Ti 24Al 11Nb Research Articles

Cyclic oxidation behaviour of Ti 3Al-based alloy with Ni–Cr protective layer

The influence of a thin 80Ni–20Cr (at.%) protective coating on the cyclic oxidation of a Ti–24Al–11Nb (at.%) alloy based on Ti 3Al at 600 and 900 °C in air was investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The results of the oxidation tests showed that deposited Ni–Cr layer provides an improved oxidation resistance due to the formation of protective oxide scale which barriers the outward Ti diffusion into the scale. In some extent surface formation of the nitride layer also prevents diffusion of alloying elements from the matrix. Although oxidation at 900 °C is faster than that at 600 °C, a remarkable reduction in mass gain of the alloy with protective coating was observed. The thickness of oxide scale on the coated samples is approximately two times less than that formed on the uncoated samples treated under the same exposure conditions (120 h).

Texture evolution during β→O→α2 and β→α2 phase transformations in a Ti3Al–Nb alloy

The alloy Ti–24Al–11Nb was hot rolled in the β phase field at 1523 K by 80% and water quenched. The material was then aged at 773 K, 873 K and 1173 K for various lengths of time leading to the formation of the O phase and the α2 phase, either directly from the β or via β to O phase transformation. The O phase shows a “tweed” microstructure while the transformed α2 phase displays the typical plate like features. The texture of the α2 phase whether formed directly or indirectly from the β phase, due to a final ageing treatment at 1173 K, is rather strong and non-basal type. By contrast, the α2 texture of the 873 K aged material is weak and again non-basal type, but it differs significantly from the texture of the 1173 K aged material.

Effect of moderate cold rolling on stability of texture in an (α 2+β) Ti 3Al-base alloy

The present study reports textural changes that occur during final cold rolling stage of a thermomechanically processed Ti 3Al base alloy as a function of rolling reduction. Textural changes have been examined for 15 and 30% cold rolling reductions. The cold rolling deformation ∼30% of a prior thermomechanically processed Ti–24Al–11Nb alloy, with initial R-type basal texture, does not change its basic character. The main texture components are found to be of (01 1 ̄ 7] [uvtw] type. No twinning is observed up to 30% rolling deformation.

Response of Ti–6Al–4V and Ti–24Al–11Nb alloys to dry sliding wear against hardened steel

Titanium alloys have been of great interest in recent years because of their very attractive combination of high strength, low density and corrosion resistance. Application of these alloys in areas where wear resistance is also of importance calls for thorough investigations of their tribological properties. In this work, Ti–6Al–4V and Ti–24Al–11Nb alloys were subjected to dry sliding wear against hardened-steel counter bodies and their tribological response was investigated. A pin-on-disc type apparatus was used with a normal load of 15–45N and sliding speed of 1.88 ms −1. In the steady state, it was demonstrated that Ti–24Al–11Nb had a substantially higher wear resistance (about 48 times) than that of the Ti–6Al–4V alloy tested under a normal load of 45 N. Severe delamination is found to be responsible for the low wear resistance of Ti-6Al-4V. In the case of Ti–24Al–11Nb, two wear mechanisms have been suggested: delamination with a lower degree of severity and oxidative wear. It is thought that the ability of Ti–24Al–11Nb to form a protective oxide layer during wear results in a much lower wear rate in this alloy.

Hydrogen evolution from cathodically charged titanium aluminide alloy Ti–24Al–11Nb

A Ti 3Al-based titanium aluminide alloy, Ti–24Al–11Nb, was cathodically charged with hydrogen in a 5% H 2SO 4 aqueous solution for various charging times, and the formation and dissociation of the hydride, the hydrogen evolution behavior and the total hydrogen uptake were investigated mainly by means of X-ray diffractometry and thermal desorption spectroscopy (TDS). The same kind of hydride phase as observed previously in Ti–25Al alloy (hexagonal hydride) was presumably formed in the Ti–24Al–11Nb alloy after cathodic charging. No damage, such as cracks, was induced by hydrogen charging. Two kinds of TDS peaks, one probably corresponding to hydride dissociation and the other to hydrogen dissolution in the normal lattice site, were found after longer hydrogen charging. It is suggested that niobium addition to Ti 3Al-based titanium aluminide alloy may reduce hydrogen susceptibility during cathodic charging.

Oxidation behavior of titanium aluminides of high niobium content

The oxidation behaviour of two titanium aluminides of composition Ti–24Al–20Nb and Ti–24Al–27Nb were studied in oxygen in the range 1125–1325 K. The parabolic rate constants of the alloys in the present investigation were higher than that of titanium aluminides of lower Nb content [Ti–24Al–11Nb and Ti–24Al–15Nb (Roy TK, Balasubramaniam R, Ghosh A. Metall Mater Trans A 1996;27A:3993)]. The major constituent of the oxides was TiO 2 while Nb 2O 5 and Al 2O 3 were the minor constituents. The formation of Nb 2O 5 as a separate phase is possibly related to the lower oxidation resistance of these alloys compared to titanium aluminides of lower Nb content.

Evolution of hot rolling textures in a two-phase ( α2+ β) Ti 3Al base alloy

Texture development during thermomechanical processing of two-phase ( α 2+ β) Ti–24Al–11Nb alloy was studied as a function of variables like initial microstructure, rolling temperature, cooling conditions, etc. The evolution of texture in different conditions has been critically analysed. It has been found that unrestricted rolling of primary α 2 at lower temperature leads to a good basal {0001}〈 uvtw〉 texture, while at higher temperatures, the α 2→ β→ α 2 phase transformation leads to weakening of the basal texture. Texture of secondary α 2 derived from rolled β is generally non-basal. However, the texture of secondary α 2 derived from recrystallized β has a basal character.

Microscopic observation of superplastic deformation in a 2-phase Ti3Al–Nb alloy

Abstract Microscopic aspects of superplastic deformation in a 2-phase Ti 3 Al–Nb alloy were studied using transmission electron microscopy. A 5-kW radiant heating system capable of heating up 200°C per min was used to minimize possible microstructural changes during heating and cooling stages. A Ti–24Al–11Nb alloy with the grain size of 3.6 μm showed the maximum elongation of 1280% at 970°C under an initial strain rate of 10 −3 /s, which is the largest elongation ever reported in open literature. Localized dislocation activity was observed mainly along α 2 /α 2 grain boundaries and/or α 2 /β phase boundaries without noticeable dislocation activity inside the α 2 grains of the superplastically deformed specimen. Severe deformation was also observed in the soft β phase. Adjacent α 2 grains, however, only contained some dislocations that were piled up at the region near the triple junctions. The observations on dislocation activities suggest that boundary sliding plays a major role in superplastic deformation of this alloy, together with a dominant accommodation mechanism via dislocation motion inside β and α 2 grains. The effects of strain rate and grain size on deformed microstructure were also investigated, and finally, a possible superplastic deformation mechanism of 2-phase Ti 3 Al–Nb alloy is proposed.

Microstructural stability and mechanisms of fatigue and creep crack growth in Ti–24Al–11Nb

Titanium intermetallics are being developed for long term applications at elevated temperatures, particularly those alloys based on the alloys Ti3Al and TiAl. Typical approaches include the design of appropriate microstructures for room and elevated temperature fatigue and creep resistance. However, a little explored area is the stability of these microstructures at elevated temperature and its effect on fatigue crack growth. The present investigation documented the microstructural stability, fatigue crack behaviour, and stress rupture of Ti-24Al-11Nb, a Nb modified Ti3Al alloy. A coarse two phase α2+β Widmanstatten microstructure was found to exhibit the best resistance to fatigue crack growth. Microstructural stability and elemental segregation were studied as a function of exposure time for up to 500 h at 800°C using transmission electron microscopy (TEM). Results indicate that the Widmanstatten microstructure is metastable and the β phase breaks up into particles. The absence of a continuous β phase surrounding the α2 phase reduces the resistance of the microstructure to fatigue crack growth at room temperature. At elevated temperature the microstructure stability does not play a role in determining the fatigue resistance. A fine Widmanstatten microstructure has the best resistance to creep deformation. Stress rupture tests were conducted in vacuum and air at 649°C and 760°C. Two types of failure mechanisms were seen in stress rupture; these include transgranular and intergranular failure within prior β grains. When tested in air at 760°C a combination of transgranular and intergranular failure occurred. Specimens that exhibited a higher proportion of transgranular failure had longer lives. When tested in vacuum at 760°C the predominant failure mode was intergranular. At 760°C extensive microstructural changes like breakup and spherodization of the β phase occurred under stress while the rate of coarsening without any stress was much slower. At 649°C the specimens tested in vacuum consistently exhibited longer lives. The creep crack growth when tested in air at 649°C was always a brittle transgranular mode while the specimens tested in vacuum always failed by an intergranular mode.

In-situ TEM observation of dissolution-enhanced dislocation emission, motion and the nucleation of SCC for Ti24Al11Nb alloy in methanol

Many experiments have showed that anodic polarization can facilitate ambient creep for various metals and alloys. Anodic polarization can decrease the yield strength, and enlarge the plastic zone on the surface ahead of a loaded crack tip. The density of dislocations in a region very close to the fracture surface of SCC was much higher than that far away from the fracture surface. All of these experiments showed that anodic dissolution facilitated the localized plastic deformation. Up to now, however, direct proof of anodic dissolution or corrosion-enhanced dislocation emission, multiplication and motion is lacking. Using a special constant deflection device, the dislocation configuration change ahead of a loaded crack tip before and after anodic dissolution together with the initiation of SCC for Ti-24Al-11Nb alloy in methanol can be in-situ observed in TEM. The results showed that the localized anodic dissolution could facilitate dislocation emission, multiplication and motion and SCC with size of nanometers would initiate in the dislocation-free zone (DFZ) or at the original crack tip after the dissolution-enhanced dislocation emission and motion reached a critical condition.

Validation of computer models for the consolidation of metal—matrix composites

Finite element method (FEM) simulation results were compared to experimental observations to establish the suitability of advanced modeling techniques for the prediction of the consolidation behavior of continuous fiber, metal—matrix composites. Two consolidation techniques were examined: hot isostatic pressing (HIP) of foil—fiber—foil layups and HIP of tapecast monotapes. In both cases, the matrix was the alpha-two titanium aluminide alloy Ti—24Al—11Nb (a/o), and the fibers were silicon carbide. Model predictions and accompanying experimental measurements revealed the important effect of the interface friction—shear factor on consolidation time for foil—fiber—foil layups. In addition, the predicted consolidation times for the foil—fiber—foil method were found to be sensitive to small variations in HIP temperature and material flow properties such as the strain-rate sensitivity, especially for low consolidation temperatures. By contrast, predicted consolidation times for tapecast monotape layups were relatively insensitive to the magnitude of the interface friction—shear factor. The kinetics of densification of the tapecast monotapes were well described using an FEM model incorporating a material-sensitive yield function and associated flow rule.

On the increase in young's modulus of Ti24Al11Nb alloy as a result of hold time during low cycle fatigue at 650 °C

Titanium aluminide Ti[sub 3]Al based Ti-24Al-11Nb alloy has been the subject of considerable interest for high temperature aerospace applications in advanced systems. Although a number of studies on the mechanical properties are reported, only a limited amount is known on fatigue behavior, particularly regarding hold-time effects on the high temperature low cycle fatigue (LCF). Here the authors report the observations made on the variation of Young's modulus during the investigation carried out to evaluate hold-time effects on the LCF of Ti-24Al-11Nb at 650 C.

Temperature dependence of interfacial shear strength inSCS-6 fibre reinforced Ti–24Al–11 Nb metal matrix composites

Experiments were carried out to study the temperature dependence of the interfacial shear strength in SCS-6 fibre reinforced Ti–24Al–11Nb (at.-%) composites. The fibre pushout technique was used with a diamond indenter and a high temperature microhardness tester. The interfacial shear strength was found to decrease with increasing temperature.MST/1893

An investigation of the effects of microstructure on the fatigue and fracture behavior of α2 + β forged Ti24Al11Nb : Soboyejo, W.O. Metall. Trans. A June 1992 23A (6) 1737–1750

We propose a surface plasmon resonance (SPR) sensor based on a single mode optical fiber with six air holes. A thin gold film and a TiO2 film are deposited on the walls of air holes. Due to high refractive index of TiO2 the proposed sensor can operate at near-infrared (IR) wavelengths. The characteristics of the sensor were numerically investigated based on the finite-element method (FEM). The numerical results indicate that the optical loss spectrum of SPR sensor can be tuned easily by changing the thicknesses of gold and TiO2 layers. The refractive-index resolution of 2.7×10−5 (sensitivity S≈370/RIU) for aqueous analytes can be achieved.

Microstructure and crack-shape effects on the growth behavior of small fatigue cracks in Ti24Al11Nb

The effects of four different microstructures in the titanium aluminide alloy Ti24Al11Nb (at.%) on the fatigue crack growth behavior of small surface cracks and large cracks have been investigated. The four microstructures were a Widmanstätten basketweave, a Widmanstätten aligned colony, an equiaxed primary α 2 in a Widmanstätten matrix and a completely equiaxed α 2 structure. Small cracks were found to develop arbitrary shapes owing to the effects of microstructure. The crack shapes (aspect ratios) were measured using a laser interferometric and photomicroscopic system, and these measurements allowed accurate calculation of crack growth rates at the surface position as well as at the depth position for the surface cracks. After accounting for the continuous variation in crack shape in crack growth rate calculations, the trends in small-crack growth rates agreed reasonably well with the corresponding large-crack growth rates. While the crack growth rates at depth positions for small cracks correlated well with large-crack data in the basketweave microstructure, crack growth rates at surface positions correlated well with the corresponding large-crack data in the other microstructures. The microstructural factors that may be responsible for this behavior are discussed.

Frequency and hold time effects on crack growth of Ti24Al11Nb at high temperature

An investigation has been conducted to evaluate the effects of cyclic frequency and hold times on crack growth in a representative α 2 titanium aluminide, Ti24Al11Nb at 49 °C. Fatigue crack propagation experiments conducted over a wide range of frequencies from 0.01 to 100 Hz indicate a strong dependence of d a/d N upon cyclic loading frequency Superposition of hold time at maximum load has a deceleration effect of the growth rate per cycle whereas superposition of a hold time at minimum load results in a higher crack growth rate which is attributed to enhanced environmental degradation. It is postulated that the hold time at maximum load results in an interaction of the environmental effect with a retardation effect due to the crack tip blunting as a consequence of creep under maximum applied load. The frequency and hold time behavior is rationalized in terms of a complex interaction between fatigue, blunting owing to creep and environmental degradation through a simple model which considers the interaction effects.

Elevated temperature fatigue behavior of SCS-6/Ti24Al11Nb

An investigation has been conducted to determine the effects of stress ratio, frequency and hold time on the elevated temperature fatigue behavior of SCS-6/Ti24Al11Nb. Load-controlled tests conducted at 650°C, and two stress ratios, 0.1 and 0.5, are compared at a frequency of 3 Hz. The baseline tests at 3 Hz are compared with data at three other frequencies: 30, 0.0083 and 0.00278 Hz. The effects of superimposed hold times were also investigated at the lowest frequency, 0.00278 Hz. Cycles to failure are compared for all the data sets, and life-fraction models were developed which take into account both the time-dependent and cyclic-dependent effects of this composite material. The model is capable of predicting cycles to failure for the frequency range investigated and is based on data resulting from the highest frequency fatigue as well as creep testing. This linear life fraction model is extended successfully to accout for hold times.

Microstructure and phase evolution in rapidly-solidified Ti24Al11Nb

The microstructure and phase evolution in a rapidly-solidified Ti24Al11Nb alloy have been studied using X-ray diffraction, and scanning and transmission electron microscopy. Two forms of this alloy, plasma rotary electrode process powder and plasma-sprayed foil, were used for this investigation. While a β phase was the only phase found in the powder, several different phases were observed in the foil. These included ordered β (B2), ordered b.c.t. (T), ordered orthorhombic (O) and ordered h.c.p. ( α 2) phases. Phase transitions occurred when the rapidly-solidified alloy was isothermally aged. The phase transition sequences that took place during isothermal aging depended upon temperature range. For T(°C) < 650, both β/ B2 a ̊ ω and β/ B2 a ̊ α 2 occurred. For 650 < T(°C) < 900, β/ B2 a ̊ α 2 was prevalent. A complex reaction sequence was found for the β/ B2 a ̊ α 2 transition: β/ B2 a ̊ T a ̊ O a ̊ α 2 . For 900 < T(°C) < 1150, a decomposition reaction β a ̊ α 2 (niobium-lean) + β(niobium-rich) took place. A preliminary time-temperature-transformation diagram for the Ti24Al11Nb alloy is proposed.

Hydrogen solubility in a titanium aluminide alloy

The dissolution of hydrogen in a titanum aluminide alloy based on Ti3Al, Ti-24 Al-11 Nb, has been measured as a function of temperature and hydrogen pressure. Both the terminal solubility and the overall solubility, or total uptake of hydrogen, were determined. The terminal solubility of hydrogen in the α2 matric phase increases with temperature, while the overall solubility decreases with increasing temperature in the high-temperature region. The partial molar heat of solution in the α2 phase was determined to be−(21.97 ± 1.46) kJ/mol [H], while the partial molar heat of formation of the hydride phase was −(70.29 ± 2.34) kJ/mol [H]. The standard partial molar free energy of hydrogen in the α2 phase in equilibrium with the hydride was −(74,060 ± 2000) + (87.8 ± 2.1) T J/mol [H].

Microstructural assessment of interaction zone in titanium aluminide/TiC metal matrix composite

Abstract Samples of Ti–24Al–11 Nb/TiC composites were heat treated at temperatures between 1000 and 1200°C for several hours and the extent of the reaction zone between the Ti–24Al–11Nb matrix and the TiC particulate was assessed using optical, scanning electron, and analytical transmission electron microscopy. It has been shown that there is an interaction zone surrounding each TiC particle caused by the diffusion of C from the TiC into the matrix and diffusion of Nb into the TiC. The extent of these interactions increases with increase of time and increase of temperature. The phases formed during heat treatment were shown to be Ti3(AlNb)C and (TiNb)C. The significance of these observations is briefly discussed in terms of recent work in which it has been shown that there is no detectable change in the matrix in similarly treated Ti–6AI–4V/TiC composites.MST/946