TiAl Intermetallic Alloy Research Articles

Phase Transition and Ordering Temperatures of TiAl-Mo Alloys Investigated by <i>In Situ</i> Diffraction Experiments

Intermetallic TiAl alloys with a significant volume fraction of the body-centered cubic β-phase at elevated temperatures have proven to exhibit good processing characteristics during hot-working. Being a strong β stabilizer, Mo has gained importance as an alloying element for so-called β/γ-TiAl alloys. Unfortunately, the effect of Mo on the appearing phases and their temperature dependence is not well known. In this work, two sections of the Ti-Al-Mo ternary phase diagram derived from experimental data are shown. These diagrams are compared with the results of in-situ high-temperature diffraction experiments using high-energy synchrotron radiation.

Isothermal and thermomechanical fatigue of titanium alloys

LCF and TMF tests were conducted on Ti6242 and on the intermetallic TiAl alloy TNB-V2 between 350–650 °C and 550–850 °C, respectively. A continuous life reduction of Ti6242 was observed with increasing temperature. On contrary, temperature does not have a systematic influence on life for TNB-V2. Life description based on Basquin-Coffin-Manson illustrates the small contribution of plastic strain to fatigue life of TNB-V2. The transition cycle number, which represents the transition from plastic strain to stress controlled fatigue life, was found to be 4 for TNB-V2 at 550 °C, while it is 409 for Ti6242. Tests in vacuum demonstrated a strong influence of the environment. TMF tests showed that TNB-V2 is very susceptible to out-of-phase loading.

Diffusion bonding of TiAl using Ni/Al multilayers

With the stimulus of temperature and pressure Ni and Al can quickly react and produce the intermetallic compound NiAl. This reaction is highly exothermic and high temperatures can be attained in the surroundings. These characteristics make Ni/Al multilayers very attractive to technological applications as localised heat sources. In this study, Ni/Al multilayer thin films are used to promote bonding between TiAl intermetallic alloys. Ni and Al alternated nanolayers were deposited by d.c. magnetron sputtering onto TiAl samples, with periods of 5, 14 and 30 nm. Joining experiments were performed at 900 °C for 60 or 30 min, in a vertical furnace with a vacuum level better than 10−2 Pa. Applied pressures of 5 MPa were tested. The microstructure of the cross-sections of the bond interface was analysed by energy dispersive X-ray spectroscopy and characterised by scanning electron microscopy. The observation of the microstructure for 14 and 30 nm period multilayers revealed sound bonding, while for 5 nm period porosity and cracks within the interlayer thin film were observed. The interface is divided into three distinct zones: one with columnar grains, another with very small equiaxed grains and the third with larger equiaxed grains. The joining process appears to depend on the diffusion of Ni and Ti across the interface and is assisted by the nucleation of nanometric grains at the interface. The mechanical strength of the joints was evaluated by shear tests. The bonds produced at 900 °C/5 MPa/60 min/14 nm exhibited the highest shear strength of 314 MPa.

Elastic properties of lamellar Ti–Al alloys

Abstract Ti–Al intermetallic alloys have a great potential for high-temperature lightweight applications. They also have rather complicated microstructure and sophisticated behaviour under thermal–mechanical loading. In many cases, Ti–Al component optimization is sufficient when using effective elastic properties of these alloys. Computation of such effective properties is difficult due to various orientations of the phases micro-constituents. Here a simple micromechanical model is applied to calculate effective stiffness and other elastic parameters of Ti–Al alloys as function of composition, orientation and temperature. The dependence of these properties is computed and possibility of simplified, quasi-isotropic or orthotropic properties application is discussed.

Quasi-static chip formation of intermetallic titanium aluminides

As a result of the development of new materials for high temperature applications the potential for mass reduction and increased process temperatures is constantly being expanded. Intermetallic γ-TiAl alloys can meet these demands to a large extent. The properties necessary for these applications have an adverse effect on the machinability however and render intermetallic titanium aluminides as difficult to machine materials. Cutting operations tend to produce damaged surfaces which are unsuitable for the intended applications. As the basis for a reliable and economic cutting technology, the chip formation of the intermetallic TiAl alloy TNBV5 has been examined in quasi-static cutting experiments. Observations showed that increased workpiece temperatures lead to a transition of the chip formation from segmented to continuous chips. By decreasing the undeformed chip thickness crack-free surfaces could be produced at low workpiece temperatures. In this case other mechanisms than the thermal activation of slip systems must be the reason for the observed large plastic deformations. The theory that hydrostatic pressure leads to this behavior is substantiated by the results of finite element simulations. This offers the possibility for damage free machining at lower cutting speeds, thus enabling the use of conventional tool materials at an acceptable tool life.

Thermomechanical Fatigue of the TiAl Intermetallic Alloy TNB-V2

TiAl is supposed to substitute Ni or Ti alloys in energy conversion systems, such as aero engines. These components are subjected to thermomechanical fatigue (TMF), whereas this mechanical behaviour may substantially differ from isothermal low cycle fatigue (LCF). Therefore, it is necessary to assess TMF properties, in order to establish a reliable and precise lifetime prediction model. In this study the γ-base TiAl intermetallic alloy TNB-V2 was subjected to fully-reversed TMF tests under total strain control at strain amplitude of 0.7%, accompanied by LCF tests in air and vacuum. Temperature ranged from 550°C to 850°C. In TMF tests a continuous built-up of compressive (in-phase) or tensile (out-of-phase) mean stresses is observed. The lifetime ratio between IP and OP is 30–200, depending on temperature range. Substantially longer lifetimes of OP-TMF tests in vacuum are observed. Fracture always occurs in a transcrystalline mode. A lifetime description at 550°C based on the Basquin-Coffin-Manson equation reveals that the fatigue behaviour is governed by the amount of elastic strain even at low cycles, and the transition lifetime was found to be at 5 cycles. A damage parameter based on the equation of Smith, Watson and Topper is able to describe LCF and in-phase TMF lifetimes reasonably.

Microstructure and properties of laser cladding TiC/TiAl composite coatings on γ-TiAl intermetallic alloy

To improve the wear resistance of the γ-TiAl intermetallic alloy, Ti–Al–TiC alloy powders were prepared on the surface of TiAl intermetallic alloy and then treated by a high power continuous wave CO2 laser. TiC reinforced TiAl matrix composite coatings have been successfully synthesised. The microstructure, phase composition, microhardness and wear resistance of the laser cladding composite coatings were investigated by optical microscope, scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and pin on disc wear test. The results show that the morphology of reinforced TiC homogeneously disperses in TiAl matrix and changes from dendritic TiC into short bar and particle shape with increasing the scanning rate. Also, the microhardness is enhanced by an increase in the scanning rate. The average microhardness of the composite coating under laser scanning rate of 600 mm min–1 is 1·8 times of the substrate. The laser cladding composite coatings have better wear resistance than original TiAl alloy, and the wear mechanisms are discussed in detail.

Erosion of a TiAl intermetallic alloy under conditions simulating plasma disruptions

Radiation erosion and thermal stability of TiAl-based intermetallic alloys produced by vacuum-arc melting, compacting of microcrystal powders with binder and impregnantion by melt, and their brazed joints with bronze have been investigated under irradiation by high-temperature pulsed hydrogen plasma flows (the flow energy density Q = 0.2−0.9 MJ/m 2, the pulse duration 15 μs, the number of pulses 1–21) which simulate the expected plasma disruptions in a fusion reactor. It has been found that the erosion coefficients and thermal stability of alloys are determined by the way of their fabrication, and compacted intermetallides have a higher thermal stability in comparison with the cast ones. The brazed joints of the intermetallic compound with bronze under irradiation by pulsed hydrogen plasma up to the energy density Q = 0.75 MJ/m 2 have a high thermal stability and formation of cracks was not observed.

Bond strength and interface energy between Pd membranes and TiAl supports

Intermetallic TiAl alloy is proposed as a promising support for Pd membranes. First principles calculations reveal that coherent Pd/TiAl interfaces possess high values of bond strengths. Calculations also show that Ti-terminated (100) Pd/(100) TiAl and (110) Pd/(110) TiAl interfaces are energetically favorable with negative interface energies of about −3.1 J/m2, and that the bond strengths of Pd–Ti are bigger than those of Pd–Al. In addition, densities of states calculations suggest that a stronger chemical bonding is formed in the Pd/TiAl interface than corresponding Pd or TiAl bulks, which agrees well with similar experimental observations in literature.

Effect of mechanical milling on Ni–TiH 2 powder alloy filler metal for brazing TiAl intermetallic alloy: The microstructure and joint's properties

A TiH 2–50 wt.% Ni powder alloy was mechanically milled in an argon gas atmosphere using milling times up to 480 min. A TiAl intermetallic alloy was joined by vacuum furnace brazing using the TiH 2–50 wt.% Ni powder alloy as the filler metal. The effect of mechanical milling on the microstructure and shear strength of the brazed joints was investigated. The results showed that the grains of TiH 2–50 wt.% Ni powder alloy were refined and the fusion temperature decreased after milling. A sound brazing seam was obtained when the sample was brazed at 1140 °C for 15 min using filler metal powder milled for 120 min. The interfacial zones of the specimens brazed with the milled filler powder were thinner and the shear strength of the joint was increased compared to specimens brazed with non-milled filler powder. A sample brazed at 1180 °C for 15 min using TiH 2–50 wt.% Ni powder alloy milled for 120 min exhibited the highest shear strength at both room and elevated temperatures.

Dual surface and bulk control by Nb of the penetration of environmental elements in TiAl intermetallic alloys

X-ray photoelectron spectroscopy (XPS) and micro-hardness measurements were combined to study the effect of Nb doping on the high temperature environmental interactions of α 2-Ti 3Al intermetallic alloys with pure oxygen and oxygen/nitrogen gas mixtures. The XPS data show a pre-oxidation stage in the first minutes of low pressure exposure to the gaseous environments at 650 °C, characterized by the absence of oxidation of the metal constituents of the alloy, the adsorption on the surface, and the sub-surface penetration of atomic oxygen and nitrogen. The XPS measurements of the surface and sub-surface concentrations reveal the effect of niobium on the surface adsorption of oxygen and nitrogen and on sub-surface penetration, showing that nitrogen is disfavored. The micro-hardness data show a bulk effect of niobium that counteracts the near-surface embrittlement caused by the penetrating environmental elements. The surface and bulk effects of niobium combine to mitigate the embrittlement of the material by nitrogen.

Advanced Coatings on High Temperature Applications

To suppress interdiffusion between the coating and alloy substrate in addition to ensuring slow oxide growth at very high temperatures advanced coatings were developed, and they were classified into four groups, (1) the diffusion barrier coating with a duplex layer structure, an inner σ−(Re-Cr-Ni) phase as a diffusion barrier and outer Ni aluminides as an aluminum reservoir formed on a Ni based superalloy, Hastelloy X, and Nb-based alloy. (2) the up-hill diffusion coating with a duplex layer structure, an inner TiAl2 + L12 and an outer β-NiAl formed on TiAl intermetallic and Ti-based heat resistant alloys by the Ni-plating followed by high Al-activity pack cementation. (3) the chemical barrier coating with a duplex layer structure, an inner* γ + β + Laves three phases mixture as a chemical diffusion barrier and an outer Al-rich γ-TiAl as an Al reservoir formed by the two step Cr / Al pack process. (4) the self-formed coating with the duplex structure, an inner α-Cr layer as a diffusion barrier and an outer β-NiAl as an Al-reservoir on Ni-(2050)at% Cr alloy changed from the δ-Ni2Al3 coating during oxidation at high temperature. The oxidation properties of the coated alloys were investigated at temperatures between 1173 and 1573K in air for up to 1,000 hrs (10,000 hrs for the up-hill diffusion coating). In the diffusion barrier coating the Re-Cr-Ni alloy layer was stable, existing between the Ni-based superalloy (or Hastelloy X) and Ni aluminides containing 1250at%Al when oxidized at 1423K for up to 1800ks. It was found that the Re-Cr-Ni alloy layer acts as a diffusion barrier for both the inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate. In the chemical barrier coating both the TiAl2 outermost and Al-rich γ-TiAl outer layers maintained high Al contents, forming a protective Al2O3 scale, and it seems that the inner, γ, β, Laves three phase mixture layer suppresses mutual diffusion between the alloy substrate and the outer/outermost layers.

Numerical prediction for columnar to equiaxed transition in solidified Ti–Al intermetallic alloy ingots by stochastic model

A stochastic model of macro- and microtype for predicting the columnar–equiaxed transition (CET) during solidification processes of Ti–Al intermetallic alloy ingots is developed in the present paper. The macroscopic part is based on a finite difference method (FDM) for modelling of heat transfer. The microscopic part consists of a cellular automaton method (CA) for modelling of nucleation and growth. The formation of a shrinkage cavity at the top of ingot is taken into account. The effects of casting variables on CET are presented and discussed. A quantitative relation between CET position and casting variables is obtained. The columnar zone is found to expand with decreasing alloy composition, mould preheated temperature and convection, and increasing thermal conductivity of mould, superheat and heat transfer coefficient at mould/metal interface. And the highly influential variables are superheat, heat transfer coefficient and convection.

Effect of Film Thickness on Fatigue Strength of TiAl Alloy Coated with TiAlN Film at Elevated Temperature

The fatigue strength of TiAl intermetallic alloy coated with TiAlN film was studied in vacuum at 1073K using a SEM-servo testing machine. In addition, three kinds of TiAlN films were given by physical vapor deposition (1, 3, and 10μ m). The fatigue strength of 3μ m was highest. Also, the fatigue strength of 1μ m was lowest. From this result, existence of optimum film thickness was suggested because the difference of fatigue strength arose in each film thickness. The justification for existence of optimum film thickness is competition of 45-degree crack and 90-degree crack. The 45-degree crack is phenomenon seen in the thin film (1μ m), and is caused by plastic deformation of TiAl substrate. The 45-degree crack is the factor of the fatigue strength fall by the side of thin film. In contrast, the 90-degree crack is phenomenon in the thick film (10μ m), and is caused as result of reaction against load to film. The 90-degree crack is the factor of the fatigue strength fall by the side of thick film. In conclusion, the optimum film thickness can perform meso fracture control, and improves fatigue strength.

Lattice imperfections in intermetallic Ti–Al alloys: an X-ray diffraction study of the microstructure by the Rietveld method

Microstructure in intermetallic Ti–Al alloys with compositions (Al=45, 50 and 55%: all compositions are in at. %) in the homogenized bulk and cold-worked powder states has been extensively studied using Rietveld whole X-ray profile fitting technique, adopting the recently developed software, MAUD (Material Analysis Using Diffraction). The analysis also considers the quantitative estimation of different phases and lattice defect related features of the evoluted microstructures, namely crystallite sizes, microstrains, stacking fault probabilities and dislocation density values, etc. in both the states of the materials (homogenized bulk and cold-worked powder). The study revealed reverse α 2→γ phase transformations due to plastic deformation in all the considered alloy compositions. Upon cold-working, high densities of stacking faults were observed in the α 2-Ti 3Al microstructure, accompanied by moderate values (∼10 11 cm −2) of average dislocation densities in the γ-TiAl microstructure. In the homogenized bulk alloys, the respective Vickers microhardness values were found to be 464, 365 and 339 HV, decreasing with increase in Al concentration. The values of all the above defect parameters have been evaluated and compared for elucidating a better structure-property relationship.

Microstructure, wear and high-temperature oxidation resistance of laser clad Ti5Si3/γ/TiSi composite coatings on γ-TiAl intermetallic alloy

Mixed NiCr–Si precursor powders on γ-TiAl intermetallic alloy (Ti–48Al–2Cr–2Nb) have been investigated for laser cladding treatment to modify the tribology and high-temperature oxidation properties of the material simultaneously. The alloy samples are pre-placed with NiCr–50% and NiCr–40%Si (wt.%), respectively, and laser treated at the same parameters, i.e., laser output power 2.8 kW, beam scanning speed 2.0 mm/s, beam dimension 1×18 mm. The treated samples underwent tests of microhardness, full dry sliding wear and high-temperature oxidation. The results showed that laser cladding with different constitution of mixed precursor NiCr–Si powders improve the surface hardness in all cases. Laser cladding with NiCr–40% Si precursor mixed powders results in the better modification of tribology and high-temperature oxidation behavior. However, with NiCr–50%Si precursor powders shows decreased wear resistance compared to the original TiAl alloy due to the existence of a large amount of hard and brittle Ti5Si3 and TiSi compounds. Optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectrometer (EDS) analyses indicated that the formation of the reinforced primary Ti5Si3, γ/TiSi eutectics and the refinement of the microstructure and both the continuous and dense Al2O3, SiO2 hybrid oxide scales are supposed to be responsible for the modification of the relevant properties. Laser cladding with NiCr–Si mixed powders is anticipated to be a promising wear- and oxidation-resisting surface modification technique for TiAl intermetallic alloy.

Transient liquid phase (TLP) bonding of TiAl using various insert foils

TiAl intermetallic alloys have been joined using the diffusion brazing technique with Ti insert foil metal combined with Cu, Ni or Fe foils. Defect-free joints can be fabricated successfully, no matter which kind of foil combination is used as insert metal. The interface structures of the joints were also investigated and results showed various interface structures in the joined zones. A mechanism for the development of the interface structure is proposed. It is suggested that the formation process of the liquid phase was responsible for the resulting interface structures. Post-bond heat treatment (PBHT) had no obvious effect on the interface structure of the joints because of the low diffusion coefficient of Cu, Ni and Fe in the parent material. Microhardness results showed a slight decrease in the joint area as a result of PBHT.

In situ hot-stage observations of structural transformation in a laser surface-melted TiAl intermetallic alloy

The structural transformation in a laser surface-melted TiAl intermetallic alloy has been observed during in situ heating. The effect of temperature on the observed metallographic changes in microstructure is discussed. The results show that during heating to certain temperatures, the dendritic structure formed by laser surface-melting can transform into an apparently fine-grain structure, which may provide a suitable substrate for superplastic/diffusion bonding.

Effect of microstructure on the characterization of creep crack growth rate and rupture in TiAl intermetallic alloys

In order to establish a standardised method of creep crack growth testing for creep brittle materials, Round Robin tests and analysis of the results were carried out using a TiAl alloy material tested at 750 °C. The TiAl intermetallic contained two different microstructures; one is fully lamellar TiAl and the other fine-grained TiAl. It is found from the results in the Round Robin tests that these materials show creep crack growth different behaviour. In the present study, the characteristics of creep crack growth rate and their rupture lives are considered. It is shown that the characterization of the creep crack growth and the specimen failure lives using the Q ∗ parameter, Larson–Miller parameter and creep ductility criteria are all possible.

Infrared brazing of TiAl intermetallic using BAg-8 braze alloy

TiAl intermetallic alloy joined by infrared brazing using BAg-8 braze alloy was investigated. The microstructural evolution of the brazed joint, shear strength and reaction kinetics across the joint was comprehensively evaluated. According to the experimental observations, silver would not react with the TiAl substrate, but copper reacted vigorously with the TiAl, forming continuous reaction layer. The consumption of copper from molten braze during infrared brazing resulted in depletion of the copper content from the braze. Therefore, chemical composition of the braze deviated from Ag-Cu eutectic into hypoeutectic with increased brazing time and/or temperature. Both AlCuTi and AlCu 2Ti phase were observed at the interface between BAg-8 and TiAl substrate for the specimen brazed at 950°C. By increasing the brazing temperature and time, the growth rate of AlCuTi phase was much faster than that of AlCu 2Ti phase. The maximum shear strength achieved 343 MPa for the specimen infrared brazed at 950°C for 60 s. Further increasing the brazing time resulted in excessive growth of brittle AlCuTi reaction layer, which greatly deteriorated the shear strength of the joint.