Titanium Aluminum Alloy Research Articles

Hydrogen induced microstructural changes in Al-Ti alloys

Among the family of transition metal high temperature alloys, Al-Ti alloys are actively being studied as a result of their attractive combination of elevated temperature properties. These materials derive their excellent strength, ductility, and creep resistance from their refined microstructure and dispersion of Al{sub 3}Ti particulates, in combination with the low solid solubility and low volume diffusivity of Ti in Al. Thermochemical Processing (TCP), or the use of hydrogen at high temperature as a temporary alloying element in titanium alloys, can significantly enhance processability and refine the final microstructure. There are no reports of the transformation of a crystalline structure into a nanocrystalline one by hydrogen charging with high fugacities at room temperature. The objective of this study is to show that hydrogen charging an aluminum titanium alloy with very high fugacities produces a very refined microstructure and may result in a microcrystalline to nanocrystalline transformation.

Ｔｉ‐５０ａｔ％Ａｌ合金の高温における析出相に関する特性

Titanium-aluminum alloys are favorable as superb light-weight and heat-resistant structure materials, and is also expected as structure material of a part of body of round-trip spaceships. We measured states of structures by investigating electric resistance and X-ray diffraction pattern. The hardness was measured in contrast with these results.

Simple Evaluation of Superplasticity Using a Tensile Test with R-type Specimens

Superplastic elongation depends heavily on a value of strain-rate sensitivity index m. The value of m is obtained by various methods, using specimens of various shapes for uniaxial tensile tests. A special technique is required to measure the m-value. In this study, R-type specimens having no smoothed region were employed instead of ordinary smoothed specimens. A series of tests for various aluminum alloy sheets and titanium alloy bars were carried out at elevated temperatures to evaluate superplasticity. It was difficult to measure the m-value using a single R-type specimen. However, the obtained value using more than three specimens was in fairly good agreement with the value of smoothed specimen. Furthermore, the total elongation of the gauge length of mm at the bottom of the R-part could replace that of the smoothed test-pieces. The obtained value was less scattered in comparison with the value of the smoothed specimens. Consequently, R-type specimen can be useful for simple evaluation of superplasticity.

Comparative study of the initial oxidation behavior of a series of titanium–aluminum alloys

Auger electron spectroscopy was used to investigate the adsorption of molecular oxygen and the growth of the initial surface oxide layer on a series of titanium–aluminum alloys. The aluminum content of the alloys varied from zero to 75 at%. The uptake kinetics, the reaction sequence of the adsorbed oxygen, the oxidation state of the metallic surface species, and the initial surface composition of each alloy were obtained from the data. All measurements were performed at room temperature (about 295 K).

On the pockets of hydrostatic pressure predicted by simulations for the plastic deformation and fracture of (alpha + gamma) titanium-aluminium alloys

Kad et al . and Dao et al . have used room-temperature flow stress and workhardening data to model the plastic deformation and fracture of two-phase (alpha+ gamma) titanium-aluminium alloys deformed to 20% strain. Here, it is suggested that their predicted pockets of hydrostatic pressure ought to give rise to X-ray line broadening and, in thin sections cut from bulk forgings, to mechanical relaxation. However, to achieve upsets of 20%, it has proved necessary to carry out the deformation at a high temperature. Subsequent examination at room temperature reveals no line broadening and no mechanical relaxation.

Effect of niobium additions on CO2-enhanced oxidation of titanium-aluminum intermetallic alloys

In a recent study, CO2 has been reported to enhance the oxidation rate of binary titanium-aluminum alloys. The detrimental effect of CO2 was not, however, observed in a ternary alloy containing niobium. In this paper, possible explanations for these observations are examined. First, results from the literature regarding the effects of niobium in improving the resistance of titanium-aluminum alloys are briefly reviewed. Second, a thermodynamic analysis which offers a possible explanation for the beneficial effect of niobium in eliminating the CO2-enhanced oxidation of titanium-aluminum alloys is presented.

SANDVIK Ti-3Al-2.5V GRADE 9

Abstract Sandvik Ti-3Al-2.5V Grade 9 titanium-aluminum alloy offers excellent corrosion resistance, especially to chloride media, and has a high strength-to-weight ratio, which is especially suitable for use in aerospace applications. Tubing can be produced having a CSR (contractile strain ratio) that enhances the fatigue endurance limit. This datasheet provides information on composition, physical properties, elasticity, tensile properties, and bend strength as well as fatigue. It also includes information on corrosion resistance as well as forming, machining, and joining. Filing Code: TI-109. Producer or source: Sandvik.

Focus control system for laser welding

We describe a focus control system for Nd:YAG laser welding based on an optical sensor incorporated into the fiber delivery system to detect light generated by the process. This broadband light is separated into two wavelength bands, and simple electronic processing gives a signal proportional to focal error as a result of chromatic aberrations in the optical delivery system. Focus control is demonstrated for bead-on-plate welds in different thicknesses of titanium alloy, aluminum alloy, mild steel, and stainless steel. The control system works for both pulsed and continuous laser radiation.

Ultrahigh speed imaging system for the measurement of solidification velocity via direct observation of rapid solidification

An ultrahigh speed imaging system has been developed and built to image rapid solidification of materials. The system collects data from a 10 by 10 array of parallel tapped photodiodes. Up to 8192, frames of data are digitized with 8-bit flash converters at frame rates ranging from 500 to 1 000 000 frames per second. The system has been used to image solidification of a gamma titanium aluminum (γ-TiAl) alloy, with composition Ti-60a/o Al, over a range of undercoolings. The data collected includes the first images of solidification from multiple solidification fronts, images of solidification of a disordered phase followed by the alloy ordering, and quantitative measurement of solidification velocity as a function of undercooling in a γ-TiAl alloy.

Thermodynamics of calcium and oxygen in molten titanium and titanium-aluminum alloy

The deoxidation equilibrium of molten titanium and titanium-aluminum alloys saturated with solid CaO has been measured in the temperature range from 1823 to 2023 K. The equilibrium constant of reaction CaO (s)=Ca (mass pct in Ti,Ti-Al)+O (mass pct in Ti,Ti-Al) and the interaction parameter between calcium and oxygen were determined for Ti, TiAl, and TiAl3. The standard Gibbs energy of reaction for TiAl was obtained as follows: $$\Delta G^\circ = 279,000 - 103TJ/mol$$ The possibilities for the deoxidation of titanium and titanium-aluminum alloys by using calcium-based fluxes are discussed.

Prediction of the pore-closing kinetics during the consolidation stage of SiC unidirectional long fibers: titanuum alloy matrix composites

Titanium-aluminium alloy matrix composites reinforced by long SiC fibers are considered as potential replacements for the conventional alloys used in aerospace applications for higher temperature service. A direct elaboration route of such composites is the hot uniaxial pressing of an assembly of metal foils stacked between uniaxial fiber mats. The kinetics of pore closing are studied through a finite-element simulation, the pressure boundary conditions, rigid-fiber rheology, sticking friction between the matrix and the fibers and a two-domain rheological constitutive equation (modelled by two Norton-Hoff laws) corresponding to both superplastic and high strain-rate behaviour ranges for the Ti-6Al-4V alloy, being considered. The influence of the distance between the fibers for two types of arrangement (rectangular and triangular) is studied. The effects on the kinetics of pore closing of both consolidation pressure and rheological laws are discussed also.

Texture of titanium self-aligned silicide (salicide)

The conventional self-aligned silicide (salicide) in IC manufacturing is made by depositing a thin (10--50 nm) titanium film on silicon wafer by physical vapor deposition (PVD) plus two rapid thermal annealing (RTA) to induce reaction between titanium and substrate. To make a via plug titanium nitride and aluminum alloy are deposited successively on the top of salicide to draw the electric current out of source or drain to interconnects. The reliability of a TiSi{sub 2}-TiN-Al tri-layer is an important issue. In particular the electromigration resistance of the aluminum alloy is heavily effected by its texture. Since the top layer nucleates on the under layer the texture of latter might influence that of the former. Therefore the texture of the aluminum alloy is expected to be correlated with that of the underlying silicide. Then, for the reliability of devices using salicide, the study of silicides` texture seems relevant to process control in wafer fabrication.

Corrosion behavior of single phase titanium-aluminum alloy in an impure helium environment

The corrosion behavior of TiAl alloy consisting of a single γ phase was examined at 1373 K for up to 3.6 Ms in an impure He gas environment (He-2′). During the test the alloy was oxidized and carburized and three kinds of corrosion products of Al 2O 3, Ti 2AlC, and Ti 3Al were formed on the specimen surface. Ti 3Al was formed also along the grain or twin boundaries in the specimen interior. The oxidation rate of the alloy followed a parabolic rate law. Its rate constant was much smaller than that of a NiCrW alloy and was comparable to that of a Ni-based ODS alloy. Due to lack of the thermodynamic data on Ti 2AlC, Ti 3Al, etc., it was impossible to construct a perfect phase stability diagram of the constituent elements of the alloy. Evaluations based on the calculation revealed that after 2.52 Ms exposure time, Ti 2AlC diminished accompanying the formation of Al 2O 3 and Ti 3Al owing to oxidation and decarburization.

Influence of substrate roughness and temperature on the adhesion/cohesion of alumina coatings

This article is devoted to the study of the adhesion/cohesion (A/C) of alumina coatings sprayed on four different substrates: aluminium alloy, titanium alloy, cast iron and mild steel. Fused and crushed alumina particles (−45 + 22 μm) were sprayed with a custom-made torch with a nozzle of internal diameter 7 mm working with an arc current of 600 A and a plasma gas mixture of 45 slm Ar, 15 slm H 2, which resulted in a voltage of 64 V and a thermal efficiency of 52%. The particles were injected internally and were almost fully molten (less than 3% α-phase in the collected powders after their passage in the plasma jet). The influence of substrate roughness and temperature before (preheating), during and after spraying was systematically studied. R a values were 6–8, 10–13 and 14–21 μm and the substrates were preheated to temperatures T p of 170, 200, 300, 350, 380 and 500 °C. The most important results were the following: 1. • for cold substrates A/C increased with R a up to a maximum of 20 MPa 2. • for substrates whose expansion mismatch with alumina was less than 4.10 −6 K, the highest A/C values (50–60 MPa) were obtained with T p between 350 and 500 °C provided the substrates were not oxidized 3. • at these temperatures the highest values of A/C were obtained with the lowest R a 4. • when particle velocity and surface temperature upon impact decreased, for example by load effect, A/C values decreased too

Identification of a new phase formed during the oxidation of γ-tttanium aluminum

In a recent study of {gamma}-titanium aluminum alloys, a new phase depleted in Al was formed underneath the oxide scale during the high temperature oxidation process. The selected area electron diffraction (SAD) patterns from this phase are inconsistent with any known phases in the binary TiAl and ternary TiAlX (X = O,N) systems. This paper concentrates on the phase identification and symmetry determination of this X-phase in the aluminum depleted layer.

Anomalous high temperature strengthening in concentrated α titanium-aluminum alloys

Tensile properties of Ti-[7 ~ 13] mol%Al have been studied at 1100 K. The steady-state deformation behavior was analyzed by a Dorn-type creep equation. Both the strain rate and the stress at steady state showed the anomalous strengthening behavior in comparison with the values expected from the deformation mechanism in Ti-[5–14] mol%Al. The anomalous high temperature strengthening may be attributable to a local chemical ordering.

Isotope effect in the diffusion of hydrogen and deuterium in titanium, Ti88Al12 and Ti3Al

Diffusion coefficients of hydrogen and deuterium in titanium, titanium–aluminium alloy Ti88Al12 and intermetallic Ti3Al have been measured in the temperature range 873–1298 K. The activation energy for diffusion has been found to be almost the same for hydrogen and deuterium and to increase, as the aluminium content in the metals increases, from ca. 0.44 eV for titanium to ca. 0.85 eV for Ti3Al. The ratio of the measured diffusion coefficient of deuterium to that of hydrogen also increases from the classical value 1/√2 for titanium to ca. 0.8 for Ti3Al. An application of the classical rate theory to the measured ratio shows that the hydrogen atom at the saddle point of the diffusion path is more tightly bound to the metal atoms and has a larger vibration energy in Ti3Al than in titanium.

Synthesis of amorphous and metastable titanium-aluminium alloys by mechanical alloying : W. Guo et al J. Mater. Science, vol 29, no 9, 1994, 2436–2444

The effect of waxes on the injection molding of carbonyl iron powder with low-density polyethylene (LDPE) have been examined. Four waxes, paraffin wax (PW), polyethylene wax (PEW), carnauba wax (CW) and acrawax (AW) were selected and compared. Experimental results indicated that the LDPErich phase was found to be separated from the wax-rich phase in the binder mixture containing AW, CW or PW. Polar AW or CW showed stronger interactions or adsorptions with iron powder than other nonpolar waxes. AW/LDPE and CW/LDPE mixtures appeared to exhibit higher viscosity, greater pseudoplasticity and lower flow activation energy than PW/LDPE and PEW/LDPE mixtures. Tensile bar specimens of all binder mixtures could be injection molded at an injection temperature of around 170–190 °C. and a pressure of around 5–6 MPa. Except for the PEW/LDPE mixture the parts derived from the other three binder mixtures showed good surface appearance following debinding and sintering. Specimens based on the PW/LDPE mixture exhibited the highest tensile strength in the resulting sintered parts, followed those based on CW/LDPE, AW/LDPE, and PEW/LDPE mixtures. The difference in tensile strength of each part results from different flow properties of each wax/LDPE mixtureL'effet des cires sur le moulage par injection de la la poudre de fer carbonylé avec le polyéthylène à faible densité (LDPE) a été examiné. Quatre cires, soit la cire paraffine (PW), la cire de polyethylene (PEW), la cire de carnauba (CW) et la cire acra (AW) ont été sélectionnées et comparées. Les résultats expérimentaux ont indiqué que la phase riche en LDPE était séparée de la phase riche en cire dans le mélange de liant contentant le AW, le CW ou le PW. Le AW ou le CW, qui sont polaires, ont montre des interactions ou des adsorptions avec la poudre de fer plus fortes que les autres cires non-polaires. Les mélanges de AW/LDPE et de CW/LDPE ont montré une plus haute viscosité, une plus grande pseudoplasticité et une activation d'énergie d'écoulement plus basse que les mélanges de PW/LDPE et de PEW/LDPE. Les éprouvettes d'essai à la traction de tous les mélanges de liant ont pu être moulées par injection à des températures d'injection de 170–190 °C et une pression de 5–6 MPa. Sauf pour le PEW/LDPE, les pièces dérivées des trois autres mélanges de liant ont montré une bonne apparence de la surface après l'enlèvement des liants et le frittage. Les spécimens basés sur le mélange de PW/LDPE ont montré la plus haute résistance à la traction parmi les pièces frittées qui en ont résulté, suivi par ceux basés sur les mélanges de CW/LDPE, de AW/LDPE et de PEW/LDPE. La différence de résistance à la traction de chaque pièce découle de propriétés d'écoulement différentes pour chaque mélange de cire/LDPE

Mechanical alloying of ternary titanium-aluminium alloys : J.H. Ahn, H. Chung (Korea Inst of Machinery and Metals, Changwon, Korea)

Mechanical alloying of ternary titanium-aluminium alloys : J.H. Ahn, H. Chung (Korea Inst of Machinery and Metals, Changwon, Korea)

Minimum-mass design of sandwich aerobrakes for a lunar transfer vehicle

A structural mass optimization study of a sandwich aerobrake for a lunar transfer vehicle was conducted. The proposed spherical aerobrake had a base diameter of 15.2 m and radius of 13.6 m. A hot thermal protection system (TPS) and cold structure were used in the design. Honeycomb sandwich aerobrake structures made up of four different materials—aluminum alloy, titanium alloy, graphite-epoxy, and graphite-polyimide—were considered. Cases of aerodynamic load, equivalent uniform pressure, and aerodynamic plus thermal load were analyzed. Both linear stress and buckling analyses were conducted for a range of skin and core thicknesses. A graphical optimization procedure was used to determine the skin and core thicknesses of a minimum-mass aerobrake. The design criteria used were material strength, global buckling, and TPS tile deformation. Among them, the TPS deformation criterion was the most critical. The graphite-epoxy aerobrake was the lightest among the four materials studied. Its total mass is about 12.3% of the LTV mass, for supports at 75% span. Equivalent uniform loading produced smaller deformations, stresses, and buckling loads than did the more realistic aerodynamic loading for the same aerobrake configuration. Thermally induced stresses countered the aerodynamically induced stresses and hence had a beneficial effect on the deformation and buckling of the aerobrake.