Strength and modulus of a molybdenum-coated Ti-25Al-10Nb-3V-1 Mo intermetallic

The three- point bend strength, Young’s modulus, and vibrational damping of a plasma- sprayed molybdenum-coated Ti- 25Al- 10Nb- 3V- lMo intermetallic were measured. The bend strength of the intermetallic samples was significantly reduced as a result of the molybdenum coating. This decrease in the strength was attributed to cracks formed in the molybdenum coating during the plasma spraying process. Experimental measurements done using the piezoelectric ultrasonic composite technique (PUCOT) indicated that the modulus and vibrational damping of the coated samples were significantly higher than for the uncoated substrates. Thermal cycling of the molybdenum- coated intermetallic between 600 °C and room temperature revealed a saturation increase in the modulus with a corresponding decrease in the mechanical damping. This behavior was attributed to crack healing occurring in the molybdenum coating during the thermal cycling process.     

Effect of enamel coating on oxidation and hot corrosion behaviors of Ti-24Al-14Nb-3V alloy

The effect of enamel coating on the isothermal and cyclic oxidation at 900 °C in air and on the hot corrosion resistance of Ti-24Al-14Nb-3V in both 85% Na₂SO₄+15%K₂SO₄ and 15%NaCl+85% Na₂SO₄ molten mixed salts at 850 °C was investigated. The results indicated that Ti-24Al-14Nb-3V alloy exhibited poor oxidation resistance due to the formation of nonprotective Al₂O₃+TiO₂+AlNbO₄ scales and poor hot corrosion resistance due to the spallation of scales formed in molten Na₂SO₄+K₂SO₄ and NaCl+Na₂SO₄. Enamel coating suppressed the migration of oxygen and corrosive ions into the substrate to improve the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy. However, the dissolution of oxides components of the coating into the molten salts degraded enamel coating and the degradation of the coating involved a process by which Cl⁻ anion penetrated into the substrate through voids in the coating to accelerate corrosion of Ti-24Al-14Nb-3V alloy.     

High temperature corrosion mechanisms of certain new TiAl-based intermetallic alloys in an aggressive H2/H2O/H2S environment at 850 °C

High temperature corrosion behaviour of three TiAl-based intermetallic alloys - Ti-44Al-8Nb-1B, Ti-46Al-8Nb-1B and Ti-48Al-2Nb-2Cr-1B (at.%) - was studied in an environment of H₂/H₂S/H₂O yielding p_S2 ∼ 6.8 × 10⁻¹ Pa and p_O2 ∼ 1.2 × 10⁻¹⁵ Pa potentials at 850 °C. The kinetic results obtained by a discontinuous gravimetric method indicate that increase in Al and Nb concentrations led to enhanced high temperature corrosion resistance, the corrosion resistance decreasing in the order: Ti-46Al-8Nb-1B > Ti-44Al-8Nb-1B > Ti-48Al-2Nb-2Cr-1B. The scale development studies using SEM, TEM, EDX, WDS and XRD confirmed the formation of a multilayered scale on all materials. An outer layer consisting of TiO₂ existed beneath which an Al₂O₃ layer was present. Then a layer of TiO₂ formed again, below which an Al-enriched NbAl₃ was observed. A TiS layer was found beneath the NbAl₃ layer. The formation of TiS led to the development of a NbAl₃ band between the multilayered scale and the substrate.     

Effect of heat treatment on the intermetallic layer of cold sprayed aluminum coatings on magnesium alloy

Dense and thick pure aluminum coatings were deposited on AZ91D-T4 magnesium substrates using the cold spray process. Heat treatments of the as-sprayed samples were carried out at 400 °C using different holding times. The feedstock powder, substrate and coating microstructures were examined using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) as well as Vickers microhardness analysis. The results demonstrate that aluminum coatings having dense and uniform microstructure can be deposited successfully using a relatively large feedstock powder. It has been identified that the intermetallics Al₃Mg₂ (γ phase) and Mg₁₇Al₁₂ (β phase) were formed at the coating/substrate interface during heat treatment. The growth rate of these intermetallics follows the parabolic law and the γ phase has a higher growth rate than the β phase. The thickness of the Mg₁₇Al₁₂ and Al₃Mg₂ intermetallic layers has reached 83 μm and 149 μm, respectively. This result is almost 45% higher than what has been reported in the literature so far. This is attributed to the fact that T4 instead of as cast Mg alloy was used as substrate. In the T4 state, the Al concentration in the Mg matrix is higher, and thus intermetallic growth is faster as less enrichment is required to reach the critical level for intermetallic formation in the substrate. The AZ91D-T4 magnesium substrate contains single α phase with fine clusters/GP-zones which is considered beneficial for the intermetallic formation as well as the intimate contact between the coating/substrate interface and the deformed particles within the coating.     

Characterisation and oxidation of LVOF sprayed Al2O3–40%TiO2 coating on superni 601 and superni 718 superalloys at 800 and 900°C

Failure of components due to high temperature oxidation is the major degradation mechanism in boiler and gas turbine industries. Superalloys having superior mechanical properties and creep resistance are used in these applications but lack resistance to oxidation under aggressive environments. Protective coatings are used to improve their oxidation resistance in such applications. In the present investigation, Al₂O₃–40%TiO₂ coating was deposited on superni 718 and superni 601 superalloys by low velocity oxy fuel process. The as sprayed coating was characterised for microhardness, surface roughness, scanning electron microscopy and X-ray diffraction analysis. High temperature oxidation behaviour of Al₂O₃–40%TiO₂ coated and uncoated superni 718 and superni 601 superalloys has been evaluated at the elevated temperatures of 800 and 900°C for total duration of 50 cycles under cyclic conditions. Each cycle consisted of keeping the samples for 1 h at the elevated temperature followed by 20 min cooling in ambient air. Al₂O₃–40TiO₂ coating in the as sprayed condition showed the presence of Al₂O₃–TiO₂, α-Al₂O₃, TiO₂ as the main phases. Al₂O₃–40%TiO₂ coating on superni 718 and superni 601 superalloys has shown a lower oxidation rate as compared to those of uncoated superalloys. However, the oxidation rate of the coating was not steady due to the occurrence of spallation/sputtering at various stages. The coating was found adherent on the substrate superalloys throughout the study.     

Experimental investigation on tool wear characteristics of PVD and CVD coatings during face milling of Ti6242S and Ti-555 titanium alloys

Ti6242S and Ti-555, as two typical titanium alloys, are often used to manufacture high-temperature aeroengine parts and landing gear components, respectively. They have different chemical composition and microstructure, which make them have different mechanical properties, and also affect their machinability. In this paper, face milling experiments were carried out to evaluate the wear performance by using CVD-Ti(C, N) + Al₂O₃ + TiN, PVD-(Ti, Al)N + TiN coated and uncoated tools. The results show that Ti-555 has the worse machinability than that of Ti6242S. When milling Ti6242S, all tools suffered adhesive wear and diffusion wear; the wear of Ti(C, N) + Al₂O₃ + TiN coated tool was more serious than that of other tools due to the blunt cutting edge; (Ti, Al)N + TiN coated tool suffered micro chipping and coating peeling with the minimal wear loss. When milling Ti-555, uncoated tool suffered serious chipping, abrasive wear and adhesive wear; Ti(C, N) + Al₂O₃ + TiN coated tool suffered serious chipping and coating peeling with short tool life; (Ti, Al)N + TiN coated tool suffered coating peeling, adhesive wear and diffusion wear. Overall, (Ti, Al)N + TiN coated tools have the longest tool life and are preferred for face milling of Ti6242S and Ti-555 titanium alloys.     

Preparation and oxidation resistance of B2O3-coated boron-modified carbon foams

To improve the oxidation resistance of boron-modified carbon foams, the B₂O₃ coating was prepared on boron-modified carbon foams by low-cost slurry method. The microstructures and phase compositions of the coated carbon foams were characterized by scanning electron microscopy and X-ray diffraction, respectively. Oxidation resistances of uncoated and coated boron-modified carbon foams were investigated at 873 K in air. The results showed that as-received B₂O₃ coating could protect boron-modified carbon foams from oxidation at 873 K. B₂O₃-coated carbon foam doped with 7% B₂O₃ (mass fraction) (BO-7) had better oxidation resistance, exhibiting mass loss of 17.40% after oxidation at 873 K for 120 min. The melting glass layer formed on the surface of BO-7 could prevent oxygen from diffusing into boron-modified carbon foams substrate during oxidation to some extent.     

The Oxidation of iridium-aluminum and iridium-hafnium intermetallics at temperatures above 1550°C

The oxidation behavior of iridium-aluminum and iridium-hafnium intermetallics has been investigated in various oxygen pressures and at temperatutes between 1550 and 1800°C. The hafnium concentration necessary for the formation of a continuous external HfO₂ scale is above 50 at.% hafnium. The aluminum concentration necessary for the formation of a continuous external Al₂O₃ scale is above 55 at. % aluminum, which is the aluminum-rich boundary of the IrAl intermetallic. Thus, it appears that the IrAl₂₅ intermetallic necessary for the formation of a protective, external Al₂O₃ scale in the iridiumaluminum system. The activation energy for the growth of Al₂O₃ on iridiuim (60 at.%) aluminum intermetallic is in agreement with that determined pre viously for the NiAl intermetallic at lower temperatures. This suggests that similar process may control the Al₂O₃ scale growth on these two intermetallics.     

Erosion wear behaviour of plasma sprayed NiCrSiB/Al2O3 composite coating

In this work, 60 wt.% NiCrSiB–40 wt.% Al₂O₃ composite coating was produced on AISI 304 substrate material using the atmospheric plasma spraying technique. The coating surface has been characterised using a scanning electron microscope (SEM), optical microscope and X-ray diffractometer (XRD). The microhardness, porosity, density and surface roughness of the coating were measured. The adhesion strength of the coating was measured using pull off adhesion tester. The erosion behaviour of plasma sprayed coating was studied at 450 °C using hot air jet erosion testing machine. The erosion rate of coated and uncoated samples was evaluated at 30° and 90° erodent impact angles. The SEM images of the eroded samples were taken to analyse the erosion mechanism. The test results reveal that the coating protects the substrate at both 30° and 90° impact angles.     

Effect of yttria(Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) coating for high temperature oxidation resistance of 9Cr-1Mo steel

This paper investigates the effect of yttria (Y₂O₃) coating on high temperature oxidation behaviour of low alloy 9 Cr–1Mo steel. The superficial coating is Y₂O₃ was prepared for experimental investigation. The isothermal corrosion study of uncoated and coated specimens was carried out in air oxidation environment at 973 K for 8 h. The corrosion rate and reaction kinetics were studied and the post corroded scales were characterized in SEM, EDS and XRD. The results clearly indicate that Y₂O₃ coated specimen improves the high temperature oxidation resistance than uncoated specimens. The improvement of oxidation resistance in presence of Y₂O₃ coating can be attributed to the changed mechanism of scale growth from outer cation migration to inner anion migration and enhancement of scale adhesion with the substrate. Further, enhancement of scale adhesion with the substrate in case of Y₂O₃ coating also improves the oxidation resistance. The detail mechanism of the oxidation of Y₂O₃ coated and uncoated specimen is further discussed in this paper.     

Spark anodizing behaviour of titanium and its alloys in alkaline aluminate electrolyte

Spark anodizing of titanium, Ti-6Al-4V and Ti-15V-3Al-3Cr-3Sn in alkaline aluminate electrolyte produces highly crystalline anodic films consisting mainly of Al₂TiO₅ with α- and γ-Al₂O₃ as minor oxide phases, irrespective of substrate composition. However, the apparent efficiency for film formation decreases in the following order: Ti-6Al-4V, titanium and Ti-15V-3Al-3Cr-3Sn. A large amount of aluminium species are incorporated from the electrolyte, probably by plasma-chemical reaction, and become distributed throughout the film thickness. This distribution indicates that the electrolyte penetrates near to the film/substrate interface through the discharge channels. Thus, the outwardly migrating aluminium ions under a high electric field can be present even in the inner part of the anodic films. Voids are developed at the film/substrate interface, particularly on the vanadium-containing alloys, reducing the adhesion of the anodic film to the substrate.     

High-Temperature Oxidation of Ti-48Al-2Nb-2Cr and Ti-25Al-10Nb-3V-1Mo

The short-term oxidation behavior of a-TiAl alloy (Ti-48Al-2Nb-2Cr) was compared andcontrasted to that of an₂-Ti₃Al base(Ti-25Al-19Nb-3V 1Mo) alloy. Oxidation ofTi-25Al-10Nb-3V-1Mo was found to occur at a moderate rate at 800°C, in aN₂ + 20% O₂ environment. A largeincrease in the oxidation rate occurred above thistemperature. This large weight increase was attributedto a breakdown in the protective oxide scale on the surface of the₂ intermetallic alloy, therebypermitting rapid diffusion of oxygen and nitrogen to thesurface of the intermetallic. The oxidation rate of thisalloy at 1200°C was not significantly higher thanthe oxidation rate at 1000°C. In contrast, theoxidation rate of Ti-48Al-2Nb-2Cr remained low up to1200°C. At this temperature, a significant increasein oxidation was observed and was attributed to acceleratedoxygen diffusion through the ₂ phaseand increased solubility of oxygen in the gamma phase ofthe intermetallic microstructure. This weight increaseoccurred despite the fact that at 1200°C, theintegrity of the oxide layer formed on the surface ofthis alloy was maintained. The results of this studyillustrate the need for developing protectiveenvironmental coatings tailored to the individualintermetallic alloy.     

Using coated ceramic particles to increase wear resistance in high-speed steels

The use of chemical-vapor-deposition (CVD)-coated ceramic particle reinforcements in metal-matrix composites allows the control of reactivity at the particle/matrix interface. Wear-resistant, high-speed, steel-based composites containing uncoatedAl₂O₃ uncoated TiC, and CVD-coated A1₂O₃ were liquid-phase sintered and characterized using pin-on-disk wear testing. TiC or TiN CVD coating of Al₂O₃ resulted in a porosity decrease at the particle/matrix interface in addition to better ceramic/metal cohesion due to improved wettability. Lower wear rates were obtained with the composites containing TiC-or TiN-coated Al₂O₃.     

Pack Cementation Coatings for High-Temperature Oxidation Resistance of AISI 304 Stainless Steel

Aluminum and titanium are deposited on the surface of steel by the pack cementation method to improve its hot-corrosion and high-temperature oxidation resistance. In this research, coatings of aluminum and titanium and a two-step coating of aluminum and titanium were applied on an AISI 304 stainless steel substrate. The coating layers were examined by carrying out scanning electron microscopy (SEM) and x-ray diffraction (XRD). The SEM results showed that the aluminized coating consisted of two layers with a thickness of 450???m each, the titanized coating consisted of two layers with a thickness of 100???m each, and the two-step coatings of Al and Ti consisted of three layers with a thickness of 200???m each. The XRD investigation of the coatings showed that the aluminized coating consisted of Al₂O₃, AlCr₂, FeAl, and Fe₃Al phases; the titanized layers contained TiO₂, Ni₃Ti, FeNi, and Fe₂TiO₅ phases; and the two-step coating contained AlNi, Ti₃Al, and FeAl phases. The uncoated and coated specimens were subjected to isothermal oxidation at 1050?°C for 100?h. The oxidation results revealed that the application of a coating layer increased the oxidation resistance of the coated AISI 304 samples as opposed to the uncoated ones.     

Synthesis and electrochemical performance of YF<Subscript>3</Subscript>-coated LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode materials for Li-ion batteries

Spinel LiMn₂O₄ cathodes were coated with 1 mol% YF₃. X-ray diffraction (XRD) analyses showed that Y and/or F did not enter the lattice of the LiMn₂O₄ crystal. Transmission electron microscopy (TEM) showed that a compact YF₃ layer of 5–20 nm in thickness was coated onto the surface of LiMn₂O₄ particles. Scanning electron microscopy (SEM) observation showed that the YF₃ coating caused the agglomeration of LiMn₂O₄ particles. The cycling test demonstrated that the YF₃ coating can improve the electrochemical performance of LiMn₂O₄ at both 20 and 55°C. Moreover, YF₃-coated LiMn₂O₄ exhibited an improved rate capability compared with the uncoated one at high rates over 5C. The immersion test in electrolytes showed that YF₃-coated LiMn₂O₄ is more erosion resistant than the uncoated one.     

Two-step plasma electrolytic oxidation of Ti-15V-3Al-3Cr-3Sn for wear-resistant and adhesive coating

Ti-15V-3Al-3Cr-3Sn (Ti-15-3) is one of the important practical titanium alloys with high cold deformability and high mechanical strength, but its wear resistance is poor. This paper reports the formation of wear-resistant and adhesive ceramic coatings on Ti-15-3 by two-step plasma electrolytic oxidation (PEO). The PEO of Ti-15-3 has been carried out first in alkaline aluminate electrolyte to form a wear-resistant oxide layer and then in acid electrolyte containing both phosphoric acid and sulfuric acid to improve adhesion of the coating. The coating formed in the alkaline aluminate electrolyte is more than 10 μm thick, and highly crystalline. The main phase is Al₂TiO₅. This coating shows high wear resistance, but is not adherent to substrate due to the development of a number of voids and pores in the oxide layer close to the substrate. A new oxide layer with amorphous structure is formed next to the substrate in the subsequent PEO in the acid electrolyte, during which the voids are filled with a new oxide formed in the acid electrolyte, reducing the porosity. As a consequence, the adhesion of the coating is markedly improved without deteriorating the high wear resistance.     

Al2O3/ZrO2涂层在热等静压致密化TiAl基合金粉末过程中对碳钢包套的影响

由于焊接性能好,成本低,碳钢材料常被用于制作热等静压致密化TiAl基合金粉末的包套。在高温高压下,碳钢包套和TiAl基合金粉末通过原子扩散在扩散区形成脆性相,导致包套失效,并降低TiAl基合金压坯的致密度。为了确保碳钢包套在热等静压致密化TiAl基合金粉末过程中的可靠性,本文利用热喷涂的方法在20#钢包套内壁添加了Al2O3/ZrO2（A-Z）涂层,然后在 。在热等静压试验中,带有A-Z涂层的20#钢包套用于热等静压致密化Ti-46Al-2Cr-2Nb-(W, B)预合金粉末,其工艺为：1523K,2.5小时,130MPa+1603K,0.5小时,130MPa。为了对比,利用没有A-Z涂层的20#钢包套在1523K,3小时,130MPa的工艺参数开展了热等静压致密化试验。利用扫描电镜、电子探针等设施对获得的压坯进行了观测和分析。结果表明：A-Z涂层的加入可以防止脆性金属间化合物的形成。在热等静压过程中,20#钢包套中的Fe原子无法通过扩散的方式与TiAl基合金中的钛原子和铝原子相遇。因此,20#钢包套在热等静压过程中的可靠性得到了保证。此外,通过利用添加A-Z涂层的钢包套获得了完全致密的TiAl基合金压坯。压坯呈现出了近全片层类型的微观组织,其室温下的抗拉强度和延伸率也分别突破了590MPa和2.0%。     

Cyclic-oxidation behavior of TiAl and of TiAl alloys

The cyclic-oxidation behavior of (in w/o) Ti-36Al, Ti-35Al-0.1C, Ti-35Al-1.4V-0.1C and Ti-35Al-5Nb-0.1C was studied between 800 and 1000° C in air. A few experiments were also performed in oxygen. Scale spallation after oxidation in air occurs during cooling on TiAl, TiAl-C, and TiAl-V at or close to the metal/scale interface when a critical scale thickness has been achieved. This process repeats and can lead to a stratified scale. These three materials form scales composed of an inward-growing fine-grain mixture of TiO₂-Al₂O₃ and an outward-growing coarse-grain TiO₂ layer or TiO₂+Al₂O₃ mixture. The TiAl-Nb alloy had a significantly different behavior. The scale on this material grew very slowly because a protective Al₂O₃ layer formed at the metal/scale interface. This behavior resulted in much better resistance to spallation because the critical scale thickness was reached only after a much longer time, and is different from the behavior of the other three alloys. Oxidation in air leads to slight nitridation of the subsurface zone beneath the scale. In comparison to oxidation in air, oxidation in oxygen improves the cyclicoxidation behavior. Whereas the scale formed in air was uniformly thick over the entire surface, the scale grown in oxygen varied locally in structure and thickness. A large fraction of the surface was covered with a thin Al₂O₃ layer, while the remaining part formed a two-layer scale similar to that formed in air. The results are discussed briefly in the light of a recently published model for scale spallation under compressive stress, however, quantitative estimations are not possible due to a lack of relevant data.     

Influence of zinc on intermetallic compounds formed in friction stir welding of AA5754 aluminium alloy to galvanised ultra-high strength steel

In this study, lap joints between AA5754 and DP1000 ultra-high strength steels were produced by friction stir welding. In order to investigate the roles of zinc on intermetallic phase formation and joint properties, steel substrates were used, two being galvanised coated and one uncoated. Joint performance has been evaluated in term of maximum tensile shear loading. The effects of the process parameter, translational speed; chemical compositions; and intermetallic phase formation on the mechanical properties have been investigated. The results show that joints with a galvanised layer exhibit higher strength as compared to the non-coated steel. A thicker galvanised layer promotes the presence of zinc in the aluminium matrix, resulting in better joint properties. The level of zinc contents in the aluminium matrix depends on process temperature and material circulation characteristics. Two stable Al-rich intermetallic phases, Al₅Fe₂ and Al₁₃Fe₄, were detected at the interface regardless of the coating conditions.     

Hot corrosion of a plasma sprayed Ni3Al coating on a Ni‐base superalloy

Ni₃Al metallic powder was prepared by the mechanical mixing of pure nickel and aluminium powders and deposited on a Ni‐base superalloy namely Superni 75 by the shrouded plasma spray process. Hot corrosion behaviour of Ni₃Al coated and uncoated superalloy specimens was evaluated in a simulated environment of fossil fuel boilers comprising of Na₂SO₄–60%V₂O₅ by accelerated corrosion tests. The accelerated testing was done under cyclic conditions for 50 cycles, with each cycle consisting of 1 h heating in the silicon carbide furnace and cooling for 20 min in air. The coated superalloy was also subjected to air oxidation for 50 cycles to get an idea about its oxidation behaviour and adherence of the Ni₃Al coating and its oxide scale with the substrate superalloy. The oxide scales were characterised by X‐ray diffraction (XRD), scanning electron microscopy/energy dispersive X‐ray (SEM/EDAX) analyses and electron probe microanalysis (EPMA). The uncoated superalloy suffered an accelerated corrosion in the form of spallation of its oxide scale, whereas the oxide scale formed on the Ni₃Al coated specimen was intact during the air as well as the molten salt oxidation. Furthermore, the coating was found successful in preventing the internal oxidation of the substrate superalloy. The coating maintained its adherence to the substrate superalloy during the exposure to both the environments of the study. The XRD analysis revealed the presence of oxides like Al₂O₃, NiO and NiAl₂O₄ in the oxide scale of the coated superalloy. The XRD results were further supported by the SEM/EDAX and EPMA analyses.