Linear Oxidation Kinetics Research Articles

Comparative study on the oxidation behavior of Si2BC3N/Ta4HfC5 ceramics at 1000 °C and 1500 °C: Microstructural evolution and kinetics consideration

The oxidation kinetics and oxide scale morphology of the Si2BC3N/Ta4HfC5 ceramics sintered by hot-pressing were explored at 1000 °C and 1500 °C in flowing air. Dense Si2BC3N/Ta4HfC5 ceramic sintered at 1900 °C with fine grain sizes shows the optimal oxidation resistance both at 1000 °C and 1500 °C. When oxidized at 1000 °C, the formation of a porous oxide layer of Ta2O5, Hf6Ta2O17 and HfO2 containing amorphous SiO2 are primarily responsible for the unsatisfying oxidation resistance of the composite. A highly oxidation resistance component like Si2BC3N can promote the oxidation resistance of Ta4HfC5 at 1500 °C, thanks to the formation of a dense and continuous tantalum-hafnium silicate glass. The oxidation kinetic curve at 1500 °C can be described by a parabolic law whilst the oxidation process is controlled by the diffusion rate of oxygen through the oxide layer; however, the oxidation reaction rate governs the oxidation progress at 1000 °C via a linear oxidation kinetics.

High-temperature oxidation behavior of laser additively manufactured AlCrCoNiSi-based high-entropy alloys at 1100 ℃

High-temperature oxidation behavior of two laser additively manufactured AlCrCoNiSi-based high-entropy alloys (Al10 and Al15) are systematically studied in this work. These two alloys exhibit similar oxidation behaviors involving initial linear kinetics oxidation and subsequent parabolic kinetics oxidation at 1100 ℃. However, Al10 alloy has a superior oxidation resistance than Al15 alloy according to the oxidation rate constant. α-Al2O3 is the predominant phase of the oxide scales grown on both alloys. Cross-sectional views of the scales show that the scales consist of surface equiaxed grains and underlying columnar grains. For all oxidized alloys, a newly-formed fcc layer forms in the subsurface Al-depletion zone due to the preferential oxidation of aluminum. Al10 alloy exhibits a much thicker newly-formed fcc layer than Al15 alloy after the same oxidation time. This work ascribes the superior oxidation resistance of Al10 alloy to its thick newly-formed fcc layer, which suppresses the outward Al diffusion from the substrate to scale-metal interface by increasing the diffusion distance.

Oxidation behavior and kinetics of (Zr, Ti)(C, N)–SiC ceramics with enhanced oxidation resistance at 850 °C–950 °C

The oxidation behavior and kinetics of (Zr, Ti)(C, N)–SiC ceramics subjected to a temperature of 850 °C-950 °C (typical operating temperatures range of Generation-IV nuclear reactors) for 1–4 h are investigated via X-ray diffraction, scanning electron microscopy, and weight gain testing. The results show that the microstructures of the ceramics after oxidation comprises primarily of m-(Zr, Ti)O2, c-(Zr, Ti)O2, and SiO2. After adding Si, the oxidation layer appears as a two-layer structure with both external and internal layers. The oxidation order of (Zr, Ti)(C, N)–SiC ceramics at 850 °C-950 °C is ZrSi > (Zr, Ti)(C, N) > SiC. Unoxidized SiC forms a “core-shell” structure with a dense external layer of SiO2. The dual effect of small SiO2 particles occupying cracks and the “core-shell” structure of SiC–SiO2 withstanding stresses can inhibit the propagation of cracks and reduce the porosity of the oxide layer. The oxidation of Z5T and Z5T5S exhibits linear oxidation kinetics, whereas the oxidation of Z5T10S and Z5T20S results in quasi-parabolic oxidation. By adding Si, the oxidation activation energy of Z5T20S (with the addition of 20 mol% Si) increases by 174 % compared to that of Z5T ceramic, thus improving the oxidation resistance of (Zr, Ti)(C, N)-based ceramics at 850 °C-950 °C.

Intermediate and high-temperature oxidation behavior of an equiatomic TaTiCr RCCA from 800 °C to 1400 °C

This study explores the oxidation behavior of a TaTiCr refractory complex concentrated alloy (RCCA) in the temperature regime of 800–1400 °C. The oxidation kinetics were found to be proportional with temperature. However, a mechanistic shift from linear oxidation kinetics at 800 °C and 1000 °C to sub-parabolic kinetics at 1200 °C and 1400 °C was observed. This was attributed to the establishment of continuous external scales of TiO2 and Cr2O3, with fairly continuous underlying complex oxide layers at the higher temperatures. While there were morphological differences in the oxide scales depending on exposure temperature, the oxide species were all similar, primarily consisting of an outer layer of TiO2, an intermediate layer of Cr2O3, and an inner layer of rutile-structured (Cr,Ta,Ti)O2. Internal nitridation was observed in all cases, with a severe degree of internal reaction at 1400 °C, reaching a depth of 750 μm. Overall, the best performance was observed at 1200 °C, where a favorable balance of kinetics and thermodynamics promoted the most continuous oxide layers, thereby enhancing oxidation resistance.

Improving oxidation resistance of TaMoZrTiAl refractory high entropy alloys via Nb and Si alloying

Here, we investigate the interplay effects of Si and Nb alloying on the oxidation behaviors of the refractory high entropy alloys (RHEAs) at 1000 ℃. The alloying of Nb and Si promotes the formation of dendritic microstructures, suppresses the transition from parabolic to linear oxidation kinetics and significantly improves the oxidation resistance. Severe cracking of oxide scales is also reduced via Nb and Si alloying. According to the scanning transmission electron microscopy analysis, ZrO2 and TiO2 are initially formed during oxidation, and oxidation proceeds with the inward diffusion of oxygen, preferentially through the interdendritic region and BCC grain boundaries.

High-temperature oxidation kinetics of a metastable dual-phase diboride and a high-entropy diboride

The processing of multicomponent (Ti0.25V0.25Zr0.25Hf0.25)B2 ultra-high temperature hexagonal transition metal diboride in dual-phase and single-phase microstructures and investigation of oxidation behavior in the air at 1000 and 1500 °C are reported. The dual-phase diboride is a metastable phase composed of Hf-Zr-rich and Ti-V-rich phases that undergo phase transformation to a single-phase high-entropy diboride after thermal annealing. At 1000 °C, a B2O3 layer was formed on the material's surface, and the oxidation kinetics followed a para-linear behavior. At 1500 °C, a porous oxide layer was formed, facilitating oxygen diffusion and reaction with the diboride, resulting in linear oxidation kinetics. The prediction of the lifetime of the materials during high-temperature oxidation suggested that the high-entropy material outperforms the dual-phase diboride, making it most suitable for related applications. The superior performance of the high-entropy single-phase diboride was associated with the high-entropy and sluggish diffusion effects.

Effect of Zr and Ti on isothermal oxidation behavior of Nb–Si based alloys

The isothermal oxidation behavior of 56Nb-16Si-(20-x)Ti–3Cr–3Al-2Hf-xZr (x ​= ​0, 2, 5, 10 ​at. %) alloys was investigated at 800 ​°C and 1250 ​°C, respectively. The results show that increasing the Zr content evidently increased the oxidation rates at 800 ​°C, accompanied by the obvious occurrence of pesting oxidation. The alloys showed alike linear oxidation kinetics at 1250 ​°C. With the increase of Zr content, the adherence and integrity of oxide scales were improved, but the overall oxidation resistance was slightly deteriorated. The observed oxidation behavior may be attributed to the composition variation of Zr and Ti in the alloys. The oxidation mechanism associated with the composition variation is discussed in this study.

On the Microstructure and Properties of the Nb-23Ti-5Si-5Al-5Hf-5V-2Cr-2Sn (at.%) Silicide-Based Alloy—RM(Nb)IC

The microstructure, isothermal oxidation, and hardness of the Nb-23Ti-5Si-5Al-5Hf-5V-2Cr-2Sn alloy and the hardness and Young’s moduli of elasticity of its Nbss and Nb5Si3 were studied. The alloy was selected using the niobium intermetallic composite elaboration (NICE) alloy design methodology. There was macrosegregation of Ti and Si in the cast alloy. The Nbss, αNb5Si3, γNb5Si3, and HfO2 phases were present in the as-cast or heat-treated alloy plus TiN in the near-the-surface areas of the latter. The vol.% of Nbss was about 80%. There were Ti- and Ti-and-Hf-rich areas in the solid solution and the 5-3 silicide, respectively, and there was a lamellar microstructure of these two phases. The V partitioned to the Nbss, where the solubilities of Al, Cr, Hf, and V increased with increasing Ti concentration. At 700, 800, and 900 °C, the alloy did not suffer from catastrophic pest oxidation; it followed parabolic oxidation kinetics in the former two temperatures and linear oxidation kinetics in the latter, where its mass change was the lowest compared with other Sn-containing alloys. An Sn-rich layer formed in the interface between the scale and the substrate, which consisted of the Nb3Sn and Nb6Sn5 compounds at 900 °C. The latter compound was not contaminated with oxygen. Both the Nbss and Nb5Si3 were contaminated with oxygen, with the former contaminated more severely than the latter. The bulk of the alloy was also contaminated with oxygen. The alloying of the Nbss with Sn increased its elastic modulus compared with Sn-free solid solutions. The hardness of the alloy, its Nbss, and its specific room temperature strength compared favourably with many refractory metal-complex-concentrated alloys (RCCAs). The agreement of the predictions of NICE with the experimental results was satisfactory.

Diverse oxidation behaviors of metal carbide solutions in high-temperature water vapor

Carbide solid solutions show diverse oxidation behaviors at 1000–1400 °C in water vapor. (Ti,Nb,Ta,Hf,Zr)C and (Ti,Nb,Ta,Hf)C exhibit parabolic or near-parabolic oxidation kinetics, while the oxidations of (Ti,Nb,Ta) and (Ti,Nb)C are mainly controlled by linear oxidation kinetics. The oxidation kinetics of (Ti,Nb,Ta,Hf,Zr)C shows a tendency of transition from parabolic to linear oxidation rate law with increasing temperature. On the contrary, a tendency of transition from linear to parabolic rate law with increasing temperature was observed in (Ti,Nb)C, which exhibits the highest oxidation resistance at investigated temperatures. The presence of dense Nb-doped TiO2 oxidation layer is responsible for the high oxidation resistance.

On the Microstructure and Properties of Nb-12Ti-18Si-6Ta-2.5W-1Hf (at.%) Silicide-Based Alloys with Ge and Sn Additions.

In this paper two Nb-silicide-based alloys with nominal compositions (at.%) Nb-12Ti-18Si-6Ta-2.5W-1Hf-2Sn-2Ge (JZ1) and Nb-12Ti-18Si-6Ta-2.5W-1Hf-5Sn-5Ge (JZ2) were studied. The alloys were designed using the alloy design methodology NICE to meet specific research objectives. The cast microstructures of both alloys were sensitive to solidification conditions. There was macro-segregation of Si in JZ1 and JZ2. In both alloys the βNb5Si3 was the primary phase and the Nbss was stable. The A15-Nb3X (X = Ge,Si,Sn) was stable only in JZ2. The Nbss+βNb5Si3 eutectic in both alloys was not stable as was the Nb3Si silicide that formed only in JZ1. At 800 °C both alloys followed linear oxidation kinetics and were vulnerable to pesting. At 1200 °C both alloys exhibited parabolic oxidation kinetics in the early stages and linear kinetics at longer times. The adhesion of the scale that formed on JZ2 at 1200 °C and consisted of Nb and Ti-rich oxides, silica and HfO2 was better than that of JZ1. The microstructure of JZ2 was contaminated by oxygen to a depth of about 200 μm. There was no Ge or Sn present in the scale. The substrate below the scale was richer in Ge and Sn where the NbGe2, Nb5(Si1-xGex)3, W-rich Nb5(Si1-xGex)3, and A15-Nb3X compounds (X = Ge,Si,Sn) were formed in JZ2. The better oxidation behavior of JZ2 compared with JZ1 correlated well with the decrease in VEC and increase in δ parameter values, in agreement with NICE. For both alloys the experimental data for Si macrosegregation, vol.% Nbss, chemical composition of Nbss and Nb5Si3, and weight gains at 800 and 1200 °C was compared with the calculations (predictions) of NICE. The agreement was very good. The calculated creep rates of both alloys at 1200 °C and 170 MPa were lower than that of the Ni-based superalloy CMSX-4 for the same conditions but higher than 10−7 s−1.

The oxidation kinetics of depleted uranium and its low-alloy molybdenum alloys in moist air

Abstract The oxidation kinetics of depleted uranium and its low-alloy molybdenum alloys (U-2wt.%Mo, U-5wt.%Mo) were measured in a moist air (75% relative humidity) at 60 and 75 ° C. Coefficients of reaction rate equations were determined for linear oxidation kinetics. In the oxidation of depleted uranium at 75 ° C, a change in reaction kinetics from linear to exponential behaviour was observed after about 2500 hours.

Improvement in the oxidation resistance of Nb-Si-Ti based alloys containing zirconium

The influence of Zr on high temperature oxidation resistance for Nb-Si-Ti based alloys has been investigated. Results show that the Zr-containing alloys exhibit an improved oxidation resistance, with reduced weight gain and decreased parabolic oxidation rate constant. In particular, the presence of Zr has suppressed the unwanted transition from parabolic to linear oxidation kinetics, therefore retaining fully parabolic oxidation kinetics. It is then demonstrated that the refined microstructure induced by the presence of Zr and the resulting continuous glassy SiO2 layer upon oxidation could be responsible for the improved oxidation resistance.

Native oxidation resistance of Al20Nb30Ta10Ti30Zr10 refractory complex concentrated alloy (RCCA)

The native oxidation resistance of an Al20Nb30Ta10Ti30Zr10 (at.%) refractory complex concentrated alloy (RCCA) was evaluated at 1000 °C in air. Oxidation tests revealed that the alloy exhibits linear oxidation kinetics out to 4 h of oxidation, with an adherence to parabolic, diffusion-controlled oxidation from 4 to 100 h. This transition was ultimately attributed to the formation of an outer oxide consisting primarily of TiO2 and Ti7Al2O15 layers. The 100 h oxide product contained multiple internal layers, but the overall oxidation kinetics was proposed to be controlled by the outer composite oxide layer. While this layer was morphologically continuous, it consisted of a non-continuous mixture of NbAlO4, ZrO2, Ti7Al2O15, and Ta2Nb4O15. The observed oxide scales are discussed relative to the oxidation behaviors of less-complex alloys.

Oxidation response of a vacuum arc melted NbZrTiCrAl refractory high entropy alloy at 800–1200 °C

An equimolar refractory high-entropy alloy NbZrTiCrAl was prepared by vacuum arc melting, the microstructure and oxidation response of the alloy at 800 °C, 1000 °C and 1200 °C were investigated. The alloy mainly consists of BCC single-phase solid solution and shows an inhomogeneous microstructure with dendritic and inter-dendritic regions. The oxidation of the alloy at 800 °C and 1000 °C follows parabolic rate law, and the protection performance can be attributed to the formation of dense and homogenous complex oxides scale composed of CrNbO4, ZrO2, TiO2, Al2O3 and ZrNb2O7. The linear oxidation kinetics of the alloy at 1200 °C is observed, the scale composed of similar oxide products but with layered structure is probably the reason of the poor resistance to long-term oxidation at higher temperatures. The formation of the oxide scale and the oxidation-resistant mechanism of the alloy at different temperatures have been discussed.

Oxidation kinetics of uranium treated by pulsed laser nitriding in air

The oxidation kinetics of uranium surfaces modified by pulsed laser nitriding in air at temperatures of 420, 440, 450 and 460 K has been investigated. X-ray diffraction (XRD), Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) were used to characterize the structure, morphology and composition of the samples. The nitrided uranium was consisted of three parts: an outer thin uranium oxide (UO2) layer in contact with air, an inner oxygen diffusion layer at the interface, which was formed by the diffusion of oxygen into the uranium mononitride (UN) and the quantity of oxygen descended gradually as the modified layer thickening, and an underlying UN layer mixed with metallic uranium. Oxide thickness measurements via in situ XRD indicate that a linear oxidation kinetics for the nitrided uranium was observed at 460 K. At temperature range 420–450 K, the oxidation kinetics can clearly be subdivided into two stages: a non-linear curve, initial region of slow oxide-film growth, which is followed by a much faster growth stage of a linear curve. The transformation critical thickness was about 300 nm. The average activation energy of oxidation was estimated to be 118.8 kJ/mol and 89.5 kJ/mol, when the oxide-film for new growth was below and above the critical thickness, respectively.

High temperature oxidation behaviors of equimolar NbTiZrV and NbTiZrCr refractory complex concentrated alloys (RCCAs)

Refractory complex concentrated alloys (RCCAs) represent an emerging class of materials that have displayed significant potential for increased mechanical performance at extreme temperatures over conventional refractory alloys and Ni-base superalloys. While limited work has been conducted in this area, several RCCAs have shown remarkable levels of oxidation resistance in comparison to the majority of uncoated commercial refractory alloys. However, the oxidation mechanisms in these types of alloys have not been well understood. This study aims to develop the foundation for understanding RCCA oxidation mechanisms through the systematic evaluation of NbTiZrV and NbTiZrCr, which display vastly different oxidation behaviors. In both cases, the predominant mechanism appeared to be internal oxidation, where a mixture of simple and complex oxides was observed after prolonged oxidation times. This resulted in extended linear oxidation kinetics for the NbTiZrV alloy and linear to near-parabolic oxidation kinetics for the NbTiZrCr alloy. Under this mindset, novel design strategies for promoting increased oxidation resistance in both RCCAs and dilute refractory alloys based on sluggish complex oxides are proposed.

Oxidation characteristics of porous-nickel prepared by powder metallurgy and cast-nickel at 1273 K in air for total oxidation time of 100 h

The oxidation behavior of two types of inhomogeneous nickel was investigated in air at 1273K for a total oxidation time of 100h. The two types were porous sintered-nickel and microstructurally inhomogeneous cast-nickel. The porous-nickel samples were fabricated by compacting Ni powder followed by sintering in vacuum at 1473K for 2h. The oxidation kinetics of the samples was determined gravimetrically. The topography and the cross-section microstructure of each oxidized sample were observed using optical and scanning electron microscopy. X-ray diffractometry and X-ray energy dispersive analysis were used to determine the nature of the formed oxide phases. The kinetic results revealed that the porous-nickel samples had higher trend for irreproducibility. The average oxidation rate for porous- and cast-nickel samples was initially rapid, and then decreased gradually to become linear. Linear rate constants were 5.5×10−8g/cm2s and 3.4×10−8g/cm2s for the porous- and cast-nickel samples, respectively. Initially a single-porous non-adherent NiO layer was noticed on the porous- and cast-nickel samples. After a longer time of oxidation, a non-adherent duplex NiO scale was formed. The two layers of the duplex scales were different in color. NiO particles were observed in most of the pores of the porous-nickel samples. Finally, the linear oxidation kinetics and the formation of porous non-adherent duplex oxide scales on the inhomogeneous nickel substrates demonstrated that the addition of new layers of NiO occurred at the scale/metal interface due to the thermodynamically possible reaction between Ni and the molecular oxygen migrating inwardly.

Oxidation and Corrosion resistance of AlCoCrFeTiHigh Entropy Alloy

Ability to form simple solid solution contrary to complex phase makes multi-component High Entropy Alloys (HEAs) promising new generation alloys. The present study deals with the oxidation and corrosion resistance of AlCoCrFeTi high entropy alloy synthesized by Mechanical alloying (MA) and consolidated using Spark plasma sintering (SPS). Elements in the alloy system were selected with small size factor to avoid the formation of amorphous phase. Powder diffraction study confirms dissolution of elemental addition into a single phase body centered cubic system. Milled powders were sintered at 900 °C, 45 MPa and with a holding time of 15 min. Spark plasma sintering results in minor traces of face centered cubic phase along with body centered cubic system. Relative density of sintered sample is 98.5%. Thermal stability of the milled powder is analyzed using Differential Scanning Calorimetry. Vickers microhardness test on sintered sample reveals a hardness value of 1074.5 HV. Synthesized alloy exhibits excellent corrosion resistance and linear oxidation kinetics with a very low weight gain.

A Review of SiC Fiber Oxidation with a New Study of Hi‐Nicalon SiC Fiber Oxidation

The linear and parabolic oxidation rate constants and activation energies for SiC fibers available in the literature are reviewed for the first time. Oxidation experiments are also conducted with Hi‐Nicalon SiC fibers over 700–1 300 °C in dry O2 with thermogravimetric analysis. Linear oxidation kinetics are observed from 700 to 800 °C and parabolic kinetics are observed from 900 to 1 300 °C. The linear and parabolic oxidation rate constant activation energies are determined to be 83 and 108 kJ mol−1, respectively. These kinetic results are compared with the literature values. Differences in oxidation behavior are assessed as a function of fiber composition.

Response of ytterbium disilicate–silicon environmental barrier coatings to thermal cycling in water vapor

A preliminary study of a promising bi-layer environmental barrier coating (EBC) designed to reduce the susceptibility of SiC composites to hot water vapor erosion is reported. The EBC system consisted of a silicon bond coat and a pore-free ytterbium disilicate (YbDS; Yb2Si2O7) topcoat. Both layers were deposited on α-SiC substrates using a recently optimized air plasma spray method. The two layers of the coating system had coefficients of thermal expansion (CTE) that were well matched to that of the substrate, while the YbDS has been reported to have a moderate resistance to silicon hydroxide vapor forming reactions in water vapor rich environments. Thermal cycling experiments were conducted between 110 °C and 1316 °C in a flowing 90% H2O/10% O2 atmospheric pressure environment, and resulted in the formation of a thermally grown (silica) oxide (TGO) at the silicon-ytterbium disilicate interface. The TGO layer exhibited linear oxidation kinetics consistent with oxidizer diffusion through the ytterbium silicate layer controlling its thickening rate. The effective diffusion coefficient of the oxidizing species in the YbDS layer was estimated to be 2 × 10−12 m2s−1 at 1316 °C. Slow steam volatilization of the YbDS topcoat resulted in the formation of a thin, partially protective, high CTE ytterbium monosilicate layer on the outside of the YbDS coating. Progressive edge delamination of the coating system was observed with steam exposure time, consistent with water vapor volatilization of the TGO edges that were directly exposed to the environment. This was aided by outward bending of the delaminated region to relax TGO and YbMS surface layer stresses developed during the cooling phase of each thermal cycle.