TiAl Research Articles

Microstructural Evolution, Intermetallic Formation, and Mechanical Performance of Dissimilar Al6061–Ti6Al4V Static Shoulder Friction Stir Welds

The present study focuses on achieving microstructural uniformity in dissimilar Al–Ti joints through static shoulder friction stir welding (SSFSW). This method restricts material mixing at the Al–Ti interface due to reduced shoulder action, resulting in peak temperatures of 317 °C (SSFSW) and 491 °C in conventional friction stir welding (CFSW). This reduction in heat input during SSFSW leads to decreased mechanical mixing, resulting in a thinner 2.169 μm intermetallic layer at interface of SSFSW compared to 5.485 μm in CFSW. The resulting weld nugget (WN) microstructure reveals the presence of Ti particles, intermetallic compounds, and fine Al‐associated grains. Rearrangement of dislocations through slip‐and‐climb mechanisms, along with distinct recrystallization processes in SSFSW, significantly reduces grain size within WN. The kernel average misorientation map highlights the presence of nonuniform dislocation concentrations at the interface of CFSW, attributed to the large Ti particles that impede the mobility of dislocations. As a result, the interface of CFSW is found to be the weakest among all the zones, eventually leading to the failure of the CFSW joint near the Ti interface after reaching an ultimate tensile strength of 259 MPa. In contrast, SSFSW welds achieve a higher tensile strength of 289 MPa.

Impact of the acceleration voltage on the processing of γ-TiAl via electron beam powder bed fusion

Electron beam powder bed fusion (PBF-EB) is an additive manufacturing (AM) technology that is maturing toward broader industrial applications. However, conventional PBF-EB machines are still limited to 60 kV acceleration voltage (Ub). Therefore, this work presents the first results of a novel prototype PBF-EB machine capable of acceleration voltages up to 150 kV. In general, a higher acceleration voltage enables larger beam powers, which shortens the pre-heating time and makes a larger pre-heating area available. Moreover, a lower beam current is required for the same power during pre-heating, enabling the processing of a gamma titanium aluminide (γ-TiAl) alloy without any process gas. γ-TiAl cuboids are built in a vacuum atmosphere (2×10–5 mbar) with 60 , 125 , and 150 kV acceleration voltage. Additionally, the deeper penetration of higher acceleration voltage should be beneficial for melting as well. Cuboids are examined for defects and aluminum content to show the influence of the acceleration voltage on the process window, melt pool formation, gas porosity, and aluminum evaporation. In short, this work aims to investigate the impact of a higher acceleration voltage on the whole PBF-EB process.

Robust γ-TiAl Dual Microstructure Concept by Advanced Electron Beam Powder Bed Fusion Technology

The dual microstructure concept for gamma titanium aluminides (γ-TiAl) processed via electron beam–powder bed fusion (PBF-EB) provides a huge potential for more efficient jet turbine engines. While the concept is feasible and the mechanical properties are promising, there are still some challenges. For an industrial application, the heat treatment window has to match the conditions in industrial furnaces. This study shows how the required heat treatment window can be achieved via advanced PBF-EB technology. Through using an electron beam with 150 kV acceleration voltage, the difference in aluminum between the designed aluminum-rich and aluminum-lean regions of the part is increased. Moreover, the aluminum content within each of these regions, respectively, is more homogenous compared to the 60 kV acceleration voltage. This combination provides a heat treatment window of 25 °C, enabling the industrial application of the dual microstructure concept for γ-TiAl.

Ti–48Al–2Cr–2Nb alloys prepared by electron beam selective melting additive manufacturing: Microstructural and tensile properties

Here, an electron beam selective melting (EBM) technique was employed to shape Ti–48Al–2Cr–2Nb alloy to combat its intrinsic brittleness and insufficient hot workability. Through process optimization, high-density samples (4.2327 g/cm3) were produced. First, the mechanism of interaction between phase transformation and microstructural evolution of EBM-formed Ti–48Al–2Cr–2Nb alloy was investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscattering diffraction (EBSD). And then, the mechanism of interaction between tensile properties and microstructural evolution of EBM-formed Ti–48Al–2Cr–2Nb alloy was discussed. Based on the results, the microstructure consists of coarse equiaxed grains and fine lamellar grains, which gradually coarsen from upper to bottom. As the thickness of the layer increases from upper to bottom, the content of the B2 phase increases continuously, with a predominant component of TiAl and a small amount of Ti3Al and B2 phases. The majority of the γ-TiAl phase does not exhibit significant texture, while the α2-Ti3Al phase shows a texture of (0001) with 19.32 times the random intensity, and its c-axis is parallel to the deposition direction. Benefiting from its unique structure features, the maximum room temperature tensile strength of as-deposited Ti–48Al–2Nb–2Cr alloy reaches 854.6 MPa with an elongation of over 2%. And the maximum high-temperature tensile strength of EBM-formed Ti–48Al–2Nb–2Cr alloy reaches 707 MPa at 650 °C with an elongation over 3.5%. According to these results, the mechanical properties of our EBM Ti–48Al–2Cr–2Nb alloy are superior to those previously reported.

Effect of scanning speed on microstructure and mechanical properties of as-printed Ti–22Al–25Nb intermetallic by laser powder bed fusion

Ti–22Al–25Nb alloy is a lightweight, high-performance, and high-temperature material with broad application prospects in aerospace. Laser powder bed fusion (LPBF) enables the rapid manufacture of Ti–22Al–25Nb (at. %) powder into complex, high-precision parts for various applications. This paper provides an in-depth analysis of how scanning speed affects the microstructure and mechanical properties of Ti–22Al–25Nb alloy prepared by LPBF. The goal of this study is to improve the mechanical properties and the printing efficiency in the future. The temperature distribution in the printing process was studied by finite element simulation, and the effect of scanning speed was analyzed by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results show that increasing the scanning speed improved the cooling rate of the liquid melt, which decreases the grain size of the β phase and increases of the dislocation density. At the same time, a significant amount of the O phase precipitates along the grain boundaries. Excellent elongation (15%) and yield strength (982 MPa) are obtained due to the pinning of the nano-O phase, a fine grain, and good relative density when the scanning speed is 0.6 and 0.8 m/s, respectively.

Unveiling the dynamic softening mechanism via micromechanical behavior for a near-β titanium alloy deformed at a high strain rate

For near-β high-strength titanium alloys, rheological softening is the key step during thermal deformation to determine the microstructure and mechanical properties. Nevertheless, the intrinsic softening mechanism during the rheological stage is still an open question due to the special complexity of the process. In this study, the deformation process of near-β titanium alloy TC18 (Ti–5Al–5Mo–5V–1Cr–1Fe, wt.%) in a dual-phase region at a high strain rate was investigated. The micromechanical behavior of TC18 alloy during thermal deformation at 785 °C and 0.1/s was revealed by using in-situ high-energy X-ray diffraction (HEXRD). The stress partitioning and microstructure evolution mechanism for this alloy were discussed in detail. It has been confirmed that the main softening mechanisms of TC18 alloy during thermal deformation are dynamic recovery (DR) and continuous dynamic recrystallization (cDRX). The stress partitioning between constituent phases featured by higher stress in the α phase than in the β phase induces an inhomogeneous strain field near α phase lamellae to promote the dislocation pile-up in subgrains. The cDRX process of β grains is composed of dislocation slipping, DR and subgrains formation, and transformation of recrystallized grains with high-angle grain boundaries (HAGB). Schmid factor and micro stress have a remarkable effect on dynamic recrystallization (DRX) behavior. The soft <200>β//LD oriented grains are more likely to undergo cDRX than the hard <111>β//LD oriented grains. This work may have implications for understanding the deformation mechanism and provide a fundamental basis for selecting appropriate processing technology for near-β titanium alloys.

Effect of Al–Ti Concentration on Alumina Inclusions Aggregation Floating Behavior in Molten IF Steel

Understanding the aggregation‐floating behavior of alumina inclusions, which is influenced by the interface properties between inclusions and molten steel, is crucial to control the purity of interstitial‐free (IF) steel in the refining process. In this work, it is attempted to investigate the effect of aluminum and titanium concentrations on the aggregation‐floating behavior of alumina inclusions in the interior of IF molten steel based on the interface properties between molten steel and alumina inclusions. The interface properties between the molten steel and alumina inclusions are measured using an improved sessile drop method. And, the aggregation‐floating behavior of inclusions in the interior of molten steel is quantitatively discussed by an aggregation model based on the attractive force between alumina inclusions, which was combined with the interface properties. In the results, it is shown that the attractive forces between alumina inclusions, ranging from 6.96 × 10−6 to 5.41 × 10−5 N, increase and decrease, respectively, with increasing aluminum and titanium concentrations. Furthermore, the terminal floating speeds of alumina inclusions, ranging from 0.0510 to 0.0873 m s−1, increase and decrease, respectively, with increasing aluminum and titanium concentrations.

Formation and aggregation behavior of inclusions in Ni-based alloys with different Mg contents

Large non-metallic inclusions affect the mechanical properties and service life of nickel-based superalloys. This study investigates the effects of magnesium elements on non-metallic inclusions in K4169 nickel-based superalloys, exploring the formation mechanisms and aggregation relationships of the inclusions. Four gradients of the magnesium content were used to simulate the erosion of MgO refractories on inclusions in the vacuum induction melting of nickel-based superalloys. The observed inclusions mainly comprised MgO, TiN, MgAl2O4, and MgAl2O4–TiN composites. To study the formation mechanism of inclusions in Ni-based alloys with different Mg contents, the predominance diagram of Mg–Al–Ti–O inclusions in nickel-based superalloy systems was calculated using Factsage thermodynamics software and classical thermodynamics separately. The aggregation behavior of MgAl2O4 and MgO inclusions on the surface of molten nickel-based superalloys was observed in situ under a high-temperature confocal laser scan microscope. The capillary force acting on inclusions in nickel-based superalloys was calculated using the Kralchevsky–Paunov model (K–P model), the calculation was verified with experimental results, and the influencing factors were analyzed. The capillary force acting on inclusions on the surface of molten nickel-based superalloys weakened in the order of MgAl2O4 > MgO > Al2O3, which is the opposite of the order for the force acting on inclusions in molten steel. Minimizing the magnesium content in the alloy is crucial to preventing the formation of large inclusions in nickel-based superalloys, and the use of magnesium-containing materials in the smelting process should thus be avoided.

Synthesis, oxidation resistance and mechanical properties of a Cr2AlC-based MAX-phase coating on TiAl

In this study, the oxidation resistance of a Cr2AlC-based MAX-Phase coating on the Ti–48Al–2Cr–2Nb (at. %) alloy was investigated at 700 °C and 800 °C in air. Coatings were synthesized by a combination of DC magnetron sputtering and ex-situ annealing in Ar. The oxidation resistance of the TiAl alloy was greatly improved due to the formation of a protective Al2O3/Cr2O3-scale at both temperatures. During oxidation Al diffuses from the coating into the substrate triggered by the higher Al activity of the Cr2AlC-phase compared to the TiAl substrate. Due to the severe Al-depletion Cr7C3, Cr23C6 as well as Cr2Al become the predominant phases in the coating. As a result of this, the very promising mechanical properties of the coated samples were negatively influenced by the brittle nature of such phases leading to a decrease in the fracture strain.

Effect of scandium on grain refinement mechanism and mechanical properties of Al–7Si–0.6 Mg alloy at different cooling rates

The effects of scandium (Sc) additions on the solidification microstructure and mechanical properties of Al–7Si–0.6Mg-0.25Ti alloy at different cooling rates have been systematically studied by thermal analysis and detailed SEM and TEM microstructure characterization of the castings. It is found that the addition of Sc has significant effects of refining the grain structure and modifying the Al–Si eutectics into fibrous structure. As a result, substantially improved strength and ductility in the as-cast state have been achieved in the Sc-added alloy. More importantly, the addition of Sc reduces the sensitivity of the alloy's strength and ductility to local cooling rates in the castings. The underlying mechanisms on the grain refinement, modification of Al–Si eutectics and strengthening by Sc addition have been studied. A new phase, DO22 structured (Al, Si)3(Ti, Sc), has been found to form in the liquid aluminium, acting as high potency nucleation sites of Al grains, which overcomes the Si poisoning effect on grain refinement for Al–Ti–B grain refiners.

Effect of in-situ post-heating on the microstructure and tensile performance of TiAl alloys produced via selective electron beam melting

Selective electron beam melting (SEBM) is an additive manufacturing technique commonly used to fabricate TiAl alloys. Changing the process parameters to adjust the alloy structure for enhanced mechanical performance is an effective means in this field. Herein, we investigated the effects of the in-situ post-heating process on the microstructure and tensile performance characteristics of SEBM Ti–48Al–2Cr–2Nb alloys. Two processes were designed: without and with post-heating after melting under identical melting process and total energy input conditions. The tensile strength of the samples with the post-heating process reached 707 ± 12 MPa and strain reached 1.8 ± 0.1%. Compared with the other process, the strain increases by 50%, and the strength decreases by 4.8%. This phenomenon was mainly caused by the more precipitation of γ-phase during the phase transformation of TiAl via an in-situ post-heating process. Even with the same energy input, different heating strategies still significantly affect the tensile performance of TiAl alloy, showing that post-heating is an effective way to improve mechanical properties of additive manufactured material.

Enhancement of room-temperature mechanical properties of TiAl alloy by trace addition of C

Lightweight and high-temperature-resistant γ-TiAl based alloys have attracted considerable research attention for application in aerospace and automotive engines. For TiAl alloys, C alloying is usually beneficial for improving the high-temperature mechanical properties, while deteriorates the room-temperature (RT) mechanical properties, particularly the plasticity. This study demonstrates that trace C alloying can significantly improve the RT mechanical properties of a cast Ti–46Al–6Nb alloy. The initiation and propagation of microscopic cracks during three-point bending with and without C microalloying were analyzed. A comprehensive effect that involves segregation, phase content, hardness, lamellar spacing, and lamellar interface improvement is proposed for the simultaneous enhancement of the strength and plasticity with trace C addition. This study offers an accurate and feasible strategy to utilize the solid-solution strengthening of interstitial C in TiAl alloys.

Effect of structure and phase composition on the physical and mechanical properties of hot extruded titanium alloy Ti–3Al–2.5V tubes

This study investigates the impact the hot extrusion process variables on the physical and mechanical properties of Ti–3Al–V alloy The research examines four tube segments extracted from various hot-extruded tubes of Ti–3Al–2.5V alloy, with an outer diameter (OD) of 90 mm and a wall thickness of 20 mm. The manufacturing process involves expanding sleeves with a horizontal hydraulic press to achieve an OD of 195 mm, followed by heating to 850–865 °C prior to extrusion. The tube segments are labeled as 1, 2, 3, and 4, corresponding to their order of production. Our findings demonstrate that an increase in the number of extrusions in the α + β area from tube 1 to tube 4 leads to a reduction in the primary α-phase volume fraction and an increase in the β-transformed structure volume fraction. These changes are attributed to the higher final extrusion temperature resulting from more intense deformation heating during hot tooling (die and mandrel) processes. Additionally, elevating the final extrusion temperature from tube 1 to tube 4 leads to a notable decrease in the residual β-solid solution volume fraction and a reduction in the “sharpness” of the α-phase tangent-oriented texture. The alterations in the structural and phase state of the alloy from tube 1 to tube 4 are found to influence the contact modulus of elasticity and microhardness. These identified relationships can be utilized to optimize the process variables for the extrusion of multiple Ti–3Al–2.5V alloy tubes.

Dissimilar friction stir butt welding of AA7075-T6 Al and Ti6Al4V Ti plates: Mechanical and metallurgical analysis

Dissimilar friction stir butt welding of AA7075-T6 Al and Ti6Al4V Ti plates: Mechanical and metallurgical analysis

Study on the magnetic viscosity of multi-step magnetized heterogeneous alloys

The multi-step magnetization behavior caused by a variety of microstructures is common in magnetic materials. The study of the effect of heterogeneous structure magnetization behavior on magnetic viscosity provides extensive guidance for the long-term stability of magnetic materials. The microstructure of Fe–Co–Ni–Al–Ti–Cu alloy was regulated by heat treatment process, and homogeneous structure (S810) and heterogeneous structure (S830) samples were prepared. In the S830 sample, there are three steps of magnetization reversal due to the presence of α1′ phases (split α1 phases). Part of the α1′ phases are reversed under a lower magnetic field, and the other part has strong interaction with the α1 phases forming a closed stable magnetic domain structure, which enhances the switch field of α1 phases. Both first-order reversal curves (FORC) analysis and electron holography have confirmed this phenomenon. The magnetic viscosity coefficient of S830 is higher, but its magnetic field dependence is better. The quantitative calculation of energy barriers shows that the energy barriers of α1′ phases are lower, which leads to the instability of S830 sample under low magnetic fields. The superposition of three magnetization reversal steps makes the variation of magnetic viscosity coefficient gentler.

Effect of the Ti volume fraction on microstructure and mechanical properties of brick-and-mortar structure Ti/Al3Ti metal-intermetallic laminate composites

The brick-and-mortar biomimetic structure Ti/Al3Ti metal-intermetallic laminate (BMS-MIL) composites were prepared by vacuum hot pressing. BMS-MIL composites with various Ti volume fractions from 45–71% were obtained by controlling the high-temperature diffusion time of Al–Ti. The phase composition of the BMS-MIL composites was analyzed, and the influence of Ti volume fraction on the microstructure was quantitatively characterized. Quasi-static compression and three-point bending tests were carried out to study the mechanical properties and fracture mechanism of the BMS-MIL composites. The results indicated that, in BMS-MIL composites, Al3Ti appears as polygonal platelets, and Ti fills the gaps between the platelets. With decreasing Ti volume fraction, the size of Al3Ti platelets increased, the number of platelets decreased, and the aspect ratio of platelets first increased and then decreased. The decrease of Ti volume fraction leads to the decrease of mechanical properties of composites. However, as a reasonable and efficient material design strategy, the brick-and-mortar structure ensured that the specific strength of the BMS-MIL composites with load perpendicular to the layer remained stable, with specific strength maintained between 345 and 353 kN m/kg. The compression properties with load parallel to the layer are significantly affected by the platelet's aspect ratio. The sample with 65% volume fraction Ti exhibited the optimal strength, specific strength, and failure strain when compressed parallel to the layer, with a strength of 1347.3 MPa, specific strength of 330.2 kN m/kg, and a failure strain of 6.32%.

Influence of chemical composition on coarsening kinetics of coherent L12 precipitates in FCC complex concentrated alloys

In this study, we report the experimental coarsening kinetics at 850, 900 and 950 °C of four complex concentrated alloys in the Al–Ti–Cr–Fe–Co–Ni senary system with different chemical compositions but a similar γ’ (L12) volume fraction (∼35 % at 950 °C) in a face-centered cubic (γ, FCC) matrix. The selected alloys were specifically designed to investigate the influence of Fe additions and Ni–Co substitutions on Ostwald ripening kinetics. Atom Probe tomography (APT) was used to determine the compositions of the FCC and L12 phases, which agree very well with Calphad calculations at thermodynamic equilibrium. Thermo-kinetic modeling of L12 precipitation was carried out using the Prisma module developed by Thermo-Calc and compared with experimental results. Apparent activation energies were determined and discussed in light of diffusion-controlled coarsening models to identify the key parameters affecting Ostwald ripening. We suggest that the abnormally high apparent activation energies results from composition-dependent parameters. When the latter are accounted for, the corrected activation energies for coarsening are in better agreement with available diffusion data.

Tailoring the Microstructure of Laser-Additive-Manufactured Titanium Aluminide Alloys via In Situ Alloying and Parameter Variation

Titanium aluminide (TiAl) alloys have emerged as promising materials for high-temperature applications due to their unique combination of high-temperature strength, low density, and excellent oxidation resistance. However, the fabrication of TiAl alloys using conventional methods is challenging due to their high melting points and limited ductility. Selective laser melting (SLM), an additive manufacturing technique, offers a viable solution for producing TiAl alloys with intricate geometries and the potential for tailoring their microstructure. This study investigates the effect of in situ copper alloying and multiple laser scans on the microstructure and mechanical properties of TiAl-based alloys fabricated using SLM. The results demonstrate that copper alloying enhances the formation of the α2-Ti3Al phase, refines the microstructure, and improves the mechanical properties of TiAl alloys. Multiple laser scans allow for the creation of distinct microstructural regions within a single component, enabling the tailoring of properties that are suitable for specific operating conditions. The findings provide valuable insights into the fabrication and optimization of TiAl intermetallic alloys with diverse applications.

Assessment of the capability of Zr, Y, La, Gd, Dy, C, and Si to enhance the creep strength of gamma titanium aluminide alloys based on their effect on flow stress and thermal activation parameters

The present work examines the capacity of the elements Zr, Y, La, Gd, Dy, C, and Si to increase the structural stability and creep strength of γ titanium aluminide alloys. The elements were alloyed for this purpose to the base material Ti-44at.%Al-5at.%Nb, and were transformed into four different states of precipitate dispersions via thermomechanical treatment. The resulting materials were deformed in compression at room temperature as well as at 1023 and 1173 K. Strain-rate cycling was performed during the tests to determine the associated flow stresses and the reciprocal activations volumes of plastic deformation. This information has been used to estimate the impact of the precipitates on the athermal component of the flow stress, which significantly influences the creep properties. The results indicate that only the elements C and Si are able to effectively enhance the creep strength in the long term. The element C forms Ti3AlC precipitates, which lead to a considerable increase of the athermal stress component, whereas Si forms Ti5Si3 precipitates leading to a moderate increase of the athermal stress component but a comparatively stable microstructure. Moreover, it appears that the strengthening effects could be increased by using higher additions of C or Si.

Improving Forging Outcomes of Cast Titanium Aluminide Alloy via Cyclic Induction Heat Treatment

The objective of this research was to improve the forging outcome of peritectic solidifying cast titanium aluminide (TiAl) 4822 alloy (Ti-48Al-2Nb-2Cr at.%) in hot isostatic pressed and homogenised (HH) condition using cyclic induction heat treatment (CHT). This study adds to research around CHT for TiAl alloys by applying industrially relevant induction heating to conduct five heating cycles at the single αphase temperatures (1370 °C) necessary for grain refinement. Two cooling rates were explored in each cycle, air cooling (ACCHT) and controlled furnace-like cooling (FCCHT), without returning to room temperature. Samples were assessed at each stage in terms of their morphologies, lamellar grain size and content, as well as phase and dynamic recrystallised fraction, and subsequent primary and secondary compression behaviour with uniaxial isothermal compression. The FCCHT process resulted in a homogeneously refined fully lamellar microstructure, and ACCHT, in a heterogeneous microstructure consisting of lamellar and feathery γ (γf) at differing fractions across the piece, depending on the cooling rate compared with HH. The results show that CHT improved forging outcomes for both compression stages investigated, resulting in uniform compression samples with higher volumes of dynamic recrystallised material compared with the instability seen with the compression of HH material.