Parabolic Rate Research Articles

Effect of oxygen pressure on the oxidation behavior of NiCoCr medium-entropy alloy at 800 °C

The oxidation behavior of NiCoCr medium-entropy alloy was investigated at 800 °C in various oxygen pressures from 10 to 105 Pa. The results showed that the oxidation kinetics of NiCoCr followed the parabolic rate law, regardless of oxygen pressure. The oxidation rate constants of the alloy steadily increased with increasing oxygen pressure. The oxidation mechanism of the MEA was mainly controlled by outward cation diffusion to form an exclusive Cr2O3 layer although minor amounts of Co- or Ni-Cr spinels were observed at PO2 = 105 Pa under TEM analyses. The main diffusing species were doubly- and triply-ionized cation vacancies.

Oxidation resistance and diffusion behavior of MoSi2/WSi2/(Nb,X)Si2 compound coating at 1400 °C

MoSi2/WSi2/(Nb,X)Si2 compound coating showed outstanding oxidation resistance at 1400 °C. After oxidation for 100 h, the specimen kept good integrality and the parabolic rate constant of the scale growth was about 1.497 µm2 h−1. NbSi2 layer could improve the bonding of MoSi2/WSi2 layers to the substrate, postpone their degradation into porous sub-silicide layer and provide adequate Si for oxidation. The intense interdiffusion between MoSi2 and WSi2 layers would result in formation of pores in WSi2 layer and then weaken its barrier effect. In addition, a small amount of alloying elements Ti, Cr and Al from the substrate could diffuse through the MoSi2/WSi2 layers along the grain boundaries during oxidation. As a result, there are TiO2 formed in the scale. After oxidation for 100 h, a gradient compound layer of (Mo,W)Si2/(W,X)Si2/(Ti,Nb)5Si4 formed eventually.

High temperature oxidation of non equi-atomic Al5Cr12Fe35Mn28Ni20 high entropy alloy

The Oxidation behavior of the Al5Cr12Fe35Mn28Ni20 high entropy alloy was investigated in air at temperatures 900 °C, 1000 °C and 1100 °C for 100 h isothermally. The oxidation kinetics at all temperatures obeys the parabolic rate law with an activation energy 100.6 kJ mol−1. The alloy was externally oxidized forming duplex layer consisted of manganese oxides that overlays Mn-Cr-Fe spinel oxides. While, it was observed that at 1000 °C and 1100 °C the samples were also oxidized internally forming Al2O3 deep in the alloy. The initial oxidation behavior of the alloy oxidized at 1000 °C for 1 and 24 h producing external scale of Mn3O4 overlaying Mn-Cr-Fe spinel oxide and Al2O3 precipitates.

High temperature oxidation behavior of quaternary ordered (Cr2/3Ti1/3)3AlC2-based MAX ceramic

High temperature oxidation behaviors of (Cr2/3Ti1/3)3AlC2-based ceramic at 800−1300 °C in air were studied. The parabolic oxidation rate constants were 3.9 × 10−13 and 1.6 × 10-11 kg2 m-4 s-1 at 800 and 900 °C, respectively. Above 900 °C, deviation of parabolic oxidation law was observed. During high temperature oxidation, the oxides of Cr2O3, (Cr,Al)2O3 and rutile TiO2 were produced, while Cr2TiO5 and (Cr0.222Ti0.778)O1.889 appeared at above 1000 °C. It was revealed that (Cr2/3Ti1/3)3AlC2 decomposed into Cr2AlC and TiC during oxidation process. The oxidation mechanism was proposed according to the evolution process of both phase composition and microstructure of oxides.

Does Rheology of Bingham Fluid Influence Upscaling of Flow through Tight Porous Media?

Non-Newtonian fluids may cause nonlinear seepage even for a single-phase flow. Through digital rock technologies, the upscaling of this non-Darcy flow can be studied; however, the requirements for scanning resolution and sample size need to be clarified very carefully. This work focuses on Bingham fluid flow in tight porous media by a pore-scale simulation on CT-scanned microstructures of tight sandstones. A bi-viscous model is used to depict the Bingham fluid. The results show that when the Bingham fluid flows through a rock sample, the flowrate increases at a parabolic rate when the pressure gradient is small and then increases linearly with the pressure gradient. As a result, an effective permeability and a start-up pressure gradient can be used to characterize this flow behavior. By conducting flow simulations at varying sample sizes, we obtain the representative element volume (REV) for effective permeability and start-up pressure gradient. It is found that the REV size for the effective permeability is almost the same as that for the absolute permeability of Newtonian fluid. The interesting result is that the REV size for the start-up pressure gradient is much smaller than that for the effective permeability. The results imply that the sample size, which is large enough to reach the REV size for Newtonian fluids, can be used to investigate the Bingham fluids flow through porous media as well.

Microstructure and oxidation behavior of an Al–Ni–Cr–Cu–MoO3–SiO2 composite coating on low-carbon steel

In the present study, an Al–Ni–Cr–Cu–MoO3–SiO2 composite coating with varying elemental compositions was deposited using the mechanical alloying technique. The composite coating structure before and after an oxidation test was studied using X-ray diffraction (XRD), three-dimensional optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). A cyclic oxidation test was conducted in an 800 °C air atmosphere for 10 cycles (200 h). Based on the XRD, SEM, and EDX analyses, the coating comprised composite-refined Al, Ni, Cr, and Cu particles as well as coarse SiO2 particles. The composite coating had a thickness in the range of 36–59 μm, with good and continuous adherence to the substrate. The cyclic oxidation curve of the composite coating followed a parabolic rate law. The sample that contained 5.3 wt% MoO3 showed the best oxidation resistance after 10 oxidation cycles at 800 °C. This sample had a lower mass gain than the other samples. Moreover, this sample had a Kp of 1.2 × 10−3 mg2/cm2·s. Protective Al2O3 oxides formed on the composite coating surface with a thickness of 0.75–1.89 μm. This oxide scale was dense and fully adhered to the coating, thereby improving oxidation resistance.

High-Temperature Air and Steam Oxidation and Oxide Layer Characteristics of Alloy 617

Nickel-based superalloys are the candidate materials for many high-temperature applications, especially in advanced ultra-supercritical coal-fired power plants and in the next-generation nuclear power plants. This paper reports the results of a comprehensive study on chemical composition, structure and kinetics of oxide film formation of a nickel-based superalloy, alloy 617, in ambient air and steam at 750 °C for a duration of 500 h. The morphology and nature of oxide scales have been characterized by scanning electron microscope with energy-dispersive x-ray attachment, grazing incidence x-ray diffraction technique and glow discharge optical emission spectrometry (GDOES). The mass gain of alloy 617 in steam was higher than that in air, and followed a parabolic rate law for both atmospheres. The depth profiles of the oxide films obtained by GDOES showed the presence of an outer chromium oxide scale with mixed oxides of nickel and aluminum at the interface. The oxidation kinetics of alloy 617 and the nature of oxide scales formed during the steam and air oxidation are discussed.

The high-temperature oxidation resistance properties of magnesium alloys alloyed with Gd and Ca

This work studied the high-temperature oxidation resistance properties of Mg alloys with Gd and Ca. The oxidation property of Mg-3.5Gd-xCa (x = 0, 0.5, 1.0 and 3.0 wt%) alloys followed parabolic oxidation kinetics at 500 °C. A stable oxide film was formed on the alloy surface and limited further oxidation. The oxide film was composed of Gd2O3, MgO and MgCO3 for Mg-3.5Gd and was mainly CaO and CaCO3 with increasing Ca content for Mg-3.5Gd-xCa alloys. The high oxidation affinity of Ca caused it to be oxidized preferentially and the fast diffusion of Ca in Mg alloys provided a source for the continuous oxidation of Ca at the surface. The parabolic rate constant, grains size and surface roughness of the oxide film increased with the increasing Ca content in Mg-3.5Gd-xCa alloy. The understanding of multi-element alloying on the oxidation behavior of Mg alloy is helpful for the design of multi-element oxidation-resistant Mg alloys.

Effect of Deposition Conditions on Oxide Parameters of Silicon

In this study, the effect of deposition conditions and the temperature thermal treatment on the oxide parameters of two structures of silicon layers were investigated. The study present the evolution of boron profiles following a dry thermal oxidation in poly-Si/SiO2/c-Si films deposited at 520°C and 605°C temperatures and thermally oxidized in dry oxygen at respectively temperature 840°C, 945°C and 1050°C for tr=1h33’duration. The results show that the deposition conditions and the temperature treatment make a very important impact on the obtained films, which affect the redistribution and localization of dopants. It has been observed that the obtained value of the linear and the parabolic rate constant, the diffusion coefficient and the oxidation thickness are higher in the films deposited at Td = 520°C than those deposited at Td = 605°C. Also, the X-ray diffraction is strongly affected by the oxide thickness deposited between poly-silicon layers and crystalline substrates.

Effect of Deposition Conditions on Oxide Parameters of Silicon

In this study, the effect of deposition conditions and the temperature thermal treatment on the oxide parameters of two structures of silicon layers were investigated. The study present the evolution of in situ boron profiles following a dry thermal oxidation in poly-Si/SiO2/c-Si films deposited at 520°C and 605°C temperatures and thermally oxidized in dry oxygen at respectively temperature 840°C, 945°C and 1050°C for duration tr=1h33’. The results show that the deposition conditions and the temperature treatment make a very important impact on the obtained films, which affect the redistribution and localization of dopants. It has been observed that the obtained value of the linear and the parabolic rate constant, the diffusion coefficient and the oxidation thickness are higher in the films deposited at Td = 520°C than those deposited at Td = 605°C. Also, the X-ray diffraction is strongly affected by the oxide thickness deposited between poly-silicon layers and crystalline substrates.

High-Temperature Oxidation of NiAlCr\u2013Ca and NiAlCr\u2013Sr Alloys in Air

In this study, interrupted oxidation behavior of synthetic NiAlCr–Ca (Ca = 0.3, 1.4, 2 at.%) and NiAlCr–Sr (Sr = 0.4 at.%) alloys in the air at 1027 °C for 192 h was investigated. Parabolic rate constants (kp) showed that the Sr-containing alloy exhibited the best oxidation resistance among the alloys investigated in this study. The oxide scale formed on the Sr-containing alloy was composed of α-Al2O3 phase with Sr-rich nodules. Increasing the Ca concentration in the alloys was found to reduce the oxidation resistance due to the formation of non-protective Ca-rich complex oxides and consumption of α-Al2O3 scale by the reaction between Al2O3 and CaO. The Ca-rich complex oxides were initially formed on the Ca-rich interdendritic region and grew with time. Very little scale spallation was observed for the Sr-containing alloy, while it was notable for 0.3 at.% Ca-containing alloy. Spallation was attributed to the coefficient of thermal expansion (CTE) mismatch arisen from the formation of CaAl4O7 phase, a compound with a very low CTE.

Oxidation behavior of melt-infiltrated SiC–TiB2 ceramic composites at 500–1300 °C in air

The oxidation behavior of melt-infiltrated SiC–TiB2 ceramic composites at 500–1300 °C in air for 32 h was investigated by the isothermal oxidation test. The phase composition and microstructure evolution were studied through examining the surficial and cross-sectional oxide layer by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The oxidized surfaces were covered by scattered TiO2 particles, a continuous B2O3 layer, a B2O3–Al2O3 layer or a mullite layer depending on the oxidation temperature. The thickness of the oxide layer increased from 10 to 30 μm as the oxidation temperature increased from 900 to 1300 °C. The analysis of the weight gain data revealed that the parabolic rate constant of melt-infiltrated SiC–TiB2 ceramic composites generally increased from 4.05 × 10−3 to 3.16 × 10−1 mg2/cm4/s, accompanied with a changed oxidation nature from parabolic to cubic, as oxidation temperature increased from 500 to 1300 °C. Formation of continuous B2O3 or Al2O3-based (Al2O3–B2O3 or Al2O3–SiO2) layer were identified on the surface of SiC–TiB2 ceramic composites after being oxidized at 700–1300 °C, which was responsible for the excellent oxidation resistance of such composites. The oxidation rate of the melt-infiltrated SiC–TiB2 ceramic composites was controlled by reaction rate of the composites at 500 °C and diffusion rate of oxygen through continuous B2O3 or Al2O3-based layer to the unaffected composites at 700–1300 °C.

Evaluation of oxidation and thermal insulation capability of nanostructured La2(Zr0·7Ce0.3)2O7/YSZ functionally graded coatings

In the present work, nanostructured La2(Zr0·7Ce0.3)2O7/YSZ functionally graded coatings were prepared by applying the atmospheric plasma spraying process. The La2(Zr0·7Ce0.3)2O7 compound was synthesized via the calcination process to get a nanostructured precursor precipitate at 600 °C. Two types of nanostructured functionally graded coatings were applied on the Hastelloy X substrates coated with NiCrAlY and 8YSZ. The agglomerates of La2(Zr0·7Ce0.3)2O7 were then used to apply the type I coating. Type II coating was then applied by the plasma spraying of the heat-treated agglomerates of La2(Zr0·7Ce0.3)2O7. The results of the phase and microstructural analysis showed that a nano-composite microstructure comprising the main phases of lanthanum zirconate with the partial solubility of cerium cation (LZ) and the solid solution of cerium lanthanum zirconate (LC) was formed in both types of coatings. Type II coating was relatively denser; so, it could be considered as a dense nanostructured coating. The results of the oxidation tests also showed that only the type II coating had higher oxidation resistance, as compared to that of the conventional YSZ coating. Parabolic oxidation rate constant of this coating was equal to 0.114 μm2/h. On the other hand, the oxidation rate constant for the type I coating and conventional YSZ coating was measured to be 0.141 μm2/h and 0.147 μm2/h, respectively. In addition, the results of the thermal insulation capability test indicated that the thermal barrier capability of the type I coating was 59% higher than that of the conventional YSZ coating. However, under similar conditions, this percentage was 29% for the type II coating.

High temperature oxidation behaviour of non-equimolar AlCoCrFeNi high entropy alloys

Oxidation properties of AlCoCrFeNi high entropy alloys, i.e. 5%Al–35%Co–25%Cr–15%Fe–20%Ni (Al5Co35Cr25Fe15Ni20) and 5%Al–25%Co–20%Cr–15%Fe–35%Ni (Al5Co25Cr20Fe15Ni35), were investigated in air atmosphere at 1273 K. Thermogravimetry (TG) investigations determined that, under these conditions, the samples demonstrate scale growth according to the parabolic rate law with rate constants the order of 10−11 g2cm−4s−1. Combined scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) surface analysis and X-ray diffraction (XRD) studies revealed that the formed protective scales consisted primarily of chromium oxide. Underneath the external Cr2O3 scale, a thin layer of the metallic substrate can be found. Below that, an internal chromium oxide scale with discontinuous Al2O3 can be observed. Al-rich precipitates can also be found in the internal layers, as well as the metallic substrates after oxidation. XRD analysis also indicated that the initial materials are single FCC phase solid solutions. From all these studies it can be concluded that both Co and Ni-rich non-equimolar AlCoCrFeNi high entropy alloys exhibit similar high temperature oxidation behaviour. Furthermore, each of the alloys has promising anti-corrosion properties that can possibly be further improved on, e.g. with higher aluminium content inside the materials.

Failure Mechanisms of APS-YSZ-CoNiCrAlY Thermal Barrier Coating Under Isothermal Oxidation and Solid Particle Erosion

The high-temperature oxidation and solid particle erosion of thermal barrier coating (TBC) system which consists of a 8 wt.% yttria-partially stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited on Inconel 718 substrate via air plasma spraying (APS) process are studied experimentally. Isothermal oxidation tests of the APS-TBCs are conducted at 1050, 1100 and 1150 °C in air for up to 1970 h. Solid particle erosion tests are also performed on both as-deposited and heat-treated APS-TBC specimens at selected particle impingement angles and velocities in room temperature. The scanning electron microscopy (SEM) analyses of the cross sections of the APS-TBC specimens after the oxidation tests show the formation of thermally grown oxide (TGO) scale due to the oxidation of CoNiCrAlY bond coat, and the oxidation kinetics of TGO growth is described by the parabolic rate equation. The failure of the APS-TBC system under isothermal oxidation is associated with the spallation of the top coat through propagation and coalescence of cracks along the coating interface, which is affected by the TGO growth. The solid particle erosion rate of the top coat is found to increase with impingement angle and reach the maximum erosion rate at normal impingement of particles. The erosion resistance of the APS-TBC is observed to increase after the APS-TBC specimen has been exposed at 1100 and 1150 °C for 72 h, probably due to the sintering effect on the top coat, which results in reduction of top coat porosity.

Oxidation of molten AZ91D Mg alloy in hexafluoropropylene/air atmospheres

The oxidation behavior of molten AZ91D Mg alloy in the mixture of 0.05–5% hexafluoropropylene and air at 670–800 °C was investigated by TG, XRD, SEM, EDS and XPS. Results show that the oxidation of the melt in 0.05 % hexafluoropropylene/air atmosphere at 700 °C obeyed a linear rate rule, while under other conditions it followed a parabolic rate law. The apparent activation energy was 82.78 kJ mo1−1 for the reaction of the melt in 0.1 % hexafluoropropylene/air atmosphere at 670–800 °C. The film formed on the melt surface mainly consisted of MgF2, MgO, C and small amounts of AlF3.

Hot corrosion behavior of plasma-sprayed NiCrAlY/TiO2 and NiCrAlY/Cr2O3/YSZ cermets coatings on alloy steel

The objective of the Present research work is to evaluate the hot corrosion resistance of plasma-sprayed 70% NiCrAlY+ 30% TiO2 and 70% NiCrAlY+ 25% Cr2O3+5% YSZ coatings on MDN 420 alloy. Hot corrosion tests are carried out under molten salt environment of Na2SO4+60 % V2O5 salt mixture at 700°C for 50 cycles. Each cycle consisting of 1 hour heating in a silicon carbide tubular furnace followed by 20 min of cooling. The thermogravimetric technique was used to determine the kinetics of corrosion. The scanning electron microscopy (SEM), energy dispersive analysis (EDAX), electron probe microanalyser (EPMA) and X-ray diffracton (XRD) techniques were used to evaluate the characterization of coatings with regard to coating bondstrength, thickness, microhardness and porosity. The parabolic rate constants of coated steels are lower when compared to the uncoated substrate. The NiCrAlY+ Cr2O3+YSZ coating is found to be more protective when compared to NiCrAlY+ TiO2 coating. The oxides of Al2O3, NiCr2O4, and Cr2O3 are formed on the outermost layer of the coatings which gives the resistance the coatings to high-temperature corrosion.

Impact of Selective Laser Melting Additive Manufacturing on the High Temperature Behavior of AISI 316L Austenitic Stainless Steel

Additive manufacturing allows production of complex geometries or customized designs that are difficult or impossible to fabricate by conventional means. However, these components have hardly ever been tested in severe conditions corresponding to real functioning at high temperature. The high temperature oxidation of AISI 316L stainless steel additively manufactured by selective laser melting (SLM) has been studied for 100 h at temperatures between 700 and 1000 °C in dry air and compared to that of wrought samples. Thermogravimetric analyses showed slower kinetics for SLM samples than for conventional coupons. In addition, SLM samples exhibit parabolic kinetics for all the studied temperatures, while conventional coupons present complete laws above 800 °C. Parabolic constant rate determined for 900 °C oxidation is one order of magnitude lower for SLM samples (1.73·10−13 g2 cm−4 s−1) than for wrought coupons (1.54·10−12 g2 cm−4 s−1). The resulting activation energy values confirm the better behavior of SLM alloys, in agreement with the formation at their surface of protective chromia Cr2O3. In contrast, additional formation of non-protective iron oxides was observed above 800 °C for the wrought samples. The different behavior could be explained by Cr depletion at the surface of conventional alloy, whereas Cr supply was still insured in the case of SLM material.

Preparation and oxidation kinetics behavior of bulk Cr3C2-20 wt % Ni cermets

In this study, bulk Cr3C2-20 wt % Ni cermets were successfully fabricated by high-energy milling and pressureless sintering in a vacuum furnace. Microstructures, elements distribution, and high temperature oxidation mechanism were researched by SEM, EPMA, and differential thermal analyzer (DTA), respectively. Oxidation kinetics regularity of bulk Cr3C2-20 wt % Ni cermets was investigated at 600–800 °C for the first time. Isothermal cyclic oxidation experiments were studied using the heat-treatment furnace for 100 h. The results indicated that the porosity decreased, while the hardness, bending strength, and fracture toughness increased with an improvement in the vacuum degree. Cr3C2-20 wt % Ni cermets displayed outstanding oxidation resistance and the dynamic oxidation curves followed the parabolic rate law. Besides, the oxidation rate constants increased three orders of magnitudes with an increase in the oxidation temperatures from 600 °C to 800 °C. The mechanism of the oxidation resistance was the generation of the protective and dense oxide layers on the sub-surface of the oxidation specimens, which hindered the diffusion of Cr3+, Ni2+, O2 and effectively protected the substrate from further oxidation.

Effect of the microstructure refinement by powder metallurgy on the oxidation behavior of Nb-6Mo-20Si-3Cr silicide-based alloy

This study investigated the effect of the microstructure refinement on the oxidation resistance of the Nb-6Mo-20Si-3Cr silicide-based alloy based on how the oxidation behavior of the alloy is severely affected by its microstructure size. Alloy powders were fabricated by a hydrogenation-dehydrogenation method, and were consolidated by a spark plasma sintering process. The sample, having a refined microstructure, could be fabricated by adopting a powder metallurgy method, and its microstructure is much finer than that of the sample fabricated by arc casting. To compare the oxidation resistance, the cast ingot and the sintered compact were oxidized at 1200 °C at various durations in an air atmosphere. The oxidation behavior of both followed the parabolic rate law. Based on the oxidation results, the parabolic rate constant of the cast ingot was 5.93 × 10−7 g2cm−4s−1, while, in the SPS compact, it was decreased to 2.31 × 10−7 g2cm−4s−1. By the microstructure and oxidation behavior results, this study confirms that the oxidation resistance of the Nb-6Mo-20Si-3Cr alloy could be improved by microstructure refinement.