sol–gel derived films

This work presents the results of the structural analysis of xNbN–(100-x)SiO2 (x=100, 80, 60mol%) thin films by X-ray absorption spectroscopy (XAS). To prepare the films, thermal nitridation of sol–gel derived coatings have been performed. The resulting films have a granular structure with NbN grains distributed in the SiO2 matrix. The size of the grains depends on the NbN/SiO2 molar ratio. A detailed X-ray absorption fine structure (XAFS) data analysis shows that in all the samples both nitrogen and oxygen atoms are present as nearest neighbours of Nb. The intra-granular phase is an ordered NbN phase, whereas the shells around the grains are formed mainly by an oxide phase and, possibly, by other niobium nitride phases (probably with low nitrogen content). Two possible origins of the inter-granular oxide phase were considered: incomplete nitridation of Nb2O5 and addition of SiO2. Both of them are connected with the sample preparation method. The obtained XAS results allowed us to correlate the thickness and stoichiometry of the films under study with the electronic structure of the Nb ions and with the local geometric structure in their environment.

Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route

Aluminum silicon alloy of composition (Al-25%Si-3%Ni-1%Fe-2%Cu) was atomized using water atomization. The powders were cold compacted in a die to produce green cylinder compacts. Four consolidation processes were applied, namely; conventional sintering at 500℃, sintering followed by hot forging to obtain pistons, one step hot forging into pistons, and hot pressing. The microstructure of the sintered specimens showed inter-granular pores and oxide layers on particle interfaces of 84% relative density. When the sintered specimens were hot forged, both the inter-granular pores and oxide layers on particle interfaces almost disappeared and the relative densities increased up to about 95%. The same microstructure is also obtained for the one step forged specimens, but the relative densities increased to about 97%. However, the hot pressing specimens showed the presence of oxide layers on particle surfaces as well as few isolated pores. The relative density of the hot pressed specimens was about 90%. Hardness and ultimate compression strength were measured. It is noted that the strongest bulk materials are those made by hot forging, followed by those made by hot pressing and the weakest bulk materials are those made by conventional sintering.

Microstructure and properties of silicon nitride ceramics with calcium aluminate additions

Silicon nitride ceramics containing calcium aluminates as sintering aids have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and the effect of sintering aid content on their microstructure, phase composition, mechanical strength, and air oxidation resistance has been studied. The results demonstrate that the Si3N4 ceramic containing 10 wt % calcium aluminates has a uniform distribution of intergranular multicomponent oxide phases and consists of densely packed silicon nitride grains. Owing to this, it offers the maximum mechanical strength (850 MPa) and is stable to air oxidation up to 1300°C.

Modifications and Growth Mechanisms of Ultrathin Aluminum Oxide Films on NiAl in Water

X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) have been combined to study the modifications and growth of alumina thin films on NiAl(001) induced by exposure to water vapor at low pressure (10−6 mbar) and by immersion in ultrapure liquid water at room temperature. The film thickness and stoichiometry, as well as the alloy composition changes, were analyzed by angle-resolved XPS measurements. Ultrathin (∼0.7 nm) hydroxylated native oxide films were compared to thicker (∼4 nm) anhydrous and oxygen-deficient thermal oxide films grown at 900 °C under low air pressure (10−4 mbar). The results show that immersion in liquid water causes the formation of one to two equivalent additional monolayers of aluminum oxide at the interface between the native oxide film and the alloy, suggesting anion transport along the oxide/alloy interface after entry at intergranular oxide sites. The hydroxylated oxide surface remains unchanged. In contrast, immersion of the thicker thermal oxide films in li...

Oxidation behavior of Kovar alloy in controlled atmosphere

Controlled oxidation experiments were performed on Kovar alloy by changing oxidation atmosphere, temperature, and exposure time to produce films with different oxide type and thickness. The results indicated that single Fe3O4 and single FeO were respectively obtained when Kovar alloy was oxidized in N2-2.31%H2O-0.95%H2 at 500°C and in N2 −2.31%H2O-0.5%H2 at 1000°C, and all kinetic curves followed linear relation; mixed oxides of FeO and Fe3O4 formed when Kovar was oxidized in N2-2.31%H2 0 at 1000°C, and parabolic kinetics were obeyed. Analysis of metal-lographic cross section of oxides indicated that oxygen diffusion inward through the oxide scale is responsible for intergranular oxide, which had formed beneath the oxide scales when the oxide products were mixed oxides of FeO and Fe3O4, and which did not occur when the oxide was single FeO or Fe3O4. The oxidation model was also established.

Roles of rare earth oxide additives in millimeter-wave sintering of AlN

Roles of rare earth oxide (RE 2O 3) additives in millimeter-wave(MM) sintering of AlN were investigated from the standpoints of phase diagram, heating characteristics of rare earth oxides, and morphology of intergranular oxide phase. In the millimeter-wave sintering of AlN, densification temperature decreased with the decrease of the ionic radius of rare earth ion and was closely related with the eutectic temperature in the RE 2O 3-Al 2O 3 binary system. The lowest densification temperature in the millimeter-wave sintering of AlN with Yb 2O 3 additive was attributed to the largest heating rate of Yb 2O 3·Al 2O 3 binary oxide under millimeter-wave radiation. Furthermore, the lowest densification temperature could be attained while selecting the Yb 2O 3 content so as to form the intergranular phase with the eutectic composition in the Yb 2O 3-Al 2O 3 binary system. The result showed good agreement with the above mentioned during the sintering of Si 3N 4 with Yb 2O 3-Al 2O 3 additive. From TEM observation, it was verified that film-like intergranular oxide phase formed under millimeter-wave radiation was favorable for attaining high thermal conductivity in the Yb 2O 3 added AlNs.

High-temperature Oxidation Behavior and Scale Morphology of Si-containing Steels

The Fe–Si alloy system is very important in steel production because silicon is often added to high-strength steel, heat-resistance steel, and other steel types. However, Si-containing steels form un-removable scales on the surface at high temperatures, and the scales allow unfavorable surface bruises in hot-rolled steels. Although many studies have been conducted on the oxidation of Si-containing steel at high temperatures, the oxidation behavior and scale morphology have not been clarified adequately.In this study, the high-temperature oxidation behavior and scale morphology of Si-containing steels (0, 0.5, 1.5, and 3.0 mass%) exposed to the air and an LNG combustion gas atmosphere at 1 373 K and 1 473 K were investigated. The external scale and internal scale formed on Si-containing steels were composed of laminated α-Fe2O3 and Fe3O4 and a mixture of FeO and Fe2SiO4, respectively. The subscale may have been composed of vitreous SiO2. The kinetics of the external and internal scale growth followed parabolic rate laws. The SEM images of the three-dimensional subscale extracted from the oxidized steels by means of an organic solvent system dissolution technique showed that an intergranular oxide formed thin walls along not only the grain boundaries but also the twin boundaries of the steel matrix, and isolated intragranular oxide particles were formed in the steel grains near the internal scale.

Ｓｉ添加鋼の高温酸化とサブスケールの形態

The oxidation behavior of Si-containing steels at high temperature has been the subject of numerous investigations. In many cases, the addition of Si to the steel in sufficient amounts to form an external film of SiO2 or other Si-rich oxide on the steel surface has resulted in extremely slow oxidation rates. However, the steel in which Si content is insufficient forms an external scale and an internal scale on the steel surface and a subscale in the steel interior which are less protective.In order to suppress the growth of the scales on the hot slab which causes surface bruise of hot rolled Si-containing steel sheets, many researchers paid attention to the de-scaling characteristics of Si-containing steels. However, the effects of Si content on the oxidation kinetics, scale composition and scale structure, especially three-dimensional scale morphology formed on the Si-containing steels at high temperatures are not necessarily understood adequately. In this study, the high-temperature oxidation behavior and scale morphology of (0, 0.5, 1.5, 3.0 mass%) Si-containing steels exposed to the flowing air and the LNG combustion gas atmosphere at 1373K or 1473K were investigated. The external scale and the internal scale formed on Si-containing steels were composed of the laminated α-Fe2O3 and Fe3O4 and the mixture of FeO and Fe2SiO4, respectively. The subscale may be composed of vitreous SiO2. The kinetics of the external and internal scale formation were followed by parabolic rate laws. The SEM images of the three-dimensional subscale extracted from the oxidized steels by means of an organic solvent system dissolution technique showed that an intergranular oxide formed thin walls along not only grain boundaries but also twin boundaries of the steel matrix and an intragranular oxide formed isolated particles in the steel grains near the internal scale.

Gas phase embrittlement and time dependent cracking of nickel based superalloys

Nickel based superalloys are critical to the safe operation of many energy conversion systems operating at high temperatures. Time dependent intergranular cracking of these alloys, under both sustained and cyclic loads, is dominated by environmental interactions at the crack tip. This review is concerned mainly with the interaction of oxygen in alloys used for combustion turbine discs, although interactions with other more aggressive species are considered. The phenomenology of this cracking is shown to be consistent with the same mechanism as that associated with oxygen embrittlement resulting from pre-exposure of uncracked material, and also with environmentally induced reduction in creep rupture life. Gas phase embrittlement (GPE), resulting from intergranular oxygen penetration, is shown to be responsible for all four streams of experimental observations. Three distinct processes of intergranular embrittlement involving oxidation reactions have been confirmed experimentally. One of these, the oxidation of intergranular sulphides, results in elemental sulphur embrittlement and subsequent local decohesion under stress. The other two, oxidation of carbon or carbides to form carbon dioxide gas bubbles and oxidation of strong oxide formers to form intergranular internal oxides, result in a reduction of the local ability to accommodate stress concentrations associated with sliding grain boundaries in an intermediate temperature range. This in turn leads to a temperature dependent minimum in ductility and maximum in crack propagation rate. Attempts to reduce the sensitivity to time dependent cracking based on chemistry (chromium level or trace element addition), microstructure control (using thermal–mechanical treatment or controlled cooling), or reversal of environmental embrittlement, are all considered. The conclusions form a basis for the development of life prediction methods for energy materials operating in diverse environments.

The Relationship between Grain Boundary Character and the Intergranular Oxide Distribution in IN718 Superalloy

The relationship between grain boundary character and the intergranular oxide distribution was investigated on the polycrystalline nickel-base superalloy IN718 by using EBSD technique. Basically, internal oxidation predominantly occurs along random grain boundaries, while, for highly symmetric grain boundaries, such as Σ3 twin grain boundaries, intergranular oxides were rarely found.

The influence of oxidation on mechanical and fracture behaviour of an air plasma-sprayed NiCoCrAlY bondcoat

The influence of isothermal oxidation on room-temperature mechanical and fracture behaviour of an air plasma-sprayed Ni-23Co-17Cr-12Al-0.5Y bondcoat was investigated by the miniaturised disc bending test (MDBT) technique. Disc specimens were extracted from the bondcoat region of both as-received and oxidised thermal barrier coating (1000 °C, 1000 h). Microstructure analysis revealed that the non-oxidised bondcoat consisted mainly of γ-phase (Ni-structure) and β-NiAl. After 500 h of oxidation no NiAl remained in the bondcoat, an effect due to internal as well as external oxidation of Al. The former resulted in the formation of an extensive oxide network and the latter in the formation of an oxide scale between the topcoat and the bondcoat. The crack propagation behaviour of the bondcoat, both in non-oxidised and oxidised condition can be characterised as intergranular with stable growth. The crack propagation resistance is substantial due to the lamellar grain (splat) orientation and the extensive intergranular oxide network, acting as crack deflection and crack branching mechanisms. As an effect of oxidation, crack propagation resistance of the bondcoat increases but the strain to crack initiation decreases.

Influence of Phase Change in Intergranular Oxides to Thermal Conductivity of AlN Sintered by Millimeter-Wave Heating

Influence of Phase Change in Intergranular Oxides to Thermal Conductivity of AlN Sintered by Millimeter-Wave Heating

Intergranular and Internal Oxidation during Hot-rolling Process in Ultra-low Carbon Steel.

Intergranular and internal oxidation in slab reheating process for hot-rolling was investigated for ultra-low carbon steel with 0.01 mass% Si. Intergranular oxidation depth significantly increased at 1 473 K because of melting of Fe–Si–O complex oxides and decreased with increasing oxidation temperature at above 1473 K. The depth increased with increasing oxidation time in the range of 1800–5 400 sec and decreasing partial pressure of oxygen in the range of 1–10%. The maximum interval between intergranular oxidation was 210μm, while austenite grain size observed at high temperature by a confocal laser microscope was over 1 mm. The size of "quasi-austenite" grains considered sites of ferrite grains was consistent with this interval. Internal oxides were formed underneath the scale and around the intergranular oxides. The thickness of internal oxidation layer extending beyond the furthest point of permeation by intergranular oxide increased with increasing oxidation temperature, time and with decreasing partial pressure of oxygen. The oxidation loss of substrate increased with increasing oxidation temperature, time and partial pressure of oxygen. The intergranular oxidation depth and the thickness of internal oxidation layer can be theoretically calculated by using oxidation loss rate of substrate and oxygen diffusion rate in austenite.

Fe-Ni合金の高温酸化におけるサブスケールの3次元的成長形態の有機溶媒系溶解法による観察

Fe-Ni base alloys form a subscale in the alloy interior by selective oxidation of the elements having a large affinity of oxygen together with the external scale formation on the alloy surface by high-temperature oxidation. Almost of all the previous papers concerning with subscale which consists of both intergranular and intragralular oxides have been on the basis of the two-dimensional observation on the cross-sections of oxidized specimens. However, it is difficult to estimate exactly the three-dimensional morphology development of the subscale because its morphology is peculiar. In order to examine directly the three-dimensional morphology we have developed a metal matrix dissolution technique for the oxidized Fe-Ni alloy without dissolving the metallic oxide phases constituting the subscale by using an organic solvent system solution. In the present study, we report here on the appropriate dissolution of the Fe-36%Ni alloy oxidized in air at high temperatrures under the applied tensile stress conditions in the organic solvent system solution which consists of bromine, tetraethyl ammonium and acetonitrile. The three-dimensional morphology of the intergranular and intragranular oxides formed in the Fe-36%Ni alloy suggests the difference of each growth mechanism.

Microstructure and Crystal Phases of Praseodymium Oxides in Zinc Oxide Varistor Ceramics

The effect of sintering temperature on the microstructure and crystal phases of the intergranular praseodymium oxides in ZnO varistor ceramics was investigated using transmission electron microscopy and high‐resolution electron microscopy. The ZnO grains were three‐dimensionally separated from the intergranular praseodymium oxides. On the basis of microdiffraction analyses of the intergranular layer, the phase transformation from fcc‐Pr6O11 into hcp‐Pr2O3 was found when the sintering temperature increased from 1300° to 1350°C. The defect reaction equation and the decrease of donor concentration with increasing sintering temperature can verify the certainty of phase transition during the liquid‐phase sintering observed by transmission elecron microscopy. Additionally, on the basis of the small variations of the breakdown voltage per grain boundary, the number of active grain boundaries is not a dominant factor for the donor concentration dependence on the sintering temperature.

Fe-Ni合金における脱ボロン現象

Effects of atmosphere, holding time and temperature on deboronization phenomena at heating of Fe-36%Ni and Fe-78%Ni alloys have been investigated. The results obtained are as follows:(1) A considerable amount of boron removal was observed during the heat treatment at 1050°C in air. Neither argon nor hydrogen (dew point -40°C) atmosphere could eliminate the deboronization.(2) Boron concentration profiles after deboronization of Fe-36%Ni alloys containing 126ppm B or less in air at the temperature higher than 1050°C are explainable by diffusion controlled mechanism.(3) The diffusion coefficient of boron in Fe-Ni alloys may be represented by the following equations, DFe-36%Ni(cm2/s)=4.6×10-3e-95, 000/RT, DFe-78%Ni(cm2/s)=6.5×10-3e-98, 000/RT, (4) In Fe-Ni alloys, the subscale composed of metal/oxide mixed zone and intergranular oxide is observed. The subscale zone in 36%Ni alloy is far wider than that in 78%Ni alloy. Oxidized boron is solved in the oxide particles of the subscale. Boron concentration is the most depleted at the boundary of subscale and matrix, and increases gradually with increase of the distance from the boundary into the matrix.

Environmental influence on the near-threshold fatigue crack propagation behaviour of a high-strength steel

The near-threshold fatigue crack propagation behaviour of a high-strength low-alloy steel has been investigated in ambient air and in vacuum so as to determine the role of the environment precisely. The analysis of the results is conducted by taking crack closure effects into account. It is concluded that fatigue crack growth rates measured in ambient air depend upon three processes: intrinsic fatigue crack propagation as observed in vacuum, adsorption of water vapour molecules on freshly created rupture surfaces, which enhances crack propagation, and a subsequent step of hydrogen-assisted cracking. The appearance of intergranular ruptures and oxide layers on rupture surfaces in ambient air is also discussed.

Cyclic oxidation of Haynes 230 alloy

The cyclic oxidation of Haynes 230 alloy (Ni-Cr-W-Mo alloy) was investigated in air at three different temperatures, 871, 982 and 1093 °C. Studies indicated that during cyclic oxidation, protective scales formed which were predominantly Cr2O3, with Kirkendall voids formed both at the scale/alloy interface and grain boundaries. Intergranular oxides were observed at temperatures above 982 °C while internal oxide particles were found above 1093 °C. Both intergranular and internal oxides were identified as aluminium oxide. A 50 μm chromium-depleted zone developed after 70 h exposure at 1093 °C and was accompanied by disastrous scale spalling. The lowest chromium concentration within the depleted zone was 14 wt% which still provided a sufficient supply of chromium for development of a continuous Cr2O3 rich scale. Decarburization was observed at the higher temperature of 1093 °C, and a carbide-free zone developed. Also, it was found that Haynes 230 is subject to a sensitization process. At the lower exposure temperature of 871 °C, large amounts of chromium carbide formed preferentially at the grain boundaries. While at the surface region chromium carbide precipitation occurred at the twin boundaries.

Joining of nickel powder grains by thermal oxidation

The subject of this work is the mechanism of intergranular oxide joint formation in uncompacted nickel powder layers oxidized in air at 400–800°C. Six kinds of nickel powder of different grain sizes and shapes were investigated. The strength of intergranular joints depends on the size of the contact area between oxide layers of adjacent grains. It is a function of time and temperature of oxidation and the size, shape, and morphology of the grains. On the interface between any two contacting oxide layers a grain-boundary-like zone is formed, which is responsible for the cohesion in the intergranular joints.