Transient Oxidation Stage Research Articles

Delayed parabolic oxidation via transient thermal exposures on a polycrystalline nickel-based superalloy

Multiple oxides were observed during the transient oxidation stage of the polycrystalline Ni-based superalloy, RR1000, before a protective Cr2O3 scale formed. Thermogravimetric analysis, synchrotron grazing incidence X-ray diffraction, and electron microscopy were performed on samples subjected to isothermal exposures at 800 °C for up to 100 h. Transient effects governed the early stages up to 40 h. NiO, spinels (NiCr2O4, (Ni,Co)(Cr,Co)2O4), Cr2O3/(Cr0.88Ti0.12)2O3, NiTiO3, and CrTaO4 formed during the initial stage with pseudo-linear kinetics. At the onset of parabolic kinetics, extensive Cr2O3 and TiO2 growth dominated scale formation with the former emerging as the major passivating oxide.

Effect of Si solution in α-Mo on oxidation behavior of Mo-Si-B alloy and oxidation kinetics model

In the present work, two Mo-12Si-8.5B alloys were prepared with α-Mo phase of a higher (2.72 at.%) and a lower (about 0 at.%) Si solid solubility. The oxidation behavior at 1100 °C indicated that a higher (2.72 at.%) Si solution in α-Mo phase improved the oxidation resistance of the Mo-Si-B alloy by enhancing the oxidation resistance of α-Mo itself and promoting the rapid coverage of protective borosilicate scale. As a result, the oxidation rate was decreased appreciably and the degeneration of the alloy was reduced by at least 77% during 30 h of the oxidation compared with that of the alloy with lower Si solution in α-Mo. An oxidation kinetics model for Mo-Si-B alloys was developed on the transient oxidation stage and it further confirmed the enhanced oxidation resistance by showing an increase decay time constant for the α-Mo phase with higher Si solution.

The influence of Cr on the oxidation behavior of a sputtered Pt+γ’ phase-equilibrium coating at 1150 ℃

Two sputtered phase-equilibrium coating (Pt+γ’ and Pt-Cr+γ’ coating) were manufactured in this work. No TCPs appeared around the interface of coating/superalloy after being oxidized at 1150 ℃ for 1000 h. The addition of chromium changes the oxidation kinetics of Pt-Cr+ γ’ coating both in transient and steady oxidation stage. In transient-oxidation stage, chromium accelerates the transition of aluminum oxide from θ-Al2O3 to α-Al2O3, which lowers the compressive stress in oxide scale. The presence of Cr in coating also refines the grain size of α-Al2O3 scale, which leads to a higher growth rate in the later steady oxidation-stage.

Microstructure and transient oxidation behavior of NiCoCrAlYSiHf coating modified via high-current pulsed electron beam

In this study, NiCoCrAlYSiHf coatings deposited via arc ion plating technique were modified by high-current pulsed electron beam (HCPEB) irradiation with different pulses. The coarse surface of the original coating was completely modified, and the modification effects of HCPEB irradiation, including surface finishing, composition optimization, and microstructural adjustment were obtained. The transient oxidation behavior of original and irradiated coatings was comparatively investigated under 1100 °C for 10 h. Experiment and characterization results showed that the remelting effects induced by HCPEB irradiation could greatly influence the growth behavior of the thermally grown oxides (TGO), presenting as the rapid formation of a complete and dense α-Al 2 O 3 layer in the transient oxidation stage, which guaranteed the protective effect of the AIP- M CrAlY X type coatings in subsequent oxidation stages. • NiCoCrAlYSiHf arc ion plated coating was modified via HCPEB technique with different parameters. • HCPEB irradiation led to a complete surface remelting, and eliminates original surface defects. • Nano-scale grains and deformation structures were obtained. • A stable, continuous and uniform α-Al 2 O 3 scale can be expected for all of treated coatings. • HCPEB technique is an effective approach to improve the performance of M CrAlY X -type coatings.

Effects of Minor Alloying Elements on Alumina Transformation During the Transient Oxidation of β-NiAl

The transient stage of alumina scale formation on alumina-forming alloys is often accompanied by the formation of metastable Al2O3 phases that may affect oxidation kinetics and scale microstructure. To clarify the role of dopants on the transient stage of NiAl oxidation, we characterized Al2O3 scales on a series of oxidized model β-NiAl samples that were un-doped or doped with additions of Ti, Y, Si, Ti + Y, and Si + Y. Our results confirm that different dopant elements (Ti, Si, and Y) have varied effects on the length of transient oxidation of alumina-forming alloys. Ti and Si accelerated the θ- to α-Al2O3 transformation via two different mechanisms. Ti promoted α-Al2O3 nucleation, while Si delayed nucleation but accelerated the lateral growth of the α-Al2O3 patches. In contrast, Y had little effect on alumina transformation, possibly due to the low Y concentration used in the current study.

The Impact of Impurities on Alloy Behavior in Supercritical CO2 at 700 °C

As part of round robin testing, specimens of commercial alloys 316, 120, 625, and 740 were exposed to 20 MPa research grade CO2 at 700 °C for up to 1500 h. The first set of specimens had higher mass gain likely due to impurities not flushed from the autoclave at startup. After this issue was corrected, an identical set of specimens exhibited lower mass gains for both the Fe- and Ni-based alloys. The differences in reaction products were characterized to understand the effect of impurities under these conditions. As suggested by the mass change, thicker oxides were formed in each case, primarily for the Fe-based alloys. For the Ni-based alloys, the difference was primarily in the transient stage of oxidation with no change in rate constant. Alloy 120 exhibited increased internal attack, and differences in the scale phases formed were characterized to better understand the effect of impurities. Alloy 740 primarily exhibited a greater depth of internal attack in the first run.

Preparation and oxidation behaviour of NiCrAlYSc coatings on a Ni-based single crystal superalloy

The NiCrAlYSc coatings with different Sc content were prepared on the Ni-based superalloy by arc ion plating. Cyclic and isothermal oxidation behaviours of all the coatings were investigated. The results show that an appropriate addition of Sc in the coating can significantly improve the scale adhesion, shorten the transient oxidation stage and promote the transformation from θ-Al2O3 to α-Al2O3. The effects of Sc addition on the oxidation performance of the coating as well as the mechanism of oxide adherence were discussed.

High-temperature oxidation response of Mo–Si–B composites with TiO2W/SiCW addition

Abstract In this study, TiO2W addition improved the oxidation resistance of the Mo–Si–B composite at 1300 °C. The TiO2 partially dissolved in SiO2 modified the network structure of the SiO2 glass and improved its fluidity at the initial oxidation stage. This favored to a continuous scale cover on the surface of the Mo–Si–B composite rapidly. The residual TiO2W promoted the formation of a passivated multilayer borosilicate scale at current temperature, which could impede the MoO3 volatilisation and O diffusion at the stable oxidation stage. The SiCW addition, compared to the TiO2W, especially could ensure the Mo–Si–B–SiC composite withstand a higher temperature such as 1400 °C. Its oxidation and the more intermetallics in the composite could increase the number of active sites of the SiO2 glass, thereby supplying the borosilicate scale with a relatively sufficient Si element. Thus, the transient oxidation stage was minimised and the initial mass loss was reduced, which indicated a continuous borosilicate scale had formed quickly at the initial stage. Finally, the improved viscosity of the borosilicate due to a lower B/Si ratio, could obviously decreased the oxygen diffusion and enabled the formation of a protective borosilicate layer at or above 1400 °C.

Variation of phase composition of Mo-Si-B alloys induced by boron and their mechanical properties and oxidation resistance

Mo-12Si-xB (x = 5, 8.5, 17 at%) alloys, doped with 0.9 wt% nanometer La2O3, were fabricated via a powder metallurgical process. The distribution of the α-Mo, Mo3Si and Mo5SiB2 phases existed in these alloys was dependent on the volume fractions of the individual phases. The Mo-12Si-5B (~52 vol% of α-Mo) and Mo-12Si-8.5B (~46 vol% of α-Mo) alloys possessed a continuous α-Mo matrix where Mo3Si/Mo5SiB2 particles distributed dispersedly and the former exhibited the highest fracture toughness. Our theoretical analysis demonstrated that the crucial contribution of crack trapping mechanism to the toughness and the α-Mo volume fraction increasing effectively promoted this mechanism. Besides, the α-Mo/intermetallic interface debonding and the formation of intragranular microcracks induced by La2O3 provided extra contributions to toughness. The Mo-12Si-17B alloy (~65 vol% of intermetallic) formed an intermetallic matrix with embedded semi-continuous α-Mo colonies and other isolated α-Mo islands, having the highest compression strengths at 800–1300 °C. Especially at 1200–1300 °C, the intermetallic matrix with low plastic deformation played a significant role in reinforcing alloy. Besides, the intragranular La2O3 particles strengthened alloy by pinning down and accumulating dislocations in the grain interior. The Mo-12Si-17B was also found to have the excellent oxidation resistance at 1000 °C owing to exhibiting the shortest duration and minimal mass loss in the transient oxidation stage. Moreover, higher B content decreased the viscosity of the borosilicate and thereby formed a dense and protective glassy borosilicate scale quickly, which substantially reduced the growth of inner oxide layers in the steady oxidation stage. In addition, the La2O3 as a blocking phase could impede the permeation of MoO3 bubbles/clusters through the flowing SiO2 glass scale.

Oxidation resistance of a Mo-W-Si-B alloy at 1000–1300 °C: The effect of a multicomponent Mo-Si-B coating

The oxidation behavior and microstructures of a three phase Mo50W20Si15B15 alloy containing (Mo, W) solid solution, (Mo, W)5Si3 and (Mo, W)5SiB2 were investigated over the 1000–1300 °C temperature range. At 1300 °C, the alloy exhibited a large initial mass loss and eventually transitioned to a steady state behavior associated with the development of an amorphous borosilica scale. The oxidation resistance at lower temperature was unsatisfactory due to the pesting of Mo and W. Grazing Incident Angle X-ray Diffraction (GIXRD) measurements during the initial transient stage of oxidation identified the evolution of surface oxide products that were responsible for the large weight loss and pesting behavior. In order to address the large initial weight loss and the low temperature pesting behavior, the alloy was coated with a Mo-Si-B based coating by employing the pack cementation process followed by a conditioning treatment at 1450 °C. The microstructures of the coatings and the interlayer between the base alloy and the coating were studied, and the oxidation resistance of the coated alloys was evaluated over the 800–1300 °C temperature range. The coated alloy showed improved oxidation resistance with nearly zero mass change for extended period of time over the entire range of temperatures studied.

A study of the determination of grain boundary diffusivity and energy through the thermally grown oxide ridges on a Fe-22Cr alloy surface

The grain boundaries (GBs) present in polycrystalline materials are important with respect to materials behaviour and properties. During the transient stage of oxidation, the higher GB diffusivity results in heterogeneous oxidation structures in the form of oxide ridges that emerge along the alloy GBs. In an attempt to delve into the more fundamental aspects of the GBs, such as GB energy, the size of the oxide ridges was quantitatively measured by atomic force microscopy on the post oxidation surface of a Fe-22 wt % Cr alloy after an oxidation exposure at 800 °C in dry air. The GB diffusivity was calculated utilising the ridge size data and the relationship between the GB diffusivity and the GB characteristics was determined. Furthermore, the GB energy was calculated from the GB diffusivity data, also to make comparison with the data available in the literature. The absolute value of the calculated GB energy was quite close to the values reported in the literature. However, compared to the extremely low temperature (0 K) data-set from the literature, the data-set obtained from this study showed much less spread. The smaller variation range may be attributed to the higher temperature condition (1073 K) in this study.

Oxidation characteristics of the electron beam surface-treated Alloy 617 in high temperature helium environments

The oxidation characteristics of the electron beam surface-treated Alloy 617, which has an Al-rich surface layer, were evaluated in high temperature helium environments. Isothermal oxidation tests were performed in helium (99.999% purity) and VHTR-helium (helium of prototypical VHTR chemistry containing impurities like CO, CO2, CH4, and H2) environments at 900°C for up to 1000h. The surface-treated Alloy 617 showed an initial transient oxidation stage followed by the steady-state oxidation in all test environments. In addition, the steady-state oxidation kinetics of the surface-treated Alloy 617 was 2-order of magnitude lower than that of the as-received Alloy 617 in both helium environments as well as in air. The improvement in oxidation resistance was primarily due to the formation of the protective Al2O3 layer on the surface. The weight gain was larger in the order of air, helium, and VHTR-helium, while the parabolic rate constants (kp) at steady-state were similar for all test environments. In both helium environments, the oxide structure consisted of the outer transition Al2O3 with a small amount of Cr2O3 and inner columnar structured Al2O3 without an internal oxide. In the VHTR-helium environment, where the impurities were added to helium, the initial transient oxidation increased but the steady state kinetics was not affected.

Degradation of a TBC with HVOF-CoNiCrAlY Bond Coat

Thermal barrier coatings (TBCs) provide both thermal insulation and oxidation and corrosion protection to the substrate metal, and their durability is influenced by delamination near the interface between the ceramic topcoat and the metallic bond coat, where a layer of thermally grown oxide (TGO) forms during service exposure. In the present work, the degradation process of a TBC with an air-plasma-spray ZrO2-8 wt.%Y2O3 topcoat and a high-velocity oxy-fuel CoNiCrAlY bond coat was studied, in terms of TGO growth kinetics and aluminum depletion in the bond coat, as well as cracking behavior. The results show that the TGO growth kinetics can be described by a transient oxidation stage with δ3 = k1t followed by a steady-state oxidation stage with δ2 = c + k2t. Significant aluminum depletion was observed in the bond coat after extended thermal exposure; however, chemical failure of the bond coat did not occur even after the aluminum content near the TGO/CoNiCrAlY interface decreased to 4.5 at.%. A power-law relationship between the maximum crack length in the TBC and the TGO thickness was observed, which may serve as the basis for TBC life prediction.

高温酸化アルミナスケールの相変態に及ぼす種々の要因

Effect of various factors on the metastable to stable phase transformation behavior of a thermally grown oxide scale of Al2O3 was reviewed and discussed on the basis of the results obtained from our recent studies. In this paper, among the factors affecting this phase transformation, particular attention has focused on the effect of Fe in order to understand the different phase transformation behavior of Al2O3 scale formed on different alloy systems in different oxidation environments. In order to explore the effect of Fe on the phase transformation, the oxidation behavior of various Al2O3 forming Fe-based model alloys was systematically investigated. The development of different oxide scales on different alloys during a high-temperature oxidation, including the initial transient stage of oxidation followed by isothermal oxidation in different oxidation atmospheres, was examined by in-situ X-ray diffraction by means of the Synchrotron radiation. Fe was found to accelerate the phase transformation to α-Al2O3. This effect can be considered to be due to an initial formation of Fe2O3, which is an isomorphous crystal structure with α-Al2O3. Further contributing factor of Fe on the transformation can be a doping of Fe3+ ion, which formed during an initial transient oxidation stage, in the Al2O3 scale. The oxidation atmospheres are considered to indirectly affect the phase transformation by changing in the formation behavior of Fe2O3 and (Al, Fe)2O3 solid solution during an initial transient oxidation stage.

Transient Oxidation of Cu-5at.%Ni(001): Temperature Dependent Sequential Oxide Formation

The transient oxidation stage of a model metal alloy thin film was characterized with in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and analytical high-resolution TEM. We observed the formations of nanosized NiO and Cu2O islands when Cu-5at.%Ni(001) was exposed to oxygen partial pressure, $$ {\text{pO}}_{ 2} = 1 \times 10^{ - 4} \,{\text{Torr}} $$ and various temperatures in situ. At 350 °C epitaxial Cu2O islands formed initially and then NiO islands appeared on the surface of the Cu2O island, whereas at 550 °C NiO appeared first. XPS and TEM revealed a sequential formation of NiO and then Cu2O islands at 550 °C. The temperature-dependent oxide selection may be due to an increase of the diffusivity of Ni in Cu with increasing temperature.

In situ ultra-high vacuum transmission electron microscopy studies of the transient oxidation stage of Cu and Cu alloy thin films

Because environmental stability is an essential property of most engineered materials, many theories exist to explain oxidation mechanisms. Yet, nearly all classical oxidation theories assume a uniform growing film, where structural changes were not considered because of the previous lack of experimental procedure to visualize this non-uniform growth in conditions that allowed for highly controlled surfaces and impurities. With the advent of vacuum technologies and advances in microcopy techniques, especially in situ, one can now see structural changes under controlled surface conditions. Here, we present a review of our systematic studies on the transient oxidation stages of a model metal system, Cu, and its alloys, Cu–Au and Cu–Ni, by in situ ultra-high vacuum transmission electron microscopy (UHV-TEM). The dependence of the oxidation behavior on the crystal orientation, oxygen pressure, temperature and alloying is attributed to the structures of the oxygen-chemisorbed layer, oxygen surface diffusion, surface energy and the interfacial strain energy. Heteroepitaxial concepts, developed to explain thin film formation on a dissimilar substrate material (e.g., Ge on Si), described well these initial oxidation stages.

Effects of water vapor on high temperature oxidation of cryomilled NiCoCrAlY coatings in air and low-SO 2 environments

Nanostructured NiCoCrAlY coatings were obtained through cryomilling and subsequent HVOF-spraying of the nanostructured powder onto copper substrates, followed by leaching of the Cu to obtain freestanding coatings. Oxidation tests in both air and an environment bearing 0.5% SO 2 with water vapor levels varying from 0 to 8% were conducted for 24 and 48 h at 1000 °C. It has been found that the oxides formed under all conditions are alumina followed by a spinel phase consisting primarily of cobalt aluminum oxide. In both air and the SO 2 environment an increase in total mass gain was seen up to 4% water vapor, followed by a decrease at 6 and 8%. This behavior is attributed to an extension of the transient oxidation stage, resulting first in a variation in scale thickness through increased spinel growth and second in a rise in internal oxidation caused by increased oxygen transport through the scale in the presence of water vapor. It would appear that above 4% water vapor the thickened spinel is able to compensate for alumina loss and create a diffusion barrier to internal oxidation, once again reducing mass gain.

Oxidation Response and Coatings for Mo-Si-B Alloys

ABSTRACTMo-Si-B alloys respond to high temperature oxidation in two distinct stages. First, there is a transient stage with an initial high recession rate that corresponds to the evaporation of volatile MoO3 due to the oxidation of the molybdenum rich phases. The steady state stage of the oxidation begins when a borosilica layer that initiated in the transient period becomes continuous and protects the alloy from further rapid oxidation. Then, the oxidation rate is limited by oxygen diffusion through the borosilicate layer. In order to improve the oxidation performance of the Mo-Si-B alloys, it is necessary to minimize the transient stage. The three phases, Mo (solid solution), Mo3Si (A15) and Mo5SiB2 (T2), composing the Mo-Si-B alloys play different roles in the transient stage. The interaction of the three phases with a reduced microstructure scale can reduce considerably the transient oxidation stage. As a further approach to inhibit the transient stage, a kinetic biasing strategy has been developed to capitalize on the reactions between different phases to develop useful reaction products and alloy compositions that evolve toward a steady state of a compatible system. In order to achieve a compatible interface coating together with enhanced oxidation resistance, a pack cementation process has been adopted to apply diffusion coatings. Two areas are highlighted for successful coating applications on Mo-Si-B alloys and robust high temperature oxidation resistance: development of metal-rich silicide + borosilicide high-temperature coating and in-situ thermal-barrier + borosilica coatings.

Abnormal Oxidation Kinetics of Fe-Cr-Al Alloys at 1273 K Induced by Cr-rich Phases

The oxidations of Fe-10Cr-10Al, Fe-15Cr-10Al, and Fe-20Cr-10Al (all in at.%) alloys were conducted in 0.1 MPa oxygen at 1273 K. After a transient oxidation stage, three alloys underwent a selective oxidation of Al incorporated with oxidation of Cr and then a steady state stage involving exclusive oxidation of Al. Compared with Fe-10Cr-10Al and Fe-20Cr-10Al, Fe-15Cr-10Al exhibited abnormal oxidation kinetics, i.e. the much higher oxidation rate during the selective oxidation of Al and the much lower oxidation rate during the exclusive oxidation of Al. The abnormal oxidation kinetics of Fe-15Cr-10Al was induced by the Cr-rich phases existing parallel to the surface of the specimens.

Transient oxidation of Mo–Si–B alloys: Effect of the microstructure size scale

The composition and phase constituency of Mo–Si–B alloys are known to be important parameters in determining the oxidation response. For three phase Mo + T 2 + Mo 3Si alloys with constant composition and phase constituency, it is observed that a refined microstructure scale provides superior oxidation resistance. The transient stage of oxidation is shortened and the recession of the alloy is decreased with microstructural refinement. In order to identify the phase interaction during the transient stage, oxidation of each of the three alloy phases, Mo, Mo 3Si (A15) and Mo 5SiB 2 (T 2) has been investigated separately. Quantification of the separate phase size distributions by image analysis was coupled with the individual phase oxidation response to evaluate the overall oxidation behavior and phase interaction effects. A kinetic model for oxidation of Mo–Si–B alloys is proposed that incorporates the key role of microstructure scale on the transient stage and provides guidance for microstructure design.