Effect of cooling rate on oxidation resistance and powder ignition temperature of AM50 alloy with addition of yttrium

This paper focused on the effect of cooling rate on oxidation resistance and ignition temperature (T_i) of AM50 alloy. Y addition of 0.0 wt%, 0.15 wt%, 0.28 wt%, 0.45 wt% and 1.00 wt%, respectively was added to the AM50 alloy. The result showed that the oxidation resistance was directly affected by the microstructure. Rapid solidification (RS) had a positive effect on improving the oxidation resistance. It is noticeable that no Al₂Y intermetallic compound was found in the microstructure after RS. Elemental Y dissolved in the solid solution increased with increasing Y addition after RS. It is confirmed that Y addition dissolved in the solid solution and phase distribution were key factors for improving the oxidation resistance.     

Long-term atmospheric corrosion behaviour of aluminium alloys 2024 and 7075 in urban, coastal and industrial environments

Atmospheric corrosion of alclad and extruded 2024 and 7075 were investigated by weight loss, loss in mechanical properties and depth of pitting over 20 years. The results demonstrated the inner cladding layer on alclad ones had higher corrosion resistance. After 20 years exposure, the cladding had not been penetrated by pitting and those alclads retained their mechanical properties well. Exfoliation occurred on extruded ones in coastal and industrial atmospheres. Especially in coastal atmosphere extruded 2024 suffered severe exfoliation and experienced rapid deterioration of mechanical properties. Furthermore, morphology and chemical compositions of corrosion products were analysed by SEM, XRD and EDS.     

Isothermal oxidation behaviour of Nb-W-Cr Alloys

A study of the effect of Cr content on the microstructure and isothermal oxidation behaviour of four alloys from the Nb-Cr-W system has been performed. Selection of specific alloy compositions has been based on the ternary isothermal sections. Oxidation experiments were conducted in air at 900 and 1300 °C for 24 h under isothermal conditions. Weight gain per unit area as function of the temperature has been used to evaluate the oxidation resistance. The phases present in the alloys and the oxide scales were characterized by XRD, SEM, and EDS. Microstructure consists of Nb solid solution and NbCr₂, Laves phase. The oxidation kinetics follows a parabolic behaviour at 1300 °C; the addition of 30% Cr resulted in the significant reduction of the parabolic oxidation rate. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates in comparison to alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄ and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content and the oxidation temperature. The characterization results delineate the effect of the Cr content on the oxidation mechanisms of these alloys that represent a promising base for high-temperature alloy development.     

A novel ultrafine-grained Ni3Al with increased cyclic oxidation resistance

An ultrafine-grained (UFG) Ni₃Al was fabricated by annealing an electrodeposited Ni–Al composite in vacuum at 600 °C for 2 h. The UFG Ni₃Al, compared to a compositional-similar but coarse-grained (CG) alloy prepared by arc-melting, exhibited a greatly increased cyclic oxidation resistance at 900 °C. Microstructural investigation showed that the CG alloy grew a scale with a high susceptibility to buckling and cracking because of the formation of large voids at the scale/metal interface, but that the UFG alloy grew an adherent scale, because its typical structure prevented the formation of the interface void during oxidation.     

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.     

Stability of MnCr2O4 spinel and Cr2O3 in high temperature carbonaceous environments with varied oxygen partial pressures

In this investigation, Cr₂O₃ and MnCr₂O₄ were comparatively tested at 1050 °C in carbonaceous environment with varied oxygen partial pressures. MnCr₂O₄ exhibits much better resistance to carbonaceous attack than Cr₂O₃. The carburization rate of MnCr₂O₄ decreases sharply with increasing oxygen partial pressures. The oxygen partial pressures have less effect on the carburization resistance of Cr₂O₃. The increased resistance of MnCr₂O₄ to carburization is attributed to the dissolution of MnO into Mn-Cr-O spinel lattices with elevated oxygen partial pressures, which retards the decomposition and carburization of Mn-Cr-O spinel. The thermodynamic equations defining the carburization stability of MnCr₂O₄ and Cr₂O₃ are modified.     

High temperature oxidation of metallic chromium exposed to eight different metal chlorides

The influence of eight different chlorides (BaCl₂, CaCl₂, KCl, LiCl, MgCl₂, NaCl, PbCl₂, and ZnCl₂) on the oxidation of metallic chromium powder was studied at four different temperatures (400 °C, 500 °C, 550 °C, and 600 °C) under dry conditions in synthetic air by using a DTA/TG-apparatus. BaCl₂, CaCl₂, and MgCl₂ did not react with chromium at any of the studied temperatures. ZnCl₂ evaporated already before the air was introduced. KCl, LiCl, NaCl, and PbCl₂ were all found to be reactive and to accelerate the oxidation of chromium. LiCl reacted only at 600 °C, whereas the other three chlorides mentioned above reacted from 500 °C upwards.     

Influence of hydrothermal treatment temperature on oxidation modification of C/C composites with aluminum phosphates solution by a microwave hydrothermal process

Carbon/carbon (C/C) composites were modified with an aluminum phosphates solution by a novel microwave hydrothermal (MH) process in order to improve their low temperature oxidation resistance. Results show that a H₃PO₄ or HPO₃ continuous molten layer with some regular, white cubic Al(PO₃)₃ crystallites are obtained on the surface of the modified composites. The anti-oxidation property of the composites after modification improves with the increase of the MH temperature from 393 to 473 K. The oxidation rate is almost constant after oxidation at 873 K for 6 h. The formation of annular structure of Al(PO₃)₃ is helpful to improve the oxidation resistance of the composites.     

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.     

Control of polymorphism in Al2O3 scale formed by oxidation of alumina-forming alloys

The selective oxidation of specific components in alumina-forming alloy such as CoNiCrAlY under precisely regulated oxygen partial pressures (P_O2) can be used to control polymorphism in Al₂O₃ scale formed on the alloy. Dense, smooth α-Al₂O₃ scale was formed rapidly by treatment at 1323 K under a thermodynamically determined P_O2, where both aluminum and chromium in the alloy were oxidized and elements such as cobalt and nickel were not oxidized. By contrast, under a higher P_O2 all the components in the alloy were oxidized, the transformation was obviously retarded, and (Co,Ni)(Al,Cr)₂O₄ was produced.     

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.     

Effect of low copper content and heat treatment on intergranular corrosion of model AlMgSi alloys

Certain 6000-series extrusions may develop susceptibility to intergranular corrosion (IGC) by improper heat treatment, especially if copper is present as an alloying element. Although occurrence of IGC in such cases is documented, the underlying mechanisms are not adequately explained. We present corrosion data for two model alloys, having different Cu content and Mg:Si ratio, showing that the susceptibility to IGC depended primarily on the Cu content and secondly on thermal processing. Low Cu samples (0.0005 wt.% Cu) were essentially resistant to IGC. High Cu samples (0.12 wt.% Cu), which were air cooled after extrusion, exhibited significant IGC. However, IGC susceptibility was reduced significantly as a result of artificial aging to peak strength. Water quenched high Cu samples were essentially resistant to IGC. However, slight IGC susceptibility was introduced after aging. Electron optical characterisation revealed Al₄Mg₈Si₇Cu₂ (Q-phase) grain boundary precipitates on all the variants susceptible to IGC. The susceptibility was attributed to microgalvanic coupling between Q-phase grain boundary precipitates (noble) and the adjacent depleted zone (active).     

Effects of temperature and the incorporation of W on the oxidation of ZrB2 ceramics

The effects of tungsten additions and temperature on the oxidation behavior of nominally pure ZrB₂ and ZrB₂ containing 4, 6 or 8 mol% of W after oxidation at temperatures ranging from 800 to 1600 °C were investigated. For pure ZrB₂, the protective liquid/glassy layer covering the surface as a result of oxidation was evaporated above 1500 °C. For (Zr,W)B₂ specimens, the liquid/glassy layer was present after exposure up to 1600 °C. The higher stability of the liquid/glassy phase in the W-containing compositions was attributed to the presence of tungsten in the liquid/glassy phase, resulting in improved oxidation resistance for ZrB₂ samples containing W.     

Experimental studies and modeling of the oxidation of multiphase niobium-base alloys

A three-phase oxidation model has been developed for study the oxidation behavior of multiphase alloys. This model has been utilized to identify the oxidation behavior of three-phase Nb-base alloys and successfully predicted the critical value for the exclusive formation of CrNbO₄. It has shown that the exclusive formation of CrNbO₄ on the Nb-base alloys can be achieved by the volume fraction increasing and the particle size decreasing for NbCr₂ phase. Moreover, the barrier effect of Nb₅Si₃ phase on the diffusion of Cr in Nb-base alloys has also been deduced in this work.     

Kinetics and mechanisms of non-isothermal oxidation of graphite in air

The non-isothermal oxidation kinetics and mechanisms of the graphite in air was studied by TG and SEM. The oxidation rate increases rapidly with the increase of temperature before 30% weight loss, then turns to steady, and decreases sharply with the increase of temperature after 85% weight loss. SEM observation shows that the oxidation starts from pores both in surface and interior, and the oxidation extent is nearly uniform through the whole body in the initial stage. Kinetics results represent the activation energy strongly depends on the weight loss. Combining SEM and kinetics analysis, the controlling mechanisms and activation energy of non-isothermal oxidation of graphite are obtained.     

Influence of water vapor on the oxidation behavior of aluminized coatings under low oxygen partial pressure

FeCrNi alloy after aluminizing was oxidized at 1000 °C in dry and humid (2.23 vol.% water) H₂. Experimental results showed that H₂ promotes the formation of θ alumina and its transformation to α alumina. The morphology of surface alumina coating does not change significantly, but the oxidation rate of the aluminized layer accelerates by the addition of water vapor. As a result, more cracks are found beneath the alumina layer when water vapor is present. The addition of water vapor seems having a favorable effect on the selective oxidation of Al and concentration of oxygen vacancy in the aluminized alloys.     

Effect of high temperature heat treatment on intergranular corrosion of AlMgSi(Cu) model alloy

Copper containing 6000-series aluminium alloys may become susceptible to intergranular corrosion (IGC) as a result of improper thermomechanical processing. Effect of cooling rate after solution heat treatment on the corrosion behaviour of a model AlMgSi(Cu) alloy of nominal composition (wt%) 0.6 Mg, 0.6 Si, 0.2 Fe, 0.2 Mn and 0.1 Cu was investigated. Slow cooling rates were simulated by isothermal treatment for predetermined times in lower temperature baths immediately after solution heat treatment. Treatment for 10-100 s at temperatures below 400 °C introduced susceptibility to IGC. Longer heat treatment at the same temperatures introduced susceptibility to pitting. A corrosion resistant time zone was found between the zones of IGC and pitting at temperatures lower than 350 °C. Quenching in water after solution heat treatment prevented IGC. IGC was related to microgalvanic coupling between the noble Q-phase (Al₄Mg₈Si₇Cu₂) grain boundary precipitates and the adjacent depleted zone. Pitting was attributed to coarse particles in the matrix. Possible mechanisms causing the corrosion resistant intermediate zone are discussed. The results indicate possible methods for obtaining increased corrosion resistance of similar alloys by proper thermal processing.     

Effects of ZrB2 and SiC dual addition on the oxidation resistance of graphite at 1600–2000 °C

The effects of ZrB₂ and ZrB₂ + SiC additions on the oxidation kinetics of graphite at 1600–2000 °C in air were investigated. The ZrB₂ + SiC dual addition improves the oxidation resistance of graphite more effectively than the ZrB₂ single addition, because the oxide scale formed on C–ZrB₂–SiC is denser and thinner due to the existence of glassy SiO₂. As the oxidation temperature increases, the oxidation rate of C–ZrB₂–SiC gradually increases and oxide scales with layered microstructures form on its surface due to the greatly enhanced active oxidation of SiC at higher temperatures.     

Influence of the surface activation and local pitting susceptibility on the AC-electrograining of aluminium alloys

AC electrograining of aluminium is strongly influenced by the surface microstructure. The mechanical and electrochemical properties of the sub-surface present in aluminium alloys affect the electrochemical reactions that prevail during electrograining. Etching pre-treatment of aluminium removes intermetallics and rolled-in oxides; as a result, the attack on the aluminium substrate starts with the initial cycles of the electrograining process. Local electrochemical investigations show differences in corrosion and passivation properties between rolled-in oxides and clean surfaces. The interface between rolled oxides and aluminium matrix acts as a weak point for pit initiation.     

Carburization behaviour of Mn–Cr–O spinel in high temperature hydrocarbon cracking environment

This investigation was initiated to understand chemical stability of MnCr₂O₄ spinel in high temperature carbonaceous reducing environments. MnCr₂O₄ and Cr₂O₃ were comparatively tested at 1050 °C for various time periods up to 100 h. It was found that the resistance to carburization of MnCr₂O₄ is better than Cr₂O₃, and the carbide conversion rate of MnCr₂O₄ was slower than Cr₂O₃. The more porous structure of Cr₂O₃ than MnCr₂O₄ after carburization was observed. MnCr₂O₄ spinel was proved to be more stable than Cr₂O₃ in carburizing environments according to the results of thermodynamic calculation. After further carburization, coke formation was built up in MnCr₂O₄ due to the catalytic effect of Mn₇C₃ converted from MnO.