High Nb-containing TiAl Alloy Research Articles

Enhancing strength and ductility in high Nb-containing TiAl alloy additively manufactured via directed energy deposition

Intermetallic TiAl alloys have found applications in the aerospace and automotive fields. Nevertheless, to satisfy the increasingly complex environmental requirements, these lightweight alloys must be improved particularly in strength and ductility. Additive manufacturing (AM) exhibits a rapid solidification rate and may strengthen intermetallic TiAl alloys. In this study, an additively manufactured (AMed) Ti-48Al-8Nb alloy with fine formability was successfully fabricated by optimizing the power used during directed energy deposition. The preferential crystal plane for the obtained alloy is {111}γ due to its lowest surface energy, and the presence of the thermal gradient during the AM process results in the alloy growing along the building direction. The TiAl alloy AMed at 500W delivers an excellent tensile strength of 880MPa that is 1.71 times higher than that of its as-cast counterpart, as well as elongation of 0.7% at room temperature. The superior properties of this alloy are due to the formation of numerous deformation twins during tensile deformation at room temperature that enables twin intersections to produce high-density nano-twins while contributing to stress release. Moreover, the AMed TiAl alloy exhibits outstanding high-temperature strength retention performance.

The Role of Boron Addition on Solidification Behavior and Microstructural Evolution of a High Niobium-Containing TiAl Alloy

This work investigates the role of boron addition in the solidification behavior and microstructural evolution during the heat treatment process of Ti-46Al-8Nb-xB (x = 0.1, 0.7, 1.4, 2.5 at.%). The results show that the solid solution boron element prefers to occupy the interstitial vacancies of the α2 phase in the alloy. However, the solid solubility of the boron element in high Nb-containing TiAl alloys is extremely low. Therefore, it does not have a significant effect on the lattice distortion of α2 and γ phases in the alloy. When the boron content is added up to 0.1%, a B27-type TiB precipitated phase is produced in the alloy. The morphology of borides mostly shows short rod-like structures, and a few show long curved shapes. And the addition of boron refines both the alloy colony size and the lamellar structure. Furthermore, it is also found that boron addition weakens the casting texture of the alloy. After a solid solution and different time aging heat treatment process, the microstructure of different boron content alloys have experienced obvious coarsening phenomenon. However, the morphology of the boride is closely related to boron content and heat treatment.

High-temperature compression deformation behavior of the high Nb-containing TiAl alloys doped with Sn

In this work, the high-temperature compression behavior of high Nb-containing TiAl–Sn alloys was investigated at deformed temperatures of 700 °C–1000 °C and strain rates of 0.002 s−1–0.2 s−1. The true stress-strain curves, microstructures, and constitutive equations of TiAl-xSn (x = 0, 0.5, 1, 1.5) alloys under different deformation conditions were systematically compared. Sn addition can increase the flow stress of TiAl alloys at the same deformation temperature, which is caused by the solid solution strengthening effect. At 800 °C/0.002 s−1, the peak flow stress of TiAl–1Sn alloy increases by 180 MPa compared with TiAl–0Sn alloy of 600 MPa. Below the peak stress, the flow stress increases with the increase of strain. Above the peak stress, the flow stress first decreases and then gradually trends to a steady state as the increase of deformation. Furthermore, the Sn addition decreases the brittle-ductile transition temperature of TiAl alloys. The TiAl-xSn (x = 0.5, 1, 1.5) alloys displays obvious flow softening behavior at 700 °C while TiAl–0Sn alloy fractures before reaching the target deformation. The relationship between of the flow stress, strain rate, and deformation temperature for TiAl–0Sn and TiAl–1Sn alloys is suitable for the hyperbolic sinusoidal equation. The dominant softening mechanism of hot deformation for TiAl-xSn (x = 0, 1) alloys is dynamic recrystallization (DRX). The activation energies Q of hot deformation for TiAl–0Sn and TiAl–1Sn alloys are 311 kJ mol−1 and 429 kJ mol−1 at condition of 700–1000 °C/0.002–0.2 s−1, indicating that the TiAl–1Sn alloy exhibits a better stability under the same application condition.

Exploration of homologous substitution element on phase ratio and high temperature properties in high Nb-containing TiAl alloy

To reveal homologous substitution and refined mechanisms, and improve mechanical properties, Ti42Al6Nb 2.5 CxTa alloys were prepared by arc melting. Element distribution, lamellar colony size, length-diameter radio of Ti2AlC phase, tensile properties, and the corresponding influence mechanism were studied. Results show that the relative contents of γ and Ti2AlC phase increase, the content of α2 phase decreases, when Ta content increases from 0 to 2.0 at%. The relative content of B2 phase increases among the lamellar colonies and the length-diameter ratio of Ti2AlC particle decreases. Lamellar colony size decreases from 24.9 to 10.6 µm. With the increase of Ta content, the supercooling at the front of the solid-liquid interface increases and the β phase is refined. The increase of the precipitated phase content leads to the increase of the number of α phase heterogeneous nucleation particles, which will refine the lamellar colony. Tensile results show that the strength increases from 238 to 513 MPa and strain increases from 2.7 % to 11.1 % of Ti42Al6Nb 2.5 C1.6Ta alloy when the tested temperature increases from 750 to 950 ℃. The high temperature tensile properties of the alloy with more Ta content are relatively higher. Ta forms large lattice distortion after solid solution into the matrix due to its large atomic size, and Ta makes more Nb dissolve into the collective to increase the effect of solid solution strengthening. Another reason is the refinement of microstructure.

Deformation and phase transformation behaviors of a high Nb-containing TiAl alloy compressed at intermediate temperatures

• The β o (ω o ) phases precipitate within the equiaxed α 2 phase during deformation. • The ω o phase forms in the lamellae and undergoes further deformation. • The precipitation of the β o phase is suppressed by DRX. In modern β-solidified TiAl alloys, the decomposition of α 2 phase is frequently observed during heat treatment or high-temperature deformation of the alloys. In this study, high-temperature deformation and decomposition mechanisms of α 2 phase in a Ti-45Al-8.5Nb-0.2B-0.2W-0.02Y alloy are investigated. In a sample deformed at 800 °C, the precipitation of β o (ω o ) phase is observed within the equiaxed α 2 phase. The nucleation of ω o particles within the β o matrix indicates the α 2 →β o →ω o transformation. In addition, numerous γ phase precipitates form within the β o (ω) areas. The α 2 lamellae decompose into ultrafine (α 2 +γ) lamellae and coarsened γ lamellae via α 2 →α 2 +γ and α 2 →γ transformation, respectively. Moreover, the ω o phase nucleates within the ultrafine lamellae via α 2 →ω o transformation. However, in a sample deformed at 1000 °C, the nucleation of β o particles is sluggish, which is caused by the efficient release of the internal stress via dynamic recrystallization (DRX). These results indicate that complex phase transformations can be introduced by the decomposition of α 2 phase in TiAl alloys with a high amount of β-stabilizing elements.

Microstructural Evolution and the Irradiation Sensitivity of Different Phases of a High Nb-Containing TiAl Alloy under He Ions Implantation at Room- and Elevated Temperatures

A high Nb-containing TiAl alloy Ti-45Al-8.5Nb-(W, B, Y) with nearly lamellar microstructure has been irradiated by 200 kV He2+ to a fluence of 1 × 1021 ions/m2 with a dose of about 1.1 dpa at 298 K and 773 K in this work. It is found that an amorphous layer formed on the surface, and no helium bubbles can be observed in the alloy after room temperature ion implantation. The surface roughness of the alloy increases significantly with the bombardment of helium ions, indicating that the ion implantation increases the surface defects. The high-temperature ion implantation leads to the phenomenon of blistering on the alloy surface, and helium bubbles are observed in both α2 and γ phases of the alloy irradiated at 773 K. The average size of the helium bubbles in the α2 (15~20nm) is larger than that in the γ (3~5 nm) phase, while the helium bubble density is opposite. Moreover, the growth mechanism of helium bubble is also investigated. By means of nanoindentation, an obvious irradiation hardening phenomenon is measured after the room temperature ion implantation. In addition, the irradiation sensitivity of different phases is also discussed in this work. The results show that the γ phase has the highest irradiation sensitivity, α2 phase second, β phase minimum. The results of this work, especially microstructure evolution and the evaluation of phase-related irradiation sensitivity during ion implantation, can be expected to provide experimental evidence for the applications of TiAl intermetallic compounds in the nuclear industries.

Physical metallurgy of high Nb-containing TiAl alloys

Abstract Intermetallic titanium aluminides exhibit attractive thermophysical properties, which give them the potential for extensive use as lightweight structural components. Novel design concepts are based on alloys with the general composition (in at.%) Ti-45Al-(5–10) Nb, which were subjected to precipitation hardening. Optimized compositions have been identified that are capable of carrying stresses in excess of 700 MPa at service temperatures of 700 °C and have superior creep properties. The alloys exhibit at room temperature yield stresses in excess of 1GPa combined with plastic tensile elongations of about 2%. Wrought alloys of this type can be an attractive alternative to the nickel-base superalloys in certain ranges of stress and temperature. The future and promise of these new TiAl alloys lies in innovative processing methods designed to achieve better performance.

Hot deformation behavior and artificial neural network modeling of β-γ TiAl alloy containing high content of Nb

In this work, the hot deformation behavior of high Nb-containing TiAl alloy with β + γ phases was investigated. Elongation, twisting and bending of the microstructure are the primary deformation phenomena at low temperatures and high strain rates, whereas high temperatures and low strain rates are beneficial for dynamic recrystallization, and the grains are equiaxed. Based on the experimental flow stress data, the Arrhenius constitutive modeling was established. In addition, an artificial neural network was innovatively utilized to propose a high temperature constitutive modeling. The results denote that the accuracy of artificial neural network is higher than the Arrhenius equation. The proposed artificial neural network modeling provides high accuracy, and the average absolute relative error and correlation coefficient are 1.28 % and 0.99988, respectively. Thus, this is a new approach to establish a constitutive modeling for TiAl alloys.

Hierarchical microstructure in high Nb-containing TiAl alloy

Plate-like η-Nb(Ti,Al)3 precipitates with hierarchical microstructure comprised of alternating arrangement of γ-(Ti,Nb)Al and η plates in the Ti-55Al-22Nb alloy have been investigated using transmission electron microscopy. In addition, the orientation relationship (OR) between η precipitates and γ matrix is determined to be tetragonal-tetragonal. The habit plane of η precipitates is along {022) crystal planes of γ matrix. And the interface structure between them can be described using the classical ‘terrace-disconnection’ theory. Furthermore, the phenomenon of phase separation responsible for the formation of this hierarchical microstructure can be well explained by the chemical free energy, that is, Al-depletion and Ti-enrichment in η precipitate drive the splitting of η precipitate. To the authors' knowledge, this is the first work which demonstrates such a hierarchical microstructure in η precipitate in high Nb-containing TiAl alloy.

Micro- and macroscopic plastic flow responses in high Nb-containing TiAl alloy by nanoindentation

High Nb-containing TiAl alloys have been widely developed and researched due to low density, high specific strength and stiffness, oxidation, and creep resistance. Nevertheless, high Nb-containing TiAl alloys demonstrate poor machinability owing to low plasticity and ductility. Accordingly, on basis of Kalidindi and Pathak's model, this paper systematically establishes a high-throughput and convenient method on extracting the macro- and microscopic plastic flow response of high Nb-containing TiAl alloy via nanoindentation experiment. And the mechanical properties of each individual microstructure were investigated by a series of indentation tests with Berkovich indenter. The results demonstrate that, i) the plastic flow behaviors of recrystallized γ grains are similar to that of fine lamellar colonies, whereas β/B2 grains are highly brittle; ii) according to the ratio of hardness to reduced modulus, the materials can present a better anti-wear capability with appropriate increase the content of β/B2 phase; iii) the Oliver-Pharr method combined with the energy method, the plastic recovery values of the recrystallized γ grains are larger than that of other microstructures, which indicate that the duplex microstructure γ-based TiAl alloy exhibits high ductile feature. Furthermore, the macroscopic plastic flow was extrapolated through the microscopic plastic flow curve combined with the volume fraction of each microstructure.

Creep-Induced Phase Instability and Microstructure Evolution of a Nearly Lamellar Ti–45Al–8.5Nb–(W, B, Y) Alloy

Microstructure degradation and stress-induced transformation of a high Nb-containing TiAl alloy with nearly lamellar microstructure during creep were investigated. Tensile creep experiments were performed at 800, 850 and 900 °C under 150 MPa in air. Microstructures before and after creep tests were examined using scanning and transmission electron microscopy (SEM and TEM). Dislocations within the lamellar structure and βo(ω) region and twin intersection in massive γ grains were investigated. Dislocation sliding played a critical role in the deformation of ωo phase, which preferentially occurred on the (0002)ωo plane. Possible deformation mechanisms were revealed. A stress-induced γ → α2 phase transformation took place during the creep test at 850 and 900 °C. α2 lamella could directly decompose into the ωo phase at 850 °C. The instability of high-temperature microstructure can weaken the creep resistance and promote the plastic deformation of the lamellar matrix, thus could be detrimental to the creep properties. The correlations between creep properties and microstructure instability were discussed.

Sintering densification, microstructure and mechanical properties of Sn-doped high Nb-containing TiAl alloys fabricated by pressureless sintering

The challenge of producing high-density TiAl alloys is hereby addressed by a simple pressureless sintering approach. A systematic study has been made on the effect of Sn addition on the sintering densification, microstructure and mechanical properties of Ti–45Al-8.5Nb-0.2W-0.2B-0.02Y alloy. The results show that Sn addition significantly reduces the alloy's melting point, directly affecting the sintering behavior and with increasing Sn content the optimum sintering temperature decreases. A high-density (≥99.0%) and fine fully lamellar microstructure with a colony size of 40–60 μm can be achieved by a small amount of Sn addition. The lamella has a specific orientation relationship of (110)γ∥(0001)α2 and [11‾0]γ∥[11‾01‾]α2. Sn element is dissolved in TiAl matrix and the soluble Sn content in α2 phase is almost two times than that in γ phase. Benefitting from the optimized microstructure and Sn solid solution strengthening, the as-sintered TiAl–1Sn alloy exhibits the best mechanical properties, with a compressive strength of 3029 ± 49 MPa, yield strength of 685 ± 57 MPa and strain to failure of 32.5 ± 0.49%, respectively. The optimal Sn content should be between 1 and 2 at.% and beyond this the samples fails prematurely.

New insights into high-temperature deformation and phase transformation mechanisms of lamellar structures in high Nb-containing TiAl alloys

The paper describes the microstructure evolution by high-temperature compression of a high Nb-containing TiAl alloy. The paper extends a previous publication [Song, et al. Intermetallics 109 (2019) 91–96], in which a unique twin-like morphology in the α2 (Ti3Al) phase was reported. However, the origin of these structures could not be clarified without doubt. The present study is focused on phase transformations that in this multiphase alloy can be associated with deformation. Particular attention is paid to local transformations of the α2 phase into O phase or ω-related phases, which, because of structural and chemical similarity of these phases with α2, can easily occur and could mistakenly be considered as a twin structure. The details of the atomic processes involved are elucidated by electron microscopy. Given the large shufflings and the atomic site interchanges required for the operation of this twinning system, it is concluded that twinning of the α2 phase is a diffusive-displacive process. Within the α2 phase, ωo is heterogeneously nucleated. The nucleation sites are defect-rich areas, which are subjected to high local stresses. The study strongly emphasizes the close relationship between high-temperature deformation and phase transformations in multiphase titanium aluminide alloys.

Effect of Sn addition on the high-temperature oxidation behavior of high Nb-containing TiAl alloys

The effect of Sn addition on the oxidation behavior of high-Nb containing TiAl alloys was investigated with isothermal oxidation tests conducted in static air at 1000 °C. Sn addition can improve the high-temperature oxidation resistance of TiAl alloys by forming a protective Ti3Sn layer to diminish the inward oxygen diffusion. Moreover, the generated Al2O3 oxide pegs provide a mechanical locking to enhance the spallation resistance. The alloy containing 3 at.% Sn exhibits superior oxidation resistance, with no spallation occurring. The lowest mass gain after being oxidized for 100 h was 3.29 mg/cm2, which is 42.1 % lower than the Sn-free alloy.

Effect of C Addition on as-Cast Microstructures of High Nb Containing TiAl Alloys

Two high Nb-containing TiAl alloys, Ti46.6Al7.5Nb0.5Si0.2B (Alloy A) and Ti46.1Al7.4Nb5C0.5Si0.2B (Alloy B), were prepared by graphite mold casting. As-cast microstructures of the two alloys were characterized to clarify the effect of carbon addition. The results show that 5 at.% carbon addition can change the primary solidification phase from β phase to α phase. The as-cast microstructure of Alloy A consists of a fully α2 + γ lamellar structure and interdendritic eutectic silicide with a volume fraction of 2.3%. However, in Alloy B, the lamellar structure only forms in the dendritic stem and the massive γ is observed in the interdendritic regions. Two types of carbides, Ti2AlC and TiC, are produced in Alloy B. A large number of randomly distributed primary Ti2AlC particles with volume fraction of 14.9% are observed in both the dendritic and interdendritic regions. Irregularly shaped TiC remains inside of the large Ti2AlC particle, suggesting TiC carbides transformed to Ti2AlC during cooling. The addition of carbon also changes the morphology of the silicides from a eutectic structure to a blocky structure in the massive γ matrix or at the interface of the Ti2AlC and the γ matrix. High level of niobium greatly increases the solid solution limit of carbon, since C content in the matrix is much higher than the solid solubility of that in the TiAl binary system. The hardness of the matrix increases from 325 HV to 917 HV caused by the addition of carbon.

Corrosion resistance and interfacial morphologies of a high Nb-containing TiAl alloy with and without thermal barrier coatings in molten salts

In this work, corrosion behaviors of the high Nb-containing TiAl alloy with and without thermal barrier coatings are compared to illustrate the factors and corresponding mechanism determining stability of TiAl alloy in molten salts. The high Nb-containing TiAl alloy with coatings suffers a mass change of −7.50 mg/cm2 in the mixture salts of 75 wt.%Na2SO4+25 wt.%NaCl at 800°C for 50 h. The mass change of TiAl alloy with coatings is only 0.20 mg/cm2 and the thickness of corrosion region is 10 ± 1 μm. Dense ceramic coating retards molten salts diffusing to TiAl substrate and corrosion products suppress self-sustaining reaction of TiAl alloy.

The nucleation of microcracks under tensile stress in multi-phase high Nb-containing TiAl alloys

A series of bending tests was conducted on specimens of different TiAl alloys to investigate the nucleation of microcracks under tensile stress. The distribution of microcracks and the deformation traces around them were characterized to study the role of plastic deformation intersecting with microstructural features in the microcrack nucleation. Results show that the microcrack nucleation plays an important role in the fracture of multi-phase high Nb-containing TiAl alloys. The microcracks tend to nucleate at colony boundaries in near fully lamellar and fully lamellar alloys, while they prefer B2/γ phase boundaries in (B2+γ) alloy. The heterogeneous deformation in different lamellar colonies was caused by plastic anisotropy of the lamellar structure. It leads to the nucleation of microcracks at colony boundaries. Meanwhile, due to the stress concentration at lamellar interfaces in the transversal deformation mode, a few microcracks nucleate inside lamellar colonies which have an orientation angle Φ near to 90°. The existence of brittle B2 phase leads to strain incompatibility and facilitates microcrack nucleation at B2/γ boundaries.

Friction and wear properties of a high Nb-containing TiAl alloy against WC-8Co, Si3N4, and GCr15 in an unlubricated contact

Friction and wear properties of a high Nb-containing TiAl alloy against WC-8Co, Si3N4, and GCr15 in an unlubricated contact were investigated under various friction and wear conditions. The counterface material plays a key role in the friction and wear properties of the high Nb-containing TiAl alloy. The Si3N4/TiAlNb pair had the highest average friction coefficient caused by the abrasive wear and adhesive wear. The applied temperature had a strong effect on the friction coefficient of the Si3N4/TiAlNb and GCr15/TiAlNb pair. The WC-8Co/TiAlNb pair has the stable average friction coefficient under various friction and wear conditions due to the abrasive wear.

Continuous cooling transformation (CCT) behavior of a high Nb-containing TiAl alloy

The continuous-cooling-transformation (CCT) diagram is significant to the microstructural design and the control of mechanical properties. The CCT behavior of a high Nb-containing TiAl alloy was experimentally investigated in the present study. Samples with the nominal composition of Ti-45Al-8.5Nb-(W, B, Y) (at.%) were isothermally held at 1500 °C (in the single β phase field) and subsequently cooled at different cooling rates. Based on the samples quenched from high temperatures, the microstructures during continuous cooling was characterized and the CCT diagram was established and evaluated. With the increase of cooling rate (5, 20, 100, 700 K/min), the phase transformation presented a lagging behavior comparing with the equilibrium phase diagram. Meanwhile, the decomposition of high-temperature β/B2 phase was incomplete at large cooling rate, indicating that a fully lamellar (FL) microstructure alloy can be obtained only below a critical cooling rate. Further, the discontinuously coarsened γ phase, the crystalline orientations of precipitated block γ and ω particles in B2 region were characterized and the phase transformation mechanisms were discussed.

Evolution of Microstructure and Microsegregation of Ti-45Al-8Nb Alloy during Directional Solidification

High Nb-containing TiAl alloys have good oxidation resistance and mechanical properties, but the microstructure and the properties are substantially affected by the segregation. To quantitatively investigate the segregation behavior of Al during solidification, microstructures of directionally solidified (DS) Ti-45Al-8Nb (in atomic percent) alloy prepared at withdrawing rates of 30 μm/s and 200 μm/s and a temperature gradient of 4200 K/m were observed by optical microscope and electronic probe microanalyzer. The microsegregations were characterized by wave dispersive spectroscopy. The results show that the DS ingots include the no melting zone, directionally solidified zone with columnar grains, mushy zone, and quenched liquid zone. The primary dendritic arm spacings are 353 μm and 144 μm, respectively, for the two ingots. But the solidified microstructures of the ingots are large lamellar colonies, which contain a few B2 patches and γ bands induced by microsegregation. From dendritic zone to columnar zone, the volume fractions of B2 patches and γ bands decrease. The segregation extents of Al and Nb decrease with the increase of solidification rate. There exists an obvious back diffusion process of Al during solidification and cooling after solidification. According to evolution of Al concentration profiles from mushy zone to columnar grain zone, interdiffusion coefficient for Al in β-Ti at near solidus temperature is semiquantitatively calculated, and the value is (6 – 11) × 10−11 m2/s.