TiAl Alloys Research Articles

Thermodynamic modeling of the Ti–Al–V system over the entire composition and a wide temperature range

Ti–Al–V alloys have received considerable attention owing to their excellent high-temperature mechanical properties. After conducting a critical review of the available experimental data for the ternary Ti–Al–V system, a thermodynamic assessment of the system was carried out over the entire composition and a wide temperature range utilizing the CALPHAD method. The extensive homogeneity ranges of the α2 and γ phases were satisfactorily reproduced. Of particular significance was the first accurate description of the phase equilibria at 1573 K. The model parameters obtained in this study well represented the thermochemical properties and phase equilibria of the experimental data. Furthermore, the enthalpies of the widely used Ti alloy Ti–6Al–4V were reproduced using the thermodynamic description established in this study. Thus, this study provides a reliable basis for guiding the development and design of Ti–Al–V alloys.

Preparation of low-oxygen Ti–Al alloy by sustainable recovery of spent SCR catalyst

The wide spread application of NH3-selective catalytic reduction (NH3-SCR) process has led to the accumulation of large amounts of spent SCR catalyst. In this study, Al reduction–electromagnetic directional crystallization was used to recycle Ti, W, and V from spent SCR catalyst to prepare W- and V- containing TiAl alloys. The effects of the slag composition, reaction time, and amount of reductant added on the recovery ratios were investigated. The maximum recovery ratios of 94.16 %, 81.85 %, and 98.14 % for Ti, W, and V, respectively, were achieved at a reaction time of 60 min, addition of 45 wt.% Al, and CaO / Al2O3 ratio of 2 / 3. Furthermore, directional crystallization was performed on Ti–Al alloy under different atmospheres (99.99 % Ar or 90 % Ar + 10 % H2 gas) to investigate their effects on deoxygenation. 90 % Ar + 10 % H2 gas could effectively remove O from the Ti–Al alloys. Si and P impurities were enriched at the bottom of the alloy and Fe impurities at the top. The Ca and Mg impurities contents are approximately 50 and 10 ppmw, respectively, and S was removed difficultly. The total impurities content of the resulting W- and V- containing TiAl alloys were less than 1.85 %. The entire process is low-carbon and environmentally friendly.

Microstructure and mechanical properties of high relative density Ti-48Al-1Fe alloy using irregular blend powder

Conventional pressing and sintering methods provide difficulties in achieving high density for PM TiAl alloys. The Ti-48Al-1Fe alloy was prepared via vacuum pressure-less sintering, and the microstructure evolution and mechanical properties during the sintering densification process of the mixed powder system were investigated. The results shows that the density of the mixed powder reached 98.3 ± 0.14% after pressure-less sintering for 3 hours under high vacuum conditions at 1300°C. The microstructure of the as-sintered alloy is a fine duplex structure composed of α2/γ lamellar colonies and the composite structure. The ultimate tensile strength (UTS) of the as-sintered alloy at room temperature is 315 MPa. The density of the as-sintered alloy reach more than 99.9% after non-capsule hot isostatic pressing (HIP). The as-HIPed alloy exhibits an UTS of 332 MPa at room temperature. At high temperatures, it maintains high strength, with an UTS of 408 MPa and yield strength of 386 MPa at 900°C, which is comparable to that of forged TiAl alloy. Furthermore, the elongation of the sample at 900°C is 20.0%. This process offers a novel method for sintering PM TiAl with high density and mechanical properties at a low cost and near-net shape.

Multiscale microstructural evolution of Fe–Ni–Al–Ti alloy with high magnetization

The Fe–15Ni–3Al–1Ti ribbons are produced by melt-spinning the annealed ingots at 10–40 m/s. The annealed ingots consist of Fe3Ni grains and α-Fe-type grain boundary phase, forming a columnar grain microstructure in the vertical direction and getting the maximum saturation magnetic polarization (Js) of 1.88 ± 0.03 T in the cross-section. Before and after the annealing, the ribbons form a cellular structure composed of the Fe3Ni intragranular phase and γ-(Fe, M), Fe7Ni3, or α-(Fe, M) cell boundary phases. Formation of enhanced <100> orientation is the primary mechanism for obtaining a high Js of 2.35 ± 0.07 T in the ribbons. The grain and grain boundary size of the ribbons is reduced by the phase transformation induced by annealing at 600 °C. The phase transformation processes of the ribbons before and after annealing are as follows: L→γ→melt−spunFe3Ni+γ1→annealedFe3Ni+Fe7Ni3 when melt-spinning at a speed of 10–25 m/s, and L→γ→melt−spunFe3Ni+Fe7Ni3→annealedFe3Ni+α−(Fe,Ni) at 40 m/s. Finally, the multiscale microstructure evolution models are used to summarize the phase transformation process of the alloys.

Evaluation of Microstructure and Mechanical Properties of TiAl/HI-TEMP 820/ SCM440H Materials Manufactured through Vacuum Brazing according to Process Temperature

The TiAl alloy is attracting attention as a lightweight and heat-resistant material, because of its high specific strength, excellent high-temperature formability, and fatigue strength. However, its applications are limited by its high unit price and low room temperature ductility. To overcome this issue, dissimilarly bonded materials have been extensively employed. This involves joining a brittle metal to a low-cost metal that possesses excellent plasticity, using various dissimilar bonding techniques. In this study, TiAl/HI-TEMP 820/SCM440H materials were fabricated using a vacuum brazing process under different temperature conditions. After the brazing process, the microstructure of the interfacial area revealed seven distinct layers resulting from chemical reactions between the base metals and the filler metal. These reaction layers consisted of a Ni solid solution, intermetallic compounds (Ti&lt;sub&gt;3&lt;/sub&gt;Al, TiNi&lt;sub&gt;2&lt;/sub&gt;Al, Ti&lt;sub&gt;2&lt;/sub&gt;Ni, FeNi), and borides (CrB, TiB&lt;sub&gt;2&lt;/sub&gt;, FeB). To analyze the effect of brazing temperature on the relationship between the microstructure and mechanical properties at the interface of TiAl/HI-TEMP 820/SCM440H materials, conventional uniaxial tests and nanoindentation tests were performed. The measured nanohardness exhibited a significantly large distribution for each reaction layer, with the highest hardness values observed in the intermetallic compounds and borides layers. Additionally, room temperature tensile tests confirmed that fractures initiated in the highhardness and brittle intermetallic compounds and borides layers.

Effect of compositional minor adjustment on localized lamellar structure coarsening of TiAl alloys with different solidification modes

Possible lamellar coarsening in the as-cast microstructure of TiAl alloys significantly affects the subsequent processing of ingot metallurgy, and a systematic study of it is necessary. In this paper, two TiAl alloys with different solidification modes (TNM-45Al alloy and TNM-0.3Si-0.7 C alloy) were obtained by compositional minor adjustment basing on TNM alloy. The effect of different adjustment routes on the localized lamellar structure coarseing behaviour and the resulting differences in the uniformity of the nanohardness distribution were investigated by comparing with TNM alloy. It is found that increasing the Al content of the TNM alloy while keeping the solidification mode as β-solidification unchanged leads to severe localised lamellar coarsening in the S-segregation zone of TNM-45Al alloy, including continuous coarsening (CC) and discontinuous coarsening (DC). In contrast, the solidification mode of the TNM-0.3Si-0.7 C alloy with the addition of the strong α stabilizer elements Si and C is transformed into hypoperitectic solidification with severe peritectic segregation, but the lamellar structure exhibits surprising full-domain stability. The results show that the thermodynamic tendency caused by high Al content and the absence of net-like β-segregation leads to severe localised lamellar coarsening in TNM-45Al alloy. CC can be accomplished through the migration of interface defects. DC is formed by the nucleation and expansion of γ-phase Shockley incomplete dislocations, and the interfaces consist of the α2 /γ interface and the first discovered γ /γ twin interface. The lamellar coarsening of TNM-0.3Si-0.7 C alloy can be effectively suppressed by the solid solution of Si/C atoms and the near net-like β-segregation in the primary β-phase region. As a result, the distribution uniformity of the nanohardness of the lamellar structure in TNM-0.3Si-0.7 C alloy is excellent, with the variance decreasing from ±0.24 of TNM alloy to ±0.20, while it increases to ±0.37 for the TNM-45Al alloy.

Enhancement of mechanical properties and oxidation resistance of TiAl alloy with addition of Nb and Mo alloying elements

In this study, small button-shaped ingots were prepared by a non-self-consuming vacuum arc melting method. The effect of the content of Nb and Mo elements on the microstructure, mechanical properties and oxidation resistance of TiAl alloys were investigated. The as-cast TiAl alloy presented laminar microstructure. When only adding Nb element, a small amount of β phase was distributed in the form of dots at the grain boundaries of α2/γ lamellar structure. When adding both Mo and Nb, the precipitated β phase started to precipitate in lines inside the α2/γ lamellar structure. The microhardness of the alloy increased with the increasing Nb and Mo content. The compressive strength of the alloys increased firstly with the increasing Nb content and decreased when the content of Nb exceeded 7 wt%. However, with the addition of Mo, the compressive strength of the alloys decreased drastically. The optimal compressive strength of the alloy, i.e. 2250 MPa, was obtained with 5 wt% Nb and 5 wt% Mo addition. The oxidation resistance of the alloys were enhanced with the addition of Nb and Mo elements and the TiAl–5Nb–5Mo alloy also showed the best oxidation resistance at 800 °C.

Structural and electrical investigation of Al/Ti/TiN/Au based N-face n-GaN contact stack

In this work, the structure of Ti/Al/TiN/Au contact layer stack on the N-face of a single-crystal n-GaN substrate is studied after heat treatment at 750 °C. Since TiN is widely regarded as a diffusion barrier in the stack, the formed structures with three different initial TiN thicknesses (15, 60 and 90 nm) in the contact layers are investigated in detail. The primary tool used for the structural investigations was a (scanning) transmission electron microscope ((S)TEM). In all three samples a low resistivity ohmic contact was formed. However, the TiN layer has not completely blocked the diffusion for any of the samples and both Al and Au diffused through the TiN layer at the temperature of 750 °C. As a result of the heat treatment, a complex AlN/TiN/Au2Al/TiN/Au + Al2.67O4 stack was formed on the surface of the GaN substrate. The measurements have not shown the often reported Ti–Al alloy phase in the samples. The formation of the AlN and TiN layers can be explained by the separation of N from GaN creating N vacancies in the GaN substrate which could help the formation of the ohmic behavior.

Numerical and experimental study of directional annealing to prepare columnar grains of TiAl alloy

Directional annealing (DA) is a novel technique to prepare colunmar grains or single crystal in TiAl alloys without melt contamination. Both numerical simulation and experiments were conducted to investigate the directional annealing (DA) of a Ti48Al alloy in this paper. The temperature distribution of the TiAl rod under different DA parameters were calculated and discussed by Finite Element (FE) method. The results showed that the temperature field was significantly influenced by hot zone temperature, moving rate of induction coil, rod diameter and cooling condition. Once induction heating started, the temperature of Ti48Al rod rose rapidly and reached a steady state after about 400 s. The effective heat area (single phase zone) enlarged with the increase of preset temperature. With the decreasing movement rate of induction coil, the α/α+γ interface became more planar, and this rule also applied to the effect of the rod diameter on α/α+γ interface. Under the guidance of calculation results, optimized parameters were proposed to directionally annealing Ti48Al alloy. Rods with a diameter of 8 mm were induction-heated under the hot zone temperatures of 1673 K and 1693 K correspondingly. In the range of α single phase, the transformation of γ→α phase occurred completely and α grains directionally coarsened with the movement of hot zone. In this way, columnar grains in Ti48Al alloy were successfully formed in a solid state.

MICRO-EDM OF TI–6AL–4V USING VARIOUS TOOL MATERIALS: MULTI-RESPONSE OPTIMIZATION AND SURFACE CHARACTERIZATION

In this paper, experimental analysis was performed during micro-electrical discharge machining (micro-EDM) of titanium alloy Ti–6Al–4V (grade 5) using three types of tools viz. copper (Cu) tool, tungsten carbide (WC) tool, and synthetic graphite (Gr) grade three tool. The main process parameters were taken as (a) pulse on time ([Formula: see text]), (b) pulse off time ([Formula: see text]), (c) voltage (V), and (d) capacitance (C). The output responses were taken as the material removal rate (MRR) and tool wear rate (TWR). Taguchi method coupled with grey relational analysis (GRA) (L[Formula: see text] orthogonal array) technique was applied to optimize the input process parameters for both the responses. Scanning electron microscopy (SEM) analysis of the workpiece and tool was also performed to investigate the morphology of the machined surface and tool surface. Energy-dispersive spectroscopy (EDS) analysis was performed to investigate the elemental composition of the machined surface. The experimental finding reveals that tungsten carbide is the most suitable tool material for machining the chosen workpiece for obtaining optimal MRR and TWR. The optimum condition for the copper tool was found as 180[Formula: see text]V, 1000[Formula: see text]pf, 10[Formula: see text][Formula: see text]s ([Formula: see text]), and 10[Formula: see text][Formula: see text]s ([Formula: see text]). Meanwhile, the optimum parametric condition for tungsten carbide and graphite tools was found to be the same as 240[Formula: see text]V, 100[Formula: see text]pf, 20[Formula: see text][Formula: see text]s ([Formula: see text]), and 5[Formula: see text][Formula: see text]s ([Formula: see text]).

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.

Leveraging a coarse yet strong matrix in L12-strengthened high-entropy alloy through simplified thermomechanical processing

High-entropy alloys (HEAs), characterized by their unique design concept without a distinct major element, offer a versatile alloy design window while ensuring outstanding properties. In this study, the FeCoCrNi–AlTi alloy system, which exploits L12 phase with slow ripening kinetics due to the coherent interface with the FCC phase, has been researched to increase strength via precipitation. This system has been widely studied to improve mechanical properties through microstructure optimization, especially in a direction that suppresses discontinuous precipitation while enhancing strength. Herein, we propose a novel processing method utilizing hot-rolling followed by direct aging for a newly designed (FeCoNi)79.5Cr10.5Al5Ti5 (at%) HEA. The coarse yet strong FCC matrix, generated via hot-rolling, proves advantageous in the subsequent direct aging, as revealed through careful microstructural characterization of both continuous and discontinuous precipitates. A high yield strength of 1065.5 ± 0.4 MPa, a uniform elongation of 14.2 ± 0.0%, and a total elongation of 27.5 ± 0.9% are simultaneously achieved, due to maintaining high dislocation density and promoting continuous precipitation, whereas discontinuous precipitation is suppressed and occupies little area fraction of ∼8%. Careful discussions are provided regarding the impact of the newly proposed processing on the precipitation mechanism, and how the resulting microstructure influences the strengthening mechanisms. This work demonstrates the effectiveness of a simplified thermomechanical process, providing valuable insights into microstructure control in HEAs.

Realizing a Superior Conversion Efficiency of ≈11.3% in the Group IV-VI Thermoelectric Module.

GeTe-based materials exhibit superior thermoelectric performance, while the development of power generation devices has mainly been limited by the challenge of designing the interface due to the phase transition in GeTe. In this work, via utilizing the low-temperature nano-Ag sintering technique and screening suitable Ti-Al alloys, a reliable interface with excellent connection performance has been realized. The Ti-Al intermetallic compounds effectively inhibit the diffusion process at Ti-34Al/Ge0.9Sb0.1Te interface. Thus, the thickness of the interfacial reaction layer only increases by ≈2.08µm, and the interfacial electrical contact resistivity remains as low as ≈15.2 µΩ cm2 even after 30 days of isothermal aging at 773 K. A high conversion efficiency of ≈11.3% has been achieved in the GeTe/PbTe module at a hot-side temperature of 773 K and a cold-side temperature of 300 K. More importantly, the module's performance and the reliability of the interface remain consistently stable throughout 50 thermal cycles and long-term aging. This work promotes the application of high-performance GeTe materials for thermoelectric power generation.

Study on atomic-scale deformation mechanism based on nanoindentation of duplex full lamellar TiAl alloys with different orientation relationships

Study on atomic-scale deformation mechanism based on nanoindentation of duplex full lamellar TiAl alloys with different orientation relationships

Microstructure and hot corrosion properties of an Al-Y coating on TiAl alloy

Microstructure and hot corrosion properties of an Al-Y coating on TiAl alloy

Low-cycle and dwell fatigue properties for a near alpha titanium alloy Ti–6Al–3Nb–2Zr–1Mo

As a new type of marine titanium alloy, Ti–6Al–3Nb–2Zr–1Mo (Ti6321) has gained attention for its comprehensive properties such as high toughness, strength, weldability, and corrosion resistance. However, there are only a few reports on fatigue properties of Ti6321. In this study, the dwell fatigue and low cycle fatigue tests were performed under various maximum load stresses with the stress ratio of R = 0. Experimental results revealed that the failure can be classified into brittle and ductile types. Fatigue life decreases with the increase of maximum load stress. The dwell time facilitates the accumulation of plastic strain in each loading-unloading cycle and significantly reduces fatigue life. The stress-strain response in the initial period of loading has a decisive effect on fatigue life for both low-cycle and dwell fatigue. The plastic strain energy density in the first loading cycle and fatigue life satisfies the power law relationship, which can be utilized to predict the load holding and low cycle fatigue life of Ti6321.

Indentation size effect in 2nd and 3rd generation advanced intermetallic TiAl alloys: theoretical and experimental estimation of dislocation density

Indentation size effect in 2nd and 3rd generation advanced intermetallic TiAl alloys: theoretical and experimental estimation of dislocation density

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.

Experimental investigation of the CaO-Al2O3-TiO2 system: Isothermal section of 1500 °C and liquidus projection

The CaO-Al2O3-TiO2 phase diagram is of great guiding significance for the comprehensive utilization of Ti-bearing mineral resources and the design of metallurgical slag for Al-Ti alloy steel. In this paper, the high-temperature phase equilibrium experimental method was adopted; the types and compositions of phases were identified by Electron Probe Micro Analysis (EPMA) and X-ray diffraction (XRD). A total of 12 equilibrium phases were determined, including CaO, Al2O3, TiO2, 3CaO2TiO2, 4CaO3TiO2, CaOTiO2, Al2O3TiO2, 3CaOAl2O3, CaOAl2O3, CaO2Al2O3, CaO6Al2O3, and Liquid; they formed a total of 16 phase equilibrium relations. The isothermal section of CaO-Al2O3-TiO2 system at 1500 °C in atmosphere was constructed for the first time. There are 10 three-phase fields (Liquid+CaO+3CaO2TiO2, Liquid+3CaO2TiO2+4CaO3TiO2, CaOTiO2+CaOAl2O3+CaO2Al2O3, CaO2Al2O3+CaOTiO2+CaO6Al2O3, Liquid+CaOTiO2+ CaO6Al2O3, Liquid+CaO6Al2O3+Al2O3, Liquid+Al2O3+Al2O3TiO2, Liquid+TiO2+Al2O3TiO2, Liquid+CaOAl2O3+4CaO3TiO2, 4CaO3TiO2(ss)+CaOTiO2(ss)+CaOAl2O3), 13 two-phase fields (Liquid+CaO, CaO+3CaO2TiO2, Liquid+3CaO2TiO2, Liquid+4CaO3TiO2, Liquid+TiO2, Liquid+CaOTiO2, Liquid+Al2O3TiO2, Liquid+Al2O3, Liquid+CaO6Al2O3, Liquid+CaOAl2O3, CaOTiO2(ss)+CaO2Al2O3, 4CaO3TiO2(ss)+CaOTiO2(ss), 4CaO3TiO2(ss)+3CaO2TiO2(ss)), 2 single-phase solid solution fields (4CaO3TiO2(ss), 3CaO2TiO2(ss)), and 2 disconnected single liquid phase fields. On this basis, the liquidus projection of the CaO-Al2O3-TiO2 system was revised. The types of relevant invariant reactions were identified by the Lever Rule; the types of relevant univariant reactions were determined by the original method proposed in this paper.

Refinement and enhancement of high-Nb TiAl alloy via in-situ precipitation of Ti2AlC and TiB2 nanoparticles

An in-situ dual morphology of Ti2AlC/TiB2 nanoparticles was refined and enhanced to fabricate Ti-47.7Al-7.1Nb-2.3V-1.1Cr composites via induction hot-pressing sintering with nano-B4C addition. The generation process of nanoparticles, lamellar refinement mechanism, and effect of nanoparticles on the microstructure evolution/mechanical properties were meticulously analysed. With 0.2 at% nano-B4C addition, Ti2AlC/TiB2 nanoparticles were uniformly distributed in the matrix following sintering. In addition, upon sintering at 1425 °C for 10 min under a pressure of 50 MPa, the lamellar colony size decreased from 280 ± 145 μm to 55 ± 16 μm, and the ultimate tensile strength (UTS) increased by 116 MPa, 142 MPa, and 176 MPa, when tested at room temperature, 800 °C, and 900 °C, respectively. The enhanced mechanical properties were attributed to the synergistic effects of grain refinement strengthening, solid solution strengthening, the Orowan mechanism, and twinning strengthening.