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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]

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 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]

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]

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]

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 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]

The chief objective of manufacturing process improvement efforts is to significantly minimize process resources such as time, cost, waste, and consumed energy while improving product quality and process productivity. This paper presents a novel physics-informed optimization approach based on artificial intelligence (AI) to generate digital process twins (DPTs). The utility of the DPT approach is demonstrated in the case of finish machining of aerospace components made from gamma titanium aluminide alloy (γ-TiAl). This particular component has been plagued with persistent quality defects, including surface and sub-surface cracks, which adversely affect resource efficiency. Previous process improvement efforts have been restricted to anecdotal post-mortem investigation and empirical modeling, which fail to address the fundamental issue of how and when cracks occur during cutting. In this work, the integration of in-situ process characterization with modular physics-based models is presented, and machine learning algorithms are used to create a DPT capable of reducing environmental and energy impacts while significantly increasing yield and profitability. Based on the preliminary results presented here, an improvement in the overall embodied energy efficiency of over 84%, 93% in process queuing time, 2% in scrap cost, and 93% in queuing cost has been realized for γ-TiAl machining using our novel approach. [ABSTRACT FROM AUTHOR]

The paper presents the structure and phase composition of Ti−Al−Si powder composites produced in different conditions, which include vacuum sintering and self-propagating high-temperature synthesis (SHS). The certain ratios of reactive components are used in experiments for the formation of two-phase composites, matching the TiAl3 + Ti5Si3 and Ti3Al + Ti5Si3 compositions. The vacuum sintering of Ti−Al−Si powder composites provides mostly the formation of the two-phase structure, but the quantitative ratio of the appeared phases can considerably differ from the calculated compositions. The lattice parameters in these phases are rather distorted. The analysis of the synthesis in the wave mode combustion of Ti−Al−Si powder composites shows that the synthesis completes only in the TiAl3 + Ti5Si3 composite. This allows us to prepare the powder from the synthesized product. The paper analyzes the behavior of the synthesized powder based on the phase composition of the TiAl3 + Ti5Si3 composite after the vacuum sintering of the powder compacts. It is found that after the 1300°C vacuum sintering of the SHS products based on the TiAl3 + Ti5Si3 composite, its qualitative phase composition remains with a small change in the quantitative phase composition. The compaction of the SHS products is observed together with the reduction in the residual porosity. [ABSTRACT FROM AUTHOR]

This paper describes the effect of preliminary mechanical activation (MA) and Ti content on maximum temperature, maximum burning rate, composite particle size, mixture yield after MA, sample elongation during combustion, and the phase composition and morphology of combustion products in the Ni–Ti–Al system. The preliminary MA of a Ni + Al + Ti mixture expands the limit of titanium content at which samples can burn without preheating, from 17 to 59% (by weight). Mechanical activation is followed by a drop in the number of phases in the products and an increase in the burning rate, while the maximum synthesis temperature decreases. The augmentation of the Ti content in the Ni + Al + Ti mixture reduces the burning rate and maximum combustion temperature of both initial and mechanically activated mixtures. Moreover, as the titanium content increases, the average size of composite particles decreases and the yield of the activated mixture becomes larger. In the combustion of mechanically activated mixtures with a high (24–52%) titanium content, the Ti2Ni phase and the Ni2TiAl Heusler phase predominate in the products, while the traces of melting are visually observed. In the activated Ni + Al + Ti mixture, the Ti content at which the Heusler phase content in the combustion products becomes maximum is revealed. Most of the observed dependences are explained. [ABSTRACT FROM AUTHOR]

The utilization of materials with high strength to density ratio enables efficiency improvements and is therefore demanded for many applications, particularly in the aerospace and other mobility sectors. However, the machining of these typically difficult-to-cut materials poses a challenge for conventional manufacturing technologies due to the high tool wear. Abrasive water jet (AWJ) machining is a promising alternative manufacturing technology for machining difficult-to-cut materials, since the tool wear is low and material independent. However, AWJ machining is limited regarding the producible geometries when conducting cuts through a material. This limitation can be resolved with AWJ milling operations which on the other hand are time-consuming. To approach this challenge, an enhanced AWJ milling operation is presented and investigated in this paper with the aim to expand the producible geometries. This operation consists of two kerfs, inserted from different sides of the workpiece, which intersect at their kerf ground. Consequently, a piece of material is separated without the cut material being entirely chipped. Thus, the operation possesses a high aggregated material removal rate. The investigations presented in this paper show and evaluate the effects that occur during the milling of kerfs with variable depths on titanium aluminide TNM-B1. Furthermore, a method to compensate these effects is introduced and thus the producible geometries for effective AWJ milling could be enhanced. [ABSTRACT FROM AUTHOR]

Synthesis is carried out in a Ti + Al mixture prepared by two methods, namely preliminary mechanical activation (MA) or preheating. Synthesis occurs in the cases of layer-by-layer combustion (SHS) and thermal explosion. The dependences of burning rates of activated Ti + Al mixtures on the MA duration are investigated. The relationships between the initial temperature and the rates and maximum combustion temperatures of initial mixtures along with the elongation of samples after combustion are determined. The phase composition of the initial mixtures after activation and of the synthesis products is described. The MA duration (12 min) at which the burning rate of the mixture is maximum is experimentally determined. For this mixture (12-min long MA), the dependence of the burning rates and maximum combustion temperatures along with the elongation of samples after combustion on the initial temperature is studied. Synthesis at which the content of the main phases (TiAl and Ti3Al) in the products is maximized is established. [ABSTRACT FROM AUTHOR]

In the present study, a NiCr–Cr 3 C 2 powder mixture was prepared by mechanical alloying and then coated on titanium aluminide substrates by the powder-fed laser cladding process using a set of optimum parameters. The high temperature oxidation behavior of the substrate and coating was studied by isothermal annealing at 900 °C for 5 h. It was found that the microstructure of the coating is composed of γ solid solution with different chromium carbide phases (Cr 3 C 2 , Cr 7 C 3 and Cr 23 C 6). The presence of different chromium carbides in the microstructure of coating can be attributed to the partial melting of primary Cr 3 C 2 and the formation of non-equilibrium carbide phases during rapid cooling of laser cladding. The NiCr-chromium carbide laser cladded coating samples showed superior oxidation resistance compared to the substrate. The oxidation mechanism of both coating and substrate follow the parabolic law, where the parabolic rate constant of the coating was 20% of that of the substrate at 900 °C. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and Grazing Angle X-Ray Diffraction (GAXRD) analysis revealed that the surface of the oxide layer formed on the NiCr-chromium carbides coating and the substrate is mostly composed of Cr 2 O 3 and TiO 2 , respectively. [ABSTRACT FROM AUTHOR]

Casting technology of thin-wall TiAl alloy turbochargers was studied by investment casting and numerical simulation. Misruns and gas holes were the main defects observed in preliminary work due to the poor fluidity of alloy, and to gas entrapment. In order to eliminate these defects, cast parameters, such as centrifugal rotation rate and mould preheating temperature, were optimized by numerical simulation, meanwhile, the structure of the shell mould was optimized to improve the filling capacity of TiAl alloy. Pouring experiments were carried out by vacuum induction melting furnace equipped with a water-cooled copper crucible based on the above optimization. The quality of the TiAl alloy casting was analyzed by fluorescent penetrant inspection and X-ray detection. The results show that a centrifugal rotation rate of 200 rpm, mould preheating temperature of 600 °C, shell preparation through organic fiber addition can dramatically improve the mould filling capacity, and integrated turbochargers were finally prepared. [ABSTRACT FROM AUTHOR]

Considering the advantages of low density and excellent high-temperature performance, titanium aluminide is identified as an ideal aerospace structural material. In recent years, twin-wire directed energy deposition-arc (TW-DED-arc), which possesses the characteristics of low cost but high deposition efficiency, has continuously attracted worldwide attention. However, obtaining equiaxed grains with small Al variation for titanium aluminide using TW-DED-arc method is still a key concern. In the present research, Ti–48Al–2Cr–2Nb alloy with equiaxed lamellar colonies and less than 1 at.% of Al content was successfully fabricated by regulating deposition current of TW-DED-arc. Also, the effect of deposition current on microstructure and mechanical properties was systematically investigated. The experimental results that focus on top and middle regions of as-fabricated TiAl-4822 alloys, indicate that increasing current favors in generation of equiaxed grains and γ lamellae, which leads to columnar to equiaxed transition and higher γ phase content, respectively. Also, elemental homogeneity can be significantly homogenized. As deposition current increases, tensile strength increases while degree of tensile anisotropy decreases significantly, benefiting from the equiaxed grains with satisfying size. Importantly, microstructure evolution, tensile anisotropy and fracture characteristics of TW-DED-arc fabricated TiAl-4822 alloys are discussed in detail. Generally, these findings provide an effective process method to optimize microstructure and mechanical properties of TW-DED-arc fabricated TiAl-4822 alloys. [ABSTRACT FROM AUTHOR]

• Propose a scheme to partition the total energy of Al 3 Ti into its atoms' energies. • Predicted the preferred orientation relationships between Al 3 Ti and Al matrix. • Obtained the definite value of surface energy on any Al 3 Ti surface. • Predicted the energy-preferred interface configuration between Al 3 Ti and Al matrix. The geometric model of coplanar reciprocal lattice points (CRLPs) was used to predict the preferred orientation relationships (ORs) between the Al 3 Ti intermetallic particle and the Al matrix. The calculation result indicates that there exist two preferred ORs, i.e. (001)Al 3 Ti//(001)Al, [100]Al 3 Ti//[100]Al and (010)Al 3 Ti//(010)Al, [100]Al 3 Ti//[10 1 ¯ ]Al. The periodic models with a specific interface structure and surface slab models on a particular crystallographic plane were proposed to partition the total energy of the Al 3 Ti compound into its constituent atoms' energies. The Ti and two kinds of Al atoms (Wyckoff notation 2b and 4d) possess the energy of -119.80325739 Ry, -5.09778518 Ry, and -5.03677683 Ry, respectively. Compared with the Al 3 Ti total energy of -269.94919247 Ry, the proposed energy partition scheme presents a satisfactory accuracy better than ∼10−7 Ry. Owing to the data of the atom's energy in the solid state, the definite value of surface energy can be obtained whether or not it is a stoichiometric or non-stoichiometric surface. The interface energy calculation, which was carried out on all the preferred ORs with all the possible atomic configurations, indicates that the energy-preferred interface possesses the orientation relationship of (001)Al 3 Ti//(001)Al, [100]Al 3 Ti//[100]Al. The atomic configuration is that the atoms on the surface of the Al slab face directly to atoms on the second layer of the Al 3 Ti slab with the Al-Ti terminated surface on account of its lowest interface energy of 0.005 J/m2. The predicted OR agrees well with the previously published experimental results, and the significantly lower interface energy explains the potency of the Al 3 Ti particle as a heterogeneous nucleation site in the aluminum solidification process. [Display omitted] [ABSTRACT FROM AUTHOR]

Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micro-mechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micro-mechanisms responsible for the highly anisotropic deformation response of TiAl on the intra-lamella and inter-lamella scales. This study focuses on two specific configurations of γ / α 2 lamellar microstructure with the γ / α 2 interfaces being aligned 25 o and 55 o to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the 25 o micropillar while the anisotropy of the 55 o micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the 25 o micropillar less localised than that in the 55 o micropillar. Moreover, the CP-FEM model successfully predicted substantial build-up of internal stresses at γ / α 2 interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins. Deformation behaviour of (ϕ = 25 o )- and (ϕ = 55 o )-pillars: in-situ observation versus CP-FEM results. Image 1 • Crystal plasticity of lamellar TiAl is studied by in-situ testing and modelling. • CP-FEM well predicts the constitutive response and localisation of lamellar TiAl. • This study reveals the micro-mechanisms of anisotropy of lamellar TiAl. • Deformation twinning significantly relieves internal stresses at γ / α 2 interfaces. [ABSTRACT FROM AUTHOR]

Lamellar γ-TiAl has a microstructure that spans multiple length scales: lamellar thickness (10 nm–1 μm), γ domain length (100 nm–10 μm) and colony size (100 μm–1 mm). Only by characterizing the deformation across the different scales can one understand how the microstructure influences plasticity and damage formation in this material. Here, we use digital image correlation and electron backscatter diffraction to map strain and lattice rotation with both sub-colony and sub-lamella spatial resolution in a polycrystalline lamellar γ-TiAl alloy with TiB 2 inclusions. Two lamellar thicknesses and temperatures are tested. It is shown that the hard mode-oriented colonies undergo minimal plastic strain. However, as the temperature is increased, macroscopic deformation bands develop across colonies of different orientations, independent of their local lamellar orientations. Increasing magnification, it is apparent that some segments of these bands occur by slip or twinning parallel to the lamellae, and others by transverse lattice rotation. In the thin lamellar condition, such macroscopic deformation bands also occur along colony boundaries; this causes a significant inhomogeneity in the strain distribution. Such strain inhomogeneities are also further characterised here at the microstructural scale of single lamellae, and locally at boride particles. • Nanoscale resolution digital image correlation (HR-DIC) strain mapping up to 700 °C. • Characterisation of plastic deformation and mechanical damage across multiple length scales in lamellar γ-TiAl. • Macroscopic bands of heightened deformation developed, focused at grain boundaries. • Methods to repress deformation bands by microstructural modifications are discussed. [ABSTRACT FROM AUTHOR]

• High cycle fatigue strains mapped by nanoscale resolution digital image correlation. • Fine bands of deformation in lamellar γ-TiAl alloy formed ∼50 nm from interfaces. • Unlike monotonic loading, high cycle fatigue strains remain highly localised. • Strain gradients develop along slip bands, increasing in magnitude with cycling. Microcompression testpieces of a lamellar γ-TiAl alloy were high cycle fatigued up to 105 cycles at stress ratios of 0.5–0.7, to maximum stresses above and below the yield stress. The localised deformation was studied by nano-scale digital image correlation strain mapping of a Pt patterned side-face of the pillars. With crystal orientation mapping, the fatigue deformation lines were found to locate at the γ/γ and γ/α 2 lamellar interfaces. The deformation lines were closely spaced slip lines in transmission electron imaging. Gradients of shear strain developed along the deformation lines, increasing in magnitude as cycling progressed. Extensive transverse twinning also occurred. [ABSTRACT FROM AUTHOR]

TiAl-based composite materials have been produced by spin-casting self-propagating high-temperature synthesis metallurgy methods using thermite-type mixtures, general relationships in the formation of their composition and structure have been investigated, the synthesis conditions have been optimized, a method for obtaining large ingots has been proposed, and their composition and structure have been determined. We have obtained cast Nb- and Cr-doped TiAl-based composite materials using mixtures of Ti, Nb, Cr, and Ca oxides with a combined reducing agent (Al and Ca) under the effect of an overload above 200g. It has been shown that partial Ca substitution for Al in the starting mixture ensures complete reduction of the TiO2, and the resultant metallic phase contains calculated amounts of Ti and Al. Mixtures of optimized composition make it possible to produce a cast composite material similar in composition to alloy 4822. [ABSTRACT FROM AUTHOR]

Abstract γ-phase based titanium aluminide has continuously been attractive because of its potential application in modern light-weight, high-temperature turbines, such as aircraft engines. However, it suffers from its poor plasticity and during manufacturing and processing. In recent years, wire-arc additive manufacturing process has been proved feasible of fabricating γ-phase based titanium aluminide structures with full density and relatively lower cost compared to traditional powder metallurgy processing. In the present research, the temperature process of a single-pass deposition process during the additive manufacturing was simulated as a linear heating (up to 1623 K) and cooling process in a vacuumed furnace, and in-situ investigated using neutron diffraction. As a result, compared to the initial as-fabricated state, the volume fraction of α 2 -phase increased by 2.54% after the heat treatment. Crystallographic aspects, specifically the α/α 2 ↔ γ phase transformation and lattice evolutions of γ-phase are discussed in detail. According to the results obtained, the γ → α 2 transition temperature in the present binary alloy is 1393 K, which largely deviates from earlier results collected from γ-phase based titanium aluminides with Nb addition. Also, the lattice evolutions of γ-phase in a function of time/temperature are linear fitted and responses of lattice strains (a , c axis and volumetric) to temperature are calculated. Highlights • Thermal phase evolutions in Ti-45Al alloy is recorded in-situ with neutrons. • α/α 2 ↔ γ phase transformation recorded by neutron is 1393 K. • Responses of γ-phase lattice strains to temperature are calculated. [ABSTRACT FROM AUTHOR]

An in situ synchrotron study of the specific features of the phase formation dynamics in mechanically activated 16 wt% Al + Ti powder composition is described, the high‐temperature synthesis being carried out under the condition of high volume inflammation by means of inductive heating. The kinetics of the phase formation were registered with an experimental complex, especially designed, constructed and adjusted for the method of dynamic diffraction analysis in synchrotron radiation beams. It has been experimentally in situ shown that increasing the time of mechanical activation of the initial powder mixture reduces the temperature at which components start to react and the time of realization of the high‐temperature synthesis. With the latter set at 1 min of mechanical activation, the temperature of the reaction in the mixture is T = 603°C; at 3 min of mechanical activation, T = 442°C; and at 7 min, T = 359°C. The maximum burning temperatures are: for 1 min of mechanical activation, Tmax = 1080°C; for 3 min, Tmax = 1003°C; and for 7 min, Tmax = 820°C. It was found that formation of both stable compounds Ti3Al, TiAl3, TiAl2, TiAl and metastable phases Ti9Al23, Ti5Al11, Ti2Al5, Ti3Al5 occurs at the stage of primary structure formation, before the system goes to thermal explosion. High‐temperature synthesis of a mixture of the studied composition takes place without formation of a liquid phase, in the solid‐phase combustion mode. It was found that the increase in the time of mechanical activation of the initial powder mixture contributes to the formation of a product with a dominant content of intermetallic compound Ti3Al. By synthesis of the powder mixture of composition 16 wt% Al + Ti, mechanically activated for 7 min, the content of Ti3Al in the final product was found to be 68%. The sequence of formation of metastable and basic phases in the process of self‐propagating high‐temperature synthesis of a mechano‐activated powder mixture of composition 3Ti + Al is established using in situ synchrotron studies. [ABSTRACT FROM AUTHOR]

Abstract The occurrence of plasticity below the macroscopic yield stress during tensile monotonic loading of nearly lamellar Ti-45Al-2Nb-2Mn(at%)-0.8 vol% TiB 2 at both 25 °C and 700 °C, and in two conditions of lamellar thickness, was measured by digital image correlation strain mapping of a remodelled Au surface speckle pattern. Such initial plasticity, not necessarily related to the presence of common stress concentrators such as hard particles or cracks, could occur at applied stresses as low as 64% of the general yield stress. For a same applied strain it was more prominent at room temperature, and located as slip and twinning parallel to, and near to or at (respect.) lamellar interfaces of all types in soft mode-oriented colonies. These stretched the full colony width and the shear strain was most intense in the centre of the colonies. Further, the most highly operative microbands of plasticity at specimen fracture were not those most active prior to yielding. The strain mapping results from polycrystalline tensile loading were further compared to those from microcompression testing of soft-mode stacks of lamellae milled from single colonies performed at the same temperatures. Combined with post-mortem transmission electron microscopy of the pillars, the initial plasticity by longitudinal dislocation glide was found to locate within 30–50 nm of the lamellar interfaces, and not at the interfaces themselves. The highly localised plasticity that precedes high cycle fatigue failure is therefore inherently related to the lamellar structure, which predetermines the locations of plastic strain accumulation, even in a single loading cycle. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract We combined a phase change material (PCM) with an Al/Al 3 Ti composite with a hierarchical open–cell structure. The porous Al/Al 3 Ti composites with different Al/Ti molar ratios (Al/Ti = 3, 5, 7, 9, and 11) were fabricated by the combustion synthesis reaction of Al–Ti system and the spacer method using NaCl particles. Open–cell structure could be obtained even when the Al/Ti molar ratio was 11. The combustion synthesis reaction did not complete but proceeded by appropriate heat treatment when the Al/Ti molar ratios were 9 and 11. Large size porous Al/Al 3 Ti composite of Al/Ti = 5 (160 mm × 100 mm × 30 mm) was also fabricated in a stainless–steel container simulating direct filling into heat storage tank. D–threitol was infiltrated into the porous Al/Al 3 Ti to obtain the Al–Ti/PCM composite, and the heat release rate was investigated. Al–Ti/PCM composite released heat in ∼4 times faster rate than PCM alone. Highlights • A porous metal was applied to improve heat release rate of a phase change material. • Porous Al/Al 3 Ti with a hierarchical open–cell structure was combined with a PCM. • The porous Al/Al 3 Ti was fabricated by the spacer method and combustion synthesis. • The effect of Al/Ti molar ratio on porous structure was systematically investigated. • Al–Ti/PCM composite released heat ∼4 times faster than PCM alone. [ABSTRACT FROM AUTHOR]

Special features of the structure of surface layers formed on billets of commercial-purity titanium VT1-0 by cladding a powder aluminum-titanium mixture with an electron beam removed into air atmosphere are studied. X-ray phase analysis and transmission electron microscopy are used to show that the material remelted by the electron beam is Ti3Al titanium aluminide. The hardness of the deposited layer is 540 – 610 HV. The behavior of the material is studied under the conditions of sliding friction and friction against fixed abrasive particles. The results reflect decrease in the friction factor and increase in the wear resistance of the clad material as compared to commercially pure titanium. [ABSTRACT FROM AUTHOR]

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+α2\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$\upgamma +{\mathrm{\alpha }}_{2}$$\end{document}) 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 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 + α2) 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]

In many modern sectors, the demand for featherweight and high-temperature materials for essential applications in automotive and aero-technology have become a novel and fast-growing trend. The use of sub-micron-sized ceramic particles to reinforce titanium alloys is notable, with the goal of enhancing hardness and wear resistance. Reduced particle sizes, on the other hand, aid in improving the ductility, creep resistance, and strength of fortified materials. Nanoceramic improves a material's mechanical characteristics significantly, making it a feasible strengthener in metal composites. Latest advances in the technology of nanocomposite necessitate new metal matrix composite (MMC) development for crucial and purposeful applications. Developing titanium matrix composites (TMCs) with ceramic reinforcements (CR) using spark plasma sintering (SPS) techniques is a unique way of improving the structural and physical properties of titanium matrix composites and other related alloys. This review discusses an adopted consolidation technique for the "cermet", ceramics-metal fusion, the spark plasma sintering, (SPS) mechanism, the necking mechanism and strengthening mechanism of the particulate dispersion in the TMC. Special emphasis is drawn to the advanced manufacturing of gamma titanium aluminide and Ti-6Al-4 V, as a typical TMC intermetallic specially used in the production of airframes and airplane engine parts, their phases, and alloy composition. The phase transformation in Ti alloys and deformation mechanism were also discussed. This helps to provide a broad overview of the scope of TMCs fabrication with dispersion strengthened technology. A tabulated review of previously published research on the dispersion strengthened TMCs, fabricated through SPS, their manufacturing parameters and mechanical properties were also a focus of this review. [ABSTRACT FROM AUTHOR]

In order to facilitate a more widespread use of intermetallic γ-TiAl based alloys in the aircraft and automotive sector, recent research focuses on the development of low-cost titanium aluminides. The strong β-stabilizing effect as well as the increased ductility upon Mn addition renders this alloying element a promising candidate to fully or partially substitute other, more expensive, alloying elements such as Mo, Ta and Nb. Mn-containing γ-TiAl based alloys, however, are prone to the formation of the undesired, brittle Ti(Mn,Al) 2 C14 Laves phase during long-term exposure at the targeted service temperatures, which can deteriorate the ductility at ambient temperatures. In this study, the transformation kinetics as well as the chemical and thermal existence range of the C14 Laves phase in the low-cost Ti–42Al–5Mn (at.%) alloy were investigated by complementary experimental and computational approaches. In situ and ex situ high-energy X-ray diffraction in combination with microstructural investigations were used to study the occurrence and transformation kinetics of possible Laves phase formation in the course of annealing treatments. The chemical stability range of the C14 Laves phase was addressed by chemical analysis in conjunction with complementary ab initio modeling. Using these combined approaches, the critical local Mn concentration of ∼16 at.% for Laves phase formation within lamellar α 2 and ∼17 at.% within β o in the Ti–42Al–5Mn alloy was determined. These results should be critically considered for future design of advanced low-cost γ-TiAl based alloys. [Display omitted] • During annealing at 800 °C Laves phase is formed in the Ti–42Al–5Mn alloy. • Laves phase in the lamellar colonies is formed from α 2 at Mn contents >16 at.%. • From β o , Laves phase is formed at Mn contents >17 at.%. • The theoretical results are in good agreement with the experimental findings. [ABSTRACT FROM AUTHOR]

Abstract Electron beam melting (EBM) has been applied to fabricate a high Nb-TiAl alloy with a fully dense microstructure and good tensile properties at both room and high temperatures. The effects of preheating and melting parameters on melting, solidification, phase transformation and resulting microstructure formation in as-EBM high Nb-TiAl alloy were investigated by performing a design-of-experiments. Results show that the limited EBM processing window can be broadened to produce different characteristic microstructures ranging from nearly fully lamellar γ/α 2 to equiaxed γ grains. Such a broadened processing window has been achieved by using stronger preheating beam current. A numerical simulation was performed to understand temperature evolution at a fixed point of interest where electron beam passed several times with a certain line offset within one build layer. Both the preheating and melting stages were considered in the model. Modelling results show that a higher preheating beam current resulted in a longer hold time within the temperature range between 1300 and 1380 °C (i.e. single α-phase region). This helped to produce fine lamellar microstructure in the high Nb-TiAl alloy. Fundamental principles are thus proposed in terms of controlling microstructure formation and fabricating fully dense high Nb-TiAl alloy in as-EBM condition. Graphical abstract Unlabelled Image Highlights • A fundamental principle is proposed to control the microstructure of Nb-TiAl alloy in EBM with competitive tensile properties. [ABSTRACT FROM AUTHOR]

Two TiAl-based alloys, Ti48Al2Mn2Nb1B and Ti50Al2Mn2Nb1B, were prepared via conventional ingot melting and unidirectional solidification (Bridgman process) to investigate their solidification behaviour and the effect of boron addition on grain refinement. Both of them are found to be α solidifying alloys. Their microstructures are refined, featuring fine and randomly oriented lamellar colonies about 150 μm in size, in conventional ingots but the unidirectionally solidified alloys still have columnar microstructures. The entirely different grain refinement behaviour in ingots and Bridgman samples gave rise to the notion that the enhanced α nucleation from the liquid is the key to grain refinement in the two α solidifying alloys although the specific mechanisms are yet to identify. [ABSTRACT FROM AUTHOR]

The present study concerns development of titanium silicide dispersed titanium aluminide composite with addition of 5 wt.% and 10 wt.% Si in Ti45Al5Nb0.5Si base alloy by direct laser cladding under varied process parameters. Direct laser cladding has been conducted using a high power (3 kW) fiber optic delivered Nd:YAG laser (with a beam diameter of 2 mm) using a 3-axis handling system in a layer by layer fashion to develop a coupon with a dimension of 10 mm × 10 mm × 4 mm. The effect of process parameters (applied power, scan speed and powder feed rate) on the depth and integrity of the clad zone, microstructures and microhardness have been studied in details to optimize process parameters. Addition of Si leads to formation of micro-cracks in the microstructures under varied parameters. There are formations of different silicides (Ti 5 Si 3 , Ti 7 Al 5 Si 12 , TiSi 2 and TiSi phases) in the microstructure of the base alloy with the presence α 2  + γ phase. The microhardness of the clad layer increases with addition of Si. However, optimization of process parameters needs to be undertaken for the formation of defect free microstructure with improved hardness. [ABSTRACT FROM AUTHOR]

This paper explores the effects of adding rare earth elements (lanthanum or erbium) on the oxidation properties of Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloy. Isothermal oxidation tests were performed under air atmosphere at 900 and 1000 °C. Mass gain was measured in several steps during the oxidation test, and the oxidized specimens were characterized by XRD and FE-SEM. The results showed that while adding 0.1 at.% rare earth elements (REEs) reduced oxidation rate of the TNM alloy, 0.2 at.% REEs addition increased the mass gain of the alloys. The oxidation curves were fitted by a power-law equation; the results showed that the oxidation kinetic curves of all alloys obeyed parabolic growth kinetics (n = 2). Meanwhile, the activation energy of oxidation was in the range of 40-50 kCal/mol, thereby suggesting that the scale growth was controlled by mass transport in the TiO2 layer. Also, the results of the scale characterization showed that addition of REEs at low level (e.g., 0.1 at.%) could reduce diffusion rate in the scale. However, addition of the higher amounts of La or Er (e.g., 0.2 at.%) due to the lower valency (+ 3) of these elements, as compared with Ti (+ 4), could lead to the increased anion diffusion, the formation of hillocks in the scale and a rise of the oxidation rate. [ABSTRACT FROM AUTHOR]

Abstract To investigate the influence of lamellar orientation and colony boundary on deformation behavior, fully lamellar Ti-43Al-2Si alloys were prepared by cold crucible continuous casting and isothermal compression tests were conducted at different temperatures in the range 1373–1523 K with a strain rate of 0.01 s−1. The as-cast microstructures were elongated colony with lamellae oriented at nearly 45° to the compression direction and equiaxed colony with various lamellae orientations, respectively. The results revealed that serious cracking or sliding traces appeared on the surface of deformed specimens with elongated colony original microstructure and that their peak stresses were lower compared to those of specimens with equiaxed colony original microstructure. The relationship between deformation behaviors and as-cast microstructure was analyzed in detail. It was found that the recrystallization/globularization level is significantly lower than that in specimens with equiaxed colony original microstructure during hot deformation owing to the preferred nucleation sites in colony boundaries. Finally, the lamellae conversion process was characterized to reveal the softening mechanisms of full lamellar TiAl-based alloy during hot compression. Highlights • TiAl alloys with different lamellar colonies were fabricated by cold crucible continuous casting. • EBSD was used to investigate the evolution of lamellae during the hot compression. • Lamellar colony boundaries promote the dynamic recrystallization process. [ABSTRACT FROM AUTHOR]

Results are provided for a study of the effect of vacuum ion-plasma nitriding on the phase composition, structure, microhardness, salt corrosion resistance, and erosion resistance of alloy Ti - 14Al - 3Nb - 3V - 0.5Zr with an original bimodal structure and different surface microgeometry. It is shown that with an increase in nitriding temperature from 550 to 650°C the content of Ti2N nitrides in the surface layer increases and Ti3AlN nitride in formed, which raises the microhardness but reduces the thickness of the hardened diffusion zone; pores appear in the surface at 650°C. Vacuum ion-plasma nitriding is shown to raise substantially the resistance of specimens of alloy Ti - 14Al - 3Nb - 3V - 0.5Zr with a polished surface to salt corrosion. Additional nitriding after deposition of a TiN coating improves ground specimens salt corrosion resistance. Vacuum ion-plasma nitriding with additional deposition of a TiN coating increases the resistance of specimens with polished and ground surfaces to erosive action. [ABSTRACT FROM AUTHOR]

In the work, the peculiarities of formation of dissimilar joints of Ti-46A1-2Cr-2Nb alloy on the base of titanium aluminide γ-TiAI with titanium alloy VТ5 in resistance butt welding, in particular, using interlayers in the form of nanolayer foils were studied. In resistance butt welding without the use of nanolayer foils it was failed to provide the defect-free joints: in the butts the presence of areas of cast metal and cracks was detected. It was found that the use of Ti/Cu and Cu-Ti/Ni-C systems as an interlayer of nanolayer foils of an eutectic type significantly influences the activation processes of the surfaces to be welded and the formation of joints in resistance butt welding. The presence of nanolayer foils in the contact zone facilitates the formation of a thin layer of the liquid phase at the initial stage of heating process, localization of heat evolution process, activation of surfaces of both alloys with the duration of heating stage of 50...60% of such at the direct resistance welding of alloys γ-TiAI and VТ5. The two-stage pressure cyclogram in RBW provides crystallization of eutectics in the process of deformation of billets and the formation of defect-free joints at the values of heating temperature, which are lower than the liquidus temperature in the system Ti-AI. According to the data of scanning electron microscopy and the ESD-analysis, , the absence of the areas of cast metal and the remnants of nanosheet foils in the zone of joints was established, which testifies the solid-phase nature of the formation of joints and the complete displacement of nanolayer foils beyond the cross-section of the billets. 12 Ref., 8 Fig. [ABSTRACT FROM AUTHOR]

Al 3 Ti-based alloys attract exceptional attention due to their high specific mechanical properties. However, their application is still insufficient due to their low ductility and fracture toughness. Several approaches were previously proposed to address these problems. The first one is stabilization of the cubic modification of titanium trialuminide by alloying. Another approach consists in fabricating metal-intermetallic laminated composites (MIL). In this study, we combined both methods to synthesize the first MIL composite with cubic Al 3 Ti interlayers. Copper additions were used to stabilize the cubic modification of Al 3 Ti and produce a novel Ti-Al 5 CuTi 2 MIL composite. First mechanical characterization by indentation tests showed that the binary Al 3 Ti intermetallic tended to crack at a load of 0.2 kg while the fracture was not observed in the Al 5 CuTi 2 layers at least at a load of 1 kg. These results are an indirect evidence of a higher ductility and fracture toughness of the composite with cubic Al 3 Ti compared to tetragonal one. The sequence of the phase transformations in the Al-Ti-Cu system was studied using in situ synchrotron X-ray radiation diffraction. The formation of Al 5 CuTi 2 occurred via several intermediate stages including eutectic melting of Al and Cu and the formation of binary AlCu and Al 3 Ti compounds. [ABSTRACT FROM AUTHOR]

The research was mainly focused on the phase transformation of Ti-42Al-5Mn (at%) alloy produced by vacuum induction melting (VIM) and vacuum arc remelting (VAR) followed by wrought process. The phase transformation kinetics of the alloy during continuous cooling under the cooling rates of 0.1 °C/s, 0.5 °C/s, 2 °C/s, 10 °C/s, 50 °C/s, and 200 °C/s were identified by the Gleeble-3800 system, which is finally used to construct a schematic continuous cooling transformation (CCT) diagram of this alloy. The related phase composition, microstructure and property of the alloy were studied using X-ray diffraction (XRD), electron probe micro analyzer (EPMA), transmission electron microscope (TEM) and Vickers hardness (HV). It shows that six kinds of phase transformation are existed during continuous cooling from 1300 °C with the cooling rates of 0.1 °C/s, 0.5 °C/s, 2 °C/s, 10 °C/s, 50 °C/s, 200 °C/s and WQ, including β→α 2 ′, β→α, β→γ, β→β o /α→α 2 , α 2 →α 2 /γ and α 2 /γ→β o,sec . It reveals that partial transformations would be inhibited when the cooling rate is relatively high. The martensitic structures (α 2 ′) obtained by β→α 2 ′ were formed under the WQ condition after holding at 1300 °C for 5min, and it will be completely restrained below the cooling rate of WQ. For the β→γ, it is restrained if the cooling rates are higher than 50 °C/s. The α 2 →α 2 /γ could be also suppressed when cooling with 200 °C/s, and the γ lath within the supersaturated α 2 -grains would be formed at lower cooling rate below 50 °C/s. For 0.5 °C/s and 0.1 °C/s, a small amount of dotted β o (β o,sec ) phases can be formed in the lamellar interfaces, which is proved the reaction products of α 2 /γ→β o,sec . Furthermore, the effect of cooling rates on the microstructure, including interlamellar spacing ( λ ) and γ grains are also identified and discussed. [ABSTRACT FROM AUTHOR]

In this work, counter-gravity investment casting was used to fabricate TiAl parts with different cross-sections. A three-dimensional finite element model of counter-gravity investment casting was established based on ProCAST simulation software. The filling capacity and solidification behaviour during the counter-gravity investment casting of the Ti-48Al-2Cr-2Nb alloy were numerically investigated, and the effects of filling pressure, melt superheating temperature and mould preheating temperature were experimentally evaluated in terms of the filling integrity and casting defects. The results show that the filling pressure plays an important role in the filling capacity. Elevating properly the mould preheating and melt superheating temperatures can also improve the filling capacity. Then, counter-gravity investment casting was performed using the parameters optimised through numerical simulations, and sound Ti-48Al-2Cr-2Nb casting parts with different section sizes and shapes were produced. The as-cast microstructure of the Ti-48Al-2Cr-2Nb alloy indicates that the phase composition and morphology are largely affected by the size effect.Counter-gravity investment casting was applied to fabricate TiAl parts. A FE model was established to calculate the filling capacity and solidification behaviour during the counter-gravity casting of the Ti-48Al-2Cr-2Nb alloy. The effects of filling pressure, melt superheating and mould temperature were evaluated. Based on the numerical results, sound Ti-48Al-2Cr-2Nb casting parts with different section sizes and shapes were produced with an optimised processing window. The as-cast microstructure of the Ti-48Al-2Cr-2Nb alloy indicates that the phase composition and morphology are largely affected by the size effect. [ABSTRACT FROM AUTHOR]

Bilayer Ti/TiN and TiAl/TiAlN coatings were deposited onto austempered ductile iron (ADI) substrates by cathodic arc deposition in an industrial device. Structure and mechanical properties of the coated samples were comparatively examined. Wear behavior of the coated samples was investigated in comparison with uncoated ADI by means of rolling contact fatigue (RCF) tests, performed in a flat washer type testing rig and using lubricated pure rolling conditions. RCF tests results were analyzed using the two-parameter Weibull distribution and the Weibayes method. The results indicate that TiN and TiAlN coatings grew with a cubic-NaCl type structure. The arithmetic average roughness of the coated samples is similar for both coating variants. The surface hardness and residual stresses are higher for the TiAl/TiAlN coated samples. The coating hardness and elastic modulus are also higher for TiAl/TIAlN. The critical loads at massive delamination and the evolution of the friction coefficients are quite similar for both coating variants. Regarding RCF, failures in coated samples were characterized by substrate spalling. No massive delamination was observed in Ti/TiN and TiAl/TiAlN coatings. The statistical analysis indicates that the deposition of Ti/TiN improves noticeably the RCF resistance of ADI while the deposition of TiAl/TiAlN does not produce significant changes. The properties mismatch between substrate and coating seems to play an important role in the RCF behavior of coated samples, since Ti/TiN coatings possess a lower mismatch with respect to ADI substrates as compared to TiAl/TiAlN. [ABSTRACT FROM AUTHOR]

Ti44Al6Nb1Cr (at%) alloys with different lamellar spacing were prepared by cold crucible directional solidification. Creep tests were conducted at 750 °C under 260 and 300 MPa, and the microstructure before and after creep testing were observed and analyzed. The results show that the prepared TiAl alloys have similar macro / microstructure except for lamellar spacing, which are different from the heat-treated TiAl alloys with obviously changed macro/microstructure. The refinement of lamellar spacing can improve creep properties, especially the steady-state of fine lamellar alloy lasted for more than 600 h with creep rate at 7.3 × 10 −9 S −1 . The improvement of creep properties by refined lamellar spacing are revealed as following two reasons. (1) Fine lamellar spacing improves the stability of γ lamellae and increases the resistance for dislocation slip in γ lamellae. (2) It disperses stress concentration and delays the formation of globular structure at colony boundary. Moreover, the alternating β and γ laths in α-segregation zone can improve microstructural stability during creep. In β-solidifying γ-TiAl alloy, the local stress concentration on β-segregation at colony boundary promotes colony boundary sliding and the formation of void with globular structure, which further accelerates the creep failure. [ABSTRACT FROM AUTHOR]

We have studied the effect of sintering temperature on the structural and phase transformations of alloys produced by the spark plasma sintering of Ti + 23.5 at % Al + 21 at % Nb powder mixtures at temperatures in the range 1100–1550°C. The sintered alloys have been characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy (elemental X-ray mapping). The alloys sintered at temperatures of 1100 and 1200°C have been shown to have a nonuniform microstructure. According to electron microscopy results, the alloys consist of grains of the α2 and Nb2Al phases and small precipitates of the O-phase (intermetallic compound Ti2AlNb). In addition, there are particles of unreacted niobium and titanium. The alloys sintered at a temperature of 1300°C have a uniform lamellar structure. [ABSTRACT FROM AUTHOR]

A mobility database for the γ-TiAl phase in the Ti-Al-Nb system was created by a critical assessment of experimental diffusion data found in literature. Optimization of mobilities and diffusion modeling was performed using the DICTRA (DIffusion Controlled TRAnsformations) software. Agreement was found between the experimental and calculated diffusion coefficients. The database was further validated with a diffusion couple consisting of 46Ti-54Al and 31Ti-55Al-14Nb alloys. The experimental data and final optimization revealed Nb-dependent effects on the diffusivities of all three elements. The Nb diffusivity increases with Nb content, and the Ti and Al diffusivities vary with composition in both binary and ternary alloys. [ABSTRACT FROM AUTHOR]

An amorphous Ti-37.5Zr-15Cu-15Ni (wt.%) ribbon fabricated by vacuum arc remelting and rapid solidification was used as filler metal to vacuum braze TiAl alloy (Ti-45Al-2Mn-2Nb-1B (at.%)). The effects of brazing temperature and time on the microstructure and strength of the joints were investigated in details. The typical brazed joint major consisted of three zones and the brazed joints mainly consisted of α 2- Ti 3 Al phase, α-Ti phase and (Ti, Zr) 2 (Cu, Ni) phase. When the brazing temperature varied from 910 °C to 1010 °C for 30 min, the tensile strength of the joint first increased and then decreased. With increasing the brazing time, the tensile strength of the joint increased. The maximum room temperature tensile strength was 468 MPa when the specimen was brazed at 930 °C for 60 min. All the fracture surfaces assumed typical brittle cleavage fracture characteristic. The fracture path varied with the brazing parameter and cracks preferred to initiate at (Ti, Zr) 2 (Cu, Ni) phase and propagation path were mainly determined by the content and distribution of α-Ti phase and (Ti, Zr) 2 (Cu, Ni) phase. [ABSTRACT FROM AUTHOR]

Electron beam melting (EBM), a powder bed fusion based additive manufacturing process, has been used to fabricate Ti-48Al-2Cr-2Nb γ-TiAl samples. The samples were evaluated for their potential use in biomedical applications in terms of in vitro wear, corrosion and biocompatibility in as-deposited (AD) and hot isostatically pressed (HIPed) conditions. The samples were found to exhibit a lamellar microstructure consisting of γ-TiAl and Ti 3 Al (α 2 ) phases in both the conditions. However, their in vitro wear and corrosion performance in Hank's balanced salt solution (HBSS), with and without fetal bovine serum (FBS), was found to be very different. The AD samples exhibited comparable passive behavior to commercially pure titanium (CP-Ti). Their corrosion potentials and currents were better than those of CP-Ti. The γ-TiAl samples exhibited wear rates of the order of 10 − 4 mm 3 /N·m in HBSS. The presence of FBS was found to increase the corrosion and the wear rate of this alloy increased by 65%. In vitro cell culture experiments, using NIH3T3 cells, demonstrated that the EBM processed γ-TiAl is non-toxic and can allow cell adhesion and proliferation as effectively as CP-Ti. [ABSTRACT FROM AUTHOR]

The digital image correlation (DIC) of speckle patterns obtained by vapour-assisted gold remodelling at 200 - 350 °C has already been used to map plastic strains with submicron resolution. However, it has not so far proved possible to use such patterns for testing at high temperatures. Here we demonstrate how a gold speckle pattern can be made that is stable at 700 °C, to study deformation in a commercial TiAl alloy (Ti-45Al-2Nb-2Mn(at%)-0.8 vol% TiB). The pattern is made up of a uniformly sized random array of Au islands as small as 15 nm in diameter, depending on reconstruction parameters, with a sufficiently small spacing to be suitable for nano-scale, nDIC, strain mapping at a subset size of 60 × 60 nm. It can be used at temperatures up to 700 °C for many hours, for high cycle fatigue testing for instance. There is good particle attachment to the substrate. It can withstand ultra-sound cleaning, is thermally stable and has a high atomic number contrast for topography-free backscatter electron imaging. [ABSTRACT FROM AUTHOR]

γ+α 2 Titanium aluminide (Ti-28Al-10Nb-3Cr-0.02B wt%) was processed through vacuum arc remelting (VAR) route using Ti sponge and pure alloying elements. Electrode for VAR melting which was designed to obtain uniform chemistry across the cross-section and throughout the height of the billet was realized through cold compaction. The alloy was hot worked in the temperature range of 1200–1100 °C. Tensile properties of the alloy at ambient temperature were evaluated at different strain rates ranging from 10 −4 s −1 and 10 −1 s −1 . The alloy exhibited ultimate tensile strength in the range of (386–553 MPa) when tested at ambient temperature and strain rates of 10 −4 s −1 and 10 −1 s −1 , whereas it varied from 630–673 MPa at intermediate strain rates of 10 −3 s −1 and 10 −2 s −1 . Percentage elongation is found to be 4.87–5.49% at strain rates of 10 −4 s −1 and at 10 −1 s −1 whereas it showed a relatively higher % elongation (5.42–7.19%) at intermediate strain rates of 10 −3 s −1 and 10 −2 s −1 , indicating intermediate strain rates are better for optimum strength as well as for ductility. The alloy is found to have predominantly two phases γ (TiAl) and α 2 (Ti 3 Al) with moderately random texture and presence of small volume fraction of β-phase. Microstructure showed presence of varying width of α 2 +γ lamellar structure. Transmission electron microscopy (TEM) confirmed presence of extensive twinning, dislocations and stacking faults with two phase structure indicating twinning has played a significant role in deformation. Presence of minor amount of β phase has been found to be contributing factor for improvement in ductility. Fracture analysis shows failure was mainly by rupture of lamellar interfaces. Occasional deformation of lamellae is also noticed in the uniaxial tensile tested samples at intermediate strain rates of 10 −3 s −1 and 10 −2 s −1 wherever small increase in ductility is observed. Marginally lower ductility at lower strain rate is due to growth of microcracks at interlamellar sites of dissimilar orientation and at higher strain rate due to faster growth of microcracks at lamellae of similar orientation. [ABSTRACT FROM AUTHOR]

We attempted to produce a porous Ti–20 at% Al alloy by pressure-sintering of a mixed powder of Ti and Al with a NaCl space holder to achieve porosity of 60%. As-sintered specimens exhibited an inhomogeneous microstructure consisting of Ti particles surrounded by a Ti–Al alloy layer. However, a nearly single-phase Ti 3 Al microstructure appeared in the porous specimen heat-treated at 1300 °C, which shows high plateau-stress level of approximately 100 MPa resulting in high absorption energy. Thus, controlling the microstructure for the formation of the Ti 3 Al single-phase would be effective to achieve high energy absorption capacity of the porous Ti–Al alloy. [ABSTRACT FROM AUTHOR]