Search

The oxidation resistance of the MCrAlY binding coat is due to the formation of protective Al2O3 oxide scale at high temperature. The oxidation behavior of NiCrAlYHf alloy in 1100 °C air and air-water vapor atmosphere was studied. The effect of water vapor on the microstructure and distribution of reactive elements was discussed. The results showed that the oxide scale in air has a double layer structure composed of columnar and equiaxed crystals, while the oxide scale in water vapor contains fine alumina grains, which provides more channels for the diffusion of reactive elements. In addition, The Cr element in the oxide scale is mainly concentrated in the outer equiaxed crystal zone, and the Hf oxide is mainly concentrated in the columnar crystal boundary. In air-water vapor atmosphere, the Cr element is uniformly distributed in the oxide scale. [ABSTRACT FROM AUTHOR]

In this work, we investigated FeCrMo coatings with 35 wt% ceramic, which were produced on a cylinder surface by an internal rotating plasma spraying and oxidized in air. The ceramic particles consisted of 80 wt% alumina and 20 wt% zirconia, which can improve the mechanical properties of the coating. For microstructural characterization, scanning electron microscopy was combined with energy dispersive X-ray analysis and electron probe micro analysis, and the nanomechanical properties were measured by a nanoindentation tester. The structure of Fe-base coating matrix consists of Al 2 O 3 , ZrO 2 and a small amount of mixed Fe–Cr oxides, and the ceramic particles exhibit a uniform distribution. During oxidation, a mixed oxide layer containing Fe 2 O 3 and Fe 3 O 4 forms on the surface of the Fe-base coating, and the thickness of the oxide layer increases with increasing oxidation temperature. Due to the in-situ oxidation, the nanohardness and Young's modulus of the Fe-base coating decreased with increasing oxidation temperature. The nanohardness evolution of Fe-base coatings oxidized at different temperatures is discussed. [ABSTRACT FROM AUTHOR]

FeCrAl-base alloys are well known for their excellent oxidation resistance due to formation of a slowly growing alumina surface scale during high-temperature service. The actual scale growth mechanism and especially adherence are strongly affected by the presence of oxygen-active elements such as yttrium, titanium or hafnium. In the present study, the effect of titanium addition on the scale growth mechanisms of an yttrium oxide dispersion-strengthened FeCrAl base alloy was studied during oxidation at 1200 °C in Ar-O2. For microstructural characterization results of scanning electron microscopy and electron backscatter diffraction were combined with X-ray diffraction data. Scale growth mechanisms were investigated by two-stage oxidation using 18O tracer with subsequent scale analyses using secondary neutrals mass spectrometry. The scale on the alloy without intentionally added titanium grew virtually exclusively by oxygen diffusion along oxide grain boundaries and exhibited a columnar structure with the grain size increasing in growth direction. The addition of titanium resulted in formation of an outer oxide zone of equiaxed grains on top of the inner columnar part. The equiaxed grains increased in size with increasing exposure time. Comparison with the tracer studies revealed that the titanium-induced equiaxed zone was the result of outer scale growth. Mechanisms for the initiation of outward aluminium transport are discussed. Indications were found that the effect of titanium on the scale growth mechanisms already occurred for titanium additions as low as 0.02 wt%. [ABSTRACT FROM AUTHOR]

In this study, we investigate the oxidation behavior of a Fe-based cylinder coating mixed with 35 wt% eutectic ceramic particles. About 250 μm thick coatings were produced on the cylinder surface by using internal rotating plasma spraying. The cylinder coatings were placed in an environment at 650 °C to experience air plus water vapor oxidation as well as air oxidation. In order to do the analysis, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), energy dispersive x-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) were used. The results show that after spraying, the Fe-based coating has less defects and a uniform and dense structure, in which Al 2 O 3 , Al 2 O 3 /ZrO 2 and a small amount of ZrO 2 are evenly distributed. In addition, Al 2 O 3 /ZrO 2 ceramic particles play a certain role in delaying the breakaway oxidation in the following stage. The oxidation behavior of the Fe-base coating is obviously different from that of traditional alloys. The oxidation rate in the aqueous atmosphere is higher than that in the air. This is mainly reflected in the diffusion of elements, the formation and volatilization of oxides, and the formation and aggregation of pores, which therefore play important roles in the breakaway oxidation. The role of ceramic particles in the oxidation of coatings, the reasons for the high oxidation rate, and the breakaway oxidation mechanism of Fe-based coatings in water vapor were analyzed. • The breakaway oxidation theory of Fe-based coating was proposed. • The breakaway oxidation behavior of the Fe-base coating is obviously different from that of traditional alloys. • Water vapor promotes the breakaway oxidation in the Fe-based coating. [ABSTRACT FROM AUTHOR]

This study investigates the influence of test atmosphere on the oxidation behavior for the CoNiCrAlYHf alloy at 1100 ℃. The oxidation rate of alloys in water vapor conditions is higher compare to that in dry air. The higher oxidation rate is attributed to an increased oxygen flux, facilitated by the transport of oxygen through voids in the oxide layer. A higher water vapor content accelerates the spinel formation by enhancing the outward diffusion of Ni and Cr through the oxide scale. Additionally, a "limpet tooth microstructure" aluminum oxide scale forms on the surface in the air-25 % H 2 O environment. Aluminum oxide with a "limpet tooth structure" may reduce the number of transverse grain boundaries, thereby decreasing the tendency for crack propagation. • A special "limpet tooth microstructure" aluminum oxide scale was characterized by EBSD and TEM. • The formation energies of oxygen vacancies in the presence of interstitial H was calculated. • The higher scale growth in water vapor is due to an increased oxygen flux across induced by transport of oxygen across voids. • Higher water vapor content means that an enhanced increase in vacancy concentration. [ABSTRACT FROM AUTHOR]

• New alumina grains formed on the TiO 2 particles surface on the ODS FeCrAl alloy. • Fe 2 O 3 formed on the surface of alumina scale on the FeCrAl alloy with Pt doping. • Pt disturbs the growth mechanism of the oxide scale formation at the initial stage. The superior oxidation resistance of oxide dispersion-strengthened FeCrAl-base alloys is due to the formation of a slowly growing alumina scale on the surface. In this study, the growth mechanism of oxide scale formation on yttria-dispersion FeCrAl alloy with and without Pt doping was studied during oxidation at 1200℃ in air. The morphologies of the alumina scales and compositions of the oxides on the surface of the FeCrAl alloys were different. Pt doping disturbs the growth mechanism of the oxide scale formation at the initial stage, and the element diffusion mechanisms of oxide formation on the ODS-FeCrAl alloys are discussed. [ABSTRACT FROM AUTHOR]

Pt modified CoNiCrAl medium-entropy alloys (MEAs) were optimized that displayed outstanding long-term oxidation resistance life at 1100 °C. Compared to air, introducing interstitial H into the Al 2 O 3 lattice changed the energy of Al vacancy formation, from 6.89 eV to 4.13 eV, indicating that water vapour accelerated the oxidation of MEAs, which was twice that of air. With the Pt increasing, the Young's modulus and hardness improved, promoting the formation of smoother Al 2 O 3 scale in 10Pt-MEA. Precipitation of Pt enrichment at the phase boundary suppressed oxygen diffusion and reduced oxidation rate. The designed MEAs expand the application of high/medium-entropy alloys. • Pt modified medium-entropy alloys were designed and optimized. • Introducing H into the Al 2 O 3 lattice reduced the formation energy of Al vacancy. • Precipitation of Pt enrichment at the phase boundary reduced the oxidation rate. • Pt solid solution in BCC phase improved the oxidation resistance. • A smoother Al 2 O 3 scale formed in 10Pt-MEA. [ABSTRACT FROM AUTHOR]

The arc-melted and vacuum-casted CoNiCrAlHf alloys with a 7YSZ top coat were designed to develop a protective alumina scale at 1100 °C in water vapor content. Arc-melted CoNiCrAlHf alloys provided substantial improvements in oxidation resistance. In this work, the oxidation behavior and void distribution of thermally grown oxide (TGO) formed on CoNiCrAlHf alloys were analyzed. The Al 2 O 3 crystal structure and void distribution were studied as a function of the grain size of the CoNiCrAlHf substrate. Numerous voids developed in the oxide scale, and coarse HfO 2 particles segregated at the Al 2 O 3 grain boundaries of vacuum-cast CoNiCrAlHf alloys. The uniformly distributed HfO 2 particles prevented void formation in the oxide scale. As a result, the phase transformation of Al 2 O 3 was effectively impeded in water vapor. • The preparation method of CoNiCrAlHf alloy affects the size of defects within the oxide scale. • The uniform distribution of HfO 2 particles suppresses the growth of void. • The high levels of water vapor inhibited the phase transition of Al 2 O 3. [ABSTRACT FROM AUTHOR]

The external atmosphere can significantly affect the microstructure within the oxide scale of alloys during high temperature oxidation. In this paper, we investigate the isothermal oxidation of CoNiCrAlHf alloys in dry air and water vapor (Ar-25%H 2 O and air-25%H 2 O). The different test atmospheres led to very different results. HfO 2 peg formed at the Al 2 O 3 /alloy interface duo to in-situ oxidation. The presence of water vapor significantly increases the oxygen potential gradient and the diffusive flux of Hf, causing the linear distribution of HfO 2. In addition, water vapor has effectively retard the phase transformation of metastable-Al 2 O 3 to α-Al 2 O 3. In high temperature water vapor, the CoNiCrAlHf alloys form a duplex-layer, consisting of external δ-Al 2 O 3 and internal γ-Al 2 O 3. The distinction can be attributed to the promotion of defect concentration due to the adsorbed water molecules. The formation of Al vacancies would promote the outward diffusion of Al and the inward diffusion of O, which greatly facilitates the formation of γ -Al 2 O 3. • The external atmosphere can significantly affect the microstructure within the oxide scale of alloys during high temperature oxidation. • Different distributions of HfO 2 were formed within the oxide scale in dry air and water vapor. • High water vapor has effectively retard the phase transformation of metastable-Al 2 O 3 to α-Al 2 O 3. • The distinction of microstructure can be attributed to the promotion of defect concentration due to the adsorbed water molecules. [ABSTRACT FROM AUTHOR]

Abstract The growth behaviour of the modified surface of NiCoCrAl alloy by Pt and Hf was investigated at 1050 °C in air atmosphere. The undoped NiCoCrAl alloy exhibited an extremely serious oxide scale spallation. Compared to the undoped alloy, the Hf and Pt depositing on the surface of NiCoCrAl can greatly increase the alumina scale lifetime. Furthermore, altering the deposition order of Pt and Hf influences the cyclic oxidation resistance. The higher content of Hf on the alumina scale of NiCoCrAl alloys resulted in the formation of spinel (NiCr 2 O 4). The spinel as a second phase in the oxide layer of NiCoCrAl + Hf alloys led to the internal oxidation. Internal defects were caused by the diffusion of Al during the oxidation process. Due to the phase transformation and grains growth, the internal stress of alumina scale increased, so that the spallation occurred on the surface of NiCoCrAl + Pt alloys. Pt particles agglomerated on the NiCoCrAl + Hf + Pt alloy surface to form irregular particles, and a lower TGO growth rate has been observed for the NiCoCrAl + Hf + Pt alloy. Graphical abstract Unlabelled Image Highlights • Altering the deposition order of Pt/Hf can improved the oxidation resistance. • Over-doping of Hf on the alumina scale resulted in the formation of spinel. • Internal defects are caused by the diffusion of Al and phase transformation. • A lower TGO growth has been observed for the NiCoCrAl + Hf + Pt alloy. [ABSTRACT FROM AUTHOR]

FeCrAl-based oxide dispersion-strengthened alloys are currently being considered for accident-tolerant fuel cladding applications. In this study, the microstructure of alumina scales that formed on the alloys after oxidation at 1100 °C and 1200 °C, in an atmosphere containing water vapor, was investigated. The alumina grain growth and grain boundary segregation were analyzed using multiscale techniques such as oxidation kinetics, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, and backscattered electron diffraction, thus evaluating the diffusion of the reactive elements and alumina grain structure at different scales. A mechanism supporting the development of the alumina grain structure was proposed. • The grain boundary segregation of reactive elements was compared and discussed. • The role of reactive elements in the growth of alumina layer was described. • A mechanism supporting the development of the alumina grain structure was proposed. [ABSTRACT FROM AUTHOR]

The oxidation behaviour of thermal barrier coatings with a NiCoCrAlY bond coat at 1050 °C in different atmospheres was investigated. Water vapour contributed to fewer defects and lesser internal growth of Al 2 O 3 in the bond coat. Additionally, the presence of excess water vapour in the air + water vapour atmosphere suppressed the local inward oxide growth. Electron backscatter diffraction and transmission electron microscopy analyses were performed to investigate the influence of water vapour on the grain growth of alumina. The grain size of Al 2 O 3 depended on the water vapour content. The driving forces for the morphological distribution of voids in various atmospheres were discussed. • Air + water vapour atmosphere contributes to fewer voids within TGO. • The grain growth of Al 2 O 3 is governed by the test atmospheres. • Local inward oxide growth is suppressed by the high levels of water vapour. • Water vapour tends to suppress the formation and/or growth of voids in TGO. [ABSTRACT FROM AUTHOR]