γ-TiAl Intermetallic Compound Research Articles

Grinding force model for ultrasonic assisted grinding of γ-TiAl intermetallic compounds and experimental validation

Abstract The introduction of ultrasonic vibration in the grinding process of γ-TiAl intermetallic compounds can significantly reduce its processing difficulty. It is of great significance to understand the grinding mechanism of γ-TiAl intermetallic compounds and improve the processing efficiency by studying the mechanism of ordinary grinding of abrasive grains. Based on this, this study proposes a grinding force prediction model based on single-grain ultrasonic assisted grinding (UAG) chip formation mechanism. First, the prediction model of grinding force is established based on the chip formation mechanism of abrasive sliding ordinary grinding and the theory of ultrasonic assisted machining, considering the plastic deformation and shear effect in the process of material processing. Second, the UAG experiment of γ-TiAl intermetallic compounds was carried out by using diamond grinding wheel, and the unknown coefficient in the model was determined. Finally, the predicted values and experimental values of grinding force under different parameters were compared to verify the rationality of the model. It was found that the maximum deviation between the predicted value of tangential force and the actual value is 23%, and the maximum deviation between the predicted value of normal force and the actual value is 21.7%. In addition, by changing the relevant parameters, the model can predict the grinding force of different metal materials under different processing parameters, which is helpful for optimizing the UAG parameters and improving the processing efficiency.

Grindability of γ-TiAl intermetallic compounds during ultrasonic vibration-assisted high efficiency deep grinding process

Grindability of γ-TiAl intermetallic compounds during ultrasonic vibration-assisted high efficiency deep grinding process

Phase transformation study during diffusion brazing of γ-TiAl intermetallic using amorphous Ni-based filler metals

The paper aims at understanding the microstructure evolution of the diffusion brazing of γ-TiAl intermetallic compound using two Ni-based filler metals, a ternary Ni-Si-B, and a quinary Ni-Cr-Fe-Si-B. It was found that the joint region comprised of two distinct zones: analogous reaction layers adjacent to the TiAl substrates and a solidified zone at the middle of the joint region. The reaction layers consisted of separate intermetallic phases regions including ternary and binary Ti-Ni-Al intermetallic compounds. The solidified zone mainly composed of the Al1-xNi3Six isostructural solid solution and athermally Ni-boride or Ni-Cr rich borides containing eutectic microstructures at the joint centerline. The maximum shear strength was 240 MPa, and obtained by using Ni-Cr-Fe-Si-B filler metal. Fracture of the joint with Ni-Cr-Fe-Si-B filler occurred in the reaction layer/solidified zone interface, but joint with Ni-Si-B filler was fractured at solidified zone.

Microstructural Evaluation during dissimilar transient liquid phase bonding of TiAl/Ni-based superalloy

The paper aims at understanding solidification phenomena and solid state precipitations during dissimilar transient liquid phase bonding of γ-TiAl intermetallic compound to IN718 Ni-based superalloy using a quinary Ni–7Cr–3Fe–4.5Si–3.2B (wt%) filler metal. The mechanisms of intermetallic formation via solidification and solid state reactions are discussed. The joint is featured by the formation of Ni-rich solid solution in an isothermal solidification zone near to IN718, Ni-rich solid solution and isostructural binary phases solid solution designated as Al1-xNi3Six in an isothermal solidification zone near to TiAl, boride/silicide and boro-silicide formation during binary and ternary eutectic-type solidification in an athermal solidification zone, fine nickel silicide precipitation during cooling in the joint centerline, and in-situ precipitation of Cr–Mo–Nb rich borides in a diffusion affected zone. On the other hand, Ni from the filler metal reacted with γ-TiAl base metal, leading to the formation of Ti2Ni, τ2-Al2TiNi and τ4-AlNi2Ti compounds on the reaction layer adjacent to TiAl substrate. The evaluation of different zones of the bond region showed that some brittle phases with high hardness are formed in the bond region. It was identified that the presence of the eutectic gamma solid solution between boride phases within the joint centerline can decrease the detrimental effect of the intermetallic phases on the joint toughness.

First-principle study of the solubility and diffusion of oxygen and boron in [formula omitted]-TiAl

Solubility and diffusion of boron and oxygen in the γ-TiAl intermetallic compound are herein investigated by first-principle calculations. On the basis of the space group of the γ phase, the accommodation of the light atoms in the various interstitial positions is studied by density functional theory. Diffusion coefficients are also obtained from ab initio calculations. Barrier energies for the boron and oxygen diffusion among the different stable interstitial sites are computed using the Nudged Elastic Band method and atomic jumping rates are obtained from the Transitional State Theory. Diffusion coefficients are obtained from the solution of the transport equation in the infinite time limit, using the analytical Multi-State Diffusion method. The methodology here applied is validated by the good agreement between the computed diffusion coefficient of boron and the experimental data available in the literature for this species.

Site Occupation of Nb in γ-TiAl: Beyond the Point Defect Gas Approximation

Microalloying is an effective approach to improve the mechanical properties of γ-TiAl intermetallic compound. Knowledge about the site occupancy of the ternary alloying element in the crystal lattice of γ-TiAl is highly demanded in order to understand the physics underlying the alloying effect. Previous first-principle methods-based thermodynamic models for the determination of the site occupancy were based on the point defect gas approximation with the interaction between the point defects neglected. In the present work, we include the point defect interaction energy in the thermodynamic model, which allows us to predict the site occupancy of the ternary alloying element in γ-TiAl beyond the point defect gas approximation. The model is applied to the γ-TiAl–Nb alloy. We show that, at low temperature, the site occupancy of Nb atoms depends on the composition of the alloy: Nb atoms occupy the Al sublattice for the Ti-rich alloy but occupy Ti sublattice for the Al-rich alloy. The fraction of Nb atoms occupying Al sublattice in the Ti-rich alloy decreases drastically, whereas the fraction of Nb atoms on the Ti sublattice in the Al-rich alloy decreases slightly with increasing temperature. At high temperature, Nb atoms occupy dominantly the Ti sublattice for both the Ti-rich and Al-rich alloys. The interaction between the point defects makes the Ti sublattice more favorable for the Nb atoms to occupy.

Study the Effects of Dielectric Type on the Machining Characteristics of γ-Ti Al in Electrical Discharge Machining

The current study surveys the results of using deionized water and kerosene as dielectrics in the machining outputs of γ-TiAl intermetallic compound obtained in electric discharge machining. Influences of these different dielectrics properties on machining speed, tool wear, surface cracks and roughness were compared. Scanning electron microscopy micrographs were prepared to investigate influences of dielectrics on the surface characteristics of electrically discharged samples. Results indicate which by kerosene dielectric; the material removal rate (MRR) is further compared to another one. But deionized water as dielectric causes higher tool wear ratio than kerosene dielectric. Electrical discharged samples in deionized water have higher surface roughness, in addition it contains surface cracks, whereas kerosene dielectric results better surface finish in low pulse current. According to XRD spectra electric discharge machining in deionized water and kerosene dielectrics produces Ti3 Al intermetallic compound on the produced surface.

Synthesis of γ-TiAl by Reactive Spark Plasma Sintering of Cryomilled Ti and Al Powder Blend: Part II: Effects of Electric Field and Microstructure on Sintering Kinetics

The current study shows the dramatic effect of an electric field (EF) and use of nanosized cryomilled grains on accelerating sintering kinetics during spark plasma sintering of blended elemental powder compacts of Ti53Al47 targeted to produce γ-TiAl intermetallic compounds. The EF had the dominating effect since it reduced the activation barrier for diffusion through Al3Ti leading to faster growth of Al3Ti; the precursor to γ-TiAl. The Avrami exponent (n) determined for the micrograin compact lies between 1.0 and 1.5, which indicates that reaction sintering is controlled by bulk diffusion in these compacts, while for cryomilled compacts this is between 0.7 and 1.0 suggesting the important role of dislocations and grain boundaries on the transformation during reaction sintering. The activation energies were found to be in increasing order as: cryomilled compacts with EF (182 kJ/mol); micrograin compacts with EF (290 kJ/mol); cryomilled compacts without EF (331 kJ/mol); and micrograin compacts without EF (379 kJ/mol). The cryomilled microstructure also enhanced the sintering kinetics because of the availability of faster diffusing paths in Al and Ti including larger grain boundary area and dislocation density.

Diffusion of chromium and manganese in γ-TiAl

The γ-TiAl intermetallic compound with suitable alloying additions has shown considerable promise as a material for high-temperature applications. Diffusion studies in this alloy system are useful in assessment of their creep behaviour and structural stability in service conditions. Tracer diffusion coefficients of 51Cr and 54Mn in a γ-TiAl intermetallic compound containing 54.1 at. % aluminium were determined in the temperature range from 1095 to 1470 K. The temperature dependence of both the diffusing species follows a linear Arrhenius behaviour and can be expressed as D Cr = 4.4 × 10−3exp(−350 kJ mol−1/RT) m2 s−1 and D Mn = 1.2 × 10−3 ×exp(−326 kJ mol−1/RT) m2 s−1. The data are analysed on the basis of empirical correlations between the diffusion and melting parameters applicable for conventional mono-vacancy diffusion mechanism in metals. It is concluded that impurity diffusion in γ-TiAl occurs through the migration of thermal vacancies via nearest-neighbour or next-nearest neighbour jumps.

Effects of Disilicade Coating on Thermal Shock of γ-TiAl Intermetallic Compounds Determined by Burner-heating Test

Effects of Disilicade Coating on Thermal Shock of γ-TiAl Intermetallic Compounds Determined by Burner-heating Test

Selected Interlayer of Diamond Deposition on γ-TiAl Intermetallic Compounds Prepared by Microwave-Plasma Assisted CVD

Several diamond coatings were performed on -TiAl substrates by a microwave-plasma assisted CVD, which were made directly to the substrate and indirectly to the TiC, Ti5Si3, Al2O3+TiO2 and Si layers on the substrate. The direct coatings suffered from severe delamination and cracks. The deposited layers on TiC and Ti5Si3 layers partially delaminated, while those on Al2O3+TiO2 and Si layers adhered well without delamination. All the diamond films deposited were characterized using scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Raman spectra showed that poly- and nano-crystalline diamond films were obtained for the coatings of -TiAl.

Evaluation of Lifetime of FGMs Disilicide Coatings for γ-TiAl Intermetallic Compound

The NbSi2/Nb/-TiAl and NbSi2/Nb functionally graded materials (FGMs) were prepared and their tolerances tested by exposing them to temperatures from 1050 °C to 1250 °C under vacuum and in air. Oxygen resistivity was estimated from metallographic investigations. The FGM lifetime was estimated by using a diffusion equation that considers the disappearance of the NbSi2 and Nb interlayer. These occurred during NbSi2 oxidation and Si diffusion from NbSi2 to Nb and interdiffusion between Nb and -TiAl. The results were validated by diffusion equations.

Microstructure and reaction mechanisms at γ-TiAl brazed interfaces

AbstractTitanium aluminide alloys based on the γ-TiAl intermetallic compound are emerging as potential materials for high performance applications on aircraft engines components, aerospace vehicles and automotive engines, owing to their low density, high specific stiffness, excellent strength retention at high temperature and good creep and oxidation resistance. The development of adequate techniques to join γ-TiAl alloys, either to themselves or to other materials, is a key issue regarding the effective use of these alloys. Understanding of the reaction mechanisms that promote bonding is mandatory when optimization of the joining procedure is envisaged.

Empirical Analysis of Lifetimes for Disilicide Coatings on a γ-TiAl Intermetallic Compound

The NbSi2/Nb/γ-TiAl and NbSi2/Nb functionally graded materials (FGMs) were prepared and their tolerances tested by exposing them to temperatures from 1323 K to 1523 K under vacuum and in air. Oxygen resistivity was estimated from metallographic investigations. The FGM lifetime was estimated by using a diffusion equation that considers the disappearance of the NbSi2 and Nb interlayer. These occurred during NbSi2 oxidation and Si diffusion from NbSi2 to Nb and interdiffusion between Nb and γ-TiAl. The results were validated by diffusion equations. The results indicate that a coating used at 1323 K for 380 h requires a NbSi2+Nb coating thickness of more than 77 μm.

Preparation of γ-TiAl intermetallic compounds through self-propagating high-temperature synthesis and compaction

Compact materials based on titanium and aluminum are prepared using self-propagating high-temperature synthesis and compaction. The parameters of the self-propagating high-temperature synthesis and compaction are optimized for the purpose of producing a dense intermetallic compound, namely, γ-TiAl.

Fabrication of TiAl Intermetallic by Spark Plasma Sintering

The γ-TiAl intermetallic compounds were produced at the temperature ranging from 850°C to 1050°C by the Spark Plasma Sintering (SPS) process. The effects of sintering temperature and holding time on the mechanical properties of γ-TiAl intermetallic compounds were investigated. The γ-TiAl intermetallic compounds sintered at 1050°C for 10 min showed a high relative density more than 98%, and had the best three-point bending strength of 643MPa, fracture toughness of 12 MPa·m1/2 and microhardness of 560MPa. The microstructural observations indicated typical characteristics of intergranular fracture, which meant the poor ductility of γ-TiAl intermetallic compounds.

Preparation and high temperature oxidation behavior of refractory disilicide coatings for γ-TiAl intermetallic compounds

Novel refractory disilicide layers were applied to γ-TiAl to enhance oxidation resistance at 1050 °C. NbSi 2 and MoSi 2 layers were prepared by joining thin Nb and Mo foils to γ-TiAl surfaces, and siliconizing the combinations (Nb/γ-TiAl, and Mo/γ-TiAl) using molten salts. The coatings and their oxidation behavior were characterized using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy techniques. Isothermal oxidation tests showed that the oxidation resistance of uncoated γ-TiAl at 1050 °C in air was insufficient, and scale spallation occurred. NbSi 2 coatings were formed and adhered firmly to the γ-TiAl substrate, whereas Mo film detached from the substrate surface causing failure of the MoSi 2 coatings. Oxidation of the NbSi 2-coated γ-TiAl (NbSi 2/Nb/γ-TiAl) at 1050 °C in air showed improved oxidation resistance at exposure times up to 100 h. Microstructural and compositional developments of the coating at prolonged time were discussed. The NbSi 2 coatings provided sufficient oxidation resistance for γ-TiAl at 1050 °C in air, and have potential use in high temperature applications.

Structures and properties of ceramic films on TiAl intermetallic compound fabricated by microarc oxidation

The ceramic oxide films with a thickness of 120 μm were prepared on γ-TiAl intermetallic compound by alternating-current microarc oxidation in silicate electrolyte. Isothermal oxidation of TiAl alloy coated with the ceramic oxide film was carried out in air at 1100 °C. Result showed that the dense layers in the oxide film were mainly composed of Al 2TiO 5 and TiO 2 rutile phase, while the loose layer contained a large amount of amorphous SiO 2 besides Al 2TiO 5 and TiO 2 rutile phase. Thermal shock tests revealed a good adhesion between the films and TiAl substrate, and the oxidation resistance of the TiAl alloy was improved.

Modelling of defects and amorphisation by ball milling of γ-TiAl

The formation and site preference energies and volume changes of single and pair of defects of ternary alloying elements in γ-TiAl intermetallic compound were studied by the density functional theory. Slight tendency to clusterization of antisite defects has been found. This may lead to disorder in the system. The V and Cr atoms prefer to reside in the Ti sublattice. The formation energy for Cr–Cr, Cr–V and V–V nearest neighbour pairs are in the 1.3–2 eV range. The Al antisite in Ti sublattice requires much less energy than the Ti antisite in Al sublattice. The amorphisation process of TiAl alloy was studied by means of high energy ball milling of Ti and Al elemental powders, which produces amorphous structure after 40 h. The amorphous states were studied by the DFT calculations of many random atomic configurations and the results were compared with the NiAl compound. Possible explanation for the amorphisation of the TiAl compound is presented.

Twin intersection in tensile deformed γ-TiAl intermetallic compounds

Intersections of deformation twins in γ-TiAl grains were examined in Ti-47at.%Al alloy specimens tensile-tested at 873 K. Transmission electron microscopy (TEM) revealed 1 / 2 < 1 1 ¯ 0 ] { 111 } dislocations and 1 / 6 < 11 2 ¯ ] { 111 } twins in deformed γ-TiAl grains. Some γ-TiAl grains contained twins on two different {111} planes, which intersect along <110] directions (type-1). No twin intersection along <101] (type-2) was observed, which was often reported in compressed specimens. Results are discussed with the Schmid factor distributions of twinning systems and slip systems. It should be noted that twinning shear in L1 0 structure is limited only in one 1/6< 11 2 ¯ ] movement on a {111}. Considering Schmid factors of possible four twin systems, both type-1 and type-2 twin intersections are expected. Tensile load direction which can activate both twinnings of type-2 intersection can also activate ordinary 1 / 2 < 1 1 ¯ 0 ] { 111 } dislocations. The absence of type-2 intersection suggests that ordinary slip systems were favored in these cases.