Ti2AlNb Alloy Research Articles

Production of superplastic Ti–6Al–7Nb alloy using high-pressure sliding process

Production of superplastic Ti–6Al–7Nb alloy using high-pressure sliding process

Vacuum brazing of Ti2AlNb and TC4 alloys using Ti–Zr–Cu–Ni and Ti–Zr–Cu–Ni + Mo filler metals: Microstructural evolution and mechanical properties

Two kinds of filler metal, Ti–Zr–Cu–Ni and Ti–Zr–Cu–Ni+Mo, were used to vacuum braze Ti2AlNb and TC4 alloys. The interfacial microstructures and the room temperature (RT) and elevated temperature shear strengths of the brazed joints were analysed. Moreover, the effects of the brazing parameters and filler metal Mo content on the microstructure and corresponding mechanical properties of the brazed joints were investigated. The results showed that the typical brazed joint mainly contained α-Ti, (Ti,Zr)2(Cu,Ni), β-Ti, and Ti-rich phases. The addition of the Mo particles was beneficial to inhibit the eutectoid transformation of β-Ti during cooling, resulting in the formation of residual β-Ti instead of α-Ti. In addition, Mo particles can also make the microstructure more homogeneous. The highest RT shear strength of the joints brazed with the Ti–Zr–Cu–Ni filler metal was 351MPa when the joint was brazed at 980°C for 10min. Under the same brazing parameters, the RT shear strength reached 437MPa with the addition of 8wt.% Mo particles to the filler metal. Moreover, the shear strengths of the joints brazed with the Mo-free and 8wt.% Mo filler metals tested at 600°C were 272MPa and 393MPa, respectively.

Microstructural Degradation of Ti-45Al-8Nb Alloy During the Fabrication Process by Electron Beam Melting

The microstructural degradation of the high Nb-TiAl alloy during the fabrication process by electron beam melting (EBM) is reported. The lamellar structure of as-EBM samples in the bottom part of the build shows significant microstructure degradation, resulting in deterioration of tensile properties at both ambient and high temperatures. Microstructural analysis has been conducted by electron backscattered diffraction and transmission Kikuchi diffraction microscopy. The results show that the degradation of the lamellar structure is not only caused by the coarsening under the high-frequency thermal cycling during the fabrication of following layers but also attributed by the discontinuous dynamic recrystallization of the unstable initial lamellar structure resulting from the rapid solidification.

Hot Deformation Behavior, Dynamic Recrystallization, and Texture Evolution of Ti–22Al–25Nb Alloy

The hot deformation behavior, dynamic recrystallization, and texture evolution of Ti–22Al–25Nb alloy in the temperature range of 950–1050 °C and strain rate range of 0.001–1 s−1 is investigated by plane‐strain compression testing on the Gleeble‐3500 thermo‐mechanical simulator. The results show that the flow stress decreases with the increase of temperature and decrease of strain rate. Besides, the flow curves appear a serrate oscillation at a strain rate of 0.1 s−1 for all the temperature ranges, which may result from instability such as flow localization or micro‐cracking. The flow behavior can be expressed by the conventional hyperbolic sine constitutive equation and the calculated deformation activation energy Q in the (α2 + B2) and B2 regions are 631.367 and 304.812 kJ mol−1, respectively. The microstructure evolution is strongly dependent on the deformation parameters, and dynamic recrystallization (DRX) is the dominant softening mechanism in the (α2 + B2) region, including discontinuous dynamic recrystallization (DDRX), and continuous dynamic recrystallization (CDRX). In addition, the ηbcc‐fiber of {110} <001> is the dominant texture component in deformed Ti–22Al–25Nb alloy. It is observed that the weakening of the deformation texture is accompanied by the occurrence of DRX, which can be attributed to the large misorientation between DRX grains and neighboring B2 matrix induced by the rotation of DRX grains toward the preferred slip systems.

Effect of Cooling Rate on Microstructure of Two Kinds of High Nb Containing Tial Alloys

In this paper, high Nb-TiAl alloys with Cr and W additions were prepared by Vacuum induction melting method, and then were heat treated under three different cooling rates of slow cooling, furnace cooling and air cooling. The phase composition of the alloy was analyzed by X ray diffraction, and the microstructure of the alloy was observed by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive analyzer. The results show that the microstructure of Ti45Al8Nb0.2Cr and Ti45Al8Nb0.2W are fully lamellar structure with the main phase composition of α+γ after 3 different heat treatment conditions. The grain size of the two alloys decreases with decreasing of cooling rate, and the grain size of the alloyed with Cr alloy is smaller than that of the alloyed with W alloy. Most of the original massive β phase at grain boundaries and lamellar interfaces dissolved after heat treatment, and the transformation of β phase is easier for Ti45Al8Nb0.2Cr.

Numerical Research on Magnetic Field, Temperature Field and Flow Field During Melting and Directionally Solidifying TiAl Alloys by Electromagnetic Cold Crucible

The electromagnetic cold crucible (EMCC) technique is an effective method to melt and directionally solidify reactive and high-temperature materials without contamination. The temperature field and fluid flow induced by the electromagnetic field are very important for melting and controlling the microstructure. In this article, a 3D EMCC model for calculating the magnetic field in the charges (TiAl alloys) using the T-Ω finite element method was established and verified. Magnetic fields in the charge under different electrical parameters, positions and dimensions of the charge were calculated and analyzed. The calculated results show that the magnetic field concentrates in the skin layer, and the magnetic flux density (B) increases with increasing of the frequency, charge diameter and current. The maximum B in the charge is affected by the position of the charge in EMCC (h 1) and the charge height (h 2), which emerges at the middle of coils (h c) when the relationship of h c < h 1 + h 2 < h c + δ is satisfied. Lower frequency and smaller charge diameter can improve the uniformity of the magnetic field in the charge. Consequently, the induced uniform electromagnetic stirring weakens the turbulence and improves temperature uniformity in the vicinity of the solid/liquid (S/L) interface, which is beneficial to forming a planar S/L interface during directional solidification. Based on the above conclusions, the TiAlNb alloy was successfully melted with lower power consumption and directionally solidified by the square EMCC.

Primerjava fizikalno-kemijskih lastnosti Al2O3 plasti, nanešenih na cpTi površine in zlitino Ti6Al7Nb z uporabo ALD metode

Primerjava fizikalno-kemijskih lastnosti Al2O3 plasti, nanešenih na cpTi površine in zlitino Ti6Al7Nb z uporabo ALD metode

High-temperature torsion induced gradient microstructures in high Nb-TiAl alloy

The high temperature torsion deformation of a duplex microstructure high Nb-TiAl alloy was investigated at 850°C. The experimental results showed that the hardness and the density of twins increased with radial distance from the center to rim of 180° torsion sample. Besides, the “recrystallized”, “substructured” and “deformed” microstructures were also presented gradient and forming sub-grains was the transition stage between recrystallization and deformation. Moreover, γ grain recrystallization at twin-twin junctions occurred and the observation of dislocation-twin interactions revealed the formation of secondary twins by dissociating of the dislocation at twin-twin boundary. The dissociation of such dislocations interacting with twin boundaries and γ grain recrystallizations at twin-twin boundaries avoided excessive stress concentration, which allowed the alloy to continue plastic deformation.

Dependence of the infrared emissivity on SiC content and microstructure of microarc oxidation ceramic coatings formed in Na2SiO3 electrolyte

High emissivity coatings have been widely used in thermal protection systems and high heat efficiency devices in furnaces. In the present work, high emissivity ceramic coatings incorporated with different SiC contents have been prepared on Ti2AlNb alloy by microarc oxidation (MAO) method. The addition of different SiC contents into the Na2SiO3 electrolyte has a distinct influence on the microstructure and normal spectral emissivity of composite coatings. Microstructure of MAO coatings was investigated by using a glancing incidence X-ray diffractometer, a scanning electron microscope, an eddy current thickness meter and a contact surface profiler. A two-step voltage-controlled technology was also used to tailor the microstructure by adjusting the microarc oxidation duration. The composite coatings formed on Ti2AlNb alloy exhibit a porous structure, and consist of R-TiO2, A-TiO2, Al2SiO5, SiC as well as amorphous phase. The normal spectral emissivity of both coated and bared Ti2AlNb samples was measured at 600°C in the infrared wavelength range of 3–20μm. The S-10C composite coating incorporated with a 10g/L of SiC in the Na2SiO3 electrolyte has a much higher emissivity in the wavelength range of 3–20μm than the bared Ti2AlNb alloy. High emissivity of the S-10C coating is attributed to its porous structure and chemical composition, especially SiC content.

Study on dynamic chip formation mechanisms of Ti2AlNb intermetallic alloy

Ti2AlNb alloy is a relatively newly developed high-temperature-resistant structural material due to its excellent mechanical properties, which is also regarded as one of the most difficult-to-cut materials. The aim of this study is to characterize the chip formation during the machining of Ti2AlNb intermetallic alloy due to its critical role in studying optimization of machining process variables. A geometry model of chip formation was established to analyze the influence of strain rate hardening effect and thermal softening effect on the flow stress in the shear zone. A series of orthogonal cutting tests and quick-stop cutting tests were conducted with varying cutting speeds and cutting depths to validate the evolutions of flow stress inside the shear band. Results associated with cutting forces, cutting temperature, chip morphology, and microhardness are discussed and compared with those of most commonly used Ti6Al4V alloy. It is shown that the cutting forces and cutting temperature of Ti2AlNb alloy are higher than those of Ti6Al4V alloy by 35.3 and 20%, respectively. The chip morphologies of Ti2AlNb alloy show very different characteristics with different cutting conditions (i.e., some very small sawtooth on the free surface of chips under low cutting speed, then transform into irregular serrated chips with increasing cutting speed, and to serrated chip with adiabatic shear band under high cutting speed), which correlate well with the evolutions of microhardness distribution. The chip morphologies and chip formation characteristics are dependent largely on the dynamic flow stress, which is sensitive to the competition of the thermal softening effect with strain rate hardening effect.

Hot Deformation Mechanism and Ring Rolling Behavior of Powder Metallurgy Ti2AlNb Intermetallics

Powder metallurgy (PM) Ti–22Al–24Nb–0.5Mo (at.%) alloys were prepared by hot isostatic pressing. In order to study the feasibility of PM + ring rolling combined process for preparing Ti2AlNb rings, thermal mechanical simulation tests of PM Ti2AlNb alloys were conducted and two rectangular PM rings (150 mm in height, 75 mm in thickness, 350 mm in external diameter) were rolled as a validation experiment. Experimental results show that the flow stress of Ti2AlNb alloys exhibited a significant drop at the very beginning of the deformation (true strain <0.1), and became stable with the increase in strain. Stress instability phenomenon of PM Ti2AlNb alloys was more obvious than that of wrought alloy. Flow stress fluctuation at the initial stage of deformation is related to phase transition of Ti2AlNb alloys which strongly depends on heat treatment and thermal mechanical deformation process. Processing windows during initial stage of ring rolling process is very crucial. A sound PM Ti2AlNb rectangular ring blank (height = 150 mm, thickness = 30 mm, external diameter = 750 mm) was successfully rolled in two passes by using the improved heat preservation method and optimized rolling parameters. Tensile properties of PM Ti2AlNb alloy were improved, and the porosity was reduced after ring rolling.

Effect of N-ion implantation and diamond-like carbon coating on fretting wear behaviors of Ti6Al7Nb in artificial saliva

The tribology behaviors of Ti6Al7Nb, its alloy with N-ion implantation, and its alloy with diamond-like carbon (DLC) coating were investigated in artificial saliva. Fretting wear tests of untreated, N-ion implanted and DLC coated Ti6Al7Nb alloys plate against a Si3N4 ball were carried out on a reciprocating sliding fretting wear test rig. Based on the analysis of X-ray diffraction, Raman spectroscopy, 3-D profiler, SEM morphologies and frictional kinetics behavior analysis, the damage behavior of surface modification layer was discussed in detail. The results indicated that the fretting wear behavior of Ti6Al7Nb alloy with N-ion implantation was increased with the dose increase of the implanted nitrogen ions. Moreover, the DLC-coated Ti6Al7Nb alloy with low ion implantation could improve the fretting wear behavior greatly. In addition, the Ti6Al7Nb with DLC coating had better corrosion resistance due to the special compact structure. All results suggested that the Ti6Al7Nb with DLC coating had better wear resistance than that with N-ion implantation in artificial saliva.

Dual structure O + B2 for enhancement of hardness in furnace-cooled Ti2AlNb-based alloys by powder metallurgy

Powder metallurgic Ti2AlNb alloys were sintered at 900°C, 990°C, 1060°C, and 1100°C (i.e. in the O+B2, α2+B2+O, α2+B2, and B2 phase region, respectively) for 12h, followed by water quenching and furnace cooling. Quenching was employed to reserve the high-temperature phase and microstructure, and furnace cooling aimed to regulate the room-temperature microstructure for the enhancement of hardness. Widmanstatten B2+O structure, which contributes to the properties, was induced from B2 crystals by sintering, unless the alloy was treated in the α2+B2 phase region. With the elevation of the sintering temperature, the content of α2 phase became lower in the furnace-cooled alloys, and the hardness was improved accordingly. The highest hardness performance, 389±23HV, was obtained in the alloy solution treated in the single B2 region, and the alloy was comprised of complete O+B2 Widmanstatten structure.

Influence of electrolyte composition on microstructure, adhesion and bioactivity of micro-arc oxidation coatings produced on biomedical Ti6Al7Nb alloy

Micro-arc oxidation is a simple, single-step surface modification method used to produce coatings on titanium alloys for biomedical applications. The formation and properties of coatings can be widely controlled by adjusting process parameters such as applied voltage, frequency, processing time and electrolyte composition. In the present study, three different electrolytes were used to obtain well adherent, rough, porous, hydroxyapatite containing, oxide coating on Ti6Al7Nb alloy. Coatings were investigated in terms of their microstructure, chemical composition, roughness, hardness, adhesion strength and biocompatibility. Among the coatings produced in different electrolytes, one of them exhibited both good mechanical properties and bioactivity, while containing crystalline hydroxyapatite on the surface, whereas the other two coatings were mostly amorphous. Thereby, we have proposed an efficient electrolyte containing calcium and phosphorous species in order to modify titanium based alloys to improve their biocompatibility and osseointegration. Such properties are achieved by formation of a rough outer layer with open pores, providing high surface area for cells to adhere, proliferate and migrate into the coating, promoting firm anchorage of the coated implant in the bone.

Continuous Casting of TiAlNb Alloys with Different Velocities by Mixing Binary TiAl Ingot and Nb Wire

A method of continuous casting is used and the research investigates microstructure and mechanical properties under different drawing velocity (R, mm/min). The results show that microstructure and composition measurement of different zones are uniform. The smallest grain size is 25.93 μm and formability is good with 0.5R. Compressive strength is higher with 0.5R and maximum value is 1697MPa. Fracture toughness with 0.5R improves about 35.7% which is 21.7MPa·m1/2. The fracture morphology is trans‐lamellar fracture and interface de‐lamination. The method is a feasible way to continuously cast the TiAl‐Nb alloys. The R is an important parameter to add high‐melting‐point element, which affects solidified temperature interval of solidification front and electromagnetic stirring.

Differences in the expression of cell cycle genes in osteoblasts and endothelial cells cultured on the surfaces of Ti6Al4V and Ti6Al7Nb alloys.

Three medically used alloys (Ti6Al4V, Ti6Al7Nb, and AISI 316 L) are compared due to proliferative potential and metabolic response of human cells (osteoblasts line Saos-2 and endothelial cells line EA.hy-926) seeded on the surfaces of these alloys. Although no statistically significant difference in the proliferative potential of the cells cultured on the surfaces of examined biomaterials was observed, it does not exclude relevant differences in metabolic response of these cells assessed as changes in genes' expression. As a result of our studies it was demonstrated that the changes in the expression of examined genes were very common. Our observation suggests the presence of the process of selective recognition of the contacted biomaterials by the cells seeded on their surfaces. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1607-1617, 2017.

Effects of Nb content in Ti–Ni–Nb brazing alloys on the microstructure and mechanical properties of Ti–22Al–25Nb alloy brazed joints

Vacuum brazing was successfully used to join Ti–22Al–25Nb alloy using Ti–Ni–Nb brazing alloys prepared by arc-melting. The influence of Nb content in the Ti–Ni–Nb brazing alloys on the interfacial microstructure and mechanical properties of the brazed joints was investigated. The results showed that the interfacial microstructure of brazed joint consisted of B2, O, τ3, and Ti2Ni phase, while the width of brazing seams varied at different Nb contents. The room temperature shear strength reached 359MPa when the joints were brazed with eutectic Ti40Ni40Nb20 alloy at 1180°C for 20min, and it was 321, 308 and 256MPa at 500, 650 and 800°C, respectively. Cracks primarily initiated and propagated in τ3 compounds, and partially traversed B2+O region. Moreover, the fracture surface displayed typical ductile dimples when cracks propagated through B2+O region, which was favorable for the mechanical properties of the brazed joint.

Enhanced Ductility of a Bimodal Grain Structure Ti–22Al–25Nb Alloy Fabricated by Spark Plasma Sintering

A bimodal grain structure Ti–22Al–25Nb alloy with enhanced ductility is fabricated from the blended powder mixture of argon gas atomized powder and mechanically alloyed powder by spark plasma sintering. The microstructure of the bimodal grain structure Ti–22Al–25Nb alloy and the effects of the argon gas atomized powder mass fraction on the compression properties are investigated. This alloy shows the bimodal microstructure, which the coarse grain regions are surrounded by the ultrafine grained matrix. Compared with the Ti–22Al–25Nb alloy sintered from 100% mechanically alloyed powder, this alloy sintered from the blended powder mixture containing 20% argon gas atomized powder exhibits the fracture elongation to improve 20.5%.

Electrochemical Behaviour of Titanium Based Biomaterials in Artificial Saliva

In the last decade, new titanium alloys such as Ti6Al7Nb and Ti6Al7NbN have been developed in different areas of dentistry. It is known that mechanical/electrochemical interactions accelerate corrosion in implant alloys, and these interactions can be reduced by improving the modular component machining tolerances or by improving the corrosion resistance. The aim of this study was to compare the Ti6Al7Nb and Ti6Al7NbN alloys corrosion resistance in AFNOR artificial saliva. Due to the nitrogen diffusion, the Ti6Al7NbN alloy presents more pronounced grain structure than Ti6Al7Nb. In the electrochemical estimations the polarisation data were converted into instantaneous corrosion rate values (jcorr). The obtained passivation properties were comparable for both alloys. The EIS spectra were best fitted using an equivalent circuit (EC) which was consistent with the model of a two-layer structure for the passive film. The electrochemical and corrosion behaviour of Ti6Al7NbN increased the occurrence of the implant alloy�s electrochemical degradation.

Diffusion Bonding of TA15 and Ti2AlNb Alloys: Interfacial Microstructure and Mechanical Properties

TA15 and Ti2AlNb alloys were joined by diffusion welding. The influence of holding time on morphology and mechanical properties of the joint was studied under two conditions of different bonding pressure and temperature. The interface structure was analyzed by BSE and EDS. The mechanical properties of joints were tested. The results show that the typical interfacial microstructure consists of lath α-phase (TA15 alloy)/flake α phase + α-interfacial phase + α2 phase/B2-rich phase/Ti2AlNb alloy. When bonding at 920 °C and 15 MPa with increasing holding time, the interface microstructure evolves into flake α phase and distributes as a basket-weave and the interfacial coarse spherical α phase distributes as a line. α2 phase and O phase disappear gradually while the content of the B2 phase increases. The tensile strength of the joints is 870, 892 and 903 MPa, for 120, 150 and 210 min holding time, respectively, while the elongation rises as well. When bonding at 940 °C and 10 MPa with increasing holding time, the interfacial area includes more Widmanstatten structure and B2 phase. The tensile strength of joints decreases from 921 to 908 MPa, while the elongation increases from 12 to 15.5%, for holding 120 and 210 min, respectively. The tendency of plastic fracture also increases with holding time for both temperature-pressure combinations.