TiAl Intermetallic Compound Research Articles

Microstructure evolution and nitriding mechanism of Ti‐6Al‐4 V alloy in alumina‐based refractories

AbstractThe aims of this study were to explore the nitriding mechanism of Ti‐6Al‐4 V alloy in Al2O3‐based refractories. Al2O3‐based composite refractories were fabricated using Ti‐6Al‐4 V and sintered alumina as main materials, followed by nitriding at different temperatures in a nitrogen atmosphere. Phase and microstructure evolution of the products was analyzed by X‐ray diffraction and field‐emission scanning electron microscopy equipped with energy‐dispersive X‐ray spectroscopy. The results reveal a multi‐stage nitridation process of Ti‐6Al‐4 V at different temperatures. At 900°C, nitridation of the Ti‐6Al‐4 V surface results in the formation of a (Ti,V)N solid solution, while Al and V migrate inward, forming intermetallic compounds Ti3Al and a‐Ti (Ti1‐x‐yVxAly) with rich Al and V. At 1100°C, nitridation of Ti3Al produces intermediate products like Ti2AlN or Ti3AlN, with continued inward migration of Al or V to form the intermetallic α2 phase of (Ti1‐xVx)3yAly in the center of the alloy particles. At 1300°C, decomposition of Ti2AlN or Ti3AlN results in the outward migration of Al atoms and the presence of AlN on the outer layer of Ti‐6Al‐4 V particles, thus forming a hollow structure. Finally, at 1500°C, the (Ti,V)N solid solution grains grow as a result of further diffusion and dissolution of nitrogen.

Microstructure, Mechanical and Fractographic behaviour of the Diffusion Welded Joints of AA2219 and Ti-6Al-4V for Propellant Tank Applications

Diffusion welding is an advanced joining process that assures the feasibility of joining dissimilar metal alloys without producing metallurgical impurities at the joint interface. Two significant aerospace alloys, AA2219 and Ti-6Al-4V, are diffusion welded for the various holding times (30-120 minutes) by keeping the bonding temperature and pressure constant. The effect of holding time on the microstructure of the diffusion welded joints is evaluated using scanning electron microscopy (SEM), and the diffusion behaviour across the joint interface is ensued by the line scan energy dispersive spectroscopy (EDS). The obtained results show that the hardness across the joint interface is increased with increase in holding time. Furthermore, the maximum shear strength of 143.58 MPa is achieved for the joint formed at the holding time of 90 minutes and reduced thereafter. The formation of a thick intermetallic layer at a higher holding time strongly affects the shear strength. It is evident from the resulting fractured surfaces that the joints predominantly failed at the bonding interface, showcasing the brittle kind of failure. The X-ray diffraction (XRD) study on the fractured surfaces substantiates the formation of AlTi, Al2Ti and Al3Ti intermetallic compounds.

Effect of pressure on microstructure and properties of Ti–Al coating on titanium alloy surface

The effect of pressure on the microstructure and properties of Ti–Al coating on titanium alloy surface is studied. Ti–Al coating is prepared on the surface of titanium alloy by pressure-assisted high-temperature thermal diffusion. The manufacturing method is simple and easy to operate. The microstructure and element distribution of the coating are analyzed under different processes, and the composition of the coating is tested. The test results show that the substrate alloy and the coating alloy are not connected when the coating is diffused at 650 °C. The coating diffused at 750 °C without pressure is mostly connected to the substrate, but there are microscopic gaps in some areas. The main components of the coating diffused at 750 °C under 80 Pa pressure are TiAl, Ti3Al, and other intermetallic compounds. The boundary between the substrate and the coating is flat, and there is no obvious micro gap, so the metallurgical connection is fundamentally realized. In addition, the maximum hardness can reach 334.9HV0.2. The thickness of the coating diffused at 750 °C under 160 Pa pressure was greatly reduced, showing a gradient form. The surface quality of the outer coating is poor, the structure density of the binding layer is also poor, and there are no obvious penetrating cracks. The bonding between the coating and the substrate is effectively enhanced by an auxiliary action under certain pressure, and then the microstructure and properties of the coating are improved.

Coupling influence of laser-Nb interlayer on microstructure and performance of Ti/Al joint

Laser fusion welding of TC4 titanium (Ti) alloy and aviation 7075 aluminum (Al) alloy was conducted with the assistance of a niobium (Nb) interlayer. The effect of laser heat input and the alloying effect of Nb on the interfacial layer was analyzed. The study also investigated the coupling influence of the laser-Nb interlayer on the microstructure and performance of the Ti/Al joint. By utilizing a 0.1 mm-thick Nb interlayer and a welding velocity of 1800 mm/min, the results reveal an initial increase in tensile-shear force followed by a decline with the increase of laser power. The tensile-shear force reached a peak of 1663 N at 1.2 kW, representing a 36.76 % improvement over the 1 kW levels. Under appropriate laser power, the enhancement in joint performance is dominated by the controlled heat input and the effective Nb-alloying effect, which reduce the thickness and brittleness of the interfacial intermetallic layer, respectively. However, metallurgical regulation of Nb will be restricted by the insufficient Nb melting amount resulting from the low laser power, leading to a decrease in joint properties. Excessive laser power can lead to the formation of hot cracks and an excessive amount of Ti-Al intermetallic compounds, which can decrease the Nb content, thereby restricting the metallurgical regulation of Nb and the overall joint performance.

The role of graded layers in interfacial characteristics and mechanical properties of Ti6Al4V/AlMgScZr-graded multi-material parts fabricated using laser powder bed fusion

Graded multi-material parts achieve a compositionally graded transition between two different materials, mitigating undesirable consequences such as cracking and delamination due to property mismatch and significantly improving the comprehensive performance of parts. In this study, the Ti6Al4V/AlMgScZr-graded multi-material parts were fabricated using laser powder bed fusion technology, introducing a composition-graded layer with 25 wt.% Ti6Al4V and 75 wt.% AlMgScZr at the interface to reduce the mismatch between the two materials. The effect of the graded layer’s laser scanning speed on the densification behavior, microstructure evolution, and mechanical properties of the Ti6Al4V/AlMgScZr-graded multi-material parts was investigated. It was revealed that the crack area at the interface reduced from 0.325 to 0.067 mm2 as the scanning speed increased from 2400 to 2800 mm/s and then increased to 0.161 mm2 at 3000 mm/s. A smooth, continuous-graded layer with good metallurgical bonding was fabricated at 2800 mm/s. The TiAl3 intermetallic compound was formed at the interface and underwent a transition from rod-like to coarse dendritic and finally to finer dendritic structure along the building direction. The Ti6Al4V/AlMgScZr-graded multi-material parts exhibited a graded decrease in microhardness from 374 HV0.2 on the Ti6Al4V side to 122 HV0.2 on the AlMgScZr side, and an excellent compressive strength of 1531 MPa was obtained at the optimal parameter of 2800 mm/s.

Investigation on Friction Stir Welding Parameters: Mechanical Properties, Correlations and Corrosion Behaviors of Aluminum/Titanium Dissimilar Welds

In industrial applications, welding of dissimilar metals such as aluminum (Al) and titanium (Ti) is a prerequisite for the development of hybrid components with improved mechanical and corrosion properties. However, dissimilar welding of the Al/Ti system is highly challenging due to differences in the physical and thermal properties of the two materials. In the present investigation, an attempt has been made to fabricate a dissimilar friction stir weld (FSW) of commercially pure Al and Ti and to elucidate the mechanism associated with superior joint formation. The process parameters, such as tool rotation speed, traverse speed and tool offset position have been optimized using Taguchi’s optimization technique. A detailed investigation of the weld with optimum process parameters has been carried out to reveal the mechanism of joint formation. The superior mechanical properties (24% higher ultimate tensile strength and 10% higher ductility than that of base Al) of the weld are attributed to the fabrication of a defect-free joint, formation of intercalated particles and an Al/Ti interlocking interface, homogeneous distribution of fine second-phase (Ti and/or intermetallics) particles in the weld nugget, reduction in the evolution of brittle Al3Ti intermetallic compounds (IMCs) and recrystallization and grain refinement of Al in the weld nugget. The potentio-dynamic polarization test indicated that the optimized Al/Ti weld has ~47% higher corrosion resistance than Al; it had a very mild corrosion attack due to the homogeneous dispersion of fine particles. The method and mechanism could have an immense influence on any dissimilar weld and metal matrix composites, improving their mechanical properties and corrosion resistance.

Cermet Powders Based on TiAl Intermetallic for Thermal Spraying

The paper presents a study of the formation process of cermet powders based on TiAl intermetallic with the addition of non-metallic refractory compounds. Non-metallic refractory compounds B4C, BN, SiC, and Si3N4 were chosen as strengthening components, improving the mechanical properties and resistance to high-temperature oxidation of TiAl-type intermetallic coatings. The composition of the initial mixtures was selected based on thermodynamic analysis of the interaction between TiAl intermetallic and non-metallic refractory compounds. As a result of the mechanochemical synthesis of powder mixtures, 73TiAl-27B4C, 69TiAl-31BN, 88TiAl-12SiC, and 83TiAl-17Si3N4 (wt. %) cermet powders are formed, consisting of titanium aluminide (TiAl, Ti3Al) phases and refractory compounds of aluminium (AlB2 and AlN) and titanium (TiB2, TiC, TiN, Ti5Si3). The conglomeration technology of produced cermet powders has been developed to enhance fluidity. Using conglomerated powders will provide their constant feed to the high-temperature jet and the formation of dense coatings during thermal spraying.

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.

Investigation of the physical properties of the metal (Co, Cr, Mn, Sc) doped TiAl3 compounds: A DFT study

In this paper, the elastic and electronic properties of Metal (M = Co, Cr, Mn, Ni, Sc) - TiAl3 are compared based on the first-principles calculation method based on density functional theory, and the Metal-TiAl3 Mechanical, thermodynamic and electronic properties of TiAl3 intermetallic compounds. The results of mechanical property calculations show that the structures of Sc-TiAl3 and V-TiAl3 are the most stable. The ability of Sc-TiAl3 to resist volume deformation and shear deformation is larger. The Poisson's ratio ν is less than 0.33, and it can be determined that the calculated compounds are brittle materials (except Ni-TiAl3). Cr-TiAl3 has the largest intrinsic Vickers hardness, while Ni-TiAl3 has the smallest intrinsic Vickers hardness. This study has a significant impact on adjusting the mechanical properties of TiAl3.

The Effect of Nb Doping on the Properties of Ti-Al Intermetallic Compounds Using First-Principles Calculations.

The crystal structures, stability, mechanical properties and electronic structures of Nb-free and Nb-doped Ti-Al intermetallic compounds were investigated via first-principles calculations. Seven components and eleven crystal configurations were considered based on the phase diagram. The calculated results demonstrate that hP8-Ti3Al, tP4-TiAl, tP32-Ti3Al5, tI24-TiAl2, tI16-Ti5Al11, tI24-Ti2Al5, and tI32-TiAl3 are the most stable phases. Mechanical properties were estimated with the calculated elastic constants, as well as the bulk modulus, shear modulus, Young's modulus, Poisson's ratio and Pugh's ratio following the Voigt-Reuss-Hill scheme. As the Al content increases, the mechanical strength increases but the ductility decreases in the Ti-Al compounds. This results from the enhanced covalent bond formed by the continuously enhanced Al-sp hybrid orbitals and Ti-3d orbitals. Nb doping (~5 at.% in this study) keeps the thermodynamical and mechanical stability for the Ti-Al compounds, which exhibit slightly higher bulk modulus and better ductility. This is attributed to the fact that the Nb 4d orbitals locate near the Fermi level and interact with the Ti-3d and Al-3p orbitals, improving the metallic bonds based on the electronic structures.

TiAl-Based Oxidation-Resistant Hard Coatings with Different Al Contents Obtained by Vacuum-Pulse-Arc Granule Melting

A method was proposed for increasing the oxidation resistance of promising wrought Ti2AlNb ortho-alloys by depositing γ-TiAl-based coatings. Using original vacuum pulse-arc melting of 100 μm thick granule layers, coatings with different Al/Ti ratios and a thickness of 50–60 µm were obtained on the surface of the Ti50Al25Nb25 alloy. Granules Ti50Al44Nb4.9Mo1B0.1 (at.%), 20–60 μm in size, were employed. To vary Al content, initial granules and their mixture with Al powder were used. Excellent adhesion of the coatings is ensured by the similar chemical composition and structure of the substrate and coatings, as well as micro-metallurgical reactions between granules and the substrate that occur during treatment. The resulting coatings had a submicron gradient structure consisting of TiAl and Ti3Al intermetallic compounds. During oxidation at 850 °C for 10 h, an oxide layer consisting of a mixture of α-Al2O3, TiO2, and AlNbO4 was formed on the coating surfaces. With an increase in the annealing duration to 100 h, a dense α-Al2O3 oxide layer, approximately 0.5 µm thick, was formed over the primary oxide mixture, the quality of which was higher in coatings enriched with aluminum.

Lightweight Al3Ti-based medium-entropy alloys with well-balanced strength and ductility

The lightweight Al3Ti intermetallic compound shows superior properties at elevated temperatures but its room-temperature brittleness largely limits its usefulness. We propose Al3Ti-based lightweight medium-entropy alloys (MEAs) with selected alloying elements, which change Al3Ti from a tetragonal D022 structure to a ductile cubic L12 one with embedded hard B2 and D8a phases. A combination of improved ductility and strength can be achieved. Four lightweight AlTiCrMnFeCu MEAs (ρ=3.65∼4.27 g/cm3) with tunable balance between hardness and ductility were fabricated. Ti and Fe promoted the formation of hard B2 phase, while Cr and Mn favored the formation of D8a phase which strengthened the alloy at the minor expense of ductility. The compressive yield and peak strengths of the MEAs e.g., Al55Ti20Cr5Mn10Fe5Cu5, can reach as high as 650 MPa and 1366 MPa, respectively, with its fracture strain around 17 %. This study provides useful information for designing Al3Ti-based lightweight MEAs with desirable properties.

Effects of laser power on the microstructural evolution and mechanical performance in laser dissimilar welding of TC4 to SiCp/6092Al composite

In this study, an overlap configuration for Ti6Al4V alloy (TC4) and SiC particle reinforced aluminum matrix composites (AMC) was achieved by laser welding process. The influence of laser power on the microstructural evolution, intermetallic compounds formation and mechanical properties of TC4/AMC joints were investigated. The presence of reinforced phase SiC complicated the metallurgical reaction in the molten pool, resulting in the formation of intermetallic compounds, such as Al4C3, TiC, TiAl, and TiAl3. The influence of laser power was more pronounced on the penetration depth compared to its impact on the weld width. The thickness of TiAl intermetallic compounds at the TC4/AMC interface increased as the laser power was raised. Abundant dislocations existed surrounding the TiAl3 phase because of the different thermal-mechanical properties between Al matrix and TiAl3 phase. The tensile shear force of TC4/AMC joints can be considerably improved by reducing the thickness of the TiAl3 phase. The maximum tensile shear force of 1675.6 N for the TC4/AMC joint was obtained at a laser power of 1500 W when the thickness of the TiAl3 layer reached the lowest value of 7.1 μm. The research results have the potential to improve laser welding techniques for TC4/AMC hybrid structures and provide valuable guidance for the design of TC4/AMC welded joints in the future.

Vacuum brazing of SiCp/Al composites and TC4 titanium alloy: Microstructure evolution and mechanical properties

In this paper, a dissimilar material consisting of 55% SiCp/ZL102 composites and TC4 titanium alloy was brazed using Al-8.0 Mg-17.0Cu-1.0Ti foil-like brazing alloy manufactured by rapid solidification technology. Brazing was completed in a vacuum of 1.0 × 10−4 Pa. Si element from Al composites accumulated at the interface of TC4 titanium alloy to form Ti7Al5Si12 intermetallic compound layer. The intermetallic compound α2(TiAl3) grew perpendicular to the brazing seam below 590 °C. The α2(TiAl3) intermetallic compound grown into long strips at 590 °C. The strip structure had a bonding interface in the form of pinning at a certain angle. The brazing interface was continuous and dense, without defects and exhibited good bonding performance. The shear strength of the brazed joint reached 98.01 MPa at 590 °C for 30 min, and the gas tightness was 1.0 × 10−10 Pa m3/s. The brazed joint failed in filler metal due to ductile-brittle mixed fracture mechanism, which resulted in shear lips, parabolic dimples, and furrows in the fracture surface.

High-temperature and interfacial oxidation of MAX phase-reinforced composite coatings deposited by laser cladding on Zr alloy substrates

MAX phase-reinforced composite coatings were successfully prepared by laser cladding in-situ reaction using TiAl–TiC–Al powder preset on R60702 zirconium alloy surface. The behavior of composite coatings under high-temperature oxidation and interfacial oxidation was investigated. Differential thermogravimetry (DTG) oxidation weight gain curves demonstrated that high-temperature oxidation resistance of composite coating was better than that of Zr alloy. Oxide phase of composite coating mainly contained TiO2 and small amount of Al2O3. Oxidation characteristics of carbides (TiC and Ti2AlC) and TiAl intermetallic compounds revealed that oxidation rates of carbides were higher than those of TiAl intermetallic compounds. In addition, reasons for their different diffusion or oxidation rates were also discussed from the perspective of stability of original microstructure before and after oxidation. At the highest experimental temperature (1100 °C), oxide film thickness of composite coating (20 μm) was smaller than that of Zr alloy (550 μm). Interfacial oxidation behavior indicated that there was bimetallic galvanic corrosion effect between composite coating and Zr alloy, which is related to high electronic energy level of Zr from the perspective of quantum mechanics.

Enhanced Properties of Ti/Al Laminated Composite Reinforced by High-Entropy Alloy Particles

Novel HEAp-Ti/Al laminated composites embedded with particles of the high-entropy alloy Al0.5CoCrFeNi (HEA) were fabricated by vacuum hot-press sintering at 730 °C. The phase composition and microstructure of the composites were studied with X-ray diffractometer (XRD), scanning electron microscope SEM, energy dispersive spectrometer (EDS), transmission electron microscope (TEM), and electron backscattering diffraction (EBSD) techniques. At this temperature, it has been observed that Al3Ti intermetallic compound is the favored phase and the reaction results in the dispersion of Al3Ti in the original Al layer. A large number of interfaces are formed between Al3Ti and Al. The deformed Al3Ti grains are concentrated in the interface near the Ti side. The mechanical properties, including tensile and compressive properties at room temperature, were analyzed. The tensile test results indicate that the composite exhibited an average tensile strength of 258 MPa and an average yield strain of 9.86%. Compression test results show that when a load perpendicular to the layer is applied, the yield strain and yield stress of the material are 9.67% and 474.09 MPa, respectively. Moreover, under a load parallel to the layer, the material fails due to interfacial debonding.

The Influence of Cu on Combustion Synthesis Mechanism of Ti–Al Intermetallic Compounds Produced From Ti and Al Elemental Powders

The Influence of Cu on Combustion Synthesis Mechanism of Ti–Al Intermetallic Compounds Produced From Ti and Al Elemental Powders

Production of spherical TiAl alloy powder by copper-assisted spheroidization

A copper-assisted spheroidization method was developed for preparing spherical intermetallic compound TiAl powder. By introducing copper into TiAl alloy, the brittle L21 Heusler structure TiAlCu2 alloy with low melting point (1050 °C) was produced. After preparing the spherical TiAlCu2 powder through solid–liquid interface de-wetting method, the porous TiAl alloy sphere with atomic ratio about 1:1 was prepared by dealloying the Cu element. Finally, the spherical TiAl alloy powder was prepared by sintering and deoxygenation. Compared with the current mature technology for preparing the spherical TiAl alloy powder, the size distribution of spherical particles can be easily regulated, which can meet the requirements for high-quality additive manufacturing technology. We believe that this method provides a new idea for preparing the spherical TiAl alloy powder and other spherical porous alloys.

Microstructure and properties of Cr–AlSi12 composite coatings pre-coated on Ti–6Al–4V alloy substrate via mechanical alloying and subsequent laser cladding treatment

Cr–AlSi12 composite coating on Ti–6Al–4V alloy substrate was fabricated by mechanical alloying (MA) and subsequent laser cladding (LC). The weight ratio of the mixed powders was Cr: AlSi12 = 60:40. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were applied to investigate the evolution of surface morphologies, cross-section microstructures and phase composition of the coatings before and after laser treatment. The results showed that Al8Cr5, AlCr2, (Si, Al)2Cr and Cr5Si3 were generated in the MA-LC coating through in-situ alloying reaction. The formation of Al3Ti and Ti5Si4 intermetallic compounds was caused by interdiffusion. Metallurgical bonding was achieved at the coating/substrate interface. Results of the scratch test indicated that laser treatment could significantly improve the bonding quality of the interface. Meanwhile, the microhardness of the MA-LC coating was about 3–4 times higher than that of the MA coating. The high-temperature oxidation resistance of the MA-LC Cr–AlSi12 coating was also studied. It was found that after 100 h of oxidation, the MA-LC coating could effectively enhance the oxidation resistance of the substrate. The oxide films and intermetallic phases formed during oxidation process could play a role of intense hindrance to the diffusion of oxygen into the internal Ti–6Al–4V substrate. The oxidation mechanism of the coating is discussed in detail.

Process optimization and property of Ti-Al-Cr-Nb coating on titanium alloy surface by laser in-situ synthesis

Ti-6Al-4V alloys are popular structural materials for the production of critical components. However, its poor surface properties, such as friction and high-temperature oxidation, further limit its application scope. Ti-Al-Cr-Nb coating was deposited on the Ti-6Al-4V alloy through laser cladding. The tendency of cracks was low when the laser energy density was 14.33-8.19 kw · s · cm −2, the corresponding effective process parameters were the laser power of 1600-1800 W matching the scanning speed of 4-6 mm/s. Due to the formation of Ti3Al intermetallic compound phase, and the lattice distortion caused by adding a small amount of Cr and Nb playing a role in solid solution strengthening, average vickers hardness of the coating was about 583.3 HV0.1, which was about 1.7 times that of the substrate.