TiAl Phase Research Articles

Microstructure, mechanical properties and formability of Ti/Al/Ti laminated composites fabricated by hot-pressing

Abstract Ti/Al/Ti laminated composites (LMCs) with different interfacial bonding strength were prepared by hot-pressing at different temperatures. The effects of interfacial morphology, reaction layer and constraint effect on the microstructure, mechanical property and formability of Ti/Al/Ti laminated composites were explored. The results indicate that micron-scale TiAl3 phases are formed at the interface. The degree of curve, the bonding strength and the micro-hardness at the interface all increase with the increasing hot-pressing temperatures. However, both the strength and elongation of laminated composites reduce with the increase of temperature. The reduction of the strength is attributed to the shear fracture tendency at the curved interface and the softening effect at high temperature, while the change of elongation is related to the interfacial debonding and constraint effect. The variation of stretch formability is nonlinear with the increase of hot-pressing temperature, which is related to the crack initiation and propagation behaviors during forming. The LMCs possess excellent ductility and Erichsen values at lower temperature, which results from the increased damage tolerance and fracture absorption energy caused by the local interfacial debonding. Nevertheless, the LMCs exhibit complex mechanical behavior at higher temperature due to two competition mechanisms: coordinated deformation by stress-strain transfer under a strong constraint and unmatched deformation by the intermetallic at the interface.

Anisotropic failure behavior of ordered intermetallic TiAl alloys under pure mode-I loading

Whether a metallic material fractures by brittle cleavage or by ductile rupture is primarily governed by the competition between cleavage and dislocation emission at the crack tip. The linear elastic fracture mechanics (LEFM) based criterion of Griffith, respectively the one for dislocation emission of Rice, are sufficiently reliable for determining the possible crack tip propagation mechanisms in isotropic crystalline metals. However, the applicability of these criteria is questionable when non-cubic, anisotropic solids are considered, as e.g. ordered intermetallic TiAl phases, where slip systems are limited and elastic anisotropy is pronounced. We study brittle versus ductile failure mechanisms in face-centered tetragonal TiAl and hexagonal Ti3Al using large-scale atomistic simulations and compare our findings to the predictions of LEFM-based criteria augmented by elastic anisotropy. We observe that the augmented Griffith and Rice criteria are reliable for determining the direction dependent crack tip mechanisms, if all the available dislocation slip systems are taken into account. Yet, atomistic simulations are necessary to understand crack blunting due to mixed mechanisms, or shear instabilities other than dislocation emission. The results of our systematic study can be used as basis for modifications of the Griffith/Rice criteria in order to incorporate such effects.

The Study of the Fine Structure of Ti-Al Coatings on the Surface of Ti, Obtained by Mechanical Alloying

The work is devoted to the study of structural-phase transformations in composite coatings (Ti-Al)+Ti during mechanical alloying. The data on the structural-phase states of (Ti-Al)-Ti coatings after mechanical alloying have been obtained, confirming the mechanism of formation of the modified layer due to deformation compaction of powder particles on the titanium surface under mechanical action. As a result of mechanochemical fusion, a TiAl3 phase with a bcc lattice (I4=mmm structure) was detected, which corresponds to the stable state of the TiAl3 alloy. Under conditions of mechanical alloying of the structure, I4=mmm transforms into the L12 structure, which corresponds to the metastable state of TiAl3

Features of the Two-Phase (α + α2) Structure Formation in the Ti–17 at % Al alloy

This work investigates the formation of an ordered Ti3Al phase (α2 phase) in Ti–Al alloys as a function of the time and aging temperature. High-temperature treatment performed in the single-phase α field and the subsequent aging at different temperatures have been shown to result in more intense precipitation and growth of disperse particles than that resulted from pretreatment in the β field. The formation of a martensite structure during quenching complicates the subsequent transformation by the homogeneous mechanism.

A study of residual Ti3Al in γ(TiAl) sheets using mismatch theory

In this paper, the γ(TiAl) sheet was prepared using Ti and Al foils through cold rolling, diffusion annealing and forging. After analyzing the microstructure and phase composition of the obtained material. The results show that the sheet is mainly composed of α2(Ti3Al) phase and γ(TiAl) phase. Ti3Al exists at the grain boundaries and in the crystal. The residual α2 phase in the crystal can significantly hinder dislocation movement, which plays a role in secondary phase strengthening. The mismatch degree was calculated through the one-dimensional and two-dimensional mismatch theories, and the two-dimensional mismatch degree of γ(TiAl) and α2(Ti3Al) was 2.12%. This means that the α2(Ti3Al) phase can effectively promote the nucleation and growth of the γ(TiAl) matrix phase and hinder the movement of dislocations.

The joining of gamma titanium aluminides via the powder interlayer bonding method

Powder interlayer bonding (PIB) is a joining technique originally developed to enable high-integrity repairs of aerospace components. The technique has previously been employed for the joining of titanium and nickel alloys utilised in the aerospace industry. This study expands on the application of the novel joining technique known as powder interlayer bonding (PIB), to the bonding of γ titanium aluminide (TiAl) material. PIB has been used to facilitate high-integrity joints in gamma titanium aluminides (TiAl), where full densification of the joint was achieved. The PIB technique described here used a metallic powder interlayer between the two faying surfaces of γ TiAl specimens. Bonds were formed in an inert atmosphere under induction heating. The PIB technique proved capable of producing high-integrity bonds in terms of microstructural evaluation, with very limited porosity retained after the bonding cycle. A brittle Ti2Al phase can be produced with heavily oxidised powder which is susceptible to cracking and will negatively affect mechanical properties.

WITHDRAWN: Review on nucleation catalysis of tetragonal D022 Al3Ti and L12 modified (Al1−xMex)3Ti intermetallic compounds for solidification of alpha-Al grains

Abstract Heterogeneous nucleation could be considered as a surface catalyzed nucleation process. It is known that Al3Ti phase in Al-Ti refiner is an effective catalytic substrate for solidification of alpha-Al. However, since the Al3Ti phase has a tetragonal D022 structure with lattice constants of a = 0.3851 nm and c = 0.8608 nm, different planes of Al3Ti phase have different lattice misfit between Al. Moreover, it is reported that the shape of Al3Ti in Al-Ti refiner is platelet and wide plane of the platelet has the relatively large lattice misfit between Al. Therefore, the dominant face of Al3Ti platelets in Al-Ti refiner is not the best plane for heterogeneous nucleation. To overcome these shortcomings, in our previous studies, high-symmetry L12 modified (Al1-xMex)3Ti phases with a lattice constant of a = 0.3927 nm for Me = Cu, a = 0.393 nm for Me = Fe, a = 0.394 nm for Me = Ni were used as catalytic substrate. In this paper, the role of lattice misfit on heterogeneous nucleation of Al will be discussed.

The Structural and Phase State of the TiAl System Alloyed with Rare-Earth Metals of the Controlled Composition Synthesized by the “Hydride Technology”

The structural state and the quantitative phase analysis of the TiAl system, alloyed with rare-earth metals synthesized using hydride technology, were studied in this work. Using the Rietveld method, the content of the major phases in the initial system Ti(50 at.%)–Al(50 at.%), as well as Ti(49 at.%)–Al(49 at.%), with alloying additions Ta, Y and Dy having a high accuracy was determined. The methods of scanning electron microscopy, transmission electron microscope and X-ray spectral microanalysis of the local areas of the structure for studying the distribution of alloying elements were used. The energies of lattices of separate phases were also determined after the full-profile specification. All the lattices of the identified structures (about 30) turned out to be stable. It was established that in the Ti(49 at.%)–Al(49 at.%) systems under study with alloying additions of metals Ta, Y and Dy, there were intermetallides composed of AlTi3, TiAl in the hexagonal, tetragonal and triclinic units. It is known that after microalloying alloys by Y and Dy metals, the mass fraction of TiAl phases increases significantly (>70%).

Effect of specific energy on microstructure and properties of laser cladded TiN/Ti3AlN-Ti3Al composite coating

Tremendous studies were focused on the influence of single parameter on the properties of cladded coating, while scarce efforts have been devoted to investigate the coating performance affected by specific energy. Herein, the effects of laser specific energy in the range 40.0–88.9 J/mm2 on the morphology, microstructure and properties of TiN/Ti3AlN-Ti3Al composite coating were analyzed. Results revealed that the dilution rate of the coating increased with increasing of specific energy, the coating at 58.3 J/mm2 exhibited the least defects with dilution rate was about 14.0%. The coating was mainly composed of TiN, Ti3AlN and Ti3Al phases, the amount of Ti3Al and TiN decreased slightly as the specific energy increased to 58.3 J/mm2, whereas the Ti3Al and TiN decreased remarkable when the specific energy increased further. Comparatively, when the specific energy reached 58.3 J/mm2, the coating exhibited uniform distribution of TiN and finer microstructure, and core-rim structure formed in the coating. As a result, the microhardness of the composite coating was approximately 3 times higher than that of the substrate and the wear resistance was improved remarkably under optimum specific energy 58.3 J/mm2. This study provided an insight for the development of high-performance Ti3Al matrix composite coating.

Effect of Al Content in Magnesium Alloy on Microstructure and Mechanical Properties of Laser-Welded Mg/Ti Dissimilar Joints.

Laser penetration welding of magnesium alloys and pure titanium TA2 with unequal thickness was performed. Mg base metal with different Al content (AZ31B, AZ61A, AZ91D) was used to investigate the influence of Al element in microstructure and mechanical properties of Mg/Ti dissimilar joints. The results revealed that the change of Mg base metal did not influence the weld appearance of the joints. Three kinds of joint all presented the best mechanical property when the laser power was 3500 W. With the increase content of Al elements in Mg base metal, a reaction layer was observed which was identified as Ti3Al. The highest enrichment of Al element was obtained and its fraction reached 19.31 at% at the AZ91/TA2 interface. The chemical potential gradient of Al from AZ91 to Ti alloy was higher than that from the other two base metals based on thermodynamic calculation. The maximum fracture load reached 3597 N when AZ61 was employed as the base metal and the fracture position was the Ti base metal. AZ31/TA2 joints failed at the weld seam without necking due to the rapid propagation of cracks at the Mg/Ti interface. The AZ91/TA2 joint failed inside the Mg fusion zone with necking at the middle area of the weld, which resulted from the precipitation of brittle phases such as Mg–Al, Ti–Al phases in the fusion zone of Mg alloys.

Microstructural Characterization of Multilayer Metal Stack on InGaAs/InP Contact

Al/InGaAs, Al/Ti/InGaAs, and Al/Ni/InGaAs contacts were prepared by conducting rapid thermal annealing at different temperatures, and the interfacial reaction was characterized using x-ray diffraction, atomic force microscopy, and transmission electron microscopy. The specific contact resistivity of the metal film contacts on n-InGaAs was determined using a circular transmission line measurement. The Al/Ni/InGaAs, Al/Ti/InGaAs, and Al/InGaAs stack contacts remained stable even after the samples were subjected to annealing at 400°C. When the annealing temperature increased to 600°C, Al began melting and clustering due to the formation of Al(Ga) to form Al/InGaAs. An AlNi phase was formed at the interfaces of an Al/Ni/InGaAs contact when it was annealed above 400°C, thus causing the specific resistance and roughness to increase dramatically. Each element of the Al/Ti/InGaAs contact began diffusing when it was annealed beyond 400°C, and an Al3Ti phase was formed above 500°C. The formation of Al3Ti led to the increase in the specific resistance and roughness. Moreover, the as-deposited Al/Ni/InGaAs, Al/Ti/InGaAs and Al/InGaAs samples had specific resistances of 1.11 × 10−5 Ω cm2, 5.11 × 10−5 Ω cm2, and 1.14 × 10−4 Ω cm2, respectively. These results reveal that to develop the Al/Ti/InGaAs and Al/Ni/InGaAs stacks on InGaAs with a low parasitic resistance, the processing temperature should be lower than 400°C.

Formation kinetics and phase stability of in-situ Al3Ti particles in aluminium casting alloys with varying Si content

The intermetallic phase Al3Ti is a promising candidate to increase the stiffness of aluminium casting alloys due to its high Young’s modulus. The phase is formed by the in-situ reaction between solid titanium particles and the liquid aluminium melt. High-shear mixing technology is used to peel off the reaction layer on the surface of the Ti particles and distribute the Al3Ti particles homogeneously in the melt. This work investigates the influence of the Si content of three different aluminium casting alloys on the in-situ reaction. Additionally, the influence of the initial titanium powder particle size and the process temperature on the formation kinetics are investigated.

Wire arc additive manufacturing of titanium aluminide alloys using two-wire TOP-TIG welding: Processing, microstructures, and mechanical properties

Titanium aluminide (TiAl) alloys are promising high-temperature structural materials in the aerospace field. Additive manufacturing is a desirable process for fabricating TiAl alloys. In the process of wire arc additive manufacturing of TiAl alloys, Al-based and Ti-based wires were used as the feedstocks. However, it is hard to ensure the two different wires melt synchronously under the heat of one single arc, so the desired microstructures with γ (TiAl) phase and α2 (Ti3Al) phase are hard to obtain. A two-wire TOP-TIG-based additive manufacturing process for TiAl alloys is proposed in this paper. The Ti6Al4V wire and pure Al wire are used as the feedstocks. The Al wire is fed in TOP-TIG mode behind the molten pool, while the Ti6Al4V wire is fed in conventional TIG mode in front of the molten pool. The two wires melt synchronously in a broad range of parameters. The compositions of the component can be controlled by adjusting the two-wire feeding speeds. The main microstructures of the as-fabricated component contain α2/γ lamellae colonies, equiaxed γ grains, and α2 grains. In the top and middle regions, when the Al content is 45 at.%, the structures are full α2/γ lamellae; as the Al content increases to 50 at.%, some equiaxed γ grains distribute at the grain boundaries; the component with 55 at.% Al content exhibits the structures consists of equiaxed γ grains with snowflake-shaped α2/γ lamellae colonies. In the bottom region, all components exhibit the coarse equiaxed α2 grains with γ laths. As the Al content increases, the α2 phase decreases, but the γ phase increases, and from the top region to the bottom region, the volume fraction of the α2 increases by about 19.4 %. The hardness value of the top and bottom regions is higher, and the middle region exhibits the lowest hardness. As the Al content increases, the hardness decreases. The component with 50 at.% Al exhibits the highest compressive strength with 1762 MPa and the largest compressive ratio with 26.1%.

Microstructure and tribological properties of laser in-situ synthesized Ti3Al composite coating on Ti-6Al-4V

Titanium intermetallics are highly significant to protect surfaces of lightweight structures against corrosion and wear. In this study, Ti and Al powders were uniformly mixed with an atomic ratio 65:35 and in-situ synthesized on Ti-6Al-4V substrate by laser cladding with Ar cooling. The microstructure and high temperature tribological behavior were studied. The results show that Ti3Al (α2) phase was first precipitated in the form of coarse primary dendrites during cladding solidification, and subsequently α-Ti (α) phase of needle-shaped martensite was formed by fast cooling. The 6:4 ratio of phase α2 to α of the composite coating provides optimal toughness and high hardness (680 HV). Under the testing up to 500 °C, the average friction coefficient of the composite coating is 0.22 ± 0.011 and a 35% lower wear rate than that of Ti-6Al-4V substrate.

Calculation and experimentation on the formation sequence of compounds at Al/Ti interface in pure Al antioxidant coatings

To investigate the precipitation order of intermetallic compounds at the Al/Ti interface, the enthalpy, entropy, Gibbs free energy, and heat capacity of Ti–Al compounds (including TiAl3, TiAl2, Ti3Al, and TiAl) were calculated using first-principles. In addition, the enthalpy of formation, entropy of formation and Gibbs free energy of formation of Ti–Al compounds were further calculated. By combining thermodynamics and diffusion dynamics, the effective free energy of formation model is proposed to predict the formation sequence of compounds at the Al/Ti interface. The results showed that the formation sequence of Ti–Al compounds followed TiAl3→TiAl2→TiAl→Ti3Al. Subsequently, the pure Al coating was prepared on the surface of pure titanium plate by arc spraying equipment and the specimens were heat-treated. The experimental results show that there was a transition zone composed of four layers of Ti–Al compounds at the interface. From the Al layer to the matrix direction, the four-layer compounds were TiAl3→TiAl2→TiAl→Ti3Al in turn, which was consistent with the calculated results. The EBSD analysis of the Al/Ti interface shows that the grain size of TiAl3 decreases gradually from the coating to the substrate direction when the heat treatment condition is 800 ℃/5 h, and the grain size and thickness of the Ti3Al layer are both larger than that of TiAl2 and TiAl layer. The Ti–Al phases at the interface presented random grain orientation. Finally, the calculation results of the preferred adsorption position of oxygen atoms on the TiAl3 (110)-Al surface show that the position of Al–Al bridge is the most preferred adsorption position of oxygen atoms and oxygen atoms are more inclined to form aluminum oxide by bonding with Al atoms.

Development of V-free porous Ti-A356-Nb alloy by liquid sintering of compacted powders and its oxidation behaviour

Porous Ti-Al-Si-Nb alloy was developed through a mixture of Ti, A356 Al alloy and Nb powders followed by compaction and sintering. The sintered Ti-Al-Si-Nb alloy was comprised of the α and β-phases after sintering. The Nb content was predominately found in the β-phase. XRD technique has detected the FCC phase (5.750 Å), the HCP (5.140 Å; c = 9.480 Å) and tetragonal Ti3Al (4.585 Å; c = 2.960 Å) phases. Upon cyclic oxidation conducted at 900 °C, the porous Ti-Al-Si-Nb alloy showed a progressive gain in weight of ∼3 wt% 100 h to ∼12 wt% after 900 h. EDS analysis confirmed the presence of O and N around the pores on the 800 °C and 900 °C annealed samples. The density of the porous Ti-Al-Si-Nb alloy increased with the holding time during cyclic oxidation.

Phase Formation and High-Temperature Stability of Very Thin Co-Sputtered Ti-Al and Multilayered Ti/Al Films on Thermally Oxidized Si Substrates.

Ti-Al thin films with a thickness of 200 nm were prepared either by co-sputtering from elemental Ti and Al targets or as Ti/Al multilayers with 10 and 20 nm individual layer thickness on thermally oxidized Si substrates. Some of the films were covered with a 20-nm-thick SiO layer, which was used as an oxidation protection against the ambient atmosphere. The films were annealed at up to 800 °C in high vacuum for 10 h, and the phase formation as well as the film architecture was analyzed by X-ray diffraction, cross section, and transmission electron microscopy, as well as Auger electron and X-ray photoelectron spectroscopy. The results reveal that the co-sputtered films remained amorphous after annealing at 600 °C independent on the presence of the SiO cover layer. In contrast to this, the -TiAl phase was formed in the multilayer films at this temperature. After annealing at 800 °C, all films were degraded completely despite the presence of the cover layer. In addition, a strong chemical reaction between the Ti and SiO of the cover layer and the substrate took place, resulting in the formation of Ti silicide. In the multilayer samples, this reaction already started at 600 °C.

Formation of Phases in Reactively Sintered TiAl3 Alloy.

This work highlights new results on the synthesis of the TiAl3 intermetallic phase using self-propagating high-temperature synthesis. This method is considered a promising sintering route for intermetallic compounds. It was found that the reactions proceed in two stages. Below the melting point of aluminum, the Ti2Al5 phase forms at 450 °C after long annealing times by a direct solid-state reaction between the aluminum and titanium, and is converted consequently to TiAl3. This is a completely new finding; until now, many authors have believed in the preferential formation of the TiAl3 phase. The second stage, the self-propagating strongly exothermic reaction, proceeds above the melting point of aluminum. It leads to the formation of the TiAl3 phase accompanied by Ti2Al5 and Ti3Al phases. The reaction mechanism was shown in the form of chemical equations, which were supported by calculating Gibbs energy. Reaction temperatures (Tonset, Tmaximum, and Toffset) were determined after induction heating thanks to recording by an optical pyrometer. This finding provides completely new opportunities for the determination of activation energy at heating rates, in which common calorimeters are not able to detect a response or even measure. Now, the whole procedure will become accessible.

Relation between heat treatment processes and microstructural characteristics of 7075 Al alloy fabricated by SLM

7075 aluminum alloy fabricated by selective laser melting (SLM) was subjected to different heat treatment processes. Microstructures and precipitates of this alloy were observed and analyzed under different treatment processes. The results indicated that there were shrinkage and porosity on the top surface of as-received alloys, and the undissolved coarse second phase distributed in the grain boundary. After a solution treatment, the grains re-nucleated and grew into some fine equiaxed crystals, and the rod-shaped Al3Ti and Mg2Si phases were also precipitated. After solid solution plus aging treatment, the grains continued to grow, and the Al3Ti phase disappeared. However, the finer θ (Al2Cu) phase and S (Al2CuMg) phase precipitate in the matrix. As a result, the shrinkage and porosity decreased significantly. After solid solution plus natural aging treatment, the grain grows and a relatively large θ (Al2Cu) phase is precipitated from the GP zone.

Comprehensive structural and mechanical characterization of in-situ Al–Al3Ti nanocomposite modified by heat treatment

Aluminum-Al3Ti reinforced MMCs were produced with a large amount (up to ~30 vol%) of Al3Ti phase via mechanical alloying, hot extrusion and heat treatment (as a supplementary process) of the Al–Ti elemental powder mixture, respectively. Samples in each stage were studied precise and comprehensive by field emission electron microscope, energy dispersive spectroscopy, X-ray diffraction, differential scanning calorimetry, densitometry, macro and micssro hardness, nanoindentation and tensile tests. The results indicated that after 4-h vibratory milling, the aluminum crystallite size reached 63 nm and Al3Ti phase formed in powder mixture. After hot extrusion, the volume percentage of Al3Ti increased by the formation of new Al3Ti nanoparticles, and a densed structure was achieved. As the milling time increased and crystallite and particle sizes of aluminum and titanium become smaller, the mechanical behavior of the hot extruded nanocomposites was improved. After heat treatments, the amount of Al3Ti phase significantly increased, and by thickening the Al3Ti layers around the titanium microparticles at high temperatures and long time periods, the voids in the structure proliferated. Increase in Al3Ti content can improve the mechanical properties, provided that volume exchanges originating from its formation do not cause widespread formation of voids in the structure. Mechanical properties varied as a function of aluminum and titanium crystallite and particle sizes and amount of Al3Ti phase and voids in the final samples. Aluminum crystallites growth during heat treatment reduces the hardness and increases the matrix ductility, and reinforcement particles growth brings faster failure for the samples.