Crystallography and phase transformation mechanisms in TiAl-based alloys – A synthesis

The variation in cooling rates of Ti–46.8Al–1.7Cr–1.8Nb (at. %) alloy from the high-temperature α domain produces lamellar, Widmanstätten, feathery-like (α₂ + γ) structures, as well as γ-massive phase, often coexisting together. Earlier reported crystallographic and morphological details of each of these structures are compiled together and a combined view on the generation of their crystallographic related possible variants and their solid phase transformation mechanisms is proposed. Sympathetic nucleation is suggested as a common mechanism for the lamellar structure at slow cooling rate, the Widmanstätten structure and the feathery-like structure.     

Texture and microstructure evolution during tempering of gamma-massive phase in a TiAl-based alloy

This work focuses on the mechanisms of microstructure and crystallographic texture evolution during gamma-massive transformation and subsequent tempering treatments, in the case of the Ti–48Al–2Nb–2Cr (at%) TiAl-based alloy. A complete massive structure is obtained by ice-water quenching. The temperature range for the destabilization of this massive structure is estimated from DSC measurement. Then, tempering is performed in both α₂–γ and α–γ phase fields. Microstructure evolution is studied by transmission and scanning electron microscopy observation of samples from different stages of the heat treatments. Isotropic samples resulting from a powder-metallurgy route are used for this purpose. Then, working on the samples from investment casting, the reduction of solidification texture through massive then tempering treatments is analyzed at the light of the mechanisms of microstructure evolution identified previously.     

Effect of composition on grain refinement in TiAl-based alloys

Grain refinement through boron addition has been investigated in a range of cast TiAl-based alloys. The alloys have 44–50 at.% Al, up to 8 at.% alloying elements, and 1 at.% boron. The observed grain size ranges from 40 to 300 μm. The grain size in as-cast TiAl-based alloys is strongly composition dependent with aluminium having the strongest effect. Decreasing Al concentration from 50 to 44% the grain size can be reduced to 50 from 300 μm with similar casting conditions. Alloying element species and concentration also have strong effect on grain size. The strong boride formers like W, Ta and Nb increase the grain size and change the prevalent titanium boride form from TiB₂ to TiB. It is elucidated that the compositional effect on the grain size could be through affecting the local boron concentration in the solidification front during casting.     

First-principles study of influence of Ti vacancy and Nb dopant on the bonding of TiAl/TiO2 interface

Influence of defects (Ti vacancy and Nb dopant) on the bonding of TiAl/TiO₂ interface was studied via first-principles calculations. It was shown that the bonding strength and the stability of TiAl/TiO₂ interface were weakened by the presence of Ti vacancy and dopant Nb. The defects could also change the relative stability of the interface with different couplings between the two compounds. Electronic structure of the interface was analyzed and the influence mechanisms of defects on the bonding of interface were presented.     

Non-equilibrium microstructures in TiAl–2Fe alloy

Microstructure evolution in Al-49.6 at% Ti-1.9 at% Fe alloy during cooling from 1400 and 1300°C leading to non-equilibrium structure is presented and discussed. ©     

Reaction behavior and pore formation mechanism of TiAl–Nb porous alloys prepared by elemental powder metallurgy

Ti–48Al–6Nb (at.%) porous alloys are fabricated by elemental powder metallurgy to study the pore formation and propagation mechanism. Reactive diffusion, pore formation process, and pore characteristics of the porous TiAl–Nb alloys are investigated at different temperatures. It is found that the porous alloys exhibit a uniform, maze-like network skeleton, viz., a typical α₂-TiAl₃/γ-TiAl fully lamellar microstructure. The reactive diffusivities between Ti and Al powders are dominant during the Ti–Al–Nb powder sintering. Gas release during sintering also plays an important role in the pore propagation and the compact expanding process. In addition, a pore-formation model is proposed to interpret the growth mechanism of pores and skeletons.     

Effect of carbon addition on solidification behavior,phase evolution and creep properties of an intermetallic β-stabilized γ-TiAl based alloy

Improving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition.     

Influence of thermodynamic activities of different masteralloys in pack powder mixtures to produce low activity aluminide coatings on TiAl alloys

Titanium aluminides are interesting high temperature materials, but show insufficient oxidation resistance as well as embrittlement at higher temperatures (>750 °C). Al-enriched coatings can be manufactured by pack cementation on many high temperature alloys to promote the formation of a protective alumina layer at high temperatures, which not only protects the alloy from oxidation but is also expected to impede embrittlement of TiAl at high temperatures. One drawback of such coatings is that Al-rich phases are very brittle. Therefore the major intermetallic aluminide phase in the coating plays a critical role for the protection behavior. Based on thermodynamic calculations different masteralloys were chosen to control the pack cementation process. Particular attention is given to the gradient between the aluminum activity of the different masteralloy powders and the aluminum activity of the substrate surface (alloy TNM^®-B1) in order to control the deposited phase at the surface. It is revealed that powder pack with Al as masteralloy provides a high Al activity and produces thick multi-layered coatings consisting of brittle TiAl₃ and TiAl₂ phase and aluminum-rich TiAl. By using different chromium aluminides as masteralloys, thinner, low-activity coatings could be produced, consisting of a bi-layer of brittle TiAl₂ phase and aluminum-rich TiAl or just the targeted pure aluminum-rich TiAl, which is known to have much better mechanical properties.     

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α₂/γ lamellar colonies at the edge of the precipitation zone, α₂ and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.     

Effect of Nb particles on the flow behavior of TiAl alloy

High temperature compressive deformation behaviors of PM-TiAl alloy containing Nb particles (Ti–45Al–5Nb–0.4W/2Nb (at. %)) were investigated at temperatures ranging from 1050 °C to 1200 °C, and strain rates from 0.001 s⁻¹ to 1 s⁻¹. The flow curves were employed to develop constitutive equations, and the apparent activation energy of deformation Q was determined as 447.35 kJ/mol. A revised processing map was constructed on the basis of the flow stress, which can accurately describe the deformation behaviors and predict the optimum hot forging condition. The addition of 2% Nb particles reduces the peak stress and increases the activation energy of TiAl-based intermetallic, however, it increases the instable domain in the processing map.     

Producing fully-lamellar microstructure for wrought beta-gamma TiAl alloys without single α-phase field

Fine-grained fully-lamellar (FL) microstructure is desired for TiAl components to serve as compressor/turbine blades and turbocharger turbine wheels. This study deals with the process and phase transformation to produce FL microstructure for Mo stabilized beta-gamma TiAl alloys without single α-phase field. Unlike the α + γ two-phased TiAl or beta-gamma TiAl with single α-phase field, the wrought multi-phase TiAl–4/6Nb–2Mo–B/Y alloys exhibit special annealing process to obtain FL microstructure. Short-term annealing at temperatures slightly above β-transus is recommended to produce the desired FL microstructure. The related mechanism is to guarantee the sufficient diffusion homogenization of β stabilizers during single β-phase annealing, and further avoid α decomposition by α → γ + β when cooling through α + β + γ phase field. The colony boundary β phase contributes to fine-grained nearly FL microstructure, by retarding the coarsening of the α phase grains.     

Microstructural instability and embrittlement behaviour of an Al-lean, high-Nb γ-TiAl-based alloy subjected to a long-term thermal exposure in air

Both ingot-cast and forged Ti–44Al–8Nb–1B alloys were exposed at 700 °C in air for up to 10,000 h. The α₂ lamellae in the two conditions are found to be thermodynamically unstable and readily decompose through phase transformations of α₂ → γ, α₂ → B2(ω) and α₂ + γ → B2(ω). Widespread B2(ω) forms throughout the lamellar structure, resulting in a significant increase in volume fraction after 10,000-h exposure. This is attributed to the composition similarity between the transformed and parent phases. The partition coefficients for Ti/Al/Nb between B2(ω) and α₂ and between B2(ω) and α₂ + γ are all measured to be close to 1. The long-term exposure has induced embrittlement owing to oxygen releasing from α₂ decomposition. Room-temperature ductility is only 1/5 and 1/3 of the original value for the two conditions, respectively. However, no clear decreasing trend in S–N fatigue strength is observed, suggesting that the embrittlement effect of B2(ω) on the surface crack initiation is difficult to detect.     

Si-modified aluminide coatings deposited on Ti46Al7Nb alloy by slurry method

Ti46Al7Nb alloy has been used as the research substrate material for the deposition of water-based slurries containing Al and Si powders. The diffusion treatment has been carried out at 950 °C for 4 h in Ar atmosphere. The structure of the silicon-modified aluminide coatings 40 μm thick is as follows: (a) an outer zone consisting of TiAl₃ phase and titanium silicides formed on the matrix grain boundaries composed of TiAl₃–type Ti₅Si₃; (b) a middle zone containing the same phase components with the matrix TiAl₃ and the silicides Ti₅Si₃, which formed columnar grains; (c) an inner zone, 2 μm thick, consisting of TiAl₂ phase. Cyclic oxidation tests were conducted in 30 cycles (690 h at high temperature) and showed a remarkably higher oxidation resistance of the Ti46Al7Nb alloy with the protective coating in comparison with the uncoated sample.     

High-temperature oxidation behavior of multi-phase Mo-containing γ-TiAl-based alloys

Intermetallic titanium aluminides are potential materials for a number of high-temperature components used in aero and automotive engines. In particular, alloys solidifying via the β-phase are of great interest because they possess a significant volume fraction of the disordered body-centered cubic β_o-phase at elevated temperatures ensuring good processing characteristics during hot-working. Nevertheless, the practical use of such alloys at a temperature as high as 800 °C requires improvement of their oxidation resistance. Various attempts have been made including alloying with additional elements such as Nb, Cr, Mo etc. or applying the so-called fluorine effect. However alloying could not provide a sufficient oxidation resistance above 850 °C whereas the fluorine effect protects the base material against environmental degradation up to over 1000 °C. This paper aims to investigate the influence of the phase composition on the oxide scale morphology without and with fluorine effect. The results refer to the oxidation behavior of three β-solidifying γ-TiAl-based alloys in the cast and hot-isostatically pressed condition at 800 °C in air. The behavior of the TNM alloy (Ti–43.5Al–4Nb–1Mo–0.1B, in at.%) was compared with that of two Nb-free TiAl alloys which contain different amounts of Mo (3 and 7 at.%, respectively) and hence a different microstructure (α₂/β_o/γ vs. β_o/γ). During testing in dry synthetic air at 800 °C a mixed oxide scale develops on all three alloys. This behavior was changed via the fluorine effect, as demonstrated for previously investigated TiAl alloys with an Al-content higher than 40 at.% based on α₂/γ and α₂/β_o/γ phases. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. The reason for this is the change in the oxidation mechanism triggered by the small additions of fluorine in the subsurface zone of the investigated alloys. The results of isothermal oxidation tests at 800 °C in air are presented and discussed in view of chemical composition and microstructure, along with the impact of the phase composition on the efficiency of the fluorine effect. From a microstructural perspective the fluorine effect leads to the formation of an even thinner oxide scale on the β-phase compared to the γ-phase.     

Interfacial reaction studies of B4C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites

The interfacial reactions of B₄C-coated and C-coated SiC fiber reinforced Ti–43Al–9V composites were investigated by scanning electron microscope and transmission electron microscope. The detailed microstructures as well as the chemical composition throughout the reaction zone were identified. For SiC_f/B₄C/TiAl composite, the reaction zone from B₄C coating to TiAl matrix is composed of 4 layers, namely, a carbon-rich layer, a mixed layer of TiB₂ + amorphous carbon, a TiC layer and a mixed layer of TiB + Ti₂AlC. For SiC_f/C/TiAl composite, the reaction zone from C coating to TiAl matrix is composed of 3 layers, namely, a fine-grained TiC layer, a coarse-grained TiC layer and a thick Ti₂AlC layer. For both kinds of composites, the reaction mechanisms of the interfacial reactions were analyzed, and the corresponding reaction kinetics were calculated. The activation energies of interfacial reaction in SiC_f/B₄C/TiAl composite and SiC_f/B₄C/TiAl composite are 308.1 kJ/mol and 230.7 kJ/mol, respectively.     

Oxidation protection of γ-TiAl-based alloys – A review

Alloys based on γ-TiAl are lightweight materials with attractive mechanical properties at high temperatures. Although these alloys reveal a superior resistance against environmental attack compared to titanium and 2-based alloys, efficient protection is required for industrial applications at temperatures between 800 and 1050 °C. Extensive research in order to solve this problem started more than 30 years ago. This review provides a summary of the different concepts based on surface modification techniques developed for the environmental protection of γ-TiAl alloys at high temperatures, including overlay and diffusion coatings, as well as the halogen effect. The discussion includes a comparison between the most promising coating types under long-term high temperature exposure and an assessment of their processing routes from a technological point of view. Therefore, a mass gain of 1 mg/cm² after at least 1000 h of exposure was set as a benchmark to evaluate these protection systems.     

Porous FeAl intermetallics fabricated by elemental powder reactive synthesis

Fabrication of porous FeAl intermetallics has been realized through the Fe and Al elemental powder reactive synthesis. The swelling behavior, synthesis process and microstructure of the porous FeAl intermetallics fabricated by reactive synthesis have been systematically studied. The pore structural parameters as a function of the sintering temperature have also been systematically investigated. It has been confirmed that the pore evolution in the porous FeAl intermetallics is attributed to the following four steps: the inter-particle pores formed during the pressing procedure, the Kirkendall pores formed during the solid-state sintering, pore formed through the liquid Al reaction, and the phase transformation during the high temperature sintering.     

High temperature deformation behavior of near γ-phase high Nb-containing TiAl alloy

High temperature compressive deformation behaviors of a high Nb-containing PM-TiAl alloy (Ti–45Al–7Nb-0.3 W, at.%) were investigated at temperatures ranging from 1050 °C to 1200 °C, and strain rates from 0.001 s⁻¹ to 1 s⁻¹. The microstructure mainly consists of γ phase. The data obtained from the flow curves were employed to develop the constitutive equation, and the apparent activation energy (Q) was determined to be 414 kJ mol⁻¹. The size of the dynamically recrystallized grains (DRX) decreased with the increasing value of Zener–Hollomon (Z) parameter. A processing map was constructed on the basis of the flow stress, and the condition of intermediate Z (1150 °C, 0.1 s⁻¹) was determined to be the optimum hot forging parameter for industrial productions. DRX was observed under all the deformation conditions. At high Z and intermediate Z condition, dislocation climbing and twinning accompanied by DRX can act as the main deformation mechanisms. At low Z condition, DRX becomes the dominant softening mechanism, accompanied by the bending of lamellar colonies as well as the broken of γ grains and α₂ grains.