Compound TiAl Research Articles

Corrosion resistance of 91W–6Ni–3Fe refractory metal, TiAl compound and iron based alloys in molten aluminum

The corrosion behaviors of 91W–6Ni–3Fe (91W) refractory alloy, TiAl intermetallic compound and two types of iron based alloys (QT700 and H13 tool steel) in a liquid aluminum were investigated. Corrosion experiments or static immersion-tests were carried out in pure molten aluminum at 750°C. The surface micro-topographies, corrosion interfaces and phase compositions of the immersed samples were investigated by 3D optical microscopy, SEM, EDS and XRD. The results show that 91W exhibits the best corrosion resistance, followed by QT700, H13 and TiAl alloy, consequently. The corrosion mass loss of the four metallic materials adheres to parabolic criterion, and the corrosion rate trends to be stable after initial acceleration. The diffusion–reaction mechanism is proposed for the dissolution of materials in molten aluminum, and the diffusion process is the rate-determining step during the dissolution of 91W in molten aluminum, while the low activation energy for the reaction between TiAl–(TiAl3)–Al couple results in poor corrosion resistance of TiAl alloy in molten aluminum.

Microstructure of Mg-Ti-Al Composite Hot Pressed at Different Temperature

Metal matrix composite comprising a multiphase magnesium matrix and titanium particles fabricated by hot pressing was characterized. Powder mixture of the Mg:Ti:Al at weight ratio equal to 10.5:6.1:3.4 was sintered at 640, 650 and 660°C whereas other parameters were held constant. Thermal effects during heating and cooling of powder mixture were measured by differential scanning calorimetry (DSC). Microstructure of composite was characterized by scanning electron microscopy (SEM) with a use of X-ray energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). For all conditions of components consolidation α-Mg, α-Ti, Mg17Al12 and Al3Ti were identified. It was revealed that dispersion and location of Mg17Al12 and Al3Ti compounds depended on sintering temperature. Measurements of hardness and density of obtained non-porous composite gave approximate results of 130 HV and 2.7 g/cm3 respectively

Microstructure, crystal structure and mechanical properties of the new ternary intermetallic alloy phase Zr2TiAl

Zr3Al is an intermetallic compound with a cubic structure whereas the isoelectronic compound Ti3Al is hexagonal. Crystallographic, microstructural and mechanical study of an alloy containing all the three elements is use in advancing our knowledge of the phase behavior across this pseudobinary alloy. A nominal composition of the Zr2TiAl alloy was prepared by arc melting and subsequently vacuum annealed at 1050 °C for 30 days and 1000 °C for 20 days. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) showed the presence of three phases in the material. Among the three phases, the major phase was the stoichiometric Zr2TiAl alloy. From the SEM micrograph, it was seen that the major phase grains varied in size from 10 μm to 50 μm. Of the two minor phases present in the specimen, one is rich in Ti and the other in Al. In order to identify the peaks due to individual phases, micro-beam XRD was carried out, enabling the identification of all the three unambiguously. The stoichiometric alloy (Zr2TiAl) has L21 structure (space group Fm3m), the Ti-rich phase has DO19 structure (space group P63/mmc) and the Al-rich phase has Aa structure (space group I4/mmm) Further, mechanical characterization of this alloy was carried out using nano-indentation and the typical hardness was found to be 8.5 GPa on the grain and 7.4 GPa at the grain boundary.

Oxidation mechanism of the intermetallic compound Ti3Al from ab initio thermodynamics

Ab initio density-functional theory and thermodynamics calculations are combined to establish a microscopic mechanism for the oxidation of the α(2)-Ti(3)Al(0001) surface. The surface energies as functions of the chemical potentials, as well as structural relaxations and electronic densities of states, are determined. The surface phase diagram (SPD) of the α(2)-Ti(3)Al(0001) systems with different defects and at various oxygen coverages is constructed. It is found that the Al antisite defect prefers to segregate on the α(2)-Ti(3)Al(0001) surface and oxygen adsorption enhances the segregation with the formation of the surface with three Al antisites per unit surface cell (i.e. the top surface layer is full of Al atoms) at the initial stage of oxidation, accounting for the aluminum selective oxidation observed experimentally. After the initial stage of oxidation, the O-α(2)-Ti(3)Al(0001) system manifests itself with a non-uniform double-phase SPD, suggesting the competition between oxidations of the Al and Ti elements in the oxidation process. This result explains the experimentally observed second regime of oxidation in which both metal elements are oxidized.

SEM and TEM characterization of the microstructure of post-compressed TiB 2/2024Al composite

In the present work, 55 vol.% TiB 2/2024Al composites were obtained by pressure infiltration method. Compressive properties of 55 vol.% TiB 2/2024Al composite under the strain rates of 10 −3 and 1 S −1 at different temperature were measured and microstructure of post-compressed TiB 2/2024Al composite was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). No trace of Al 3Ti compound flake was found. TiB 2–Al interface was smooth without significant reaction products, and orientation relationships ( [ 1 1 2 ¯ 0 ] TiB 2 // [ 1 1 0 ] Al and ( 1 1 ¯ 0 0 ) TiB 2 // ( 1 ¯ 1 1 ¯ ) Al ) were revealed by HRTEM. Compressive strength of TiB 2/2024Al composites decreased with temperature regardless of strain rates. The strain-rate-sensitivity of TiB 2/2024Al composites increased with the increasing temperature. Fracture surface of specimens compressed at 25 and 250 °C under 10 −3 S −1 were characterized by furrow. Under 10 −3 S −1, high density dislocations were formed in Al matrix when compressed at 25 °C and dynamic recrystallization occurred at 250 °C. Segregation of Mg and Cu on the subgrain boundary was also revealed at 550 °C. Dislocations, whose density increased with temperature, were formed in TiB 2 particles under 1 S −1. Deformation of composites is affected by matrix, reinforcement and strain rate.

Solidus surface of the Al–Ti–Pd system in the Al–AlPd–TiPd–Ti region

The Al–Ti–Pd system has been studied by microstructural analysis, electron microprobe analysis, x-ray diffraction, and differential temperature analysis in the composition range 0–50 at.% Pd (Al–AlPd–TiPd–Ti partial system) at subsolidus temperatures. The solidus surface of the system is constructed for the first time. At subsolidus temperatures, there are two ternary compounds, τ1 (Al61Ti26Pd13, AuCu3 type) and τ3 (Al44Ti35Pd21, MgZn2 type), a continuous solid solution (β0 phase) between isostructural (CsCl type) high-temperature modifications of AlPd and TiPd equiatomic phases, and terminal solid solutions based on β-Ti and α-Ti and TiAl, Ti2Al5, TiAl3, and Al3Pd2 binary compounds and e phase. The above-mentioned phases participate in 11 four-phase invariant equilibria involving the liquid phase and are represented by isothermal tie-line triangles in the temperature range between 630 and 1205°C.

Formation of the Intermetallic Layers in Ti-Al Multilayer Composites

Commercially pure aluminum and commercially pure titanium plates have been explosively welded and annealed at temperature of 630 °C for 5, 20, 50 and 100 hours. The investigation of intermetallic formed during explosion welding and heat treatment processes has been carried out. The metallographic studies showed variation in the intermetallic volume fraction according to the deformation degree of different interfaces. Moreover the relation between the intermetallic layer thickness and time of explosively welded “Al-Ti” composite annealing has been found. The X-ray analysis reviled that intermetallic layer formed during the heat treatment process consisted of Al3Ti compound.

Low temperature (down to 450 °C) annealed TiAl contacts on N‐type gallium nitride characterized by differential scanning calorimetry

AbstractThis work reports on Differential Scanning Calorimetry (DSC) measurements performed on Ti‐Al metallic layers stacks deposited on n+‐GaN. The aim is to get better understanding of the mechanisms leading to ohmic contact formation during the annealing stage. Two exothermic peaks were found, one below 500 °C and the other one around 660 °C. They can be respectively attributed to Al3Ti and Al2Ti compounds formation. The locations of these peaks provide clear evidence of solid‐solid reactions. Lowest contact resistance is well correlated with the presence of Al3Ti compound, corresponding to Al(200nm)/Ti(50nm) stoichiometric ratio. Subsequently, Al(200 nm)Ti(50 nm) stacks on n+‐GaN were annealed from 400 °C to 650 °C. Specific Contact Resistivity (SCR) values stay in the mid 10‐5 Ω.cm² range for annealing temperatures between 450 °C and 650 °C. Such low‐temperature annealed contacts on n+‐GaN may open new device processing routes, simpler and cheaper, in which Ohmic and Schottky contacts are annealed together. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

FP-LAPW investigation of structural, electronic, and thermodynamic properties of Al 3V and Al 3Ti compounds

A theoretical study of structural and electronic properties of the intermetallic compounds Al 3V and Al 3Ti is presented; using the full-potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). In this approach, both the local density approximation (LDA) and the generalized gradient approximation (GGA) were used for the exchange-correlation potential calculation. The calculated total energy for both compounds shows that the distorted D0 22 structure is energetically more stable than the L1 2 structure. The electronic structure exhibits a pseudo-gap near the Fermi level. The quasi-harmonic Debye model is used to predict the temperature and pressure effects on the molecular volume, Debye temperature, heat capacity, and thermal expansion coefficient.

Nucleation Effect of Ti-6Al-4V Powder on Al–Si Eutectic Alloy

In this research nucleation effect of Ti-6Al-4 V powder on grain size and tensile strength of Al-12.1% Si eutectic alloy was studied. Metal powder produced from Ti-6Al-4 V machining chips by meshes of 50 and 140 and weight percentages of 0.1, 0.15, 0.2, and 0.25 were used. The microstructure by mesh 50 with 0.15 wt% consists of grain with average size of 6 µm, tensile strength and elongation equal to 492 MPa and 7.25%, respectively. By mesh 140 with 0.15 wt% the considered values are equal to 10 µm, 540 MPa, and 8.82%, respectively. Before refinement the needle-plate-like eutectic silicon is distributed arbitrarily. The morphology of the eutectic silicon is changed from needle-plate to a fibrous form with addition of 0.15 wt% of mesh 50 and finally changes its morphology from a needle-plate to a globular form after adding 0.15 wt% of mesh 140, which is fully modified. SEM/EDX micro-analyzing shows that TiAl3 compound has been formed, which is the major factor in grain size modification and tensile strength increasing. This phase is formed prior to solidification of Al–Si alloy makes it an efficient nucleation site for aluminum grains upon solidification.

Effects of mechanical alloying on the characteristics of a nanocrystalline Ti–50at.%Al during hot pressing consolidation

Two different powder metallurgy processes, i.e., reactive and non-reactive sintering, were investigated for the production of titanium aluminides. Ti–Al intermetallics have been successfully produced by mechanical alloying and hot pressing of powders with a nominal composition of Ti–50 at.%Al. A Ti(Al) solid solution was formed at the early stage of milling that transformed to an amorphous phase at longer milling times. On further milling, the amorphous structure transformed to a supersaturated hcp-Ti(Al) solid solution with trace amounts of TiAl after 80 h of milling, which was completely transformed to TiAl, Ti3Al, and TiAl3 intermetallic compounds after additional milling up to 100 h milling with particles of about 200 nm. Blended elemental powders and 100 h MA-ed powders were used for the reactive and non-reactive sintering processes, respectively. The results showed that the HP-ed pre-alloyed powder had better properties in terms of density, hardness, homogeneity of microstructure, yield stress, and ductility than that produced by reactive sintering. The major contribution to the yield stress of the sintered pre-alloyed powder comes from the nanometer-sized grains of intermetallics in accordance with the Hall–Petch relation.

Diffusion of Au in the Intermetallic Compound Ti3Al

The Au diffusion in the Ti3Al compound was investigated at six compositions from 25 to 35 at. pct Al by using the diffusion couples (Ti-X at. pct Al/Ti-X at. pct Al-2 at. pct Au; X = 25, 27, 29, 31, 32, and 35) at 1273 to 1423 K. The diffusion coefficients of Au in Ti3Al $$ \left( {D_{\text{Au}}^{{{\text{Ti}}_{3} {\text{Al}}}} } \right) $$ are relatively close to those of Ti. The $$ {D}_{\text{Au}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}} $$ slightly increase with Al concentration within the same order of magnitude. The activation energies of Au diffusion, $$ Q_{\text{Au}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}}, $$ evaluated from the Arrhenius plots were relatively close to those of Ti diffusion, $$ Q_{\text{Ti}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}}, $$ rather than those of Al diffusion, $$ {Q}_{\text{Al}}^{{{\text{Ti}}_{3} {\text{Al}}}} {\text{s}}; $$ therefore, it was suggested that Au atoms diffuse by the sublattice diffusion mechanism in which Au atoms substitute for Ti sites preferentially in Ti3Al and diffuse by vacancy mechanism on Ti sublattice. The influence of the D019 ordered structure (hcp base) of Ti3Al on diffusion of Au and other elements is discussed by comparing the diffusivities in Ti3Al and α-Ti.

Comparison of properties of titanium aluminium nitride film produced by using two different types of Ti–Al target

AbstractThis work aims to study the influence of two different types of Ti–Al target (sintered and melted) which have same nominal composition (50 at% Ti, 50 at% Al) on the properties of thin films coated by using arc ion plating (AIP) method. The hardness of the melted target was of higher value than that of the sintered target. The property of the film is related to the microstructure and phase analysis of both types of target that have quite different phases. Ti and Al metals were found as major phases and TiAl2 as minor phase in the sintered target but only TiAl compound was found in the melted target. After film coating at the same operation parameters, it was found that although the phase structures of the sintered and melted targets were quite different, the coated thin films using these targets showed the same phase structure of Ti0.5 Al0.5 N. However, the microstructure of films coated by using the sintered target showed higher density of droplets and pores than those of film produced by using the melted target. As a result, the hardness and adhesion strength of thin film produced by using the melted target were slightly higher than those values of film produced by using the sintered target. Copyright © 2009 John Wiley & Sons, Ltd.

Structural transformations in the deuterium-containing intermetallic compound (Ti3Al)D1.2 induced by high-pressure torsion

The intermetallic compound Ti3Al (which is a very brittle material) with a high deuterium concentration (x = 1.2) is fabricated in a monolithic state by high-pressure torsion at room temperature. The structure of the intermetallic samples is studied by X-ray diffraction and transmission electron microscopy after deformation at various degrees. Under certain conditions, the main volume of the material is found to transform into an amorphous state, and areas 1–2 nm in size with an atomic arrangement close to the initial arrangement are also present in the material. The possible causes of the deformation-induced amorphization of the material alloyed with interstitial atoms are discussed.

EFFECT OF AL CONTENT ON MICROSTRUCTURE AND PROPERTIES OF AN INTERMETALLIC Ni-Ti (Al) COMPOUND/Ni GRADED COATING DEPOSITED ON COPPER SUBSTRATE

Copper and its alloys with high electrical and thermal conductivity are a group of widely used engineering materials in numerous applications. In order to improve the tribological properties of copper substrate, an electroplating nickel layer was firstly deposited on copper substrate, subsequently these electroplated specimens were treated by slurry pack cementation process at 900°C for 12 h using a slurry mixture composed of TiO 2 as titanizing source, pure Al powder as aluminzing source and also a reducer for titanizing, an activator of NH 4 Cl and albumen (egg white) as cohesive agent. The effect of Al content on the microstructure and the properties of the coating has been studied. The results showed that an intermetallic Ni - Ti ( Al ) compound/ Ni graded layer was formed on copper substrate after slurry pack cementation process. With the rise of Al content in slurry mixture, the microhardness of the graded coating increased and the friction coefficient decreased from 0.35 to 0.18, at the same time, the slurry pack process gradually transited from the titanizing process to an aluminizing one. Correspondingly the main phases of the coating were changed from Ni - Ti intermetallic compounds into Ni - Al ones.

Catalytic effect of titanium nitride nanopowder on hydrogen desorption properties of NaAlH4 and its stability in NaAlH4

Single crystalline titanium nitride (TiN) nanopowder is synthesized by a mechano-chemical reaction between titanium chloride (TiCl3) and lithium nitride (Li3N) by means of high-energy ball milling. The TiN nanopowder has an average particle size of 6 nm and is introduced into sodium alanate (NaAlH4) as a catalyst. During hydrogen sorption cycles, TiN-catalyzed NaAlH4 exhibits a greater hydrogen desorption rate and higher hydrogen capacity than TiCl3-catalyzed NaAlH4. Contradicting thermodynamic predictions, in situ X-ray diffraction results reveal that TiN nanopowder remains stable and produces no by-products (e.g., Ti–Al compounds) in the reaction with NaAlH4 during hydrogen desorption. In situ Raman spectroscopy also confirms the stability of TiN nanopowder in NaAlH4. This implies that the sustained hydrogen sorption kinetics and hydrogen capacity of TiN-catalyzed NaAlH4 originate from the structural and chemical stability of TiN nanopowder in NaAlH4 for the given conditions of the hydrogen cycle test.

Elimination of inclusions and evaporation control during melting of Ti-48Al-2Cr-2Nb-1B intermetallic alloy

Titanium aluminide intermetallic compounds have an excellent capability for use in engineering structures at high temperatures. In the present work the formation of Nb rich inclusions in microstructure and evaporation of Al during melting of γ-TiAl based alloy (Ti-48Al-2Cr-2Nb-1B (at %)) was studied. The results show that the inclusions cannot be removed even with a four-stage melting process, when elemental Nb is used as raw material. However, by replacing Nb with NbAl3 and using a three-stage melting process, the inclusions were removed from microstructure and also evaporation of Al was reduced remarkably. Otherwise, with removing elemental Al from raw material by using TiAl compound, evaporation of Al will be very low. Increasing vessel pressure from 400 to 600 mbar will not influence evaporation of Al.

Fabrication of high wear resistant Al/Al 3Ti metal matrix composite by in situ hot press method

In this study, hot press method has been used to fabricate Al/Al 3Ti composite. In situ reacted Al 3Ti compound is formed through Al and TiO 2 powder blend in order to enhance mechanical properties and mainly wear resistance. After homogeneous blending of Al and TiO 2, hot pressing was performed at 580 °C at different durations and pressures. Scanning electron microscopy was used for microstructural survey. Density measurements using Archmede’s method and Brinell hardness test were carried out for all the samples. By increasing the hot pressing pressure, both density and hardness increased significantly. On the other hand, by increasing the time, hardness increased and density decreased due to the formation of Al 3Ti. Samples produced at higher pressures and longer durations had higher wear resistance at longer wear distances. The three observed dominant types of wear mechanisms were abrasive, delamination, and adhesive.

Hard amorphous Ti–Al–N coatings deposited by sputtering

Ti–Al–N coatings were deposited by direct current reactive magnetron sputtering using two titanium and two aluminum targets. Two series of films with Al/(Al + Ti) atomic ratios of ≈ 23.5 and ≈ 34.5% were studied. The amount of nitrogen in the films was varied from 0 to 44at.%. The incorporation of N atoms led to a change of the α-Ti lattice preferential orientation from <100> to <001>, a decrease in the degree of crystallinity, and subsequently to the collapse of the crystalline structure. Annealing at 975K promotes the formation of the Ti 3Al compound. The hardness increases smoothly with the nitrogen content. The high hardness values (31 and 41GPa) measured for the films with the highest N contents may be explained by the deposition of a nanocomposite phase. For the Ti–Al–N film deposited with Al/(Al + Ti) atomic ratio of 34.5% the α-Ti structure was completely transformed to TiO 2 upon oxidation. The high oxidation resistance of the film deposited with 44at.% N at 1075K is characteristic of Ti–Al–N films.

Effect of niobium and aluminium additions in TiAl prealloyed powders during high-energy ball milling

Mechanical alloying (MA) of TiAl/TiAl– xNb– yAl–1Cr ( x = 4, 8 and 12, y = 2.5, 4 and 6.5 in at.%) was carried out with optimized ball milling parameters. MA of prealloyed TiAl powder resulted nanocrystalline structure at early stages of MA and sustained up to 50 h. TiAl–Nb–Al–Cr systems exhibited amorphous structure in lower Nb and Al content and formation of Nb 2Al nanocrystalline compound with increasing Nb and Al additions. Stability of TiAl covalent bonded intermetallic compound was weakened by dissolution of Nb in the matrix and resulted amorphous structure. Formation of brittle Nb 2Al and presence of TiAl compounds favoured fracturing during MA which resulted particle refinement to submicron size and spherical morphology. DTA scans exhibit no phase transformation in nanocrystalline TiAl and nanointermetallic composites, but amorphous powder exhibits a sharp exothermic peak.