γ-TiAl alloys have attractive properties such as low density, high creep resistance, high modulus, good oxidation and burn resistance. The main concern for the fabrication and use of TiAl alloys is the low deformability at room and high temperatures. Alloying and processing have been developed during the last three decades to improve the high-temperature deformability. Fabrication methods have been developed, including powder metallurgy and ingot metallurgy, to provide sheet materials for aerospace applications. This paper reviews the current status of TiAl sheet materials in terms of fabrication methods, microstructural control and mechanical properties. Future challenges and opportunities are also discussed.

•Disparity in cyclic deformation behavior due to strain amplitude was investigated.•TEM observations and EBSD techniques revealed the phase transformations.•B2 phase transformation and γ recrystallization are detrimental to fatigue life.•Twin boundaries can promote discontinuous dynamic recrystallization process.•The fracture mode is a combination of ductile fracture and intergranular fracture.The cyclic stress-strain (CSS) behavior of high Nb containing TiAl alloy with a duplex microstructure was investigated at 850 °C. Transmission electron microscope (TEM) observations and electron backscattered diffraction (EBSD) techniques were carried out to obtain insight into the microstructure evolution governing this behavior. At low strain amplitude, the material exhibits a rapid saturation of stress amplitude. At intermediate and high strain amplitude, the CSS behavior is characterized by generally cyclic softening. The changes of microstructure are strain-induced phase transformations and dynamic recrystallization, which lead to a degradation of lamellar microstructure. Twin boundaries can promote discontinuous dynamic recrystallization of γ phase, for processing relatively high energy. α2 + γ → B2, α2 → B2, and α2lamellae → γ phase transformations are detrimental to fatigue life of the material, because fracture propagation are usually along B2 phase boundaries and γ grain boundaries, thus, the fracture mode is a combination of ductile fracture and intergranular cleavage fracture.

•The isothermal section at 1300 °C in the Ti-Al-Nb system has been determined.•The isothermal section consists of 4 three-phase regions and 9 two-phase regions.•The β phase region expands and the σ and δ phases move to higher Al contents.•The single-phase regions α, γ, σ, δ and ε extend along the isoconcentrate of Al.Phase equilibria in the Ti-Al-Nb system at 1300 °C were studied using X-ray diffraction, scanning electron microscopy and electron probe microanalysis. Using the results, an isothermal section at 1300 °C for this system was constructed. No ternary compound was observed in this work. The isothermal section is characterized by four three-phase regions (α + β + γ, β + γ + σ, σ + δ + β and σ +γ + ε) plus the corresponding two-phase regions. The locations and areas of the tie-triangles are different from the previous results. The homogeneity range of α, γ, σ, δ and ε is located along a constant Al concentration.

•The quality of coating can be controlled by adjusting the parameters of square wave pulsed current.•A linear regression model is established for deposition effect.•The deposition rate is affected by the interval time between neighboring pulses.Cathodic plasma electrolysis (CPE) has been used as an environmentally-friendly process to deposit metal coating on metal substrate. In this work, a coating of zinc was deposited via CPE on a low carbon steel wire using direct current (DC) and square wave pulsed current (SWPC) power. The experimental results showed that application of continuous DC power did not produce zinc deposition, but a dense coating of zinc was easily applied to the substrate with the application of SWPC power at 120 V, 4000 Hz and an 80% duty cycle. The resulting coating was porous with some imperfections on the surface, but the morphology and quality of the coating could be controlled by adjusting the parameters of the SWPC power. Increasing both the duty cycle and frequency of the power resulted in a lower deposition rate, but the coating was denser. The steel wire surface was cleaned using plasma generated in the CPE process and the Zn coating was found to accumulate during the rise in the power pulse. A linear regression model was established to describe the relationship between coating thickness and power parameters.

Cathodic plasma electrolysis (CPE) is used to deposit Zn coating on the surface of steel wire. The relationships between power parameters and coating characteristics were investigated in this study to determine the best way to control the coating process according to the CPE procedure and pulsed DC power cycle. We found that voltage should be greater than the critical voltage for the formation of plasma. Deposition coating is difficult to establish under DC supply, however, continuous coating is rather easily prepared under pulsed DC power of 120 V, 4000 Hz, and 80 % duty cycle. We adopted pulsed DC power to successfully facilitate Zn cations approaching the cathode surface as well as to prevent wire melting under high voltage by reducing the duty cycle. Decreases in voltage, frequency, or duty cycle did not contribute to plasma stability, but did increase the deposition rate and porosity. Our experimental plasma formation process showed that the role of plasma formation is to clean the cathode surface by melting and shocking, which produces deposition at the interval between two neighboring pulses.

The large scale ingot of Ti-45Al-8.5 Nb-(W, B, Y) alloy, which was manufactured by plasma arc cold hearth melting process, was successfully forged by a two-step canned forging process with a total deformation of 85%. The pancake is crack-free with smooth surface and presents recrystallized duplex microstructure. Additionally, the fraction of β/B2 phase in the as-forged microstructure has dropped. The high angle grain boundaries are dominant among recrystallized grains. After the heat treatments at different temperatures, the fine duplex, nearly lamellar and fully lamellar microstructures can be obtained respectively. Compared with the as-cast condition at room temperature, the ultimate tensile strength increases from 633 MPa to 897 MPa and the elongation improves from 0.23% to 2.2%. The strengths of as-forged TiAl alloy are relatively high at high temperatures. Moreover, superplastic behavior appears above 900 °C.

The microstructure and orientation relationship (OR) of the α2 and ωo phases in the Ti–34Al–13Nb (at. %) alloy were investigated. The α2 segments nucleated at the ωo boundaries after aging at 900 °C for 2 h. The ORs between the two phases were studied by selected area diffraction patterns using a transmission electron microscope and interpreted by the superimposed stereographic projections. Two ORs were defined as follows: \( \left[ {1\bar{2}10} \right]\upalpha_{2} //\left[ {000\bar{1}} \right]\upomega_{o} \); \( (0002)\upalpha_{2} //\left[ {\bar{1}2\bar{1}0} \right]\upomega_{o} \) and \( \left[ {\bar{1}010} \right]\upalpha_{2} //\left[ {2\bar{4}2\bar{3}} \right]\upomega_{o} \); \( \left( {0002} \right)\upalpha_{2} //\left( {01\overline{12} } \right)\upomega_{o} \). Two α2 laths with parallel \( \left\langle {\bar{1}102} \right\rangle \) directions were observed in this alloy, which were explained by the different α2 variants nucleated at the ωo boundaries according to ORI and ORII.

γ-TiAl alloys, including two categories (the conventional TiAl and the high Nb-containing TiAl (high Nb–TiAl)), are technologically intriguing because of their applications at high temperatures. Specifically, the service temperature of the high Nb–TiAl alloys is 60–100 °C higher than that of conventional TiAl alloys. Recently developed TiAl alloys, for example TNB, TNM, β-γ alloys, belong to the high Nb–TiAl alloys, displaying similar behavior in phase transformation, strengthening, oxidation at high temperatures, and relationships between composition, microstructure, and mechanical properties. This work presents an in-depth review of the high Nb–TiAl alloys regarding the advances in phase diagram, formation mechanism of the new γ1 phase, microsegregation induced by adding a high content of alloying element Nb, and the mechanism of the B2/ω phase formation. Some challenges in developing the high Nb–TiAl alloys are also discussed.

Microstructure and microsegregation of directionally solidified Ti–45Al–8Nb alloy were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalyzer (EPMA). For the alloy solidified at the solidification rates (v) ranging from 10 to 400 μm·s−1, the microstructure of the mushy zone exhibits a cellular-dendritic structure at lower growth rate (v = 10–50 μm·s−1) and a typical dendritic morphology at higher growth rate (v = 100–400 μm·s−1). The relationship between primary dendrite arm spacing (λ1) and v is λ1 = 1.08 × 103v−0.35. Al and Nb elements segregate at interdendritic zone and in the dendritic core, respectively. In solid of mushy zone, a relatively flat concentration profile is observed for the typical dendrite structure, and Nb enriches in B2 phase induced by β → α + β transformation. The content of B2 phase is hardly affected by v. The extent of microsegregation in steady-state zone decreases at a lower growth rate because holding the samples at higher temperature after solidification for a long time can homogenize the solid effectively.

•The α2 laths in lamellar colonies are composed of the modulated α2 + B19 phases.•Thin γ plates can nucleate within the B19 phase to release the lattice misfit.•The B19 phase can be eliminated at high temperature.•The pure α2 phase observed at room temperature is not in the equilibrium state.The B19 phase is widely observed in the α2 laths in Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y alloy, forming a modulated structure together with the α2 phase. The distortion between the α2 and B19 phases exists only along the [11¯00]α2 direction and is very difficult to detect. Thin γ laths precipitate regularly within the α2 + B19 structure to accommodate the lattice misfit, whereas the sizes of the γ laths are limited to a small scale. The B19 phase, however, is unstable and was eliminated after annealing at 900 °C for 30 min, followed by air-cooling. The pure α2 phase observed in the air-cooled sample is not in the equilibrium state. The B19 phase can be a stable phase in some temperatures below 900 °C but be repressed under rapid cooling.

•Strain rate effect on mechanical properties was investigated in Ti-based MGM composites.•The mechanical properties of the composite have a week dependence on quasi-static strain rates.•In contrast to quasi-static case, the dynamic yielding strength increased but plasticity decreased.•The dynamic mechanical behaviors related with the dendrite size and volume fraction.Compressive tests were conducted on metallic glass matrix composites at a series loading rates. It was found that mechanical properties of the composite, e.g. yielding stress and plasticity, have a week dependence on strain rates of 4.0 × 10−4 s−1–4.0 × 10−1 s−1. Four composites were tested at a constant strain rate of 2.3 × 10 s−1 to uncover the dynamic deformation behaviors. Compared with the quasi-static case, the yielding strength increased under dynamic loading rate, but the plasticity decreased significantly. On the other hand, the dynamic compressive has closely relation with the dendrite size and volume fraction. The decreasing of the dendrite size and volume fraction leaded to the dynamic yielding strength increased but the plasticity decreased. For a same composite, e.g. T1 alloy, the yielding strengths increased slightly but fracture strain decreased with increasing of dynamic strain rates.

•The βo phase in as-cast high Nb containing TiAl alloys is composed of numerous ordered ω particles.•The βo(ω) area decomposed into large B82-ωo grains and small γ particles after annealing at 850 °C.•The direct α2.→ωo transformation in the lamellar colonies is experimentally observed.•The βo phase is substituted by α2 after annealing at 1250 °C through coarsening of the α laths.The ordered ω phase in as-cast Ti-45Al-8.5Nb-0.2B alloy and its phase transformation during heat treatment are investigated. Ordered ω variants are observed to uniformly precipitate within the βo area in as-cast Ti-45Al-8.5Nb-0.2B alloy. After annealing at 850 °C for 500 h, the βo areas are replaced by large B82-ωo grains. Small γ precipitates are observed at the grain boundaries of the ωo phase and are thought to be transformed from the βo phase. Moreover, the ωo precipitates directly transformed from the parent α2 laths are found within the lamellar colonies. The orientation relationship between the ωo phase and the lamellar structure is < 110]γ//[0001]ωo//[112¯ 0]α2; (111)γ//(112¯ 0)ωo//(0001)α2. The interfaces between the ωo and γ are semi-coherent. The ωo phase is an equilibrium phase at 850 °C in high Nb-containing TiAl alloys. When annealed at 1250 °C, the ordered ω is eliminated in a short time, and the βo phase is substituted by the coarsened α2 laths in the lamellar colonies after 12 h annealing.

•Gravity and centrifugal investment casting processes were comparatively studied.•Good correlation between simulation and experimental.•The defects of turbine blades were comparatively studied and discussed.•Microstructures of castings were comparatively studied and discussed.Gravity and centrifugal investment casting processes of low-pressure turbine blades with high Nb–TiAl alloy were simulated by Procast software. Actual blade components were poured by vacuum induction suspended furnace with Ar protection. The experimental verification indicated that the simulation results were in good agreement with the experimental results. Comparative results had shown that the surface of centrifugal casting blade was more complete than that of gravity casting one. In gravity casting process, molten metal filled the thinnest trailing edge at last, resulting in the generation of misrun defects. Furthermore, the shrinkage porosity and crack defects of gravity casting were much more and dispersive. The internal and external quality of centrifugal casting was much better than that of gravity casting. Microstructures from edge to center of gravity casting blade had no significant change. The microstructure for centrifugal casting blade is finer than that for gravity casting blade, however, a large number of dentritic γ segregation appeared in the blade edge of centrifugal casting, which resulted from the fast cooling rate of centrifugal casting surface.

•The yielding strength increased but the strain decreased with a decrease temperature.•The sharp ductile to brittle transition occurred at 100 K.•The MGM composite exhibits the large work-hardening behavior at 298 K.•All sample display the work-softening behavior below 298 K.•The nominal work-hardening parameter was employed to express the dependence.Systematic mechanical behaviors were investigated in a Ti-based metallic glass matrix (MGM) composite containing the in-situ β-dendrite phase at 100 K–298 K. We found that the yielding strength increased but the plastic strain decreased with a decrease temperature. The sharp ductile to brittle transition occurred at 100 K. The MGM composite exhibits the large work-hardening behavior at 298 K, but all sample display the work-softening behavior below 298 K. The nominal work-hardening parameter was employed to express the dependence of mechanical properties on temperatures including the brittle failure, the work-hardening and work softening behaviors. It may provide a useful way to evaluate the dependence of mechanical properties on temperatures of MGM composite.

The ordered ω phases in high Nb containing TiAl (Nb-TiAl) alloys have been garnering increasing attention in the recent years. However, the investigations on the Nb dependence on the ωo precipitation are scarce. In this study, the effect of Nb content on the ωo precipitation in high Nb (6–10 at%) containing TiAl alloys after long-time annealing at 850 °C has been studied. The results show that small ordered ω particles in the retained βo phase cannot be discerned under scanning electron microscope (SEM) but can be observed using transmission electron microscopy (TEM). Although the Nb segregation can be eliminated after the homogenization heat treatment, the ωo phase precipitated in all the alloys studied after annealing at 850 °C. TEM examination reveals that the orientation relationship between the ωo and α2 phases can be derived as: [0001]ωo//[112̄0]α2; (112̄0)ωo//(0001)α2, which indicates that the ωo phase is directly transformed from the parent α2 phase. Small γ particles are also observed within the ωo areas. The α2→ωo+γ decomposition process is expected during annealing. It is concluded that ωo phase is an equilibrium phase in high Nb-TiAl alloys at 850 °C.

•The phase transformation process in the Nb-segregation area is studied.•The ωo phase undergoes B82-ωo → D88-ω → γ transformation during annealing.•The βo(ω) → γ transformation can be realized in several ways.•The ωo particles act as pinning points during the βo(ω) → γ transformation.High Nb containing TiAl (Nb–TiAl) alloys are thought to be used at higher temperatures than the conventional TiAl alloys, typically up to 900 °C. However, the βo and ω-related phases are usually induced by Nb-segregation in cast alloys. It is important to reveal the transformation mechanisms of the βo and ω-related phases at elevated temperatures for better design of the alloy and optimizing the heat treatment process. In this study, the interconversion mechanism between the ordered ω and βo phases and the decomposition process of the βo phase in the as-cast Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at.%) alloy are studied. The ωo particles grow up by richening Nb and rejecting W to the surrounding βo matrix. The βo and ωo phase are separated after annealing at 900 °C, whereas the ωo phase dissolves into the βo matrix after annealing at 950 °C. The βo and ωo phases undergo βo → γ and ωo (B82 structure) → D88-ω (D88 structure) → γ transformation respectively during annealing, and the βo → γ transformation can be achieved in several ways. The ωo particles with a high Nb content act as pinning points at the βo/γ boundaries during the βo → γ transformation. The corresponding mechanisms of the transformations mentioned above are discussed.

•TiAl–Nb porous alloys are successfully fabricated by elemental powder metallurgy.•Due to exothermal reaction, expansion of gas results in expansion of the compact.•Volume expansion is restricted owing to vacancy migration and atomic diffusion.•Phase transformation and atomic diffusion promote coalescence of pores.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.

•The compression of two Ti-based MGM composites was tested at different loading rates.•Both composites exhibited the high strength and large plasticity at quasi-static compression.•A TZ1 alloy with fine dendrites demonstrated a catastrophic failure under dynamic loading.•The strength of TZ2 alloy (coarse dendrites) decreased at dynamic loading, but the strain is over 7%.•The possible mechanism has been discussed by analyzing the effects of the dendrites.The compression behaviors of Ti-based metallic glass matrix composites with dendrites scale were tested at different loading rates. It was found that the composites exhibited not only high strength, but also large plasticity under quasi-static compression. Under the dynamic loading, however, the TZ1 alloy with fine dendrites demonstrated a catastrophic failure. Although both the strength and plasticity decreased for the TZ2 composite sample with coarse dendrite, the total strain is over 7%. Discussions on the strain rates and dendrite scale are provided by analyzing the effects of dendrite, which can present the possible deformation mechanism of the composites.

In situ Ti-based metallic glass matrix composites are fabricated by the Bridgman solidification, and the mechanical properties are investigated. The fine dendrites about 2 to 10 μm are uniformly distributed in the glass matrix. The compressive results show that the composites have high strength and large plasticity. The fracture strength for the composite at the withdrawal velocity of 1.6 mm/s is as high as 3000 MPa and the total fracture strain is up to 31.5 pct. Particularly, the dendrite size of the current composite would decrease with the increasing of the withdrawal velocity, which leads to the higher yield strength.

This article focuses on the tensile and compressive characteristics of a Ti-based bulk metallic glass composite (BMGC). It is found that the yield stress, maximum strength, and fracture strain are 1380 MPa, 1516 MPa, and 4.3% for uniaxial tension, but 1580 MPa, 4010 MPa, and 29% for uniaxial compression, respectively. The composite displays a linear “work hardening” capacity under compression; however, the “work softening” behavior is observed in the true engineering stress-strain curve upon tensile loading. The fracture surfaces of specimens also exhibit dissimilar properties under the different loadings.

A Ti–48Al–6Nb porous microfiltration coating was fabricated by cold gas spraying (CGS) followed by reactive sintering. The porous coating possesses a porosity of 20.0% and interconnected pore structure. The skeletons in the coating were consisted of typical γ-TiAl/α2-Ti3Al fully lamellar microstructure, and firmly bonded with substrate. Uniform pores in the range of 0.5–3 μm were formed, attributing to the diffusion reactions and skeletons growth during the reactive sintering process. The bending strength and gas permeability of the supported coating with a thickness of 604.4 μm, are 89.33 MPa and 5.39 × 10− 13 m2, respectively. The porous coating has high permeability and sufficient strength for microfiltration applications in kinds of extreme environments.Highlights► An effective approach for fabrication of Ti-48Al-6Nb porous coating. ► The average pore size of coating is 1.8 μm, and the porosity is 20.0%. ► Phase transformation is followed with the pore formation. ► The coating has sufficient strength and permeability for microfiltration.

The synergistic effect of Y and Nb addition on the long time isothermal and cyclic oxidation behaviors of Ti–45Al–(6–9)Nb alloys containing 0.1–0.4 at.% Y was investigated in air at 900 °C. The results showed that the high temperature oxidation resistance was improved with an increase of Nb content. However, even 9 at.% Nb content was not sufficient to form the perfect Al2O3 layer. Scale spallation still occurred under hundreds of cyclic exposure. Y addition remarkably enhanced the long time oxidation resistance of high Nb containing TiAl alloys, and only a proper amount of Y addition could improve the long time oxidation resistance under both isothermal and cyclic oxidation. In this work, it was concluded that the principle of Y addition was choosing the minimum amount on the premise of meeting enough spallation resistance. Moreover, the optimum amount of Y changed regularly with the variation of Nb content. The reasonable addition of Y decreased from 0.4 at.% to 0.2 at.% with Nb content increasing from 6 at.% to 9 at.%. Based on synergistic effect of Y and Nb, the best oxidation resistance of alloys was achieved.

•Modifying grain boundary of La2/3−xLi3xTiO3 (LLTO) with a Li2O-B2O3 (LIB) glass.•Possessing grain boundary conductivity of over 10−4 S/cm for LLTO/LIB composites.•Alleviating interfacial barriers of ionic migration between electrolyte & cathode.•Explaining LIB contributions to ionic transfer along LLTO grains.This work is devoted to coating perovskite-type La2/3−xLi3xTiO3 (LLTO) grains with a Li2O-B2O3 (LIB) glass, considering good wetting properties, low melting point, and no grain boundary of LIB, to obtain novel LLTO/LIB composite electrolytes used for all-solid-state batteries. Perspectives with regard to the effect of LIB glass on mitigating the LLTO-related issues, such as low grain boundary conductivity, have been illustrated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and other techniques. From the characterization observations, the LLTO/LIB composites with the LIB phase filling at grain boundaries of LLTO show great densification, local lattice distortion and Li-ion redistribution. Obvious improvement on conduction properties both along LLTO grains and across LLTO electrolyte/LiCoO2 (LCO) electrode interface occurs by using appropriate amount of LIB materials. Specifically, LLTO/1.5 wt% LIB composite electrolytes achieve a grain boundary conductivity of 1.06 × 10−4 S/cm at room temperature, which is more than one order of magnitude higher than that for pure LLTO. The favorable conduction behaviors have been also discussed on the basis of microstructure and Li-ion distribution of the composites, as well as intrinsic attributes of LIB glass, such as isotropic homogeneity and existence of non-bridging oxygens.