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An metal–intermetallic material based on the combustion products of the layered system (Ti–Al–Mg)/Ti was first obtained using the self-propagating high-temperature synthesis (SHS) combined with pressing. Exothermic synthesis from elemental powders was carried out at a pressure of 10 MPa, and the hot product of synthesis was pressed at a pressure of 250 MPa. It was demonstrated that the SHS pressing process results in an inseparable bond between the "intermetallic/metal" layers. The main features of microstructure formation, phase composition, and strength properties of the transition zones at the interface between the reacting SHS compositions Ti–Al–Mg and the Ti substrate were investigated. The thickness of the transition zone between the layers was at least 15 μm. Energy-dispersive analysis (EDA) showed that Mg in the synthesized alloy is mainly located in intergranular layers, in the form of a small amount of Ti–Al–Mg compound, indicating incomplete reaction diffusion between the Ti–Al and Al–Mg layers. The microhardness of Ti–Al grains in the synthesized alloy is 5820 MPa on average, while that of the matrices based on Al–Mg is 3980 MPa. The hydrostatic density is 3.3 g/cm3, with a porosity of less than 13%. The porosity of the Ti–Al–Mg alloy obtained by the SHS pressing method was reduced by three times. [ABSTRACT FROM AUTHOR]

The morphology of microstructure and tensile properties of Ti-48Al-2Cr-2Nb (at％) alloy rods fabricated by the electron beam powder bed fusion (EB-PBF) process were investigated with a particular focus on the influence of scan speed of the electron beam. Homogeneous near lamellar structure composed of the α2 and γ phases can be obtained in the rod fabricated under the slowest scan speed. The fine lamellar spacing which contributes to the high strength of the alloy is derived from the fast-cooling rate of EB-PBF. On the contrary, a layered microstructure comprising a duplex-like region and an equiaxed γ grain layer (γ band) perpendicular to the building direction is obtained when increasing scan speeds. We observed for the first time that an increase in the scan speed results in a narrow width of the γ band. We also found that these microstructural changes have a significant influence on the mechanical properties of the rods. The near lamellar structure exhibits higher strength compared to the layered microstructure. Whereas, the rods with the layered microstructure show large ductility at room temperature. The elongation of each rod strongly depends on the width of the γ band owing to the preferential deformation of the γ band. [ABSTRACT FROM AUTHOR]

The low ductility of titanium aluminide at room temperature has been one of the factors that severely limit its applications. In this study, Ti–6Al–4V (Ti64) alloys were combined with Ti–47Al–2Cr–2Nb (TiAl) alloys to make micro-laminated composites with optimal ply configurations to eliminate crack formation in TiAl and improve the ductility. A dual-material laser powder bed fusion (LPBF) system was used to fabricate the TiAl/Ti64 micro-laminate samples. The presence of cracks, elemental composition and mechanical properties were experimentally characterised and analysed to understand the effect of ply configuration and the mechanism of eliminating cracks. It was found that smaller TiAl ply thickness and lower TiAl volume fraction result in fewer cracks formed in the micro-laminates. Elemental composition analysis reveals that the Al content in the first two powder layers (i.e., 200 μm thick in total) of a TiAl ply is reduced to 10%–35% due to the partial mixture of the TiAl ply and the proceeding Ti64 ply in the LPBF. The reduced Al content leads to combined α-Ti, β-Ti and α2-Ti3Al phases instead of γ-TiAl phases, thus eliminating the formation of cracks within the first 200 μm thick region of the TiAl ply. This study demonstrates that crack-free and dense TiAl/Ti6 micro-laminated composites can be produced with the TiAl ply thickness reduced to 200 μm and the TiAl volume fraction reduced to 25% (Ti64 ply thickness 400 μm). Ductility of the crack-free micro-laminates at room temperature is comparable to or even exceeds the properties of TiAl/Ti64-type laminates made by other manufacturing processes, which may imply good machinability. Note that further research is required to investigate the properties and applications of the micro-laminates at elevated temperatures.

The influence of furnace annealing on the structure of cast metal in ingots of 200 mm diameter of the intermetallic Ti-28Al-7Nb-2Mo-2Cr titanium alloy produced by the method of electron beam melting and its welded joints produced by the method of electron beam welding was determined. It was determined that the EBM metal produced from the ingots of 200 mm diameter is satisfactorily welded under the conditions of applying such additional technological measures as preheating and local heat treatment. It is shown that annealing at the temperature of 1260 °C for 10 h led to the formation of a uniform microstructure in the base metal, HAZ and weld metal, decomposition of the duplex structure and absence of regions with a two-phase (γ+α2)-lamellar structure. The room temperature strength of welded joints after annealing is equal to 746 MPa or 98 % of the base metal strength. [ABSTRACT FROM AUTHOR]

This study examines the influence of hot isostatic pressing and heat treatment on the microstructure and mechanical properties of specimens manufactured by selective electron beam melting (SEBM) of the metal powder composition (MPC fraction 40–100 μm) of a new six-component intermetallic beta-solidifying TiAl alloy Ti–44.5Al–2V–1Nb–2Cr–0.1Gd, at % (Ti–31.0Al–2.5V–2.5Nb–2.5Cr–0.4Gd, wt %). It is shown that SEBM with a high line energy input (EL= 285 J/m) produces a fine-grained microstructure in the as-built material with a grain size of 5–14 μm and residual porosity of less than 0.5 vol %. An increase in the electron beam current (I) from 9.5 to 19.0 mA intensifies Al evaporation, as a result, the fraction of large columnar grains (d = 30–100 μm in width, h = 150–400 µm in height) formed mainly in Al-depleted regions (layers) increases. Heat treatment of the as-built SEBM specimens by two-stage annealing in the (α + γ)- and (α2 + γ + β)-phase fields or by thermal cycling in the (α + γ)-phase field leads to complete or partial fragmentation of columnar grains. Combined postprocessing of the specimens produced at lower I by hot isostatic pressing in the α-phase field and two-stage annealing completely eliminates residual porosity and transforms the columnar structure into a fine-grained one with the grain size less than 150 μm. As a result, the achieved short-term mechanical characteristics at 20°С (UTS = 525 ± 5 MPa, δ = 1.1%) and 750°С (UTS = 405 ± 10 MPa, δ = 3.8%) are comparable to those of the studied TiAl alloy in the as-cast state. [ABSTRACT FROM AUTHOR]

This article evaluates the fabrication technologies of titanium aluminide (Ti-Al) and its practical applications by comparing it with the well-known Ti-Al binary phase diagram and US patents. Meanwhile, by analyzing and discussing the various patented Ti-Al fabrication technologies and applications, this article discusses the applications of Ti-Al-based alloys, mainly in the aircraft field. The improved fabrication processes and new application technologies are under patent protection. These technologies are classified into six categories: basic research on Ti-Al-based alloys, powder metallurgy of Ti-Al-based alloys, casting and melting of Ti-Al-based alloys, PM and AM manufacturing methods for aircraft applications, other fabrication technologies by Ti-Al-based alloys, and self-propagating high-temperature synthesis (SHS) of Ti-Al-based alloys. By comparing the principles and characteristics of the above techniques, the advantages, disadvantages, and application fields of each are analyzed and their developments are discussed. Based on the characteristics of Ti-Al, new fabrication and application technologies can be developed, which can overcome the existing disadvantages and be used to form new aircraft components. [ABSTRACT FROM AUTHOR]

The influence of furnace annealing on the structure of cast metal in 200 mm ingots from intermetallic titanium alloy Ti–28Al–7Nb–2Mo–2Cr produced by electron beam melting and of its welded joints produced by the method of electron beam welding was determined. It is established than the metal of 200 mm Ti–28Al–7Nb–2Cr–2Mo ingots based on titanium aluminide is readily welded under the conditions of application of such technological measures as preheating and local heat treatment. It is shown that annealing at the temperature of 1260 °C for 10 h led to formation of a uniform microstructure in the base metal, HAZ and weld metal, decomposition of the duplex structure and absence of regions with a two-phase (γ+α2)-lamel structure. Room temperature strength of welded joints after annealing is equal to 746 MPa or 98 % of base metal strength. [ABSTRACT FROM AUTHOR]

Works have been performed on optimization of the technological scheme of producing ingots for consumable electrodes with a stable chemical composition and properties. The paper presents the results of studying an ingot of 195 mm diameter from a titanium aluminide-based alloy of Ti-28Al-7Nb-2Mo system, made by double electron beam remelting. Further ingot remelting in the arc furnace was performed, which allowed producing a homogeneous and defect-free ingot of an optimal composition of Ti-28Al-7Nb-2Mo-0.3 (Y, Re, B). The influence of modifying on the structure and properties was studied. It was determined that addition of surfactants promotes refining of the structural components and improvement of the alloy mechanical properties. [ABSTRACT FROM AUTHOR]

Work has been performed on optimization of the technological scheme of producing ingots for consumable electrodes with a stable chemical composition and properties. The paper presents the results of studying an ingot of 195 mm diameter from a titanium aluminide-based alloy of Ti–28Al–7Nb–2Mo system, made by double electron beam remelting. Further ingot remelting in the arc furnace was performed, which allowed producing a homogeneous and defectfree ingot of an optimal composition of Ti–28Al–7Nb–2Mo–0.3 (Y, Re, B). The influence of modifying on the structure and properties was studied. It was determined that addition of surfactants promotes refining of the structural components and improvement of the alloy mechanical properties. [ABSTRACT FROM AUTHOR]

The microstructure of conventionally solidified and rapidly solidified Ti–46Al–8Nb-2.5V alloys was analyzed with phase transformation geometric models, revealing the phase transformation mechanisms. The β→α→γ phase transformation processes follow Burgers and Blackburn orientation relationships (ORs), respectively, from which the calculated transformation between the β/β0 and γ phases follow K–S OR. Whlie, the direct β/β0→γ phase transformation can be directly observed in the microstructure, which follows a variety of ORs, mainly following the K–S OR but also following the N–W or Bain ORs, most of which deviate from the strict ORs. In cases where N–W and Bain ORs are followed, there is a large dislocation spacing between the habit planes of the β/β0 and γ phases, resulting in low static energy at the interface and narrow elongated morphology of precipitated phases. Conversely, when following K–S OR for β/β0 → γ phase transformation, interfacial migration energy is relatively lower and grain precipitated phases is larger. The rapidly solidified microstructure of Ti–46Al–8Nb-2.5V alloy consists of the massive α2 phase, massive γ phase, and basket-weave β0 phase closely resembling those found in β-type titanium alloys. After annealing at service temperature, the rapidly solidified alloy forms a microstructure highly similar to that of conventionally solidified TiAl alloy. Various metastable phases, such as the Ti2Al, L12 and Ti1.4Al phases, precipitate in the blocky γ phase, which provides a theoretical basis for studying the phase transformations of TiAl alloys in service.

Currently, there is a request from aerospace and aircraft for the construction materials with sufficiently high mechanical strength, thermal creep, corrosion and oxidation resistance. The conventional alloys used for these purposes are too heavy. At the same time, alternative light materials such as Ti–Al-based alloys have many flaws, when they are produced by conventional methods. This work considers the possibility to produce the Ti–Al-based alloys by the method of a wire-feed electron-beam additive manufacturing (EBAM). We study the chemical and phase compositions, microstructure and microhardness of a bimetallic Ti–Al alloy, obtained by this method. It is found the formation of five characteristic regions between titanium and aluminum parts of the bimetallic billet. The mixing zone consists of TiAl and TiAl3 intermetallics, that is confirmed by the investigation of microstructure, chemical and phase compositions. According to XRD (X-ray diffraction) and EDS (energy-dispersive X-ray spectroscopy) analyses, it can be assumed that TiAl intermetallic prevails over TiAl3 one. The average microhardness of the mixing zone equals to 450 HV (≈4.4 GPa). This zone has developed dendritic microstructure, and even distribution of the phases without link to dendritic and inter-dendritic zones. The cracks appearing in this area are filled with the material of the upper layers, so the whole material is poreless and defect-free. Thus, the results of this work have shown a fundamental possibility to produce the intermetallic Ti–Al alloys with the use of the EBAM.

Al–3Ti–1B and Al–5Ti–1B master alloys represent the most popular modifiers for the majority of aluminum alloy groups. To date, several methods, requiring various conditions and resource costs, have been applied for producing these master alloys. The paper presents the results of a comparative analysis of methods for producing Al–3Ti–1B master alloy using both a Ti sponge and salts (K2TiF6 and KBF4) as the starting components. The degree of Ti and B recovery for three methods of the master alloy production was experimentally established. The phase and element compositions of slags were studied; the amount of fluorine gas, emitted during the master alloy production, was calculated. A method, using a titanium sponge and molten KBF4 + KCl mixture, was identified to ensure a recovery of Ti and B equal to 88.10 and 72.20%, respectively. The modifying ability of the produced master alloys was studied both according to the TP-1 method and using the laboratory and industrial unit of semi-continuous casting. [ABSTRACT FROM AUTHOR]

Effect of N addition (0.5, 1, and 2 at. %) on solidification behavior and microstructural features of the cast and hot isostatically pressed (HIPed) Ti–48Al–2Cr–2Nb (4822) intermetallic alloy was investigated. Alloys were fabricated using vacuum arc re-melting followed by HIPing. The results showed that N addition changes the primary β phase dendrites to α phase. The morphological uniformity of the as-cast microstructures was notably increased by N addition. In addition, N addition beyond its solubility limit led to the formation of primary Ti2AlN precipitates located within lamellar colonies. A possible mechanism for nucleation of the Ti2AlN precipitates is proposed. Furthermore, N addition caused much more microstructure stability during HIPing. Although a duplex structure was formed after HIPing in the 4822 alloy, N addition decreased the formation of this structure. Moreover, highly extended network of secondary Ti2AlN precipitates were formed in almost all of (α2+γ) lamellas interfaces in the N-bearing alloys.

This article evaluates the fabrication technologies of titanium aluminide (Ti-Al) and its practical applications by comparing it with the well-known Ti-Al binary phase diagram and US patents. Meanwhile, by analyzing and discussing the various patented Ti-Al fabrication technologies and applications, this article discusses the applications of Ti-Al-based alloys, mainly in the aircraft field. The improved fabrication processes and new application technologies are under patent protection. These technologies are classified into six categories: basic research on Ti-Al-based alloys, powder metallurgy of Ti-Al-based alloys, casting and melting of Ti-Al-based alloys, PM and AM manufacturing methods for aircraft applications, other fabrication technologies by Ti-Al-based alloys, and self-propagating high-temperature synthesis (SHS) of Ti-Al-based alloys. By comparing the principles and characteristics of the above techniques, the advantages, disadvantages, and application fields of each are analyzed and their developments are discussed. Based on the characteristics of Ti-Al, new fabrication and application technologies can be developed, which can overcome the existing disadvantages and be used to form new aircraft components.

The effect of the linear energy input (EL), varied from 120 to 300 J/m due to a change in the electron beam current (I) from 4 to 20 mA and the scanning speed (v) from 2 to 8 m/s, on residual porosity, aluminum content and local distribution, and microstructure has been studied on as-built samples of a new six-component intermetallic beta-solidifying TiAl alloy Ti–44.5Al–2V–1Nb–2Cr–0.1Gd, at.% (Ti–31.0Al–2.5V–2.5Nb–2.5Cr–0.4Gd, wt.%) produced by selective electron beam melting (SEBM). It has been established that layer-by-layer processing at EL ≥ 250 J/m allows producing dense material with a residual porosity of less than 1 vol.%. It has been revealed that an increase in EL results in the average aluminum content reduction by ΔAl = 1.7 at.% at EL = 300 J/m and distribution inhomogeneity thereof in the vertical cross-section of the samples (building direction Z), with the formation of layered structure consisting of Al-enriched and Al-depleted alternating layers. It has been shown that a fine-grained equiaxed microstructure is formed after processing at EL < 200 J/m and I < 8 mA; an increase in EL and I results in the formation of columnar grains, predominantly located in Al-depleted layers, as well as an increase in their size and volume fraction. After all studied SEBM modes, both columnar and equiaxed grains exhibit lamellar intragranular ( γ + α 2 ) colonies with γ and β0/B2 phases at grain boundaries, which corresponds to the sequence of phase transformations predicted by thermodynamic simulation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The microstructures of intermetallic γ-titanium aluminide (TiAl) alloys are subjected to a certain degree of Al evaporation when processed by electron beam powder bed fusion (EB-PBF). The magnitude of the Al-loss is mainly correlated with the process parameters, and highly energetic parameters produce significant Al evaporation. The Al-loss leads to different microstructures, including the formation of inhomogeneous banded structures, thus negatively affecting its mechanical performance. For this reason, the current work deals with creating EB-PBFed TiAl capsules with the inner part produced using only the pre-heating step and melting parameters with low energetic parameters applying high beam speed from 5000 to 3000 mm/s. This approach is investigated to reduce the Al-loss and microstructure inhomogeneity after hot isostatic pressing (HIP). The results showed that the HIP treatment effectively densified the capsules obtaining a relative density of around 100%. After HIP, the capsules produced with the inner part melted at 3000 mm/s presented a lower area shrinkage (around 6.6%) compared to the capsules produced using only the pre-heating step in the core part (around 20.7%). The different magnitudes of shrinkage derived from different levels of residual porosity consolidated during the HIP process. The HIPed capsules exhibited the presence of previous particle boundaries (PPBs), covered by α2 phases. Notably, applying low energetic parameters to melt the core partially eliminates the particles' surface, thus reducing the PPBs formation. In this case, the capsules melted with low energetic parameters (3000 mm/s) exhibited α2 concentration of 3.5% and an average size of 13 µm compared to the capsules produced with the pre-heating step in the inner part with an α2 around 5.7% and an average size around 23 µm. Moreover, the Al-loss of the capsules was drastically limited, as determined by X-ray fluorescence (XRF) analysis. More in detail, the capsules produced with the pre-heating step reported an atomic percentage of Al of 48.75, while using low energetic melting parameters led to 48.36. This result was interesting, considering that the massive samples produced with standard parameters (so more energetic ones) revealed atomic Al percentage from 48.04 to 47.70. Finally, the recycled small particles showed a higher fraction of α2 phases with respect to the coarse particles, as determined by X-ray diffraction (XRD). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have studied the technological properties, chemical composition, and structure of a metal powder composition (MPC, 40–100 μm) made from a new six-component intermetallic β-solidifying TiAl-alloy Ti–44.5Al–2V–1Nb–2Cr–0.1Gd, at. % (Ti–31.0Al–2.5V–2.5Nb–2.5Cr–0.4Gd, wt.%) by crucibleless induction melting inert gas atomization and intended for additive manufacturing of dense parts by selective electron beam melting (SEBM). It is shown that the main elements of the composition of the MPC are the same as in the electrode, and the phase composition of the MPC corresponds to a rapidly solidified metastable state (γ + α + β). High flowability and sphericity of the MPC were achieved. The Al content in the starting electrode reduces by 1.1–1.3 at.% (1.0–1.2 wt.%) after SEBM due to evaporation. The microstructure of the as-SEBM-built testpieces exhibited intragranular lamellar (γ + α2)-colonies and some γ- and β0/B2-phases at grain boundaries. It was found out that the cross-sectional distribution of Ti and Al along the building direction (Z) is nonuniform, resulting in nonuniform distribution of phases and grain sizes. No areas with duplex or coarse-grained γ-phase, which are characteristic for the 2nd-generation electron beam melted TiAl-alloy Ti-4822, were identified. [ABSTRACT FROM AUTHOR]

The mechanism of pore formation associated with intermetallic reaction is studied in sintered Ti50Al50 alloy with two different heating rates i.e. 1 °C/min and 10 °C/min. The heating rate significantly influences the pore parameters and the intermetallic reactions. The pore size and porosity increase with the extent of exothermic reactions during sintering. The intermetallic Ti2Al5 phase forms by a low-temperature solid-state diffusion of Al at the surface of Ti skeletons, while TiAl3 and TiAl2 intermetallic phases form at a high temperature owing to Ti2Al5 phase reaction with Al and Ti, respectively. A considerable change in thermal and volume expansion take place in the alloy heated with 10 °C/min compared to 1 °C/min. Therefore, it is found that the heating rate during sintering greatly influences the porous properties of Ti50Al50 by controlling the degree of exothermic reactions.

Tin (Sn) can be used as an effective TiAl powder sintering aid, but it is unclear whether Sn will deteriorate its properties as a micro-alloyed element in TiAl intermetallic. This work focuses on the influence of Sn on the mechanical properties and oxidation resistance of TiAl alloys. The effect of Sn on the mechanical properties of TiAl alloys is investigated by experiments and first-principles calculations. Sn atoms preferentially occupy the Al sites in γ-TiAl alloys and increase the lattice volume, which is consistent with the XRD pattern. The elastic constants of TiAl alloys with different Sn contents are also systematically presented. It is found that 1.85 at.% Sn addition improves the ductility of TiAl, but a higher Sn content reduces the ductility. This result is also verified by room temperature compression test. The density of states is also used to prove the ductility improvement of TiAl alloys with a certain amount of Sn. Last, the diffusion barrier of O and Al/Ti–O bond strength were calculated to evaluate the oxidation resistance of TiAl–Sn alloys. The addition of Sn markedly increases the O diffusion barrier and Al–O bond force constant, indicating that Sn can improve the oxidation resistance of TiAl alloy.

Herein, an innovative powder‐cored wire arc additive manufacturing (PC‐WAAM) process is proposed to fabricate γ‐TiAl thin‐walled intermetallic alloy. The metallography, phase composition, and mechanical properties at different thin‐wall locations are characterized. The results show that the alternatively distributed layer‐like microstructure composed of α2 (Ti3Al) and γ (TiAl) phases is obtained along the building direction. The content of α2 phase exhibits the tendency of decreasing from the bottom to top region. This unique microstructure characteristic is closely related to the typical thermal cycling history during deposition. Moreover, the tensile strength and microhardness of the top region are lower than the middle and bottom region. In general, the current PC‐WAAM technique shows promising capability of fabricating γ‐TiAl intermetallic alloy with low cost. This work becomes a valuable reference for understanding the evolution mechanism of microstructure and paves the way for the flexible and customized additive manufacturing of γ‐TiAl alloy. [ABSTRACT FROM AUTHOR]

The morphology of microstructure and tensile properties of Ti48Al2Cr2Nb (at%) alloy rods fabricated by the electron beam powder bed fusion (EB-PBF) process were investigated with a particular focus on the influence of scan speed of the electron beam. Homogeneous near lamellar structure composed of the ¡2 and £ phases can be obtained in the rod fabricated under the slowest scan speed. The fine lamellar spacing which contributes to the high strength of the alloy is derived from the fast-cooling rate of EB-PBF. On the contrary, a layered microstructure comprising a duplex-like region and an equiaxed £ grain layer (£ band) perpendicular to the building direction is obtained when increasing scan speeds. We observed for the first time that an increase in the scan speed results in a narrow width of the £ band. We also found that these microstructural changes have a significant influence on the mechanical properties of the rods. The near lamellar structure exhibits higher strength compared to the layered microstructure. Whereas, the rods with the layered microstructure show large ductility at room temperature. The elongation of each rod strongly depends on the width of the £ band owing to the preferential deformation of the £ band. [ABSTRACT FROM AUTHOR]

The products of the combustion synthesis in titanium, silicon, and aluminum powder mixtures under a wave combustion mode are studied using X-ray diffraction analysis and optical metallography. The synthesis products contain titanium silicide Ti5Si3 and titanium aluminide TiAl3, the ratio of which depends on the content of aluminum powder in the reaction mixtures. The combustion temperature of the mixtures decreases with an increase in the aluminum content owing to the substitution of titanium aluminide in the synthesis products for titanium silicide, which has a multiply less negative enthalpy of formation compared to that of the silicide. The effect of the combustion temperature on the dispersity of the structure of the synthesis products is considered using the concepts of the features of the growth of silicide and aluminide nuclei in a liquid metal solution. [ABSTRACT FROM AUTHOR]

In this work, an ultrafine-grained high-Nb–TiAl alloy with a nominal composition of Ti–45Al–8Nb–0.2W–0.2B (at%) was prepared by cryomilling and subsequent spark plasma sintering (SPS) technique. The chemical composition, particle size, morphology and crystallite size of cryomilled powder were studied. It is found that cryomilling can effectively reduce the particle size and enhance grain refinement. The ingots sintered at 900 and 1000 °C show an equiaxed near-γ microstructure with grain sizes < 700 nm, while the sample sintered at 1100 °C exhibits duplex microstructure. Especially, the one sintered at 1000 °C has excellent mechanical properties, whose compression yield strength, fracture strength, bending strength and plastic strain achieve 1310, 2174, 578 MPa and 16.8%, respectively. The reasons for the effect of cryomilling and the mechanical behavior of sintered ingots were discussed. It is suggested that cryomilling in combination with SPS is an effective way to synthesize high-Nb–TiAl alloy with ultrafine-grained structure. [ABSTRACT FROM AUTHOR]

Twin-wire plasma arc additive manufacturing (TW-PAAM) is an innovative process for titanium aluminide manufacture which is low in equipment cost and high in efficiency. However, as an in situ alloying process, the composition homogeneity is typically a source of concern. The present work investigates this subject by observing the element input mode at various welding wire positions with a high-speed camera system and particle tracking method. It is found that when the welding wires are in a high position, the droplet enters the molten pool from the tip of the titanium wire, and the alloy element input interval is long, resulting in the formation of bands with high aluminium content at the edge of the deposition layer. When the welding wires are close to the workpiece surface, the droplet transfer mode switches to a mixture of dual wire-bridge transfer and Ti wire-bridge transfer. The diameter and transfer interval of the droplets decrease, as well as the heterogeneity of the deposition layer. The conclusion establishes a correlation between droplet transfer mode and composition homogeneity, which is beneficial for optimizing the TW-PAAM titanium aluminide fabrication process. [ABSTRACT FROM AUTHOR]

In this article, an attempt was made to investigate the effect of the addition of silicon on the titanium aluminide compound formation from TiO2 and Al raw materials. Silicon has a eutectic with aluminum and can reduce its melting temperature, which can have effective impact on the formation of titanium aluminide compounds. On the other hand, due to the production of Ti5Si3 compound, it can be effective in increasing the oxidation resistance of compounds and composites containing titanium aluminide. With this aim in mind, the present paper has strove to determine the impact of this element on the formation of titanium aluminides and the type of titanium silicide phases by adding different amounts of silicon to TiO2 and Al raw materials. The key findings emerged, showed that when silicon is added to the system in small amounts (0.1), it not only does not have a positive effect on the reactions, it also prevents the formation of titanium aluminide compounds, but with an increase in its percentage (0.28), it also causes the formation of these titanium aluminide compounds and leads to the formation of Ti5Si3 phase. This phase changes from a discrete morphology to a continuous state as the percentage of silicon increases (0.6). With a further increase in the amount of silicon (1), the TiSi2 phase is formed, which is dispersed throughout the sample with a string-like structure. [ABSTRACT FROM AUTHOR]

Titanium aluminide (TiAl) alloy is attracting attention in the automobile and aviation industries as a promising lightweight heat-resistant alloy material. Parts used in high-temperature environments must have ductility and toughness at room temperature in addition to high strength at high temperatures, however, some alloys, such as TiAl alloys, are known to be difficult materials for casting and machining. Recently, manufacturing processes using additive manufacturing (AM) have been studied to solve these problems. In the present research, the effect of energy density during the AM process on surface roughness and on the strength of the built parts was studied by changing build parameters. As the energy density increased, the porosity and surface roughness decreased. Additionally, higher energy density tended to increase the fraction of lamellar structure. It was found, however, that an increase in lamellar structure does not necessarily lead to any improvement in tensile or creep strength at 750°C. We found that a constant energy density of 15 J/mm³ showed better tensile and creep properties than those of the standard parameter. Our results suggest that it is important to optimize the parameters according to the required properties of the parts. [ABSTRACT FROM AUTHOR]

Microstructure and tensile properties of Ti-8Al-2Cr-2Nb (at%) rods fabricated by electron beam powder bed fusion (EB-PBF) process were investigated by changing input energy density (ED) which is one of the important factors affecting formation of the melt pool. We found that unique layered micro structure consisting of an equiaxed γ grain layer (γ band) and a duplex region can be formed by EB-PBF with ED in the range of 13 to 31 J/mm³. It is interesting to note that the width of the γ band and the volume fraction of the γ phase in the duplex region decrease with increasing ED. On the other hand, the α2/γ lamellar grain in the duplex region increases with increasing ED. These morphological changes in the layered micro structure are attributed to variation of temperature distribution from melt pool caused by increasing ED. Moreover, we also found for the first time the strength of the alloys can be improved by decreasing width of the γ band and increasing of the α2/γ lamellar grain in the duplex region. Whereas, the width of the γ band and the fraction of the equiaxed γ grain in the duplex region should be increased to enhance ductility of the alloys. [ABSTRACT FROM AUTHOR]

Recently, additive manufacturing for titanium aluminide has received sustained attention. Considering the extensive applications on low pressure turbine blades in aerospace field, dynamic mechanical properties of titanium aluminide, especially the fatigue properties, are of great importance. In present work, fatigue test at ambient temperature was conducted for the first time on twin-wire directed energy deposition-arc (TW-DED-arc) fabricated Ti-48Al-2Cr-2Nb alloy with equiaxed lamellar colonies. More detailed researches on fatigue fracture characteristics and deformation modes are also investigated. The experiment results indicate that TW-DED-arc fabricated Ti-48Al-2Cr-2Nb alloy exhibits flat S–N behavior with a good resistance to fatigue. Fatigue life fluctuates widely at same stress level, but such fluctuations gradually weaken as stress decreases. Furthermore, γ/α 2 interface and lamellar colony boundary as well as special microstructures of as-fabricated Ti-48Al-2Cr-2Nb alloy are weak areas during fatigue process, which easily become crack nucleation sites. As stress level decreases, deformation mode of as-fabricated Ti-48Al-2Cr-2Nb alloy translates from twinning and dislocation slip to predominantly dislocation slip. In general, these findings provide an important reference for engineering applications of titanium aluminide. • Ti-48Al-2Cr-2Nb alloy with equiaxed lamella colonies and small Al fluctuation was successfully fabricated via TW-DED-arc. • Ambient temperature fatigue test was conducted on TW-DED-arc fabricated Ti-48Al-2Cr-2Nb alloy for the first time. • During fatigue process, γ/α 2 interface, lamellar colony boundary and special microstructures are weak areas. • Fatigue deformation mode of TW-DED-arc manufactutred Ti-48Al-2Cr-2Nb alloy consists of mechanical twinning and dislocation slip. [ABSTRACT FROM AUTHOR]

Current research work emphasizes on the development of particulate titanium aluminide (TiAl), an intermetallic class of material, through mechanical alloying (MA) from elemental powders and usage of same as reinforcement in AA6061 aluminum matrix to enhance its mechanical and tribological performance. TiAl, being a light weight, strong and abrasion-resistant material, is found to be a suitable candidate to replace dense ceramic-based reinforcements in aluminum composite. Friction stir processing (FSP), a well-established surface severe plastic deformation tool, is used to develop the composite in this study. Several strategies on improving the stirring action and particle distribution during FSP is employed and their scientific effect is studied in detail. Microstructural evolution was studied through scanning electron microscopy (SEM) combined with electron backscattered diffraction (EBSD) analysis which revealed uniform particle distribution of reinforcement in the friction stir processed (FSPed) zone with 88.23 % reduction in grain size of composite as compared to base material. Hardness studied showed an increase in the hardness by 33 % for composite material as compared to un-reinforced FSPed material, showing the positive effect of reinforcement alone on the mechanical strength of the material. Tensile results showed an improvement in yield tensile strength from 156 MPa in base to 186.8 MPa in the composite material without significant compromise in the ductility 26.86 % for composite and 28.14 % for base. Mechanisms aiding uniform particle distribution, and the reasons for improvement in mechanical performance of the developed composite is discussed in detail with the help of microstructural studies and post-deformation fractograph. • Titanium Aluminide is developed from elemental powders and detailed mechanism of phase evolution is presented. • Developed TiAl is used as reinforcement to develop AA6061/TiAl MMC through FSP. • Defect free surface composite is successfully developed with a refined microstructure (88.23 % grain size reduction). • Effect of refined microstructure on mechanical performance is evaluated through tensile and compressive testing. [ABSTRACT FROM AUTHOR]

The effect of mechanical activation of TiAl powder on the structural and phase transformations in mechanochemical synthesis of the powder mixture of the composition 60.8Fe + 39.2TiAl, intended to produce Fe3Al intermetallics, alloyed with titanium was studied. Using a semi-empirical Miedema’s model, the changes of Gibbs energy for the binary systems Ti-Al, Fe-Ti and Fe-Al were calculated. The results showed that the driving force for the formation of intermetallic phases for all the binary systems is higher as compared to the formation of solid solutions and amorphous phases. A range of compositions was established, in which the formation of amorphous phases in the binary systems Ti-Al and Fe-Al is possible. Carrying out the mechanical activation of TiAl intermetallic powder in a high-energy planetary mill allowed reducing the size of a coherent scattering from 280 to 9 nm with a partial amorphization. The formation of particles with a homogeneous microstructure consisting of intermetallic phase (Fe, Ti)3Al and Lavhes’s phases – Fe2 Ti was established in the process of mechanchemical synthesis of a powder mixture 60.8Fe+39.2 TiAl when using nanostructural TiAl powder. The change in the area of coherent scattering of powders of 60.8Fe+39.2TiAl mixture was evaluated, obtained by the method of mechanochemical synthesis, and it was established that interaction of a nanostructural powder TiAl with Fe begins to occur when the size of the coherent scattering area is <70 nm. The resulting product has an amorphous-nanocrystalline structure with the size of the coherent scattering area of <15 nm. The use of the developed powder in thermal spraying technologies will allow producing coatings based on Fe3Al intermetallics with a nanocrystalline structure, a higher modulus of elasticity and ductility. [ABSTRACT FROM AUTHOR]

The development of process parameters for electron beam powder bed fusion (PBF-EB) is usually made with simple geometries and uniform scan lengths. The transfer to complex parts with various scan lengths can be achieved by adapting beam parameters such as beam power and scan speed. Under ideal conditions, this adaption results in a constant energy input into the powder bed despite of the local scan length. However, numerous PBF-EB machines show deviations from the ideal situation because the beam diameter is subject to significant changes if the beam power is changed. This study aims to demonstrate typical scaling issues when applying process parameters to scan lengths up to 45 mm using a fourth generation γ-TiAl alloy. Line energy, area energy, return time, and lateral velocity are kept constant during the additive manufacturing process by adjusting beam power and beam velocity to various scan lengths. Samples produced in this way are examined by light microscopy regarding lateral melt pool extension, melt pool depth, porosity, and microstructure. The process-induced aluminum evaporation is measured by electron probe microanalysis. The experiments reveal undesired changes in melt pool geometry, gas porosity, and aluminum evaporation by increasing the beam power. In detail, beam widening is identified as the reason for the change in melt pool dimensions and microstructure. This finding is supported by numerical calculations from a semi-analytic heat conduction model. This study demonstrates that in-depth knowledge of the electron beam diameter is required to thoroughly control the PBF-EB process, especially when scaling process parameters from simply shaped geometries to complex parts with various scan lengths. [ABSTRACT FROM AUTHOR]

The effect of mechanical activation of tial powder on the structural and phase transformations in mechanochemical synthesis of the powder mixture of the composition 60.8fe + 39.2-i-1, intended to produce fe3al intermetallics, alloyed with titanium was studied. using a semi-empirical miedema's model, the changes of Gibbs energy for the binary ti-al, fe-ti and fe-al systems were calculated. the results showed that the driving force for the formation of intermetallic phases for all the binary systems is higher as compared to the formation of solid solutions and amorphous phases. a range of compositions was established, at which the formation of amorphous phases in the binary Ti-al and fe-al systems is possible. carrying out the mechanical activation of tial intermetallic powder in a high-energy planetary mill allowed reducing the size of a coherent scattering region from 280 to 9 nm with a partial amorphization. the formation of particles with a homogeneous microstructure consisting of intermetallic phase (Fe, Ti)3al and laves phases -- Fe2ti was revealed in the process of mechanochemical synthesis of a powder mixture 60.8fe + 39.2 Tial when using nanostructural tial powder. the change in the region of coherent scattering of powders of 60.8fe + 39.2Tial mixture, produced by the method of mechanochemical synthesis, was evaluated and it was established that interaction of a nanostructural powder tial with fe begins to occur when the size of a coherent scattering region becomes <70 nm. the resulting product has an amorphous-nanocrystalline structure with a size of the coherent scattering region of <15 nm. the use of the developed powder in thermal spraying technologies will allow producing coatings based on Fe3al intermetallics with a nanocrystalline structure, a higher modulus of elasticity and ductility. [ABSTRACT FROM AUTHOR]

In this work, titanium aluminide alloy have been fabricated via the laser deposition technique. The effect of some selected deposition parameters on the microstructure and mechanical properties of produced deposits were studied. The relationship between the laser power, and the microhardness of deposited samples on laser preheated substrate showed an incremental change in laser power from 200 to 600 W. This led to an overall decrease in microhardness of deposited samples from 426 to 373 HV. Sample deposited at 500 W gave the lowest Icorr of 1.8 x 10-8 and the highest Ecorr of -0.138 V. It is evident from the nanoindentation results that indentation modulus and stiffness of sample deposited at 600 W laser power had a lower value compared with 400 W laser power. However, the modulus of both samples fell within titanium alloy modulus range between 105-120 GPa. The microstructures of the deposits are mainly characterized with γ-TiAl and α2-Ti3Al phases and an improved hardness property almost two times higher than that of commercially pure titanium were achieved. It was concluded that changes in the laser power directly causes changes in the microstructure, hardness, stiffness, modulus of elasticity and corrosion resistance of the deposits. [ABSTRACT FROM AUTHOR]

Due to the inherent temperature brittleness and poor workability, the forming and fabrication of TiAl alloy is extremely difficult. Thus, in recent years, an innovative twin-wire-based plasma arc additive manufacturing (TW-PAAM) technique has been developed to fabricate the Ti-48Al alloy with low cost. In this research, the Ti-48Al alloys are fabricated by the TW-PAAM and the tungsten inert gas welding-based wire and arc additive manufacturing (TIG-WAAM). Afterward, the microstructure, residual stress and fatigue properties are characterized subsequently. The microstructure of the TiAl alloy was found to consist of a dendritic grain region and a fully lamellar colony region. The fully lamellar colonies composed of α2 and γ phases, and the size of the lamellar colonies tends to increase from the upper to the lower. The residual stress value in the TiAl alloy of lower part is higher than the upper part. Additionally, the mean residual stress value of TW-PAAM TiAl alloy (57.6 MPa) is lower than the TIG-WAAM TiAl alloy(68.4 MPa), decreasing by 15.7%. And the fatigue strength of TiAl alloy in the lower part shows poor fatigue properties compared to the upper part, which is mainly attribute to the effect of residual stress and the size of lamellar colonies. [ABSTRACT FROM AUTHOR]

TiAl alloys are difficult to obtain high density through traditional pressing and sintering process. Sn is a promising liquid-phase sintering addition for TiAl alloys; however, the further studies have elaborated that the liquid phase sintering of Sn is limited due to poor wettability between TiAl and melt Sn. Based on it, this study adopts Sn–Al powder addition instead of pure Sn powder. The results describe that a small amount of active Al enhanced the wettability of the Sn droplets to the TiAl matrix, thereby further promoting the liquid phase sintering effect of the addition of Sn. The sample sintered at 1320–1350 °C has a density excess of 95%, near γ or duplex microstructure with small proportion of lamellar colonies. Through subsequent non-encapsulation HIP, the fine-grained structure is maintained at the same time to reach full density, which enables the as-sintered samples to obtain moderate tensile properties at room temperatures (UST = 520 ± 5 MPa, YS = 401 ± 7 MPa, EL = 1.33 ± 0.1).

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.

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.

Electron beam melting (EBM) made feasible γ-titanium aluminide (TiAl) alloys. However, the surface roughness resulting from EBM is poor, and the high mechanical properties make γ-TiAl difficult to be machined. Electro-Chemical Machining (ECM) is gaining interest as a finishing process to achieve low surface roughness. Based on the anodic dissolution mechanism, ECM realises a contactless material removal and results particularly suitable for difficult-to-cut materials. The paper investigates the use of ECM for the finishing of Ti-48Al-2Nb-2Cr, a γ-TiAl alloy made by EBM. The effects of current, voltage, feed rate and duty ratio have been accounted for under pulsed machining conditions. [ABSTRACT FROM AUTHOR]

The aim of this research was to study the tribological behaviour of Ti–48Al–2Nb–2Cr alloy under dry sliding reciprocating conditions. A reciprocating wear machine was employed to evaluate the typical abrasion resistance of the alloy. Infrastructure assessment of worn surfaces was performed by scanning electron microscopy (SEM) analysis equipped with Energy Dispersive X-Ray Spectroscopy (EDS). The average surface roughness of initial specimen was 32 nm. According to the SEM observations of worn surfaces and debris, the dominant wear mechanism under defined tribosystem was identified to be abrasive and delamination wear. The weight loss measurements showed that by increasing the applied load from 10 to 40 N, the weight loss was significantly increased from 5.4 to 18.5 µg after passing 1000 m slip distance. [ABSTRACT FROM AUTHOR]