Cyclic oxidation of aluminized Ti-14Al-24Nb alloy

Titanium aluminides are considered as replacements for superalloys in applications in gas turbine engines because of their outstanding properties. Ti₃Al has a superior creep strength up to 815° C, but has poor oxidation resistance above 650° C. Two approaches can be followed to improve the oxidation resistance of Ti₃Al above 650° C. One is alloying and the other obtaining a protective surface coating. Niobium was found to improve the oxidation resistance, when added as an alloying element. Recent investigations showed that a TiAl₃ surface layer considerably improves the oxidation resistance of titanium. In the present work, a TiAl₃ layer was obtained on a Ti-14Al-24Nb (wt%) alloy using a pack aluminizing process. The cyclic oxidation behaviour of aluminized and uncoated samples was evaluated.     

Comparative Evaluation on the In Vitro Biological Performance of Ti45Al8.5Nb Intermetallic with Ti6Al4V and Ti6Al7Nb Alloys

The corrosion resistance of Ti45Al8.5Nb intermetallic alloy in artificial saliva and its cytocompatibility was studied via electrochemical tests, scanning electron microscopy, ion release measurement, and MTT assay, with contemporary biomedical Ti6Al4V and Ti6Al7Nb alloys as comparison. The results demonstrate that the corrosion potential (E_corr) and the corrosion current density (i_corr) of the three experimental alloy samples are similar and there is no statistically significant difference among them (p > 0.05). The Al³⁺ ion releasing concentration for Ti45Al8.5Nb intermetallic and Ti6Al7Nb alloy after anodic polarization are close. The relative cell proliferation rates of the three experimental alloy extract groups are all over 90% at various cultivation periods (1, 3, and 5 d), and there is no obvious difference for the MG63 cell morphologies comparing with that of the negative group, reaching confluence after 5 d culture and showing well stretched, which indicates that Ti45Al8.5Nb intermetallic alloy has a good cytocompatibility with the Grade 1 RGR value (no toxicity) according to ISO 10993‐5: 1999.     

Oxidation resistance of diffusion coatings formed by pack-codeposition of Al and Si on γ-TiAl

Oxidation resistance of the aluminide and silicide diffusion coatings pack-deposited on -TiAl were studied in air over the temperature range of 800 and 850°C for up to 4596 h. The oxidation kinetics of the coatings was monitored by intermittent weight gain measurement at room temperature. The XRD and SEM/EDS techniques were used to identify the oxide scales formed during the oxidation process and to assess the thermal stability of the coatings at the oxidising temperatures. It was revealed that the TiAl₃ coating underwent preferential Al oxidation to form the Al₂O₃ scale in the early oxidation stage, which resulted in Al depletion and formation of TiAl₂ in the subsurface of the coating. The Al depletion could not be sufficiently compensated by Al diffusion from the inner layer of the coating and eventually, in the late oxidation stage, led to the Ti oxidation and formation of the TiO₂ phase in the scale. The preferential Si oxidation was the main oxidation mechanism for the coatings with an outer silicide layer and an inner TiAl₃ layer with the formation of SiO₂ as the stable oxide scale. The thermal stability of the coatings over the temperature range up to 850°C was discussed in relation to the high-temperature stability of diffusion couples of different coating layers.     

Influence of sol–gel derived Al2O3 film on the oxidation behavior of a Ti3Al based alloy

Al₂O₃ thin films were deposited on a Ti₃Al based alloy (Ti–24Al–14Nb–3V–0.5Mo–0.3Si) by sol–gel processing. Isothermal oxidation at temperatures of 900–1000 °C and cyclic oxidation at 800–900 °C were performed to test their effect on the oxidation behavior of the alloy. Results of the oxidation tests show that the oxidation parabolic rate constants of the alloy were reduced due to the applied thin film. This beneficial effect became weaker after longer oxidation time at 1000 °C. TiO₂ and Al₂O₃ were the main phases formed on the alloy. The thin film could promote the growth of Al₂O₃, causing an increase of the Al₂O₃ content in the composite oxides, sequentially decreased the oxidation rate. Nb/Al enriched as a layer in the alloy adjacent to the oxide/alloy interface in both the coated and uncoated alloy. The coated thin film decreased the thickness of the Nb/Al enrichment layer by reducing the scale growth rate.     

Microstructure evolution and oxidation behaviour of Tif/TiAl3 composites

In this paper, Ti_f/TiAl₃ composites were fabricated by infiltration–in situ reaction method and its oxidation behaviours were investigated by cyclic oxidation testing at 700 °C, 800 °C and 900 °C. The microstructure evolution and oxidation of Ti_f/TiAl₃ composites were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray diffraction (EDX). The reaction between Ti₃Al particles and Al was more violent than that of Ti fibres and Al. Ti₃Al/Al reaction consumed a large amount of Al and inhibited the reaction of Ti fibres indirectly. Reactant of Ti fibres was TiAl₃ at 700 °C, and four reaction layers surrounding Ti fibre (Ti₃Al, TiAl, TiAl₂ and TiAl₃ from inner to outside) were observed above 800 °C. The thickness of the total reaction layers increased little with temperature and time, while the thickness of inner reaction layers increased remarkably. A model corresponding to the microstructure evolution process was drawn schematically. Oxidation resistance of Ti_f/TiAl₃ composites decreased with increasing of temperature, and changed from cubic law at 700 °C to parabolic law at 900 °C. The oxidation weight gain of Ti_f/TiAl₃ composite was dominated by the exposed Ti fibres. Due to outward diffusion of Ti and Al element, the oxide of Ti fibre at 900 °C changed to mushroom-shape. Fortunately, when TiAl₃ was oxidized, a thin and continuous Al₂O₃ layer was formed, protecting matrix from further oxidation.     

Microstructure of interfacial reaction zone in SCS-6 SiC fibre reinforced super α2 composites

Abstract

The microstructure of the interfacial reaction zone in SCS-6 SiC/super α2 composites heat treated at 700°C for 3000 h was investigated by means of analytical transmission electron microscopy. The very fine grained reaction layer adjacent to the carbon coating of the SiC fibre was found to consist of two sub layers, determined to be (Ti, V)C and (Ti, V,Nb)₅Si₃. The second layer is (Ti,Nb)C with large equiaxed grainsfollowed by the third layer consisting of the (Ti,Nb)₃(Al,Si)C phase. This layer is separated from the matrix by a fourth layer with the phase composition (Ti,Nb)₅(Si,Al)₃. At some interface positions, the two layers of(Ti,Nb)C and (Ti,Nb)₃(Al,Si)C are separated by an additional layer of the (Ti,Nb)₃(Si,Al) phase. The thickening of the interfacial reaction zone at 700°C is mainly due to the layers of (Ti, Nb)₃(Al,Si)C and (Ti,Nb)₃(Si,Al). The growth of these two layers is probably responsible for the degradation of the mechanical properties of the composites.     

Nickel aluminide coating produced on γ-TiAl alloy by high activity aluminising process

Abstract

Ni aluminide diffusion coatings on the surface of γ-TiAl alloy were produced by electroplating a Ni layer followed by a single step high activity aluminising carried out in Ar+H₂ atmosphere with a mixture of Al, NH₄Cl and Al₂O₃ powders at 1000°C for 5 h. The effect of initial thickness for Ni layer on microstructure of produced Ni aluminide coating was highlighted. The thickness of initial Ni layer was changed to 4–20 μm. In the case of the Ni layer with thickness of 4 μm, only a little amount of NiAl phase was formed in a TiAl₃ matrix. However, the microstructure of coating, in the case of the Ni layer with thickness of 8 μm, consisted of an outer layer of two phases (NiAl+TiAl₃), an intermediate layer of TiAl₃ and an interdiffusion layer. For thicker initial Ni layers (16 and 20 μm), beside the latter coating microstructure, a continuous surface layer of NiAl phase was observed. Isothermal oxidation tests on these aluminide coatings reveal that the oxidation resistance of the aluminide coatings increases with increase in initial thickness of Ni layer.     

Effects of Electromagnetic Stirring,Shearing, and Extrusion on TiB2 and TiAl3 Particles in Al–5Ti–1B(wt.%) Alloy

Electromagnetic stirring induced metal flow and led to homogenous dispersion of TiAl₃ particles. Fragmentation mechanism induced by electromagnetic stirring also contributed to TiAl₃ particle refining. TiAl₃ particle size decreased with the increases of stirring temperature and time. Shearing force among different melt layers under the shear action of roll in the roll-shoe gap increased with the decrease of casting temperature and the increase of melt viscosity, and the fragmentation of TiAl₃ phase became obvious correspondingly. Under the optimal process parameters, Al–5Ti–1B(wt.%) alloy wire with excellent inner microstructure and high surface quality was produced. The average sizes of TiAl₃ are less than 20 µm, and TiB₂ phases are less than 0.5 µm, respectively. Al–5Ti–1B(wt.%) alloy wire manufactured by present method has a high refining ability on pure aluminum and an excellent ability of refining effectiveness.     

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract

The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO₂, intermediate Al₂O₃, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.     

Pack formation and long term oxidation kinetics of TiAl3 coating on γ-TiAl

Abstract

The aim of this study is to produce microcracking free TiAl₃ coatings on γ-TiAl alloy by the pack cementation process and to determine the long-term oxidation kinetics and thermal stability of the coating at high temperatures. It was shown that microcracking free coatings could be prepared in the AlCl₃ activated packs containing 4 wt-%Al as the depositing source. The conditions required for the formation of a microcracking free coating are discussed in relation to the pack chemistry at high temperatures. The TiAl₃ coatings formed were oxidised in air for more than 6200 h, during which weight gains were measured at regular intervals. The major oxide in the scale was Al₂O₃. It was observed that a TiAl₂ phase zone formed in the subsurface of the scale as a result of preferential oxidation of Al in the TiAl₃ coating. It was found that a linear relationship existed between the weight gain and logarithm of time of oxidation at 800°C: Δm _t = k _lln(αt+ 1). The thermal stability of the coating was assessed by measuring the growth kinetics of the TiAl₂ interlayer at the boundary between the TiAl₃ coating and substrate, which was determined to be d^1·4 = 0·1t (d and t in μm and h respectively) at 800°C using the experimental data measured over a diffusion annealing period of more than 6200 h.     

Effect of Vacuum Heat Treatment on the Microstructure and Tribological Property of Ti–6Al–4V Alloy with Cu Coating

The effect of vacuum heat treatment on the interface microstructure and tribological property of Cu-coated Ti – 6Al – 4V alloy is investigated herein. After the vacuum heat treatment process, a diffusion layer is formed at the interface between the Cu coating and the Ti – 6Al – 4V substrate. The formed intermetallic compounds at the interface between the Ti – 6Al – 4V substrate and Cu coating are CuTi₂, CuTi, Cu₄Ti₃, and β-Cu₄Ti. The activation energy of intermetallic compound growth in the diffusion zone of Cu-coated Ti – 6Al – 4V is 126.0 kJ mol⁻¹, and the pre-exponential factor is 0.1 m² s⁻¹. The tribological properties of the Cu-coated Ti – 6Al – 4V alloy are best when subjected to diffusion treatment at 700 °C for 300 min, with weight loss reduced by 58.2% compared to the Ti – 6Al – 4V alloy. The wear resistance of the Ti – 6Al – 4V alloy can be enhanced by Cu coating and vacuum diffusion heat treatment, and the formation of the Cu – Ti intermetallic compound contributes to this improvement. These findings offer new insights for further advancements in the tribological properties of titanium alloys.     

Properties of (Ti,Nb)Al–(Ti,Nb)5Si3 eutectic composite

Ti–40Al–5Si and Ti–39Al–5Si–2Nb (in at.%) alloys were studied as prospective high-temperature structural composites consisting of γ-(Ti,Nb)Al + α₂-(Ti,Nb)₃Al matrix and Ti₅Si₃ reinforcement. The alloys were prepared by arc melting under helium. Oxidation resistance was studied at 900 °C in air. Thermal stability of alloys was investigated by measuring room temperature hardness and compressive strength after long-term annealing at 900 °C. To prepare oriented composites, directional crystallization at rates of 5–115 mm/h was carried out by the floating zone technique. It was observed that the addition of 2% Nb to the Ti–40Al–5Si alloy does not modify eutectic structure. Niobium is almost uniformly distributed in all present phases. Both alloys show excellent oxidation resistance at 900 °C in air. The Nb-addition causes significant improvement of oxidation resistance due to the doping effect and increase of Al activity in the scales. Room temperature hardness and compressive strength of both as-cast alloys are similar – about 500 HV and 1600 MPa, respectively. Room temperature mechanical properties do not reduce significantly after 300 h annealing at 900 °C, due to a high morphological stability of eutectic silicides. Directionally solidified alloys consist of columnar Ti–Al grains elongated in crystallization direction and silicides. Niobium refines both Ti–Al grains and Ti₅Si₃ silicides. As a consequence, orientation and elongation of silicides in the Nb-containing alloy are reduced. In the Ti–Al–Si alloy directionally crystallized at 5–115 mm/h, the silicide interparticle spacing λ (in mm) is related to the crystallization rate R   (in mm/h) by a following expression: λ^1.33·R=0.32${\lambda }^{1.33}·R=0.32$. In the Nb-containing alloy, silicide interparticle spacing does not depend on the crystallization rate.     

In situ chemical vapour co-deposition of Al and Si to form diffusion coatings on TZM

Multilayer alumino-silicide and silicide coatings were formed by in situ chemical vapour co-deposition of Al and Si on TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy for improving its high-temperature oxidation resistance. MoSi₂ and Mo (Si, Al)₂ layers were formed in the inner and the outer layers, respectively in the case of alumino-silicide coating. Whereas silicide coating consisted of Mo₅Si₃ and MoSi₂ phases in the inner and the outer layers, respectively. 24–100-μm thick coatings were formed by optimizing the pack mixture of Al and or Si, NH₄F and Al₂O₃ powders and conducting the experiments at 1000 °C for 8–36 h. MoSi₂ layer showed a faster growth rate and presence of columnar grains. A small weight gain at the initial stages was observed during the oxidation tests of the coated samples under continuous or cyclic heating at 1300 °C in air. Neither cracks nor peeling of the coating layers were noticed after oxidation tests.     

Microstructure evolution of directionally solidified Nb–Si alloy

Abstract

By taking the method of liquid–metal cooled directional solidification, alloys with a nominal composition of Nb–14Si–24Ti–10Cr–2Al–2Hf (at-%) were prepared under different conditions. Alloys were initially directional solidified with different withdrawal rates (R?=?1·2, 6, 18 mm min^?1) at 1750°C and subsequently heat treated at 1450°C for 10 h. These processes aimed to investigate the microstructure of directionally solidified (DS) and heat treated (HT) alloys by XRD, SEM, and EDS. The microstructure of DS alloy was composed of (Nb,Ti)_SS, (Nb,Ti)₅Si₃, and Laves phase Cr₂Nb, and the former two components formed (Nb,Ti)_SS+(Nb,Ti)₅Si₃ eutectics. In addition, (Nb,Ti)₅Si₃ laths only presented in DS1·2 alloy. With the increasing withdrawal rates, the microstructure of alloy altered from hypereutectic into pseudo-eutectic, accompanied with the eutectic morphology transformation from petaloid into coupled. Also, the dimension of constituent phases reduced. However, after heat treatment, the constituent phases did not change. The petaloid morphology of eutectics in DS specimens disappeared and coupled eutectic transferred into network. The block or needle-like Cr₂Nb gathered along the boundary between (Nb,Ti)₅Si₃ and (Nb,Ti)_SS, and the overall alloy composition became homogenisation.     

Effect of TiAl<Subscript>3</Subscript> particles size and distribution on their settling and dissolution behaviour in aluminium

The effect of TiAl₃ particle size and distribution on their settling and dissolution behaviour in molten aluminium during grain refinement has been studied. For this purpose Al–5Ti master alloys containing blocky TiAl₃ particles of different size and distribution are synthesised at reaction temperatures 750, 800 and 850 °C for 60 min and used for grain refinement. The extent of fading and the recovery due to stirring is calculated from the measured grain size and used to judge the dissolution and settling behaviour of TiAl₃ in molten Al, which is greatly attributed to its size and distribution in Al–5Ti master alloy. Fine TiAl₃ particle dissolve faster in the melt and cause fading. Larger size TiAl₃ particles exist for longer time in molten Al and act as a nucleating site even when added in hypoperitectic concentration (0.05 wt% Ti).     

Improving interfacial properties of a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications

Dissimilar welds of aluminium alloy AA6056 and titanium alloy Ti6Al4V were produced by a novel technique. AA6056 sheet was machined at one end to a U-slot shape, enabling the intake of the Ti6Al4V sheet. The Al-alloy U-slot was then butt welded by split laser beam without using a filling wire, thus making a weld by melting only the Al-alloy. Thereby the intermetallic brittle phase TiAl₃ formed at the weld interface and affected mechanical properties. As a continuation of the previous work, the joint design was modified by chamfering Ti6Al4V to reduce the formation of interfacial TiAl₃. It is shown in this work how this seemingly insignificant joint modification has refined microstructure and increased hardness and strength. The most impressive feature was the improved resistance to fatigue crack propagation whereby the fracture type in the fusion zone of AA6056 adjacent to the weld interface changed from partially intercrystalline to completely transcrystalline. Possible metallurgical processes leading to the property improvements are discussed.     

High temperature strength,fracture toughness and oxidation resistance of Nb–Si–Al–Ti multiphase alloys

Nb–Si–Al–Ti quaternary phase diagram around three-phase region, which consists of niobium solid solution (Nb_ss), Nb₃Al and Nb₅Si₃,is constructed in this study. The three-phase region exists up to titanium content of about 20 mol%. Based on the quaternary phase diagram, three-phase alloys containing Nb_ss from about 50 to 75% in volume are prepared to improve high temperature strength, room temperature fracture toughness and oxidation resistance simultaneously.

When microstructure and composition are optimized (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloy with the addition of titanium exhibits higher compressive strength than nickel-based superalloys at room temperature to 1573 K. Fracture toughness at room temperature of (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloys is increased to over 12 MPa m^1/2 by the addition of titanium without sacrificing high temperature strength. Oxidation resistance of (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloys is improved by the addition of titanium.     

Nb掺杂对Ti-Al合金化层抗高温氧化性能的影响

为提高钛合金TC4的抗高温氧化性能,采用激光表面合金化技术在钛合金表面制备不同Nb掺杂量的Ti-Al合金化层。采用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、能谱仪(EDS)、箱式电阻炉等对合金化层的组织结构和高温氧化行为进行分析测试。结果表明,合金化层主要组成物相为TiAl以及少量的Ti_3Al相。Nb主要以置换溶质原子的形式固溶于合金化层中。合金化层组织均匀,与基体呈典型的冶金结合,在不含Nb的Ti-Al合金化层中发现大量的表层裂纹及少量的贯穿性裂纹,而在Nb掺杂的合金化层中未发现明显的宏观裂纹。合金化层在800℃保温1000h的氧化增重显著低于基体,表现出优异的抗高温氧化性能。相比而言,随着Al含量和Nb掺杂量的提高,合金化层的抗高温氧化能力也随之提高。Nb掺杂提高Ti-Al合金化层抗高温氧化性能的作用机理包括减少TiO_2中的空位缺陷、细化氧化物颗粒及促进Al_2O_3的形成。     

Effect of Nb,Mo, V contents on oxidation resistance of Ti3Al based alloys

In the present paper, the effect of the contents of Nb, Mo, V on the oxidation properties (700°C, in air) of Ti₃Al based alloys has been studied. It has been shown that the alloys were oxidized rapidly as exposed at 700°C in the air. After 100 h exposure, oxygen-affected alloy surface layer of about 10 thickness has been formed on account of the poor protection of the oxide film. An addition of (11–13%)¹ Nb enhanced the oxidation resistance. The addition of Mo and V in the Ti₃Al–Nb system alloy reduced the oxidation resistance significantly.     

High temperature oxidation behavior of Ti0.5Al0.5N coating and Ti0.5Al0.4Si0.1N coating

Good oxidation resistance of hard coatings is important for cutting tools. Ti_0.5Al_0.5N coating and Ti_0.5Al_0.4Si_0.1N coating were deposited by cathodic arc evaporation and their oxidation behavior at 850 °C, 900 °C and 1000 °C was compared. The effect of Si addition on the oxidation resistance of Ti_0.5Al_0.4Si_0.1N was investigated. Results show that the oxidation resistance of Ti_0.5Al_0.4Si_0.1N coating at 850-1000 °C is superior to Ti_0.5Al_0.5N coating. The improved oxidation resistance of Ti_0.5Al_0.4Si_0.1N coating can be ascribed to the combined action of Al₂O₃ and SiO₂ barrier layer, the reduction of columnar structure and the refinement of grains. In particular, Si addition increases the diffusion coefficient of Al and promotes the preferential formation of Al₂O₃ barrier layer.