Ti2AlNb Alloy Research Articles

Optimizing the Surface Treatment Processes to Enhance the Bioactivity of Ti–6Al–7Nb Alloy

The current research is devoted to assessing the bioactivity of different surface-treated Ti–6Al–7Nb samples by assessing the formation of calcium phosphate compounds. Three surface modification processes were performed, followed by dynamic immersion in Hanks’ solution for 32 days at the body temperature. The first set of samples was treated by oxidation at 400, 500 and 600 °C for 1 and 2 h to study the optimum oxidation condition. The second group was alkaline-treated using different concentrations of NaOH and aged at 400 and 600 °C. The third group was treated by a strong acidic solution (H2SO4 and HCl) for 0.5–1.5 h and then aged at 400 and 600 °C. Growth of calcium phosphate compounds (CPC) was generally enhanced with the formation of NaCl dendritic network, as it worked as an adhesive layer. Formation of the oxide layer enhanced the affinity to precipitate CPC, and the optimum oxidation condition was 500 °C for 1 h; higher temperatures and/or longer periods tended to dissolve back the built-up regions. Alkaline treatment prior to aging resulted in the formation of uneven build-up of CPC. Increasing the aging temperature at constant concentration encouraged the formation of dendritic structure (NaCl) and increased the width of the branches resulting in increased precipitation of CPC. Alkaline treatment and aging for 1 h using 2 M solution at 400 °C was the best condition to grow dense flower-shaped precipitates of CPC. However, increasing the aging temperature to 600 °C using the same solution resulted in the formation of TiO2 which created more sites for CPC growth, and hence more homogenously distributed particles with less concentration were obtained. In case of the acidic treatment, increasing the immersion time in the acidic solution beyond 1 h dissolved back NaCl built-up regions and thus decreased the surface ability to grow CPC. A palette of colors was obtained on the samples’ surfaces after the different treatments, which was beneficial in improving the esthetics of implant components and implant prosthesis substructures.

Interface characteristic and mechanical performance of TiAl/Ti2AlNb diffusion bonding joint with pure Ti interlayer

Solid-state diffusion bonding (DB) of TiAl alloy and Ti2AlNb alloy was carried out using pure Ti as an interlayer at 1000 °C under 20 MPa for 60–120 min. The effects of bonding times on the interfacial microstructure and mechanical performance of the TiAl/Ti/Ti2AlNb bonded joints at room temperature (RT) were investigated detailly. The results demonstrated that the diffusion layers (DLs) mainly consisted of four characteristic layers, (I) single coarse α2 phase adjacent TiAl alloy, (II) single refined α2 phase at the bonding interface, (III) equiaxed/acicular α2 phase embedded in β phase adjacent Ti2AlNb alloy and (IV) both equiaxed α2 phase and acicular O phase embedded in β phase adjacent Ti2AlNb alloy, respectively. The thickness of the four layers increased with the increasing of the bonding time. The growth of DLs is controlled by diffusion and the reaction rate constant k for region I, II, III and IV are 1.22 × 10−6, 1.27 × 10−6, 2.6 × 10−7 and 7.7 × 10−7 m·s−1/2, respectively. Meanwhile, the interface α2 grain grows up without texture. The maximum tensile strength of 281 MPa was maintained at 1000 °C for 90 min under the pressure of 20 MPa. Consequently, the phase transformation and dynamic recrystallization behavior of the DLs were discussed.

Preparation and Tribological Behavior of Ti2AlNb/Ag Self-Lubricating Composites

In this paper, Ti2AlNb-based composites with different concentrations of Ag were fabricated by ball milling and spark plasma sintering (SPS). Microstructural evolution and phase compositions of the composites were further analyzed. The MS-T3001 friction wear tester was used to evaluate the friction performance of the Ag/Ti2AlNb self-lubricating composites. Results showed that the composites exhibit better anti-friction and anti-wear performance comparing with pure Ti2AlNb alloy. As the Ag wt.% increased, the coefficient of friction (COF) and wear rate of the composites were all decreased. Compared with pure Ti2AlNb, the COF and wear rate of Ti2AlNb-15 wt.%Ag self-lubricating composites at room temperature were decreased approximately 42 and 46%, respectively. The self-lubricating mechanisms of the composites were further discussed, and the silver-rich friction film formed on the friction surface under compression stress was regarded as the main anti-friction and anti-wear factors.

Preparation and Performance of an Abradable NiCrFeAlBN-YSZ-NiCrAl Layered Seal Coating for Aircraft Engines

Ti2AlNb alloys are often used to fabricate high-pressure compressor blades and casings in aircraft engines with high thrust/weight ratios. In this study, a three-layered seal coating, NiCrFeAlBN-YSZ-NiCrAl, was prepared by thermal spraying on a titanium alloy (Ti2AlNb) substrate. The microstructure, surface hardness, and bond strength of coatings as well as their thermal cycling performance and abradability under simulated working conditions were investigated. The coating surface hardness was 61 HR15Y and the bond strength was 7.13 MPa. The coating maintained a good bonding to the substrate with no cracks after 40 cycles of water-cooling from 750 °C to room temperature. The abradability of the as-sprayed coating and the coating heat-treated at 750 °C for 200 h was tested against a Ti2AlNb blade at 750 °C, with a tip line speed of 300 m/s, and feed rates of 5, 50 and 480 m/s. The coating wear mechanisms involved micro-ruptures. The Cr2O3 phase formation and a decrease in the number of micropores in the heat-treated coating decreased its abradability.

Microstructure and optical properties of TiO2- and HA-coated Ti6Al7Nb alloy by sol–gel method

In this study, the combination of titanium dioxide (TiO2) and/or hydroxyapatite (HA) coatings on Ti6Al7Nb titanium alloy to be used as an implant was performed in different solution retention times by the sol–gel dip-coating method and the characterization and optical properties of these coatings were investigated. Two different coatings were made on the substrate, the first group of samples was first coated with TiO2 and then with HA, while the second group of samples was coated with TiO2 alone. Before sol–gel coating, the samples were rinsed in distilled water and dried at 200°C for 2 h. The samples were immersed perpendicular to the coating solution. Thin films were obtained at a draw rate of 2 mm s−1. Microstructure investigations of the prepared coatings were examined under optical and scanning electron microscopes, while solid-state elemental fractions were determined by energy-dispersive X-ray spectroscopy analysis. Fourier-transform infrared spectroscopy analysis was performed to determine the functional groups placed on the surface of the material. The optical properties of the material were determined by UV analysis.

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.

Influence of surface modification of Ti6Al7Nb alloy on adhesion of poly (lactide-co-glycolide) coating (PLGA)

The paper presents the results of studies on the influence of surface topography and wettability of a Ti6Al7Nb alloy substrate on the adhesion of a PLGA coating. The alloy surface was modified using a mechanical pre-treatment including: grinding, vibration treatment, mechanical polishing, sandblasting and anodic oxidation. The polymer coating was applied to the metal substrate by a dip-coating method. The scope of the research included microscopic observations of the substrate and polymer coating using SEM and acoustic microscopy. In addition, studies on the wettability and topography of the polymer coating and the metal substrate, the thickness of the polymer coating as well as qualitative and quantitative testing of the adhesion of the polymer coating to the substrate were carried out. Coating adhesion tests were conducted for samples in the initial state and after 6 weeks exposure to Ringer's solution.Analysis of the results indicates the influence of the method used to modify the metal substrate on its topography and wettability. These parameters affect the thickness of the obtained polymer coating. Regardless the parameters of the metal substrate, a qualitative analysis of the adhesion of the coating applied to the substrate of the Ti6Al7Nb alloy did not show any delamination for both samples exposed and non-exposed to Ringer's solution. On the other hand, quantitative scratch-test studies showed different adhesion of the polymer coating to the substrate depending on the surface topography obtained by various modification methods.The cytotoxicity test conducted by the indirect method using extracts confirmed that the surface modification does not affect cell growth.The complex methods of surface pre-treatment of the alloy together with the kind of polymer selected for the study allowed to develop well adhered PLGA layers on Ti6Al7Nb intended for short term implants. The lack of delamination of the layer during 6 weeks was proved, what allows for maintaining the protection function of the layer during this period and contribute to improving biocompatibility.

Spark Plasma Diffusion Bonding of TiAl/Ti2AlNb with Ti as Interlayer

To solve the problem of poor weldability between TiAl-based and Ti2AlNb-based alloys, spark plasma diffusion bonding was employed to join a TiAl alloy and a Ti2AlNb alloy with a pure Ti foil as interlayer at 950 °C/10 KN/60 min. After welding, slow cooling was carried out at a rate of 5 °C/min, followed by homogenization at 800 °C for 24 h. The microstructural evolution and elemental migration of the joint were analyzed via a scanning electron microscope equipped with an energy dispersive spectrometer, while the mechanical properties of the joint were assessed via microhardness and tensile tests. The results show that the spark plasma diffusion bonding formed a joint of TiAl/Ti/Ti2AlNb without microcracks or microvoids, while also effectively protecting the base metal. Before heat treatment, the maximum hardness value (401 HV) appeared at the Ti2AlNb/Ti interface, while the minimum hardness value (281 HV) occurred in the TiAl base metal. The tensile strength of the heat-treated joint at room temperature was measured to be up to 454 MPa, with a brittle fracture occurring in the interlayer. The tensile strength of the joint at 650 °C was measured to be up to 538 MPa, with intergranular cracks occurring in the TiAl base metal.

Effect of nitriding conditions of Ti6Al7Nb on microstructure of TiN surface layer

Presented experiments were aimed at comparing microstructure of the TiN layer produced on Ti6Al7Nb alloy being gas nitrided (GN) or glow discharge nitrided with either active screen (GD-ASN) or at cathode potential (GD-CPN). They were treated at 620 °C, 680 °C, 740 °C and 830 °C for 6 h. The transmission electron microscopy investigations showed that thickness of the TiN layer depends less on average temperature of nitrided piece, but more on temperature of its surface, being the highest for the GD-CPN process. The growth of TiN layer during the GN treatment proceeds mainly towards the core and to a lesser extent at the surface. The former process is controlled by chemisorption and inward diffusion of nitrogen atoms, while the latter by outward diffusion of titanium. The factor controlling the growth of TiN during the GD-ASN treatment is exclusively the flux of the titanium atoms sputtered from the active screen. The thickness of the TiN layer produced during the GD-CPN process in the temperature range between 680 °C and 830 °C is controlled in the same way as during the GN, except the fact that bombardment of the processed material with nitrogen ions strongly raises the surface temperature.

Tribological Properties of Ti2AlNb Matrix Composites Containing Few-Layer Graphene Fabricated by Spark Plasma Sintering

Ti2AlNb alloys with few-layer graphene were fabricated by spark plasma sintering (SPS) to enhance the tribological properties (TP) of the composite materials. Microstructure characteristics of the original few-layer graphene (FLG), Ti2AlNb powders, and the sintered composites were characterized by X-Ray Diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. The experimental results indicated that FLGs were homogeneously distributed in the composites. Tribological results indicated that the coefficient of friction (COF) of the composites was reduced as the content of FLG increased. Compared with the pure Ti2AlNb alloy, the average COF of the composite with 1.0 wt.% FLG was decreased by 9.4% and the wear rate was decreased by 36%. Meanwhile, the microstructures of the worn surface showed that TiC particles and friction layers formed by residual FLGs were present on the surface of the composites after tribological test. It is proposed that Ti2AlNb alloys with FLGs presented the enhanced wear resistance.

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.

Microstructure evolution and mechanical properties of Ti2AlNb/TiAl brazed joint using newly-developed Ti–Ni–Nb–Zr filler alloy

Joining of Ti2AlNb alloy to TiAl intermetallics was conducted by the newly-developed Ti–Ni–Nb–Zr brazing filler alloy. The microstructure evolution of the joints was investigated by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and electron backscatter diffraction (EBSD). The macro-micro mechanical properties were studied by shear test and nano-indentation test. Typical interfacial microstructures across the brazing seam were Ti2AlNb substrate, α2-Ti3Al+β-Ti, γ-TiAl+Ti2Ni+TiNi+α2-Ti3Al, α2-Ti3Al+β-Ti, TiAl substrate. The Ti2Ni phase were firstly dissolved in the joints brazed at 1000 °C for 10 min and then precipitated after a prolonged holding time of 15 min. The nano-indentation test revealed that Ti2Ni phase exhibited the highest hardness of 12.60 GPa. The joints brazed at 1000 °C/15 min presented the maximum shear strength of 271 MPa. The dissolution and precipitation behavior of Ti2Ni phase was also discussed.

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%.

Effects of pure Ti or Zr powder on microstructure and mechanical properties of Ti6Al4V and Ti2AlNb joints brazed with TiZrCuNi

Ti6Al4V and Ti2AlNb alloys were brazed with three kinds of filler metals, typical commercial filler of TiZrCuNi, TiZrCuNi mixed with Ti or Zr powder. The microstructure evolution, production and growth mechanisms of reaction phases and mechanical properties of the brazing joints were investigated. The results demonstrated that primary dendrites formed by IMCs (Ti,Zr)2(Cu,Ni) and a layer of eutectic β+(Ti, Zr)2(Cu, Ni) products were observed in the TiZrCuNi joint. And the number of IMCs significantly in the joint increased when Zr powder was added in the filler. But if the addition was Ti powder, IMCs can be eliminated a lot. The suppression of ICMs production in the TiZrCuNi + Ti joint was mainly due to two reasons: firstly, the more β phase were firstly formed in the molten filler as the solid solubilities of Cu or Ni in Ti was higher than those in Zr; secondly, owing to the increased melting point of filler by adding the Ti powder, the time spent on the solidification of liquid filler can shorten and the segregation of Cu/Ni then partially repressed. Resulted from the efficient suppression of the detrimental IMCs, the highest tensile strength of 738.7 MPa was obtained by TiZrCuNi + Ti joint, which was higher than those of the TiZrCuNi joint (~136.1 MPa) and TiZrCuNi + Zr joint (~200.1 MPa).

Microstructure evolution and mechanical properties of LaB6-modified Ti2AlNb alloy fabricated by blended elemental powder metallurgy

This study was conducted to evaluate the microstructure evolution of the blended elemental powder metallurgy Ti2AlNb alloy with a 2.0 wt% LaB6 addition and the effects of LaB6 doping on the microstructure and mechanical properties of the alloy. The results showed that the microstructure evolution could be divided into two stages. During the first stage, the reaction between Ti and Al powders was dominant. During the second stage, the Nb-enriched area disappeared gradually and microstructural homogeneity was enhanced with the diffusion of Nb. It was demonstrated that the LaB6 phase could be transformed into La2O3, (Ti, Nb)B and the (Ti, Nb)3B2 transition phase in the sintering process. LaB6 addition inhibited the precipitation of the brittle α2 phase and refined the microstructure. Compared with the alloy without LaB6, the Ti2AlNb alloy with 2 wt% LaB6 had 31.5% and 38.4% improvements in the fracture strain at 25 °C and 650 °C, respectively, and its yield strength was also increased.

Seeded growth of Ti–46Al–8Nb polysynthetically twinned crystals with an ultra-high elongation

Large size polysynthetically twinned crystals of Ti–46Al–8Nb alloy with a parallel lamellar microstructure were successfully prepared using a Ti–43Al–3Si seed by our new operation. A large amount of columnar B2 phase paralleling to the growth direction was found in the final lamellar microstructure. Higher growth rate (>30 mm/h) led to the failure of seeding process. Based on these results, a new mechanism is proposed to describe the seeding process of the hypo-peritectic TiAl alloys. The peritectic α phase is suggested to directly nucleate from the melt, and then act as nucleus for transformed α phase in the subsequent β to α transformation. At the higher growth rate, the appearance of β phase secondary dendrites and homogeneous nucleation lead to the failure of seeding process. High Nb addition leads to a large amount of residual β phase, and these β dendrites finally evolve into B2 phase. The room temperature tensile elongation was measured to be 11.9–18.5% for Ti–46Al–8Nb PST crystals, which is the highest ever reported value for TiAl based alloys.

Electrochemical and Biological Performance of Biodegradable Polymer Coatings on Ti6Al7Nb Alloy.

The inhibition of the corrosion of metal implants is still a challenge. This study aimed to increase the corrosion resistance of Ti6Al7Nb alloy implants through surface modification, including grinding, sandblasting, and anodic oxidation followed by the deposition of a polymer coating. The aim of the work was to determine the influence of biodegradable polymer coatings on the physico-chemical properties of a Ti6Al7Nb alloy used for short-term implants. Biodegradable coatings prepared from poly(glycolide-caprolactone) (P(GCap)), poly(glycolide ε-caprolactone-lactide) (P(GCapL)), and poly(lactide-glycolide) (PLGA) were applied in the studies. The dip-coating method with three cycles of dipping was applied. Corrosion resistance was assessed on the basis of potentiodynamic studies. The studies were carried out on samples after 30, 60, and 90 days of exposure to Ringer’s solution. Surface topography, wettability, and cytotoxicity studies were also carried out. The degradation process of the base material was evaluated on the basis of the mass density of the metal ions released to the solution. The results indicated the influence of the coating type on corrosion resistance. In addition, a beneficial effect of the polymer coating on the reduction of the density of the released metal ions was found, as compared to the samples without polymer coatings. The obtained results provide basic knowledge for the development of polymer coatings enriched with an active substance. The presence of ciprofloxacin in the coating did not reduce the corrosion resistance of the metal substrate. Moreover, the cytotoxicity test using the extract dilution method demonstrated that the implants’ coatings are promising for further in vitro and in vivo studies.

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

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Effect of synovial fluid temperature on the corrosion resistance of Ti6Al4V, Ti6Al7Nb, and CoCrMo alloys

AbstractThe frictional heat generated during the operation of the joint prosthesis in vivo could change the corrosion resistance of the prosthesis material. In this paper, the corrosion behavior of three medical alloys at different synovial fluid temperatures was analyzed using electrochemical measurement technology. Furthermore, the scanning electron microscope and energy‐dispersive spectrometer were used to characterize the surface morphology and composition of the alloys after long‐term immersion. The results show that increasing temperature causes the open‐circuit potential of titanium alloy to shift negatively and the corrosion tendency to increase. The increasing temperature leads to the decrease of activation energy of titanium alloy, which in turn results in the increase of corrosion current density and accelerated corrosion. The results of Nyquist curves confirmed that the radius of the capacitive arc decreased with the increase of temperature, indicating the deterioration of corrosion resistance. The CoCrMo alloy shows the same regularity as the titanium alloy in 0.9% NaCl, though no obvious regularity in 25% newborn bovine serum; this may be related to the complexity of the corrosion system.