Ti Al Nb Research Articles

Creep behavior and effect factors of a TiAl–Nb alloy at high temperature

The effect of solution treatment on the microstructure and creep properties of forged TiAl–Nb alloys was investigated. The results showed that the microstructure of forged alloy mainly consisted of γ/α2 lamellar colonies and fine equiaxed recrystallized γ/α2 grains. During the solution treatment the microstructure of the alloy transformed into a fully lamellar structure due to the lamellar colonies growth by consuming equiaxed grains. Compared with the forged alloy the creep life of the solution treated alloy at 800 ​°C/220 ​MPa increased from 116 ​h to 339 ​h. The better creep resistance may be attributed to the transform of fine equiaxed γ/α2 grain to the lamellar colonies with serrated grain boundaries due to the solution treatment. The deformation mechanism of the solution treated alloy during creep is considered to be dislocation slipping within the lamellar γ/α2 phases, and the dislocation movement may be hindered by the γ/α2 interface and the formation of dislocation tangles. The interaction of the dislocations with the tangles may increase the resistance of the dislocation motion and hence improve the creep resistance of the alloy. It was found that during the creep of the forged alloys the cracks mainly initiated at the equiaxed grain, and in the solution treated alloy the cracks initiated at the grain boundaries. As creep continued the cracks propagated and connected to each other, leading to the damage and rupture of the forged and solution treated alloys.

Structural and mechanical properties of Ti–Al–Nb–Mo–B alloy produced from the SHS powder subjected to high-energy ball milling

Ti–Al–Nb–Mo–B alloy was successfully produced via high-energy ball milling (HEBM), self-propagating high-temperature synthesis (SHS) and hot isostatic pressing (HIP) techniques. HEBM was used to prepare powder mixtures with different types of microstructure and homogeneous distribution of the components. After SHS in thermal explosion mode powder grains with a microgradient structure were synthesized: their central part consists of α2-Ti3Al and the peripheral part consists of γ-TiAl. Compacted samples with duplex fine-grained structure and high thermomechanical properties (σ0.2 = 1245 MPa and 524 at 20 °C and 800 °C, respectively) have been obtained using the HIP technology. It was found, that mechanical twinning is the preferential deformation mechanism of the studied Ti–Al–Nb–Mo–B alloy.

Electron beam melted TiC/high Nb–TiAl nanocomposite: Microstructure and mechanical property

To enhance the process stability and densification, semi-melt step has been introduced when fabricating the TiC/high Nb–TiAl nanocomposite via electron beam melting. The homogenous TiAl matrix microstructure with dispersed nano-scale carbides was realised. During the EBM melt, most TiC nanoparticles dissolved and Ti2AlC formed with near-spherical and rod-like shapes. The particles had an influence on solidification behaviour and the subsequent microstructural degradation. High Nb–TiAl nanocomposites with 1.2 wt% TiC addition exhibited a duplex microstructure with dispersed carbides, while a nearly lamellar microstructure (carbide-free) was found in samples with 0.6 and 0.8 wt% TiC. Furthermore, a lower scanning speed resulted in higher relative density, greater Al loss, increased α2-phase but reduced carbide fractions. The microhardness of 433 ± 10 HV0.2, ultimate tensile strength of 657 ± 155 MPa and fracture toughness of 8.1 ± 0.1 MPa√m in 1.2 wt% TiC/high Nb–TiAl nanocomposite processed by EBM are very promising. In addition, the compressive yield strength of 1085 ± 55 MPa, fracture strength of 2698 ± 34 MPa and strain to fracture of 26.1 ± 1.0%, are superior to those processed by conventional means. The strengthening and toughening mechanisms have been interpreted on the basis of crack path observation.

Study of dry sliding wear behaviour of Ti-48Al-2Nb-2Cr alloy

ABSTRACT The aim of this research was to study the tribological behaviour of Ti–48Al–2Nb–2Cr alloy under dry sliding reciprocating conditions. A reciprocating wear machine was employed to evaluate the typical abrasion resistance of the alloy. Infrastructure assessment of worn surfaces was performed by scanning electron microscopy (SEM) analysis equipped with Energy Dispersive X-Ray Spectroscopy (EDS). The average surface roughness of initial specimen was 32 nm. According to the SEM observations of worn surfaces and debris, the dominant wear mechanism under defined tribosystem was identified to be abrasive and delamination wear. The weight loss measurements showed that by increasing the applied load from 10 to 40 N, the weight loss was significantly increased from 5.4 to 18.5 µg after passing 1000 m slip distance.

Microstructure refinement assisted by α-recrystallization in a peritectic TiAl alloy

Fine-grained nearly-lamellar (FGNL) microstructures and ultrafine α2/γ lamellar microstructure were successfully obtained by means of α-recrystallization in a Ti–48Al–2Nb–2Cr alloy. The dynamic recrystallization of α phases and subsequent cooling control contributed to microstructure refinements, which were governed by small-strain α-deformation. Lamellar grain refinement is due to dominating continuous dynamic recrystallization (CDRX). Lamellar structure refinement is mainly achieved by the increase in γ lamellae precipitation sites by α-deformation and fast cooling. Finally, the microstructure evolution and control mechanisms were illustrated, suggesting a promising method for improving the mechanical properties of TiAl alloy.

Tensile behavior and deformation mechanism for Ti–22Al–25Nb alloy with lamellar O microstructures

In this research, the tensile behavior and deformation mechanism of two microstructures for Ti–22Al–25Nb alloy are studied by in-situ tensile test and electron backscatter diffraction (EBSD) experiment. Due to the higher B2 phase content and the long and continuous slip line, the solution-treated microstructure has better plasticity. For the aged microstructure, the slip line is short and scattered due to the obstruction of acicular O phase, which leads to the higher strength. In addition, the slip mode of B2 phase includes single system slip, double system slip and cross slip. {110}<111> slip and {112}<111> slip for B2 phase can be activated at room temperature. Meanwhile, the potential slip systems of O phase are summarized. The slip mode of O phase includes single system slip and double system slip, and the former is the most important one. All three types of slip systems (prism ‘a’ slip, basal ‘a’ slip, and pyramidal ‘c+a’ slip) for O phase can be activated at room temperature. Moreover, there is a good slip transmission between B2 and O phases. B2 phase slip can induce single system slip and double system slip of O phase. The Burgers orientation relation (BOR) between B2 and O phases is conducive to the slip transmission.

Corrosion resistance of TiAl–Nb coating on 316L stainless steel in liquid zinc

In order to improve corrosion resistance of the 316L stainless steel in liquid zinc, four different kinds of TiAl–Nb coatings were deposited by high velocity oxygen fuel spraying. The results indicated that the TiAl–Nb coatings have dense and uniform microstructure and good mechanical properties. The TiAl–Nb coatings exhibit excellent corrosion resistance. Four different kinds of TiAl–Nb coatings have a incubation time, which do react with liquid zinc. The Ti28.15Al63.4Nb8.25Y0.2 coating possesses the longest lifetime, which can reach up to 1080 h. The TiAl–Nb coatings can provide a long-term corrosion protection for 316L stainless steel. The Al content has an important effect on the corrosion resistance of the coating, and the addition of the Y element may benefit the corrosion resistance to some extent. The corrosion process of the coating consists of the following steps: physical barrier of the TiAl–Nb coatings (incubation period), the corrosion of bond coat and the failure of the coatings.

The partial liquidus projection and solidification sequences in Ti-(40–60) Al-(5–30) Nb alloys

The properties of Ti–Al–Nb alloys strongly depend on alloy compositions and microstructures. Proper design of these important aspects requires detailed knowledge of the solidification sequences and liquidus projection. However, there are still some conflicts between previous experimental and calculated results about the liquidus projections in the composition range of Ti-(40–60)Al-(5–30)Nb and this has hindered Ti–Al–Nb alloy development. In this present work, we explicitly investigated the as-cast microstructures and phase constitution of 50 alloys with the composition range of Ti-(40–60)Al-(5–30)Nb and clarified the solidification sequences. We re-constructed the partial liquidus projection and found that the primary β phase region extend to just below 55 at.% Al content and the primary α phase region extend to just below 24 at.% Nb content. Moreover, it was observed that the invariant reaction between Liquid, β, α and γ phases is a peritectic reaction (Liquid+β+γ→α) and all the Liquid+β+γ, Liquid+β+α and Liquid+α+γ three-phase regions are of peritectic type. The present work provides essential information necessary for optimizing the Ti–Al–Nb thermodynamic database to accelerate alloy development.

Phase and Structural Transformations in a VTI-4 (Ti–22Al–25Nb) Alloy during High-Rate Hot Compression

The behavior of a VTI-4 (Ti–22Al–25Nb) alloy during hot compression at a strain rate of ~2.5 s–1 is studied. Hot compression to a strain of 65% is carried out in the temperature range 850–1020°C, covering two phase regions β + Ο and β + α2. Hot compression results in the dissolution and spheroidization of lamellar O and α2 phases in the severe deformation zone. X-ray diffraction analysis does not reveal any qualitative changes in the phase composition after deformation. Energy dispersive X-ray spectroscopy (EDS) analysis conducted on a transmission electron microscope indicates niobium depletion of the O phase when the hot compression temperature increases or during compression. Niobium-depleted zones form near interphase boundaries during the dissolution and spheroidization of lamellar-shaped O and α2 phases. The dissolution and spheroidization process includes two stages. The first stage involves defect accumulation in the O-phase and α2-phase lattices, and it is accompanied by a simultaneous increase in the temperature due to deformation-induced heating. The second stage icludes niobium redistribution between the β matrix and the phase in defective sites, which leads to phase dissolution and spheroidization.

Creep behaviour of equiaxed fine-grain γ-TiAl-based alloy prepared by powder metallurgy

In this study, γ-TiAl-based alloy with chemical composition of Ti–45Al–5Nb (in at.-%) fabricated by powder metallurgy method was crept at 700°C under 200–500 MPa. The creep properties and the microstructure after creep tests were investigated. The results showed that the γ-TiAl-based alloy was composed of equiaxed γ-TiAl grains and α 2-Ti3Al grains with average sizes of 1.4 and 0.5 μm, respectively. The creep resistance deteriorated generally with increased applied stresses. The typical intergranular fracture characteristics were observed though the grains were small. The calculated stress exponent and activation energy revealed the main creep mechanism of grain boundary sliding. Furthermore, twinning and dynamic recrystallisation also led to the creep deformation.

Room and high temperature mechanical behavior of Ti–Al–Nb–Mo alloy reinforced with Ti2AlN ceramic particles

In order to improve the room and high temperature mechanical properties of Ti46Al4Nb1Mo alloy, Ti46Al4Nb1Mo-xN (x = 0, 0.4%, 0.8%, 1.2%, 1.6%, atomic percent, hereafter in at.%) alloys were prepared by adding TiN powders. Results showed that when the N addition was more than 0.8%, the grain morphology changed from columnar dendrites to equiaxed grains due to the formation of Ti2AlN particles, and the equiaxed grain size decreased with further increasing of the N addition. With 1.6% N addition, the compressive strength increased by 16.8% at room temperature, which was due to the collective strengthening effect of lamellae refinement and Ti2AlN particles. While lamellae refinement and Ti2AlN particles had opposite effects and competed with each other at high temperature, strengthening by Ti2AlN particles had the dominant effect on improving the peak stress of the Ti46Al4Nb1Mo-1.6 N alloy, which increased by 21.8% and 24.1% at 900 °C and 1000 °C, respectively. In addition, compared with the fracture microstructure at room temperature, the deformed microstructure showed that the load transfer effect of Ti2AlN particles was more effective at high temperature, which was due to micro-damages in the Ti2AlN particles and the absence of cracks along irregular Ti2AlN particles.

Comparison of high-temperature deformation behaviors for Ti–Al–Nb–V alloy with different initial microstructures at the strain of 0.7

The Ti–Al–Nb–V alloy with the Widmanstätten B2+α2 structure was prepared by induction skull melting, and it became single-B2-phase alloy with the equiaxed structure by further solution treatment and quenching. The compression properties and the deformation behaviors of the Ti–Al–Nb–V alloy with the two types of microstructures were investigated in the temperature range of 500–900 °C. When the alloy was loaded at 500–800 °C, the yield strength of the specimen with the equiaxed structure was higher than that of the one with the Widmanstätten structure, whereas the opposite trend was exhibited at 900 °C. For the specimen with the Widmanstätten structure, the flow softening occurred at 600–900 °C upon compression, and the initiating strain of the flow softening stage decreased with the increasing deformation temperature. For the specimen with the equiaxed structure, the flow softening occurred at 500–900 °C. By comparing the undeformed and deformed microstructures of the specimens at the strain of 0.7, it was concluded the deformation mechanism of the specimen with the Widmanstätten structure was dislocation glide below 600 °C, and above this temperature, dynamic recrystallization in the B2 phase became dominant. For the specimen with the equiaxed structure, the deformation mechanism was the dislocation glide that formed the slip bands at 500 °C, while recrystallization occurred in the slip bands at 600–900 °C, leading to the flow softening. When the deformation temperatures were 800 °C and 900 °C, the deformation mechanism also referred to the phase transformation of B2→α2. According to the detailed microstructure observations, the deformation mechanisms were schematically illustrated for the Ti–Al–Nb–V alloy with different initial structures.

Fracture toughness of Ti–22Al–25Nb alloy at room and high temperatures

In this research, the fracture toughness of Ti–22Al–25Nb orthorhombic alloy with bimodal size lamellar O phase microstructures was investigated at room temperature and 650 °C. In addition, the fracture toughness model and crack tip plastic zone model were established by simplifying the shape of the crack tip plastic zone. The fracture toughness model is used to quantitatively calculate the intrinsic and extrinsic contributions to fracture toughness, and the crack tip plastic zone model is used to calculate the real height of the crack tip plastic zone. The results show that the fracture toughness KIC at high temperature is twice that at room temperature. The fracture mechanisms at room and high temperatures are the quasi-cleavage fracture and dimple fracture, respectively. The increase of temperature is conducive to the significant increase of the fracture toughness by improving the inherent property of the material (intrinsic contribution) and the crack propagation tortuosity (extrinsic contribution). The fracture toughness at room temperature is dominated by intrinsic contribution while the fracture toughness at high temperature is dominated by both the intrinsic and extrinsic contributions. Besides, whether at room temperature or high temperature, the intrinsic contribution is the most important factor to the total fracture toughness. The increase of temperature significantly improves the size of the crack tip plastic zone, which leads to a significant increase in fracture toughness.

Anti-correlation between solution strengthening and precipitation strengthening in lamellar O microstructures of a Ti2AlNb based alloy

Anti-correlated change between yielding strength (YS) and ultimate tensile strength (TS) was found in heat-treated lamella O microstructures of Ti–22Al–25Nb (at. %) alloy. The B2-phase-region-forged microstructure contained lamella O precipitates and residual B2 matrix. The initial forged lamella O microstructure was solution-treated in B2+O phase region and fine O precipitates transformed into B2 phase. The YS of the lamella O microstructure increased significantly while the TS decreased dramatically after the solution treatment. On the one hand, electron probe microanalyses indicate that the composition of the strong strengthener Al in the B2 matrix got enriched in the solution-treated microstructure and the composition of the weak strengthener Nb in the B2 matrix got diluted. Higher Al composition resulted in stronger solution strengthening effect (SSE) in the B2 matrix of the solution-treated microstructure. Since the B2 phase in such lamella O microstructures dominated the yielding behaviors, higher Al composition further resulted in the higher YS. On the other hand, transformation from O precipitates to B2 matrix during the solution treatment caused fewer O/B2 boundaries, which resulted in weaker precipitation strengthening effect (PSE) and led to the lower TS of the solution-treated microstructure. Inversely, the YS got decreased while the TS got increased when the solution-treated microstructure was subsequently aged at a lower temperature, where abundant acicular O re-precipitated from the B2 matrix. It is concluded that the anti-correlated change between YS and TS is attributed to the anti-correlation between SSE and PSE due to the appropriate composition of strong strengthener Al and weak strengthener Nb.

High Nb–TiAl Intermetallic Blades Fabricated by Isothermal Die Forging Process at Low Temperature

In this study, the isothermal die forging process of high Nb–TiAl (Ti-44Al-8Nb-0.2W-0.2B-Y, at.%) alloy blades was simulated using the ABAQUS V6.11 software and the blades were fabricated successfully. The influence of a low forging temperature (lower than 1000 °C) and strain rate on the distributions of effective strain and stress were analyzed. The results indicate that the effective strain exhibits negative temperature sensitivity and positive strain rate sensitivity. The stress exponent (n = 3.02) and the apparent activation energy (Q = 293.381 kJ/mol) of the present alloy suggests that this as-forged high Nb–TiAl alloy exhibits good deformability at low temperatures. With the reduction in strain rate and the increase in forging temperature, the effective stress decreases. Finally, high-quality high Nb–TiAl alloy blades were fabricated using an isothermal die forging technology at a rate of 0.01 mm/s and temperature of 950 °C, chosen on the basis of the simulations results. Scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD) results indicated that the center of the TiAl alloy blade possessed a duplex microstructure, consisting of remnant lamellar colonies and recrystallized γ/B2 grains. The refined α2 laths showed a typical forging flow line feature in the edge position, whereas the γ laths had broken down and recrystallized.

Microstructure, oxidation behavior and adhesion of a CoNiCrAlTaY coating deposited on a high Nb–TiAl alloy by plasma surface metallizing technique

A CoNiCrAlTaY coating was deposited on the Ti–45Al-8.5Nb substrate by plasma surface metallizing technique. The microstructure, morphology and composition of CoNiCrAlTaY-coated specimens were studied by X-ray diffractometer, scanning electron microscope, and transmission electron microscopy. The as-prepared coating consisted of mixed phases including Al13Co4, CrCo, and Ni3Al, resulting in an increased microhardness. The Al2O3, CoO and Cr2O3 phases formed during initial oxidation at 900 °C. However, Al2O3 accompanied with CoCr2O4, NiCr2O4 and Cr2TiO5 phases generated during isothermal oxidation, which was responsible for the oxidation resistance. These oxides leaded to the increased surface hardness of the coated specimens. The adhesion strength between the CoNiCrAlTaY coating and Ti–45Al-8.5Nb substrate were improved caused by the interdiffusion of involved elements.

Thermal stability and mechanical properties of Ti–22Al–25Nb alloy with different initial microstructures

Abstract In order to better understand the thermal stability and mechanical properties of the nominal Ti–22Al–25Nb alloy with different initial microstructures, microstructural evolution after long-term aging, and mechanical properties at room temperature and 650 °C of four differently heat-treated Ti–22Al–25Nb alloy were investigated. The investigation results indicated that α2 platelets formed by aging at (α2+B2) region were not as stable as previous research claimed and spontaneously decomposed into fine (O+β/B2) mixture during the annealing at 650 °C. As a result, the alloy's steady-state creep strain rate dropped dramatically at the secondary creep stage and tensile strength grew. On the contrary, long time aging exposure resulted in the connection of original dispersed lenticular O precipitation and thus degraded the ductility of all specimens. Besides, the discontinuous precipitation B2→β+O caused by supersaturation of solutes in the matrix also harmed the mechanical properties. Under the condition of 650 °C/250 MPa, the steady-state creep strain rates of the Ti–22Al–25Nb specimens show a significant dependence on their grain size. Crept samples showed that most dislocation motion was restrained in the β channels under a small strain and then extended into O phase when creep strain grew over 10%. Finally, by comparing the room temperature tensile property and creep resistance of four samples, supertransus solution and furnace cooling ensured the alloy most balanced mechanical properties among lamellar samples. The supertransus then furnace-cooled sample did not only have an excellent creep resistance but also kept a moderate room temperature elongation to failure of nearly 6%.

Microstructure and elevated-temperature mechanical properties of in situ Ti2AlNb-reinforced TiAl-matrix composite prepared by powder metallurgy

An in situ Ti2AlNb-reinforced Ti–45Al–5Nb–0.3 W-based intermetallic composite was prepared using 2 vol. % Nb powder and TiAl pre-alloy powder. The microstructure evolution and elevated-temperature mechanical properties of the Ti2AlNb/TiAl composite were investigated. The results show that during the hot isostatic pressing (HIP) process, Nb diffused into the TiAl matrix, and formed Nb-rich particles in which an Nb-rich-region and the coarse rod-like Ti2AlNb phase existed. After hot extrusion, the Nb-rich particles elongated along the direction of extrusion. Further, the mean diameter of the Ti2AlNb phase regions decreased, and the Ti2AlNb content increased. After heat treatment at 1280 °C for 1 h, the rod-like Ti2AlNb phase regions in the extruded specimen transformed into needle-like and fine lath-like structures, and the proportion of Ti2AlNb in the Nb-rich-regions increased. The extruded Ti2AlNb/TiAl specimen shows a high tensile elongation (TE), and the heat-treated specimen shows a high yield strength (YS) and ultimate tensile strength (UTS) at the test temperatures of 750 °C–900 °C. In particular, the extruded specimen shows excellent strength–ductility product (UTS × TE) values of 38.8 GPa% and 46.9 GPa% at 800 °C and 850 °C, respectively. The high toughness of TiAl intermetallic composites was because of the fibrous tough Nb-rich-regions that contained the fine Ti2AlNb phase.

Construction of Constitutive Model and Strain-Rate Sensitivity Coefficient Distribution Map of Ti2AlNb

The hot deformation behavior of Ti–22Al–25Nb was studied by the high temperature compression over a range of temperatures (950–1050 °C) and strain rates (0.001–10 s−1) in this paper. The work-hardening (WH) and softening deformation behaviors of Ti–22Al–25Nb were analyzed. Obvious linear decreasing regimes of WH rate curves can be found before the dynamic recrystallization (DRX) onset, which indicates WH + DRV (dynamic recovery) stage. And WH rate decreased significantly with strain rate reduced and temperature elevated. A physically-based constitutive model was established, which can well predict the flow behavior of Ti–22Al–25Nb. Additional, strain-rate sensitivity coefficient distribution map was established. The higher values of m appeared at the low strain rate. When the strain rate exceeded 0.1 s−1, the values of m were lower than 0.25.

WITHDRAWN: Characterization of the hot deformation behaviour of powder metallurgy Ti–22Al–25Nb alloy by using 3D processing maps

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