Ti2AlNb Alloy Research Articles

Electrochemical and Tribological Performance of Ti–Al with xNb Addition Synthesized via Laser In situ Alloying

Additive manufacturing is a growing technique of producing 3D parts directly using metal powders or wires melted with a high-powered intensity beam or laser. It is still a challenging process as to how laser processing parameters such as gas flow rate and powder flow rate can profitably be adopted to significantly produce Ti–Al-based materials from elemental powders to synthesize alloys that are defect-free and have good mechanical properties. The density of titanium aluminide (Ti–Al) intermetallic alloys makes it gain lots of interests due to its potential ability to substitute nickel-based superalloys in gas turbine engines. This work aims to investigate the effects of Niobium (Nb) additions on Ti–Al–xNb ternary alloys created via the use of 3D printing technology, specifically looking at microstructural evolution, microhardness, electrochemical behavior, and tribological properties. Ti–Al–Nb alloy was synthesized at scan speed of 26 in/min and laser power of 450 W. The structural morphology of the alloys produced was investigated using scanning electron microscopy equipped with energy dispersive spectroscopy and the electrochemical studies of the in situ alloyed Ti–Al–xNb were studied using potentiodynamic techniques. Using an Emco microhardness tester, the microhardness characteristics of the produced TiAl–xNb alloys were examined. From the results obtained, it was observed that the microstructure showed not much substantial cracking or crack initiation. The micrographs are evident of refined microstructure associated to increase in Nb feed rate with α-Ti3Al, γ-TiAl and precipitates of β-TiAl phases as the distinctively identified in the microstructure. The highest recorded microhardness value of 679.1 HV0.5 was achieved at Nb feed of 0.5 rpm and gas carrier of 2 L/min. The fabricated Ti–Al–Nb alloys showed good corrosion resistance behavior in HCl and appreciable wear characteristics with coefficient of friction of 0.412, 0.401, and 0.414 µ at B1, B3, and B5, respectively.

High temperature oxidation behaviors of Al/Cr composite coating on Ti2AlNb alloy

The Al/Cr coatings were deposited on Ti2AlNb alloy by magnetron sputtering and double glow plasma alloying technology to improve the oxidation resistance. The morphology of the protective coating was investigated by scanning electron microcopy and the phase composition of the coating was analyzed using x-ray diffraction. The results suggested that the protective coating with multilayer structure was dense and homogeneous. The formation of Cr inner diffusion layer was beneficial to the bonding strength between the coating and substrate, which was attributable to the double glow technology. The oxidation behaviors of Al/Cr coating were investigated by the isothermal oxidation tests for 100 h at 700 °C, 800 °C and 900 °C, respectively. The results indicated that the protective coating showed a lower oxidation rate. At 700 °C, the Al2O3 oxidation products were formed on the surface of coating at high temperature, which were dense and homogeneous against oxygen diffusion. With the increase of oxidation temperature, the external diffusion of Cr element reacted with oxygen to form Cr2O3, which provided the further protection. As for 900 °C, the volatile CrO3 was formed by the reaction of the external diffusion of Cr and Cr2O3, and the oxidation products Al2O3 and Cr2O3 continued to provide protection for Ti2AlNb alloy.

Stability of the microstructure and properties of Ti2AlNb alloys under thermal exposure conditions

In this paper, the microstructure and property stability of Ti2AlNb alloy specimens treated by thermal exposure at 800°C for 0~12h were investigated using universal tensile tester, X-ray diffractometer, scanning electron microscope, electron backscattering diffraction, and in-situ tensile scanning electron microscope. The results show that with the extension of thermal exposure time, the tensile strength and yield strength of the alloy slightly increased, the B2-phase of the alloy decreased while the O-phase increased, and the content of the α2-phase also slightly decreased. It has been found that the properties changes are related to the changes of microstructure. The tensile cracks expanded along the grain boundaries between the O-phase and B2-phase as well as along the O-phase grain boundaries. The slight changes in microstructure and properties indicate that the Ti2AlNb alloy is stable and reliable for short-term service at 800 °C.

Effect of hydrogen on the fretting wear mechanism of a high Nb-TiAl alloy

In this paper, the effect of hydrogen placement on the microdynamic wear mechanism of high niobium-titanium-aluminum alloys is investigated. Corresponding decreases and increases in loading force and displacement amplitude cause the microslip behavior of the alloy to change from partial slip to mixed slip. Slip type in mixed fire complete slip when you, the friction coefficient fluctuates. The average friction coefficient of hydrogen-placed alloys is small compared with that of non-hydrogen-placed alloys, the maximum wear marks are not obvious, and the oxidative wear is weak. The main wear mechanisms of non-hydrogenated alloys are adhesive wear, abrasive wear and oxidative wear. The main wear mechanisms of hydrogen-placed alloys are adhesive wear and abrasive wear.

Properties of biodegradable polymer coatings with hydroxyapatite on a titanium alloy substrate.

Purpose: Titanium alloys are among the most widely used materials in medicine, especially in orthopedics. However, their use requires the application of an appropriate surface modification method to improve their properties. Such methods include anodic oxidation and the application of polymer coatings, which limit the release of alloying element ions. In addition, biodegradable polymer coatings can serve as a carrier for drugs and other substances. The paper presents the results of research on the physical properties of biodegradable polymer coatings containing nanoparticle hydroxyapatite on a titanium alloy substrate. Methods: A PLGA coating was used in the tests. The coatings on the substrate of the anodized Ti6Al7Nb alloy were applied by ultrasonic spray coating. The tests were carried out for coatings with various hydroxyapatite content (5, 10, 15, 20%) and thickness resulting from the number of layers applied (5, 10, 15 layers). The scope of the research included microscopic observations using scanning electron microscopy, topography tests with optical profilometry, structural studies using X-ray diffraction, as well as wettability and adhesion tests. Results: The results shows that with the use of ultrasonic spray coating system is possible to obtain the continuous coatings containing hydroxyapaptite. Conclusions: The properties of the coating can be controlled by changing the percentage of hydroxyapatite and the number of layers of which the coating is composed.

Dual-droplet transition control for improving forming quality and composition homogenizing in dual-wire additive manufacturing of Ti2AlNb alloy

Dual-wire additive manufacturing (AM) couples traditional wire-based AM for part fabrication and the molten pool metallurgy for material-preparation with high deposition efficiency and material utilization. However, compared with traditional single-wire AM technology, it has a more complex and sensitive dual-droplet transition distance (TD), which not only affects the forming quality but also the metallurgical quality. Therefore, it is necessary and urgent to monitor and control its TD value online. In this study, we systematically investigated the sensing, controlling, and influential mechanism of the TD value in dual-wire AM technology, and Ti2AlNb was taken as the target alloy owing to its great application prospects in the aerospace field. Specifically, a deposition experiment with different initial TD value was conducted to study the effect on the morphology and composition distribution of the as-printed part. Based on the optimal distance, the related image extraction algorithms and closed-loop control methods are developed. The closed-loop controlled verification experiment on the slope and step substrate, as well as the multi-layer deposition test, were carried out and analyzed. The results indicate that the developed system can control the TD to the desired value with good robustness. In addition, the controlled deposited multi-layer part exhibited good morphology and composition homogenizing in the post-characterization experiment. This study is of great significance for the intelligent and industrial development of dual-wire AM technology.

Structural transformation behavior in a high Nb–TiAl alloy additively manufactured by laser powder bed fusion

High Nb–TiAl alloys fabricated using laser powder bed fusion (LPBF) exhibit a metastable α2 phase due to the rapid cooling rate of the LPBF process. However, the understanding of the structural transformation behavior in LPBF-fabricated high Nb–TiAl alloys remains limited. In this work, a high Nb–TiAl alloy with a nominal composition of Ti–48Al–2Cr–8Nb (at. %) was additively manufactured using LPBF. The structural transformation from metastable α2 phase to γ phase was then studied by the in-situ high-temperature x-ray diffraction (XRD), in-situ heating transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Furthermore, the phase composition and microstructure of the samples annealed at different temperatures were determined by XRD and electron backscatter diffraction (EBSD). Our findings demonstrate that the α2 phase in LPBF-fabricated high Nb–TiAl alloy remains stable up to 873 K, after which it decomposes and forms fine γ lamellar colonies along with a disordered γ′ phase as an intermediate phase. Notably, the transition from α2 to γ′ phase is reversible. In contrast, the formation of the γ phase is irreversible and persists down to room temperature. It is also found that the changes in grain size and phase composition of the LPBF-fabricated high Nb–TiAl alloy have a significant impact on its hardness. This study provides valuable insights into the structural transformation behavior of additively manufactured high Nb–TiAl alloys by LPBF, offering guidance for regulating their structure and properties.

Heterostructure manipulation of Ti2AlNb alloy through directional induction heating: investigation of multi-scale deformation mechanisms in the O phase

In this study, a heterostructure of Ti2AlNb alloy (Ti-22Al-26Nb) was fabricated using directional heating treatment (DHT) technology. The microstructure of the alloy was examined before and after DHT treatment using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The primary objective of the study is to investigate the deformation mode and strain hardening mechanism in the multi-scale O phase. The results indicate that DHT treatment leads to a significant refinement in the grain size of the alloy, resulting in a 14.2% increase in ultimate tensile strength (UTS) and a 29.2% increase in elongation. However, the treatment also causes a decrease in yield strength. The soft phase region, characterized by the microstructure with micron-sized O phase precipitates, and the hard phase region, characterized by the microstructure with submicron-sized O phase precipitates, exhibit different modes of strain adjustment. To mitigate the internal stress arising from the deformation mismatch between the soft and hard phases, the micron-sized O phase activates slip on the {110} and {010} planes during the tensile process at room temperature. Additionally, twinning behavior was observed on the {110} plane. On the other hand, the submicron O-phase activates slip on the {041} plane and occurs a crystal rotation of approximately 90° along the 〈012〉 and 〈100〉 orientations. These high-angle grain boundaries (HAGBs), which form continuously during deformation, effectively store dislocations and contribute to a continuous strain hardening effect in the alloy. Additionally, the alloy exhibits a high strain hardening ability due to the accumulation of geometrically necessary dislocations caused by the strain gradient resulting from the uneven plastic deformation of the soft and hard phase components, as well as the potential hetero deformation-induced (HDI) hardening. Ultimately, a Ti2AlNb alloy with a favorable balance of UTS and elongation was achieved.

In-situ investigation of tensile anisotropy mechanism in an advanced Ti2AlNb-based alloy associated with CRSS ratio and damage model

Combined with in-situ tensile test and electron backscatter diffraction technology, the critical resolved shear stress (CRSS) ratio of O phase and the damage model of grain boundaries were proposed to reveal the tensile anisotropy mechanism of Ti–22Al–25Nb alloy isothermally forged in B2 phase region. In-situ observation suggests that the single slip mode of lamellar O phase is the major deformation behavior, and its preferential slip dominates the yield of the material. Based on the types, number, Schmid factor (SF) and orientation of slip activation, the CRSS ratio of basal <a>, prism <a>, first-order and second-order pyramidal <c+a> slips for O phase is estimated to be 1.19: 1: 1.37: 1.39. Meanwhile, basal and prism <a> slips mainly appear the radial and axial directions (RD and AD) samples with [012] and [100] texture, while the [001], [010] and [212] texture of OD (45° to RD) sample shows an increase in pyramidal <c+a> slips. Thus, the slip activation of O phase depends on the grain orientation and CRSS, resulting in significant strength anisotropy. Furthermore, the microcracks and secondary cracks at grain boundaries are significantly affected by the slip accumulation of lamellar O phase at the O/O and O/B2 interfaces. Considering the stress state of the grain boundary, a damage model about crack nucleation and propagation is proposed to explain the ductility anisotropy. From this, the RD sample has a higher crack nucleation and propagation resistance than the AD and OD samples due to the flattened morphology of B2 grains.

The Impact of Al2O3 Particles from Grit-Blasted Ti6Al7Nb (Alloy) Implant Surfaces on Biocompatibility, Aseptic Loosening, and Infection.

For the improvement of surface roughness, titanium joint arthroplasty (TJA) components are grit-blasted with Al2O3 (corundum) particles during manufacturing. There is an acute concern, particularly with uncemented implants, about polymeric, metallic, and corundum debris generation and accumulation in TJA, and its association with osteolysis and implant loosening. The surface morphology, chemistry, phase analysis, and surface chemistry of retrieved and new Al2O3 grit-blasted titanium alloy were determined with scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and confocal laser fluorescence microscopy, respectively. Peri-prosthetic soft tissue was studied with histopathology. Blasted retrieved and new stems were exposed to human mesenchymal stromal stem cells (BMSCs) for 7 days to test biocompatibility and cytotoxicity. We found metallic particles in the peri-prosthetic soft tissue. Ti6Al7Nb with the residual Al2O3 particles exhibited a low cytotoxic effect while polished titanium and ceramic disks exhibited no cytotoxic effect. None of the tested materials caused cell death or even a zone of inhibition. Our results indicate a possible biological effect of the blasting debris; however, we found no significant toxicity with these materials. Further studies on the optimal size and properties of the blasting particles are indicated for minimizing their adverse biological effects.

Simultaneously enhancing strength and ductility of high-Nb-containing TiAl alloy via extrusion-assisted microstructure tailoring

In most metallic materials, the strength-ductility trade-off serves as an inevitable scenario. Therefore, improving the room/high-temperature strength/ductility of high Nb-TiAl alloys based on a lamellar microstructure is important for practical applications. In this work, a novel microstructure with fine lamellar colonies and a certain number of residual γ grains was developed through hot extrusion in the near α phase region. The results indicated that the lamellar colony size and spacing of the as-cast alloy were significantly refined by hot extrusion. The as-extruded Ti-45Al-8Nb-(W, B, Y) alloy demonstrated a good combination of high strength and ductility. The tensile strength and ductility were 1129 MPa and 2.04% at room temperature (RT), respectively, and 744 MPa and 51.20%, respectively, at 850 °C. A simultaneous improvement in strength and ductility below ductile-brittle transition temperature (DBTT) could be attributed to the abundant deformation twins and their intersections, as well as prominent strain working caused by high-density dislocations in the γ phase. Due to dynamic recrystallization, the ductility was significantly enhanced when stretched over DBTT. This extrusion-assisted microstructure tailoring strategy could be extended to other TiAl systems and may serve as a useful solution for addressing the significant engineering constraints posed by the low RT ductility and poor hot-working capabilities of high Nb-TiAl alloys.

Regulating the interfacial microstructure in TiAl/Ti2AlNb vacuum diffusion bonded joints for superior mechanical performance

Dissimilar material joining between TiAl and Ti2AlNb intermetallic alloys is the key to integrating the attractive properties of these lightweight structural materials in one hybrid component. However, some intractable problems, such as brittle reaction layers, residual thermal stress at the interface, and poor plasticity at ambient temperature, hamper its application. To overcome these deficiencies, diffusion bonding of intermetallic TiAl alloy to Ti2AlNb alloy using a series of newly designed Ti100-xMox (x = 2.56 ∼ 14.26 at. %) interlayers was investigated. By regulating the Mo content in the interlayer, a fine α-Ti layer and a gradient α-Ti + β-Ti composite layer were regulated in the reaction zone when the Mo content was 8.09 at. %. Such elaborately tailored interfacial microstructure led to superior mechanical performance for which the plasticity markedly exceeded the TiAl substrate while maintaining a considerable tensile strength (402 MPa). The improvement of mechanical performance of bonded joints could be attributed to the competition and collaboration between initiation of cracking in the TiAl/Ti91.91Mo8.09 interface and stress-induced martensitic transformation in the interlayer. Tensile testing showed that the bonded joints finally ruptured at the brittle α2 reaction layer. The results in this paper provide important insights into fabricating high-quality TiAl-based hybrid joints by diffusion bonding.

Study on improving the fluidity of Ti2AlNb alloy

In this paper, the effects of main elements like Al, Nb and trace elements like Fe, Mo, W, Co, B and Si on the fluidity of Ti–22Al–25Nb alloy were investigated. The alloy composition which is beneficial to improve the fluidity was selected by calculating the thermal property parameters affecting the fluidity of the alloy through thermodynamic software, numerical simulation test of the fluidity of the alloy and verification test of the fluidity of the preferred alloy. Finally, the improvement mechanism of the alloy fluidity was discussed. Results showed that the appropriate reduction of Nb element was better than Al element for the improvement of fluidity. The addition of trace Fe, B and Si elements was beneficial to the improvement of fluidity, the improvement effect of B element was the best, while the addition of trace Mo, W, and Co was not conducive to the improvement of fluidity. The cessation mechanism of Ti2AlNb alloy is the cessation mechanism of the alloy with a wide crystallization temperature range. The composition which was the most beneficial to improve the fluidity was Ti–22Al–24Nb-0.1B. The main reasons for the improvement of the fluidity were that on the one hand, reducing 1 at% Nb and adding 0.1 at% B not only increased the superheat and crystallization latent heat but also reduced the melt viscosity and thermal conductivity, thus improving the fluidity. And on the other hand, the TiB phase refined the grains, and the fine grains prevented the dendrites from growing into a well-developed dendritic network, inhibited the adverse effect of the increase in the width of the solidification zone on the fluidity, and further improved the fluidity.

Microstructure evolution and mechanical properties of pulse high current diffusion bonding γ-TiAl alloy to Ti2AlNb alloy

The development of lightweight high-temperature materials and their processing methods are receiving increasing attention as the need for improved performance and energy efficiency in aero-engines increase. In this study, the γ-TiAl alloy was successfully soundly bonded to Ti2AlNb alloy by pulsed high current (PHC) diffusion welding. The effect of different high level (HL)/low level (LL) of composite pulse current on the microstructure and mechanical properties of the joints have been studied. The representative interface microstructure of PHC-950 °C TiAl/Ti2AlNb joint was γ-TiAl substrate/diffusion bonding zone/Ti2AlNb impacted zone/Ti2AlNb substrate. The diffusion bonding zone was composed of continuous equiaxed α2 grains and a small amount of B2 and O phases. The α2 phase in the diffusion bonding zone was mainly transformed from the γ phase in γ-TiAl substrate and the B2 and O phase in Ti2AlNb substrate. The composite pulse current has a significant promoting effect on the diffusion of elements and joint formation. The diffusion coefficients of DAl and DNb in hot-pressing (HP, without current effect) diffusion welding joint at interface were 2 × 10−14 m2/s and 0.92 × 10−14 m2/s, respectively. In the representative PHC-950 °C joint with HL/LL = 12/2 cycles (single cycle duration 3.3 ms), diffusion coefficients of DAl-12/2, DNb-12/2 at interface were 19.62 × 10−14 m2/s, 13.34 × 10−14 m2/s, respectively. Increasing the duration of LL would weaken the current effect. The current waveform parameters were tuned to HL/LL = 12/4 and 12/6 cycles, the diffusion coefficients of DAl-12/4, DNb-12/4DAl-12/6 and DNb-12/6 were reduced to 15.69 × 10−14 m2/s, 12.49 × 10−14 m2/s, 15.31 × 10−14 m2/s and 10.99 × 10−14 m2/s, respectively. The PHC-950 °C joints have the highest shear strength of 257.3 MPa at HL/LL = 12/2 cycles, reaching 83.3% of the base material strength. The HP-950 °C TiAl/Ti2AlNb joint has a shear strength of 230.5 MPa, reaching 74.6% of the base material strength. The shear strength of PHC-950 °C joints at HL/LL = 12/4 and 12/6 reduced to 205.3 MPa, 195.8 MPa due to insufficient element diffusion and damages the bonding quality. This work provides a strategy that contributes to reduce the thermal damage of base metal and improve the joint forming efficiency.

Enhancement of Microstructure and Mechanical Properties of High Nb-TiAl Alloys Prepared Using Laser Additive Manufacturing by Annealing Treatment

The microstructure, phase composition, hardness, and tensile properties of the Ti-48Al-2Cr-5Nb alloy have been systematically investigated using laser additive manufacturing technology. Results indicate that both the as-deposited and annealed microstructures contain both the α2 (Ti3Al) and γ (TiAl) phases. As the annealing temperature increased, the structure changed significantly from a large block structure to a fine equiaxed structure and finally to a large lamellar structure. Nevertheless, the amount and distribution of precipitation of α2 phase are obviously different, especially during the annealing at 1260 °C, where the fine α2 phases are evenly distributed on the γ phase matrix. The hardness value of the as-deposited sample is the highest, with a HV value of 484 at the room temperature, while the hardness value of the annealed sample at 1260 °C is the smallest, with a HV value of 344. An annealed sample at 1260 °C exhibits the highest tensile strength and elongation at room temperature, with values of 598 MPa and 2.1%, respectively. These values are increased by 1.15 times and 1.4 times compared to the as-deposited sample (519 MPa, 1.5%).

Numerical Simulation of Fatigue Behavior of Four Ti2AlNb Alloy Structural Parts

Abstract In aerospace engineering, many titanium alloy structures are subjected to fatigue loads and thus fail. Based on the Seeger fatigue life theory and the improved Lemaitre damage evolution theory, the fatigue behavior of four Ti2AlNb alloys is investigated. First, finite element models of four structural parts are established by ABAQUS software. Meanwhile, the fatigue life of four Ti2AlNb alloys is predicted by referring to the damage model parameters determined by previous work. Under the same initial conditions, the average errors of the predicted fatigue lives of the four structural parts are 20.1, 19.8, 20.9, and 19.5 %, respectively. The effects of load amplitude, temperature, and structural characteristics on the fatigue properties of Ti2AlNb alloy structural parts are studied. The stability of the two fatigue life simulation methods is analyzed. By comparing fatigue data of Ti2AlNb structural parts from various literature, the rationality of the simulated data is confirmed. Finally, the application of the Ti2AlNb structural fatigue database to machine learning is illustrated. These results provide a numerical simulation method for evaluating the fatigue life of various Ti2AlNb alloy aviation structural parts.

Suppression of texture and enhancement of high-temperature tensile property in heavily deformed gradient Ti2AlNb alloy

Hot isostatic pressed Ti2AlNb alloys with trapezoid-shaped sections were hot rolled at 1000 °C to obtain the specimens with the gradient of deformation. The effect of the rolling reduction on the microstructure of the Ti2AlNb alloys was investigated by this high throughput method, and the mechanical properties of the gradient alloys were studied. With the increasing rolling reduction, the morphology of the orthorhombic (O) phase changed from well-arranged herringbone and palisade structures to a randomly distributed lamellar structure. By the electron backscatter diffraction characterization, the textures of the B2 and O phases changed with the increasing rolling reduction, and the texture of the O phase was depressed. The elevated-temperature tensile properties of the Ti2AlNb and Mo-modified Ti2AlNb gradient specimens were measured, and the deformation gradient was 36.0–43.0% and 43.0–50.0%, respectively. The heavily deformed gradient specimens exhibited higher strength and ductility than the lightly deformed ones, due to the increasing low-angle grain boundary and uniform distribution of local strain.

Effect of scanning speed on microstructure and mechanical properties of as-printed Ti–22Al–25Nb intermetallic by laser powder bed fusion

Ti–22Al–25Nb alloy is a lightweight, high-performance, and high-temperature material with broad application prospects in aerospace. Laser powder bed fusion (LPBF) enables the rapid manufacture of Ti–22Al–25Nb (at. %) powder into complex, high-precision parts for various applications. This paper provides an in-depth analysis of how scanning speed affects the microstructure and mechanical properties of Ti–22Al–25Nb alloy prepared by LPBF. The goal of this study is to improve the mechanical properties and the printing efficiency in the future. The temperature distribution in the printing process was studied by finite element simulation, and the effect of scanning speed was analyzed by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results show that increasing the scanning speed improved the cooling rate of the liquid melt, which decreases the grain size of the β phase and increases of the dislocation density. At the same time, a significant amount of the O phase precipitates along the grain boundaries. Excellent elongation (15%) and yield strength (982 MPa) are obtained due to the pinning of the nano-O phase, a fine grain, and good relative density when the scanning speed is 0.6 and 0.8 m/s, respectively.

Microstructure evolution of a forged TiAl-Nb alloy during high-temperature tensile testing

A forged TiAl-Nb alloy was investigated by means of measurements of properties related to performance at high temperatures, and the microstructure was observed by scanning electron microscopy (SEM) with electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The brittle-ductile transition temperature of the alloy was between 800 °C and 820 °C. The alloy had high strength in the brittle stage, and the main deformation mechanisms of the alloy were dislocation movement and dynamic recovery (DRV) in the γ phase. In the ductile stage, the alloy had good deformation ability and the main deformation mechanism of the alloy was continuous dynamic recrystallization (CDRX) in γ phase and α2 phase, the integrity of the lamellar colonies was destroyed. At the same time, the phase transformation occurred.

Structure and High-Temperature Oxidation Performance of Si-Co Diffusion Coatings Prepared on a TiAl-Nb Alloy

Si-Co diffusion coatings were prepared on a Y-modified TiAl-Nb alloy using the pack cementation process. The structures of the coatings prepared at different temperatures (1050, 1080, and 1120 °C) and pack Co contents (5, 10, and 20 wt.%) were comparatively studied. The coatings possessed the typical structure of a (Ti,X)Si2+Ti5Si4 (X represents Nb and Al elements) outer layer with a Co-rich superficial zone, a Ti5Si4+Ti5Si3 middle layer, and a TiAl2 inner layer. Increasing the co-deposition temperature in the range of 1050–1120 °C led to a larger coating thickness but a more-porous coating structure, while increasing the pack Co contents in the range of 5–20 wt.% caused a lower coating growth rate. The formation of the Si-Co diffusion coating followed an orderly process of depositing Si first and then Co. The Si-Co diffusion coating had much better anti-oxidation performance than both the TiAl-Nb substrate and pure silicide coating. After undergoing oxidation at 1000 °C for 100 h, the oxidation parabolic rate of the Si-Co diffusion coating was approximately 6.16 × 10−3 mg2/cm4h1, which was lower than those of the TiAl-Nb substrate by about two orders of magnitude and pure silicide coating by about one order of magnitude.