γ-TiAl Alloy Research Articles

Effect of forging on the microstructure and texture of a high Nb containing γ-TiAl alloy

Effect of forging on the microstructure and texture of a high Nb containing γ-TiAl alloy

Microstructure Characteristics and Elevated Temperature Mechanical Properties of a B Contained β-solidified γ-TiAl Alloy

Microstructure Characteristics and Elevated Temperature Mechanical Properties of a B Contained β-solidified γ-TiAl Alloy

Effect of crystal orientation on the scratching behavior of γ-TiAl alloy nanowires by molecular dynamics simulation

In micro-and-nanomachining, crystal orientation effects significantly influence the plastic deformation, mechanical response, and temperature profile of nanowire materials. This study utilizes molecular dynamics simulation to investigate the process of surface scratching on γ-TiAl alloy nanowires under six different crystal orientations. The results reveal that the evolution of internal microscopic defects during the surface scratching process is heavily affected by crystal orientation, resulting in distinct dislocation motion-induced shear deformation and extrusion removal by the tool. These factors determine variations in both the shape of the nanowire's surface pile-up and the overall degree of workpiece deformation. Among all cases examined, (001)[1‾1‾0] crystal orientation exhibits easier removal of atoms with minimal internal defects, while (110)[11‾0] crystal orientation shows maximum atom removal without significant overall deformation, both cases demonstrate superior surface morphology and subsurface quality. Furthermore, stress and temperature profiles within abraded atoms and inside the workpiece depend on crystal orientation. These findings provide valuable insights for characterizing surface damage in γ-TiAl alloy nanowires as well as facilitating the preparation of surface nano-channels.

Preliminary study of the effect of Sn addition on microstructure and creep resistance of a β-solidifying TiAl alloy

In the present work, two β-solidifying γ-TiAl based intermetallic alloys doped with Nb, Zr, Hf (designated TNZ γ-TiAl alloy) and additionally Sn (designated TNZ-Sn) were investigated. Sn is known to be widely used as an alloying element in titanium alloys but almost has not been used for γ-TiAl alloys. In contrast to Nb, Zr and Hf atoms occupying only or preferentially the Ti sublattice, Sn atoms occupy the Al sublattice because Al and Sn belong to the same group of post-transition metals. Two microstructural conditions per alloy, duplex and fully or near lamellar, were obtained in the alloys by upset forging and heat treatments. Microstructure examination by SEM and XRD analysis revealed two basic phases in both alloys, γ-TiAl and α2-Ti3Al. The feature of the Sn-containing alloy was the presence of a small amount (0.5–1.5 vol%) of the phase enriched with Sn, Zr and Hf and located along grain/colony boundaries. After upset forging and heat treatment this phase had the average chemical composition 60(Ti + Nb + Zr + Hf)-40(Al + Sn) (at.%) and was identified as the α2 phase enriched with Sn, Zr and Hf and depleted with Al. The creep tests were performed for the produced microstructural conditions of the alloys. It was revealed that doping with Sn led to a strong increase in the creep resistance at 800 and 850 °C. The obtained effect was attributed to solid solution hardening due to substitution of Al by Sn and probably to a more ionic interatomic bonding taking into consideration an appreciably higher electronegativity of tin as compared to that of other alloying elements.

Surface characteristics of additively manufactured γ-TiAl intermetallic alloys post-processed by electrochemical machining

Electron beam melting (EBM) process is the most preferable powder bed fusion process for high melting point alloys. γ-TiAl alloys are intermetallic chemical compounds, which are lightweight and resistant to oxidation and heat, and have low ductility as well. EBM is the best manufacturing process for such alloys to produce aerospace parts, but poor surface quality is the main drawback of this process. Thus, surface post-processing is needed after the EBM process. This study presents the surface characteristics of EBM γ-TiAl alloy surfaces post-processed by electrochemical machining (ECM). The surfaces were ECM and the surface roughness values (Sa, Sq and Sz) have been reduced significantly. The mean reduction values of the experiments are 98.6 %, 98.5 % and 95.3 % for Sa, Sq and Sz, respectively. Additionally, it is observed that the surface roughness is inversely proportional to the electrolyte conductivity up to a limit value (115 mS/cm), and after that the roughness increases via the stray current transition. Sq values decrease with the increased feed rate via the increased current density. However, an increase in feed rate negatively affects the Sku values, which statistically describe the sharpness or spread of surface roughness. Also, XRD results showed that Ti2AlN layer is formed on the surface via the HIP operation. This layer inhibits the electrochemical dissolution of γ-TiAl alloy and caused surface defects.

Influence of heat treatment on microstructure of a new β-solidifying γ-TiAl alloy

Influence of heat treatment on microstructure of a new β-solidifying γ-TiAl alloy

Effect of repetitive nano-cutting tool parameters on surface quality and subsurface damage of γ-TiAl alloy

In this paper, the molecular dynamics simulation of the repeated nano-cutting of single crystal γ-Tial alloy was carried out by selecting different geometric parameters of the second cutting tool by single factor experiment. The cutting force, friction coefficient, subsurface defects, dislocation evolution and surface roughness of the second cutting were analyzed systematically. The results show that when the tool rake angle is 15°, the surface roughness is lower and the surface quality is better. The influence of different second cutting tool rake angle on the surface roughness is not strong. When the rake angle of the second cutting tool and the radius of edge are constant, the average normal cutting force decreases with the increase of the clearance angle of the tool. Under the machining parameters in this paper, the critical clearance angle of the second cutting of single crystal γ-TiAl alloy is between 10° and 15°. When the tool clearance angle is greater than the critical clearance angle, the average cutting force and the machined-surface roughness no longer change significantly. With the increase of the radius of the second cutting tool, the chip decreases, the subsurface defect increases, and the surface roughness of the machined surface also increases with strong regularity.

Wear mechanism of diamond-cutting tool in nano-cutting polycrystalline γ-TiAl alloy based on molecular dynamics simulation

The machining of γ-TiAl is susceptible to tool wear, and investigations into tool-wear mechanisms can contribute to enhancing machining efficiency and optimizing the surface integrity of workpieces. In this study, a molecular-dynamics method is employed to simulate the cutting process of polycrystalline titanium–aluminum alloys using a single-crystal diamond tool. The findings suggest that tool wear is induced by cutting forces and plastic deformation, with amorphization and diffusive wear as the primary mechanisms. Plastic deformation within the matrix facilitates interactions between internal dislocations in the grains, thus resulting in work hardening at the surface layer, increased cutting forces, and heightened susceptibility of the cutting edge to wear due to plastic deformation. Moreover, elevated temperatures and pressures in the cutting contact area induce atomic amorphization, which reduces both the strength and hardness of the tool while triggering structural phase transitions for certain atoms. Consequently, lattice distortion occurs within this region, thus causing matrix atoms to diffuse into the tool. The combined effect of amorphization and diffusion significantly exacerbates tool wear. This investigation provides valuable insights into wear mechanisms and guidance for enhancing the quality of γ-TiAl alloy.

The influence of induction-assisted milling on the machining characteristics and surface integrity of γ-TiAl alloys

The γ-TiAl alloys are characterized by low density, high specific strength, and excellent high-temperature performance, making it an ideal material for achieving lightweight and high-temperature durability in high-pressure compressors. However, due to its intrinsic embrittlement, conventional machining (CM) methods often lead to machining defects, making it difficult to meet industrial requirements. Induction-assisted machining (IAM) provides a new approach to improve the machinability of γ-TiAl alloy as an emerging machining technology. In this study, finite element analysis was first used for thermal-electromagnetic coupling analysis to determine the depth of the thermal affected zone and preheating time. Subsequently, the influence of two key parameters, preheating temperature and feed rate, on machining characteristics and surface integrity was investigated, and compared with traditional milling methods. Finally, the fracture mechanism of the γ-TiAl alloys lamellae was revealed through microstructural analysis. The results indicate that IAM is a promising machining method that can significantly improve the machining performance and surface integrity of γ-TiAl alloys.

Oxidation behavior of Ti2AlC MAX phase-based coating on a γ-TiAl alloy TiAl48-2-2 produced by DC magnetron sputtering

The application of intermetallic γ-TiAl-based alloys is limited by the deterioration in strength and creep resistance as well as reduction in oxidation resistance above 750 °C. The deposition of protective coatings is a promising opportunity to enhance the oxidation resistance or the lifetime with regard to the sustainability by several orders of magnitude. Moreover, additive manufacturing processes could enable the production of components with more complex geometries in the future. Thus, designs of γ-TiAl turbine blades with internal cooling systems could be feasible in the near future. In this context, the deposition of thermal barrier coatings and, therefore, oxidation protective bond coatings became important. MAX phases are of increasing interest as coating materials for high temperature applications due to their unique combination of metallic and ceramic properties. Especially the alumina forming MAX phases such as Cr2AlC, Ti2AlC or Ti2AlN are promising coating materials.In the present work a Ti2AlC MAX-phase based coating was deposited by DC magnetron sputtering. Using three pure elemental target materials - Ti, Al and C and a two-fold rotation, a homogenous all-around coating was applied on the γ-TiAl alloy TiAl48-2-2 with a coating thickness of 10 μm. After the deposition process the stochiometric Ti2AlC coating was X-ray amorphous, therefore a post-heat treatment at 800 °C for 1 h was performed to achieve the desired hexagonal phase. Finally, the MAX-phase coated TiAl48-2-2 alloy was subjected to a cyclic oxidation test at 850 °C in laboratory air. The Ti2AlC MAX-phase coated TiAl48-2-2 alloy exhibits an excellent oxidation behavior, due to the formation of a thermally grown alumina top layer with the desired hexagonal Ti2AlC MAX-phase below. Extensive TEM analyses also showed the dependence of Al2O3 formation on post-heat treatment. θ-Al2O3 with a lower protective effect and the concomitant formation of TiN under the TGO was observed with post-heat treatment in Ar, while post-heat treatment in air lead to the formation of a protective α-Al2O3 layer.

Tribological Properties of γ-TiAl Alloy Fretting against Nickel-Based Superalloy at Elevated Temperature

The tribological properties of γ-TiAl alloys under sliding condition have been well established, but the mechanisms for fretting wear, especially at elevated temperature, remain to be further clarified. In the present study, the tribological behaviors and mechanisms of a γ-TiAl alloy were investigated under an elevated temperature fretting condition. The wear scars, debris, and tribolayers were studied with scanning electron microscope, energy dispersive spectrometer, and Raman spectrometer. The structural and mechanical properties of the tribolayers were investigated utilizing transmission electron microscope and nanoindentation tests. The results demonstrated that the severe adhesive wear, delamination, and abrasive wear in the early stage of fretting resulted in high wear rate of the γ-TiAl alloy. As fretting progresses, the formation of tribolayers with nanograins and excellent mechanical properties improved the fretting wear resistance of the γ-TiAl alloy. Due to the protective effect of the adhesively transferred γ-TiAl, the wear rate of the nickel-based superalloy counterpart was significantly lower.

Strong compressive γ-TiAl alloy produced by dual-wire-fed electron beam directed energy deposition: Microstructural evolution and strengthening mechanisms

Dual-wire-fed electron beam directed energy deposition (EB-DED) of TiAl alloy is of increasing interest due to its potential to produce high performance components at low cost. However, the strengthening mechanisms involved, in particular the complex relationship between microstructure and mechanical properties, are still unclear. This study uses a combination of experimental fabrication and numerical simulation. The aim is to investigate the microstructural evolution and strengthening mechanisms of a fully columnar γ-TiAl alloy produced by the dual-wire-fed EB-DED technique. The results show that cyclic heating significantly increases the dislocation density in the TiAl alloy. In addition, dislocation networks, twinning intersections and dislocation-twin interactions were observed. Compression test results show that the dual-wire-fed EB-DED prepared γ-TiAl alloy exhibits 26% higher yield strength and fracture toughness, as well as 25% higher work-hardening stress compared to the traditionally processed TiAl alloy of the same composition. The nanoscale twinning and interaction structures are identified as the unique strengthening mechanisms, contributing 194 MPa to yield strength and 1196 MPa to work hardening. This study provides valuable insights into the microstructural characteristics and strengthening mechanisms of TiAl alloy produced using the dual-wire-fed EB-DED.

Atomic-Scale Dislocation Structure Evolution and Crystal Ordering Analysis of Melting and Crystallization Microprocesses in Laser Powder Bed Melting of γ-TiAl Alloys

Laser Powder Bed Fusion (LPBF) technology exhibits significant advantages in the manufacturing of components with high dimensional accuracy and intricate internal cavities. However, due to the inherent room-temperature brittleness and high-temperature gradient induced by the laser forming process, the LPBF fabrication of γ-TiAl alloy is often accompanied by the initiation and propagation of defects. The aim of this study is to investigate the forming process of γ-TiAl alloy by the LPBF method through molecular dynamics simulation, and to explain the microparticle arrangement and displacement evolution of the melting and crystallization processes, thus elucidating the link between the variations in the laser process parameters and defect generation during microscopic laser heating. The results show that during the melting process, the peaks of the radial distribution function (RDF) decrease rapidly or even disappear due to laser heating, and the atomic disorder is increased. Although subsequent cooling crystallization reorders the atomic arrangement, the peak value of the RDF after crystallization is still 19.3% lower than that of the original structure. By setting different laser powers (200–800 eV/ps) and scanning speeds (0.2–0.8 Å/ps), the effects of various process parameters on microforming and defect evolution are clarified. When the laser power increases from 200 to 400 eV/ps, the stable value of atomic displacement rises from 6.66 to 320.87, while it rises from 300.54 to 550.14 when the scanning speed is attenuated from 0.8 to 0.4 Å/ps, which indicates that, compared with the scanning speed, the atomic mean-square displacements are relatively more sensitive to the fluctuation of laser power. Dislocation analysis reveals that a higher laser power significantly increases the cooling rate during the forming process, which further aggravates the generation and expansion of dislocation defects.

The mechanism of element inhomogeneity in TW-DED-arc fabricated γ-TiAl alloy

The mechanism of element inhomogeneity in TW-DED-arc fabricated γ-TiAl alloy

Metastable microstructure evolution and grain refinement in a Low-Ta containing γ-TiAl alloy through heat treatment

The paper comprehensively describes the evolution of metastable microstructures in a cast low-Ta-containing γ-TiAl alloy during cooling-tempering-lamellarization heat treatments. The behavior and mechanism of microstructure refinement are discussed. The rapid cooling generates metastable microstructures, including Widmanstätten, feathery, and massive structures, with many γ variants in small sizes and different orientations. It significantly refines the coarsened colonies and randomizes the texture. The tempering can effectively stabilize the metastable microstructures. During tempering, the γ→α phase transformation is more active at higher temperatures while coarsening and recrystallization dominate at lower temperatures. After tempering, a refined microstructure with spherical grains and convoluted structures forms. Subsequent lamellarization treatment optimizes the microstructure and obtains a fine-grained nearly-lamellar microstructure with small colonies (∼70 μm) and dispersed fine γ grains (∼10 vol%), which yielded a 1.0 % elongation with a 487 MPa yield strength (0.2%) and a 589 MPa ultimate tensile strength at room temperature. Finally, the unique promotion effects and mechanism of a small amount of Ta addition on the nucleation, growth, evolution, and stabilization of metastable microstructures are concluded and discussed, which is crucial for the refinement of cast γ-TiAl alloy by heat treatment alone.

Experimental Study on Surface Microscopic Characteristics of scCO2-MQL Milled γ-TiAl Alloy

γ-TiAl alloy has high specific strength and excellent low-temperature resistance, but its cutting performance is poor. In this study, the four aspects of residual stress, plastic deformation, surface phase transformation, and work hardening were analyzed via experiments involving supercritical carbon dioxide minimum quantity lubrication (scCO2-MQL) and dry milling of γ-TiAl alloy. The results show that the residual stresses generated during the milling process can be significantly improved under the scCO2-MQL condition and correspondingly reduced in depth from the surface, while the plastic deformation layer is significantly reduced, and the maximum reduction of β-phase volume fraction is 12.43% under the scCO2-MQL condition compared with the dry cutting condition. With increasing cutting speed in the range of 30–90 m/min, the depth of the hardened layer gradually decreased, while increasing the feed rate caused increased microhardness.

Role of temperature in tribolayers in fretting wear of γ-TiAl alloy

The formation of tribolayers may play significant influences on fretting wear. At elevated temperature, the adhesion among wear debris and the increased diffusion rate facilitate the formation of tribolayers. However, the intensification of oxidation at elevated temperature and the low diffusion rate in oxides may play an adverse role. The present study aims to investigate the role of temperature in tribolayers in fretting wear using a γ-TiAl alloy. Scanning electron microscope, energy dispersive spectrometer, Raman spectrometer, transmission electron microscope and nanoindentation were utilized to investigate the wear debris, tribolayers, and wear scars. The fretting tests showed that, compared with that at room temperature (RT) and 350 °C, significant reduction in wear rate and decrease in the fluctuation of friction coefficient occurred at 550 and 750 °C. It was further revealed that when temperature raised from room temperature (RT) to 750 °C, the oxidation of the wear debris increased slightly and the diffusion coefficients increased prominently, which facilities the formation of well tribo-sintered tribolayers. The well tribo-sintered tribolayers presented homogenous structure, nanocrystalline grains with excellent mechanical properties, and resulted in the improvement in the fretting wear resistance of the γ-TiAl alloy at 550 and 750 °C.

Effect of Cooling Rate on Powder Characteristics and Microstructural Evolution of Gas Atomized β-Solidifying γ-TiAl Alloy Powder

Effect of Cooling Rate on Powder Characteristics and Microstructural Evolution of Gas Atomized β-Solidifying γ-TiAl Alloy Powder

Microstructures and mechanical properties of a γ-TiAl alloy modified by Gd additions

The microstructural evolutions and room-temperature tensile properties of Ti-45Al-2.5Nb-1.5Mo-(0-0.2)Gd were investigated in details. Experimental result revealed that there were precipitated particles, involving Gd2TiO5 and GdAl3, distributing within the microstructure of the Gd-containing TiAl alloys, and the increase in the Gd addition resulted in the aggregation of Gd-containing particles, forming a network structure along grain boundary and colony boundary. The addition of Gd also had a significant refining effect on the average inter-lamellar spacing. The ultimate/yield strength and elongation increased first and then decreased with the content of Gd. Gd addition did not change the mixed fracture mode of crack propagation along and across the lamellae. In the plastic deformation process of Gd-containing sample, except for dislocation gliding, twinning-controlled deformation mode occurred, contributing to the improvement of mechanical properties. Moreover, dislocations could directly cut through the small-size Gd-containing particle, while some dislocations would continue to slip via bypassing the large-size Gd-containing particle.

Investigation of Survived Solidification Features After Hot Isostatic Pressing in Third Generation γ-TiAl Alloys: Ti–45Al–(5, 10)Nb–0.2B–0.2C

Investigation of Survived Solidification Features After Hot Isostatic Pressing in Third Generation γ-TiAl Alloys: Ti–45Al–(5, 10)Nb–0.2B–0.2C