Effect of lamellar microstructure on oxidation kinetics of Fe3Al sintered by hot isostatic pressing

The present paper focuses on the investigation of the relationship between microstructure of Fe₃Al prepared by hot isostatic pressing (HIP) and kinetics of alumina layer formation during oxidation at 900 °C, 1000 °C and 1100 °C. As prepared HIPed Fe₃Al sample reveals lamellar microstructure with inhomogeneous Al distribution which originates from the preliminary mechanical activation of Fe-Al mixture. At 900 °C, Fe₃Al oxidation is characterized by selective growth of very rough alumina layer containing only transient aluminium oxides. In addition to these transient oxides, α-Al₂O₃ stable phase is formed at 1000 °C. At the highest temperature (1100 °C), continuous and relatively smooth alumina layer mainly contains fine crystallites of α-Al₂O₃. The initial lamellar structure and phase inhomogeneity in as-HIPed Fe₃Al samples are supposed to be the main factors that determine observed peculiarities after Fe₃Al oxidation at 900 °C and 1000 °C.     

Early high temperature oxidation behaviour of Ti(C,N)-based cermets in air

Isothermal oxidation behaviour of two Ti(C,N)-based cermets (TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32-x)Ni-xCr, x = 0 and 6.4 wt%) was investigated in air at 800-1100 °C up to 2 h. Mass gains exhibited neither linear nor parabolic law during isothermal oxidation. The oxide scales formed at 800-1100 °C were multi-layered, consisting of NiO outerlayer, NiTiO₃ interlayer and TiO₂-based innerlayer. The internal oxidation zones formed at 1000-1100 °C consisted of Ti-based, Ni-based and Mo-based complex oxides. Cermet with 6.4 wt% Cr exhibited superior oxidation resistance, due to the presences of Cr_0.17Mo_0.83O₂ in TiO₂-based innerlayer of the oxide scale and Cr-rich Ti-based complex oxide in the internal oxidation zone.     

Comparative investigation of Al- and Cr-doped TiSiCN coatings

The aim of this work was a comparative investigation of the structure and properties of Al- and Cr-doped TiSiCN coatings deposited by magnetron sputtering of composite TiAlSiCN and TiCrSiCN targets produced by self-propagating high-temperature synthesis method. Based on X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy data, the Al- and Cr-doped TiSiCN coatings possessed nanocomposite structures (Ti,Al)(C,N)/a-(Si,C) and (Ti,Cr)(C,N)/a-SiC_xN_y/a-C with cubic crystallites embedded in an amorphous matrix. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum at 1000, 1100, 1200, and 1300 °C or in air at 1000 °C for 1 h. The results obtained show that the hardness of the Al-doped TiSiCN coatings increased from 41 to 46 GPa, reaching maximum at 1000 °C, and then slightly decreased to 38 GPa at 1300 °C. The Cr-doped TiSiCN coatings demonstrated high thermal stability up to 1100 °C with hardness above 34 GPa. Although both Al- and Cr-doped TiSiCN coatings possessed improved oxidation resistance up to 1000 °C, the TiAlSiCN coatings were more oxidation resistant than their TiCrSiCN counterparts. The TiCrSiCN coatings showed better tribological characteristics both at 25 and 700 °C and superior cutting performance compared with the TiAlSiCN coatings.     

Effect of dysprosium addition on the cyclic oxidation behaviour of CoNiCrAlY alloy

The cyclic oxidation behaviour of a Co32Ni21Cr8Al0.6Y (wt.%) alloy with and without the addition of 0.2 wt.% dysprosium was investigated at 800 and 1100 °C in static laboratory air. The Dy-containing alloy showed a faster θ- to α-alumina transformation and significantly less weight gain than Dy-free alloy. Under cyclic oxidation at 1100 °C, Dy addition produced a continuous and protective Al₂O₃ scale. The Dy-free alloy exhibited poor oxidation resistance. Scale spallation led to the development of a complex oxide scale and internal precipitation: (Al,Cr)₂O₃ on the surface, followed by a Al₂O₃ layer, then (Al,N) precipitates alone beneath the external scale.     

Effects of electrical discharge surface modification of superalloy Haynes 230 with aluminum and molybdenum on oxidation behavior

The effects of the electrical discharge alloying (EDA) process on improving the high temperature oxidation resistance of the Ni-based superalloy Haynes 230 have been investigated. The 85 at.% Al and 15 at.% Mo composite electrode provided the surface alloying materials. An Al-rich layer is produced on the surface of the EDA specimen alloyed with positive electrode polarity, whereas, many discontinuous piled layers are attached to the surface of the EDA superalloy when negative electrode polarity is selected. The oxidation resistance of the specimen alloyed with positive electrode polarity is better than that of the unalloyed superalloy, and the effective temperature of oxidation resistance of the alloyed layer can be achieved to 1100 °C. Conversely, the oxidation resistance of the other EDA specimen alloyed with negative electrode polarity is even worse than that of the unalloyed superalloy.     

Preparation and cyclic oxidation of gradient NiCrAlYRe coatings on Ni-based superalloys

Arc ion plating (AIP) and chemical vapor deposition (CVD) aluminizing were combined together to produce a gradient NiCrAlYRe coating. Elements in this coating were chemically graded with Al enrichment in the outer zone and Cr enrichment in the intermediate zone. Cyclic oxidation tests were performed at 1100 °C for up to 22 cycles. The results showed that the gradient NiCrAlYRe coatings performed better resistance to spallation and Al depletion than the conventional NiCrAlYRe coatings. This favorable oxidation behavior was attributed to the high Al distribution in the near surface region of the gradient coating.     

Cyclic oxidation behaviour of electrodeposited Ni3Al-CeO2 base coatings at 1050 °C

This paper presents the cyclic oxidation behaviour of electrodeposited pure, nano CeO₂ (9-15 nm)- and micron CeO₂ (5 μm)-modified Ni₃Al coatings on Fe-Ni-Cr substrate at 1050 °C for periods up to 500 h. The pure Ni₃Al coating had a marginal resistance to cyclic oxidation at 1050 °C, while the CeO₂-dispersed Ni₃Al coatings showed much better cyclic oxidation resistance. This difference was attributed to many beneficial effects of CeO₂ including changing the growth mechanism of α-Al₂O₃ scale, reducing the growth rate of the scale, improving mechanical properties of the scale, and reducing void formation at the scale/coating interface and at the scale-grain boundaries.     

Effect of alloying with Al on oxidation behavior of MoSi2 coatings at 1100 °C

MoSi₂ - 0, 15.3, 22, and 29.3 at.% Al coatings were prepared on the nickel-based super-alloy substrates by electro-thermal explosion ultrahigh speed spraying technology. The analysis showed that the coatings had fine microstructure with grain sizes ranging from 0.5 to 2 μm. The bonding between coating and substrate was typically metallurgical cohesion. The oxidation resistance of the coating was further studied at 1100 °C in air. Al alloyed in MoSi₂ coatings increased the oxidation resistance of the coatings, and the oxidation resistance of MoSi₂-15.3 at.% Al coating was higher than the other two MoSi₂–Al coatings. This suggested the oxidation resistance might have close relations to the obtained grain size.     

Ti基与Cr基氮化物涂层的抗氧化性能

选用TiN,TiAlN,CrN和CrAlN 4种涂层材料,使用电阻炉对试样加热并保温,进行抗氧化性能实验,利用SEM、EDS和XRD获得了氧化结果。结果表明:Ti基涂层的氧化机制以O原子向涂层内部扩散为主;Cr基涂层的抗氧化机制为N原子和Cr离子向涂层表面的扩散所形成的微孔诱发的氧化;Cr基涂层比Ti基涂层具有较好的抗氧化性;Al的加入使得TiAlN与CrAlN涂层的氧化性能和高温后硬度提高,特别是CrAlN氧化后生成的致密Cr2O3和Al2O3混合氧化物使其抗氧化性能达到最优;氧化及涂层与基体的热涨失配使得几类涂层最终开裂失效;四种涂层的抗氧化能力为CrAlN>TiAlN>CrN>TiN。     

Effect of cracking in diffusion aluminide coatings on their cyclic oxidation performance on Ti-based IMI-834 alloy

Diffusion aluminide coatings improve the high temperature oxidation resistance of Ti-alloys. This study evaluates the oxidation resistance of a Al₃Ti type aluminide coating and a Pt-aluminide coating on Ti-alloy IMI-834, at 650, 750 and 850 °C under cyclic oxidation conditions in air. Both coatings provide good oxidation resistance, however, the extent of through-thickness cracking in coating and localized oxidation degradation of substrate increases with thermal cycling. At high temperature of 850 °C, TiO₂ outgrowths emanate from these cracks, resulting in a prominent mud-crack pattern on the surface. The possible effect of such cracking on long-term oxidation properties of coatings has been discussed.     

Synthesis and oxidation behavior of platinum-enriched γ + γ′ bond coatings on Ni-based superalloys

Simple Pt-enriched γ + γ′ coatings were synthesized on René 142 and René N5 Ni-based superalloys by electroplating a thin layer of Pt followed by a diffusion treatment at 1150-1175 °C. The Al content in the resulting γ + γ′ coating was in the range of 16-19 at.% on superalloys with 13-14 at.% Al. After oxidation testing, alumina scale adherence to these γ + γ′ coatings was not as uniform as to the β-(Ni,Pt)Al coatings on the same superalloy substrates. To better understand the effect of Al, Pt and Hf concentrations on coating oxidation resistance, a number of Ni-Pt-Al cast alloys with γ + γ′ or β phase were cyclically oxidized at 1100 °C. The Hf-containing γ + γ′ alloys with 22 at.% Al and 10-30 at.% Pt exhibited similar oxidation resistance to the β alloys with 50 at.% Al. An initial effort was made to increase the Al content in the Pt-enriched γ + γ′ coatings by introducing a short-term aluminizing process via chemical vapor deposition or pack cementation. However, too much Al was deposited, leading to the formation of β or martensitic phase on the coating surface.     

High-Temperature Oxidation of an Aluminized NiCr Alloy formed by a Magnetron-Sputtered Al Diffusion Coating

Aluminium diffusion coatings were obtained on Ni–20Cr substrate by sputtering an aluminium film, followed by a two stage diffusion treatment in an argon inert gas atmosphere (first stage at 600°C, second at 900 or 1100°C). Aluminides obtained at 900°C and 1100°C are close to those obtained by pack cementation process with high aluminium activity. These diffusion coatings are able to develop alumina scales during isothermal oxidation at high temperatures, whereas the untreated substrate had a chromia-forming behaviour. The weight gain recorded at 1100°C on coated sample is then smaller than the one of uncoated NiCr at 950°C. Presence of chromium was detected in the diffusion coating and Cr-rich precipitates were observed at the diffusion coating/substrate interface. After oxidation at 900°C and 1100°C, only α-Al₂O₃ was revealed by XRD. An intermediate scale with a “whiskered” morphology could however be observed after 48 hr oxidation at 900°C. After 100 hr of oxidation at 1100°C, the Ni_xAl_y diffusion phases were no longer detectable and the upper part of the oxide scale spalled away during cooling. Large cavities appeared at the initial location of the diffusion coating/substrate interface.     

Cyclic oxidation of high temperature coatings on new γ′-strengthened cobalt-based alloys

An emerging class of cobalt-based γ′-strengthened alloys promises higher temperature capabilities compared to current Ni-base superalloys commonly used in aerospace and power generation applications. As with Ni-base alloys, high temperature coatings that enhance environmental resistance are desirable. Single crystal samples of Co–9.2Al–9.0W and Co–7.8Al–7.8W–4.5Cr–2.0Ta (at.%) were coated with vapour phase nickel aluminide and MCrAlY. Samples were subjected to cyclic oxidation at 1100 °C with 300–450 1-h cycles. Compared to NiAl-based coatings, the MCrAlY coatings exhibited superior adherence and an interdiffusion zone free of detrimental intermetallic phases. Evolution of microstructure during cycling is discussed with reference to the available thermodynamic data.     

Ion-plated Al-Al2O3 films as diffusion barriers between NiCrAlY coating and orthorhombic-Ti2AlNb alloy

Ion-plated Al-Al₂O₃ cermet films were fabricated as diffusion barriers between NiCrAlY coating and orthhombic-Ti₂AlNb alloy. The oxidation and interdiffusion behaviour of coatings with and without diffusion barrier were investigated in isothermal and cyclic oxidation tests at 800 °C. The results indicated that substantial interdiffusion and rapid oxidation degradation occurred in the coated specimens without diffusion barrier. With Al-Al₂O₃ diffusion barriers, deferred interdiffusion and improved oxidation resistance was observed. Among them, duplex coating containing 1Al-Al₂O₃ interlayer exhibited the best performance. Coefficient of diffusion hindering and factor of reaction hindering were proposed to compare and quantify the efficiency of the diffusion barriers.     

The isothermal and cyclic oxidation behaviour of two Co modified aluminide coatings at high temperature

The isothermal and cyclic oxidation behaviour of two Co modified aluminide coatings together with the simple aluminide coating were performed at 1000 °C and 1100 °C. All the three coatings show a much lower oxidation rate compared with the bare alloy. Results also indicate the addition of Co to the aluminide coating decreases the oxidation resistance slightly. It can be ascribed to that Co is easier to be oxidized than Ni at high temperature, and the Cr(W) rich phases which could act as a diffusion barrier are less in the coating with higher Co content.     

The structure and high temperature corrosion performance of medium-thickness aluminide coatings on nickel-based superalloy GTD-111

Simple and Si-modified aluminide coatings having medium-thickness (40-60 μm) have been applied on the superalloy GTD-111 by a slurry technique. Hot corrosion and cyclic oxidation performance of the uncoated and the coated superalloy were investigated by exposing samples to a molten film of Na₂SO₄-40 %wt NaVO₃-10%wt NaCl at 780 °C and 1 h cyclic oxidation at 1100 °C in air, respectively. The presence of silicon in the aluminide structure increased the oxidation resistance by a factor of 1.7 times. In addition, a SiO₂-containing scale, which formed on the Si-containing coating surface, was stable during of the hot corrosion testing.     

Cyclic oxidation and mechanical behaviour of slurry aluminide coatings for steam turbine components

The excellent steam oxidation resistance of iron aluminide coatings on ferritic steels at 650 °C has been demonstrated both by laboratory tests and field exposure. These coatings are formed by the application of an Al slurry followed by diffusion heat treatment at 700 °C for 10 h. The resulting microstructure is mostly composed of Fe₂Al₅ on top of a much thinner FeAl layer. This coating exhibits perpendicular cracks due to thermal expansion mismatch between coating and substrate. However, these stress relieving cracks do not seem to have an effect on the mechanical properties of the substrate. Cyclic oxidation, creep resistance and TMF testing of these coatings at 650 °C indicate excellent performance.     

Preparation and oxidation behaviour of an AlSiY diffusion coating on a Ni-based single crystal superalloy

An AlSiY diffusion coating was developed by a combined approach of arc ion plating and subsequent diffusion treatment. Cyclic and isothermal oxidation behaviour of the coating specimens was tested in a muffle furnace at 1000 °C and 1100 °C. The results confirmed an enhanced performance of the diffusion coating due to the presence of β-NiAl enriched outer layer. The microstructure change and the oxidation behaviour of the diffusion coating are discussed.     

The effect of Si additions on the high temperature oxidation of a ternary Ni-10Cr-4Al alloy in 1 atm O2 at 1100 °C

The oxidation of four Ni-10Cr-ySi-4Al alloys was studied at 1100 °C to examine the effects of Si additions (from 2 to 6 at.%) on the behavior of the alloy Ni-10Cr-4Al. Addition of 2 at.% Si prevented completely nickel oxidation, but could not form alumina scales. Larger Si additions produced alumina only over part of the alloy surface (about 20% with 4 at.% Si and 30% with 6 at.% Si), but could not prevent completely the internal oxidation of Al. The results are interpreted by extending to quaternary alloys the mechanism of the third-element effect already proposed for ternary alloys.     

Effect of chromium on the oxidation resistance of TiAl intermetallics

The effect of 10 at.%Cr on the oxidation resistance of TiAl intermetallic compound at 800–1100°C in air was investigated. The results indicated that 10 at.%Cr equally substituting for Ti and Al in TiAl alloy had duplex effects on the isothermal kinetics of DAL At lower temperatures (800–900°C), Cr increased the oxidation rates as a result of the doping effect of Cr in the scale and at higher temperatures (1000–1100°C), especially at 1100°C, Cr significantly reduced the oxidation rates as a result of the formation of a continuous Al₂O₃ film on the surface. 10 at.%Cr only substituting for Ti in TiAl alloy remarkably reduced the oxidation rates at all temperatures by about two orders of magnitude. Moreover, 10 at%Cr significantly improved the cyclic-oxidation rsistance of TiAl alloy.