Ti2AlNb Alloy Research Articles

Flow characteristics and deformation mechanisms for TiAl/Ti2AlNb diffusion bonded joint

The hot deformation behavior and deformation mechanism of TiAl/Ti2AlNb diffusion bonded (DB) joint in the temperature range of 900 °C–1000 °C and strain rate range of 0.001 s−1∼0.1s−1 were investigated by isothermal compression tests. The deformation may be related to the coordination deformation of the two alloys，with the deformation of Ti2AlNb section coming firstly, followed by TiAl due to the existence of the DB interface. The flow behavior of DB joint can be expressed by the hyperbolic sine constitutive equation, and the calculated deformation activation energy Q is 343.5 kJ/mol. The Q value is almost equal to lattice diffusion coefficient (265–331 kJ/mol) in O + B2 Ti2AlNb alloy, which suggests the deformation mechanism could be controlled by lattice diffusion. Microstructure observation shows that, When deformed at 900°C-0.1s−1, the deformation mechanism is grain boundary sliding accommodated by O-DRX in Ti2AlNb, γ-DRX in TiAl, deformation twin and β→α2 phase transformation. While the deformation mechanism is dislocation slip accommodated by β+O→ε phase transformation，ε-DRX，β-DRX in Ti2AlNb and deformation twin when deformed at 1000°C-0.001s−1. Besides, the 3D processing map of TiAl/Ti2AlNb DB joint were obtained with deformation temperature and strain rate. There are no flow instability regions of DB joint and the optimum processing parameters were determined to be 1000°C-0.001s−1, favoring development of a new structural component.

Effect of W addition on phase transformation and microstructure of powder metallurgic Ti-22Al-25Nb alloys during quenching and furnace cooling

Powder metallurgic Ti2AlNb alloys with W addition are sintered at 900, 1000, 1070 °C, and 1150 °C (i.e., in the O + B2, α2 + B2 + O, α2 + B2, and single B2 phase regions, respectively) for 12 h, followed by water quenching and furnace cooling. Comparisons of phase and microstructure between quenched and furnace-cooled W-modified alloys are carried out to illustrate the phase transformation and microstructure evolution during the cooling process. Furthermore, a comparison is also made between W-modified and W-free alloys, to reveal the function of the W alloying. W addition accelerates the solutions of α2 and O phases during the high-temperature holding, and a Widmannstätten B2 + O structure, which contributes to the properties, is induced by furnace cooling from all the phase regions. The Widmannstätten structure includes a B2 matrix, primary O, and secondary O precipitates. However, W alloying refines the Widmannstätten structure only when the alloys are solution-treated and then cooled from the single B2 phase. Although the hardness of the W-modified alloys is lower than that of the W-free alloys sintered in the same phase region, an enhancement of hardness, 489 ± 18 HV, is obtained in the alloy solution-treated in the single B2 phase region for only 0.5 h.

Improved superplasticity and microstructural evolution of Ti2AlNb alloy sheet during electrically assisted superplastic gas bulging

The electrically assisted forming has been a potential technique in the field of metallic sheet processing. Due to the demand of aerospace industry, Ti2AlNb alloy sheet combined with electrically assisted bulging method is studied. The results indicate that the superplasticity characterized by height/diameter ratio of the electrically assisted sample is 0.64 whereas that of the furnace-heating sample is 0.18. Because of the decrease in temperature during electrically assisted bulging and uneven temperature distribution, the lamellar O-phase precipitates from the matrix B2-phase with different sizes at typical positions of bulging sample, which is considered to simultaneously give rise to the transformation induced superplasticity. Subject to the high heating rate and short elevated-temperature exposure time compared with furnace-heating bulging, the O-phase is not coarse attributed to the deficient diffusion, while the O-phase has markedly grown up in the furnace-heating bulging sample. Moreover, no cavity has been found in the electrically assisted bulging sample. The impacts of temperature, strain, and electric current during electrically assisted bulging on precipitation and growth of O-phase were individually discussed in this work.

Oxidation behavior of a novel multi-element alloyed Ti2AlNb-based alloy in temperature range of 650–850 °C

The oxidation behavior of a novel multi-element alloyed Ti2AlNb-based alloy (Ti–22Al–25Nb–1Mo–1V–1Zr–0.2Si) was studied in the temperature range of 650–850 °C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with an energy-dispersive spectroscopy (EDS) were used to identify the phase constituents and microstructure of the scales formed on the specimens after oxidation at different temperatures. Isothermal oxidation tests show that the oxidation rate of the alloyed Ti2AlNb-based alloy is obviously reduced at all temperatures, and the mass gains are very low for this alloy in comparison with those of Ti–22Al–25Nb alloy. The alloying elements Mo, V, Zr and Si have an obvious affect on the oxidation products of the alloys. The improved oxidation resistance for the alloy is ascribed to the introduction of Mo, V, Zr and Si elements, which are beneficial to the selective oxidation of Al to form protective oxides while are disadvantage of the formation of poor oxidation resistance oxides such as AlNbO4.

Investigation on electrically-assisted diffusion bonding of Ti2AlNb alloy sheet by microstructural observation, mechanical tests and heat treatment

The process of diffusion bonding involves long time heating and needs bulky heating furnace. A long production cycle severely limits the commercial application on a large scale due to capital and energy costs. In an attempt to shorten the heating time in a cost-feasible manner, a novel electrically-assisted diffusion bonding (EADB) was proposed in this study. EADB is a result of applying electrically assisted concept to the conventional diffusion bonding process. Firstly, an EADB set-up was assembled to study the joining of Ti2AlNb alloy sheet that is recently studied as an excellent high-temperature structural material. The temperature distributions under different electric current densities were experimentally studied and the Ti2AlNb alloy sheets were successfully bonded without micro-cavity under 4.58 A/mm2. The microstructural evolution of EADB samples was discussed. Detour effect of electric current was found around the micro-cavity, which was determined by finite element analysis. Moreover, the mechanical tests of EADB samples showed that the highest shear strength was 587.24 MPa and the maximum hardness was 472.88 ± 6.36 HV/0.5/10. After that, heat treatment was used to homogenize the microstructures of EADB samples and the final samples showed basket-weave microstructures. Finally, comparisons with conventional processes were discussed in detail.

Interface characteristics of Ti6Al4V-TiAl metal-intermetallic laminate (MIL) composites prepared by a novel hot-pack rolling

Ti6Al4V (wt%)/Ti-44Al-8Nb-0.2W-0.2B-0.03Y (at.%) (high Nb-TiAl) metal–intermetallic laminate (MIL) composite with a defect-free interface was successfully prepared by hot-pack rolling and the interface microstructure was characterized by XRD, SEM, EBSD, and TEM techniques. The results showed that the interface was about 260 μm thick and consisted of four different microstructure regions. The thickness of each region was about 60–80 μm and the microstructure of the four regions consisted of acicular O, α2, and β/B2 phases; acicular α2 phase; acicular α2 and β/B2 phases; and acicular γ, α2, and β/B2 phases respectively. The resulting microstructure of the interface region was attributed to the interdiffusion of Ti atoms from the starting Ti6Al4V alloy layer to the high Nb-TiAl alloy layer and Al and Nb atoms from the high Nb-TiAl alloy layer to the Ti6Al4V alloy layer. The formation of acicular α2 and γ phases was diffusion-controlled and the Nb segregation at the interface caused the formation of O phase which had specific relationships with α2 and β/B2 phases. The detailed interface microstructural evolution of the Ti6Al4V/high Nb-TiAl MIL composite and the Vickers hardness of the composite are also discussed in this paper.

Superplastic Deformation Behavior of As-Received and Hydrogenated Ti<sub>2</sub>AlNb Alloy

In this paper, the effects of temperature and initial strain rate on the superplasticity of as-received Ti2AlNb alloy were studied by uniaxial tensile tests. Temperature from 870°C to 1030°C with an interval of 40°C and initial strain rate range of 10-2s-1to 10-4s-1were selected. The optimal superplasticity of 190.3% was obtained at 990°C with initial strain rate of 10-3s-1. The superplastic properties were deteriorated at 1030°C due to serious grain coarsening. In order to improve superplastic properties, the as-received alloy was hydrogenated with different hydrogen contents. It was found that hydrogen addition can significantly decrease flow stress and increase elongation. A higher elongation occurs at 910°C in hydrogenated alloy.

Application of Ti6Al7Nb Alloy for the Manufacture of Biomechanical Functional Structures (BFS) for Custom-Made Bone Implants.

Unlike conventional manufacturing techniques, additive manufacturing (AM) can form objects of complex shape and geometry in an almost unrestricted manner. AM’s advantages include higher control of local process parameters and a possibility to use two or more various materials during manufacture. In this work, we applied one of AM technologies, selective laser melting, using Ti6Al7Nb alloy to produce biomedical functional structures (BFS) in the form of bone implants. Five types of BFS structures (A1, A2, A3, B, C) were manufactured for the research. The aim of this study was to investigate such technological aspects as architecture, manufacturing methods, process parameters, surface modification, and to compare them with such functional properties such as accuracy, mechanical, and biological in manufactured implants. Initial in vitro studies were performed using osteoblast cell line hFOB 1.19 (ATCC CRL-11372) (American Type Culture Collection). The results of the presented study confirm high applicative potential of AM to produce bone implants of high accuracy and geometric complexity, displaying desired mechanical properties. The experimental tests, as well as geometrical accuracy analysis, showed that the square shaped (A3) BFS structures were characterized by the lowest deviation range and smallestanisotropy of mechanical properties. Moreover, cell culture experiments performed in this study proved that the designed and obtained implant’s internal porosity (A3) enhances the growth of bone cells (osteoblasts) and can obtain predesigned biomechanical characteristics comparable to those of the bone tissue.

Effect of Initial Microstructures on Hot Deformation Behavior and Workability of Ti2AlNb-Based Alloy

In order to study the effect of initial microstructures on the hot deformation behavior and workability of Ti2AlNb alloy, the isothermal compression experiments of as-rolled and solution-treated Ti–19Al–23Nb–0.5Mo alloys were conducted in the temperature range of 900–1100 °C and strain rate range of 0.001–10 s−1. Subsequently, the processing maps of different state materials were established based on dynamic material model (DMM) and Prasad’s instability criterion. The suitable regions for hot working were determined in the processing maps, which was verified through high-temperature tensile test. The results show that although the solution-treatment could be used to improve the ductility of as-rolled Ti2AlNb alloy at room temperature, the as-rolled microstructure exhibited better hot workability at high temperature compared to the solution-treated microstructure.

Physically-based constitutive model for flow behavior of a Ti-22Al-25Nb alloy at high strain rates

The objective of the study is to understand the dynamic flow behavior of Ti–22Al–25Nb alloy at high temperatures using a modified Zerilli–Armstrong (Z-A) model. To investigate the dynamic mechanical behavior of titanium alloy, the material was subjected to quasi static and dynamic compression testing. It is found out that the modified Z-A models possess better predictability in evaluating the flow behavior of the Ti–22Al–25Nb alloy at elevated strain rates and temperatures. The model well established the quasi-static and dynamic behavior of the titanium alloy, mainly the effects of strain hardening and thermal softening. The regimes of high strain rate deformation behavior represented by dynamic recrystallization (DRX), lamellar globularization and the instability flow have been discussed. Using transmission electron microscopy (TEM) studies, it is found that the dislocation density is directly associated with strain rate.

Hot Deformation Behavior and Microstructural Characteristics of Ti–46Al–8Nb Alloy

Hot deformation behavior, microstructural evolution and flow softening mechanism were investigated in Ti–46Al–8Nb alloy via isothermal compression approach. The true stress–strain curves exhibited typical work hardening and flow softening, in which the dependence of the peak stress on temperature and strain rate was obtained by hyperbolic sine equation with Zener–Hollomon (Z) parameter, and the activation energy was calculated to be 446.9 kJ/mol. The microstructural analysis shows that the alternate dark and light deformed ribbons of Al-rich and Nb-rich regions appeared and were associated with local flow involving solute segregation. The Al segregation promoted flow softening mainly arising from the recrystallization of γ phase with low stacking fault energy. The coarse recrystallized γ and several massive γ phase were observed at grain boundaries. While in the case of Nb segregation, β/B2 phase harmonized bending of lamellae, combined with the growth of recrystallized γ grains and α + β + γ → α + γ transition under conditions of temperature and stress, leading to the breakdown of α2/γ lamellar colony. During the hot compression process, gliding and dissociation of dislocations occurred in γ phase that acted as the main softening mechanism, leading to extensive γ twins and cross twins in α/γ lamellae and at grain boundaries. In general, homogeneous microstructure during the hot deformation process can be obtained in TiAl alloy with high Nb addition and low Al segregation. The deformation substructures intrinsically promote the formability of Ti–46Al–8Nb alloy.

O Phase Precipitation and Variant Selection in Ti–22Al–25Nb Alloy during the Hot Shear Spinning

O phase precipitation and variant selection in Ti–22Al–25Nb alloy during the hot shear spinning are investigated using scanning electron microscope (SEM), electron backscattered second diffraction (EBSD), and transmission electron microscope (TEM). The results show that the random O phase precipitates from the B2 matrix during the hot shear spinning. With the increase of reduction ratio, the average thickness of O precipitates increases from ≈0.18 to ≈0.50 μm. Meanwhile, the O precipitates show various morphologies, including lamella, short rod, and sphere. Twelve O variants simultaneously form in the parent B2 phase and the orientation relationship between the B2–O phases is confirmed as [−111] B2//[1–10] O and (110) B2//(001) O. Different patterns of dislocation are also found in the B2 matrix, such as dislocation tangles, dislocation nets, and dislocation arrays. They play an important role in the variant selection of O precipitates, resulting in the different angled combinations of O variants. In addition, massive dislocation piles up along the O/B2 interface and some dislocation wall generates within the O phase, which are benefit for the globularization of O precipitates.

Dynamic spheroidisation behaviour of the lamellar Ti–22Al–25Nb alloy during hot compression

ABSTRACTThe dynamic spheroidisation behaviour of the lamellar Ti–22Al–25Nb alloy during hot compression at 970°C/0.01 s−1 and strain of 30%, 50% and 70% was investigated. The results showed the lamellar-O phase that participated in the B2 matrix could be obtained by hot pressed sintering from pre-alloy powders. The corresponding scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy observations indicated that the dynamic spheroidisation process can be divided into three stages, the first is the primary interphase high-energy defect stage, introduced by dislocation migration twinning and localised shearing. The second is the separation, and the last is the spheroidisation stages, which could be explained by boundary splitting and termination migration.

Diffusion bonding of Ti2AlNb alloy using pure Ti foil as an interlayer

In this work, the vacuum diffusion bonding process was employed to robustly join Ti2AlNb alloy using pure Ti foil as an interlayer. The effects of bonding parameters on the interfacial microstructure evolution and mechanical properties of the Ti2AlNb bonded joints at both room temperature and elevated temperatures were investigated in detail. The results showed that the joint consisted mainly of two characteristic zones: diffusion zone I and central zone II, and both these zones were composed of α-Ti and β-Ti phases. The thickness of diffusion zone I as well as the formation of lamellar α and β phases in the central zone II was dependent primarily on the diffusion of the Nb and Al elements from the Ti2AlNb alloy into the Ti interlayer, which was controlled by the modification of bonding parameters. The joint diffusion bonded at 960 °C for 60 min under a pressure of 20 MPa showed a maximum room temperature shear strength of 664 MPa, and its shear strengths at 500 °C, 650 °C, and 800 °C were 442 MPa, 384 MPa, and 305 MPa, respectively. With increases of diffusion bonding temperature and holding time, the shear strength of the Ti2AlNb bonded joints improved owing to the enhanced elemental interdiffusion across the interface. Meanwhile, the fracture path changed from the interface between diffusion zone I and center zone II to center zone II. The fracture morphologies of the joints after the shear tests were characterised mainly by a dimple-like structure, which corresponded to a ductile rupture.

Insights into structure and high-temperature oxidation behavior of plasma electrolytic oxidation ceramic coatings formed in NaAlO2–Na2CrO4 electrolyte

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti2AlNb alloy in various NaAlO2 electrolytes containing 2 g L−1, 4 g L−1, and 6 g −1 Na2CrO4 additive, respectively. The influence of Na2CrO4 additive in NaAlO2 electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na2CrO4 additive in the NaAlO2 electrolyte not only promotes the formation of γ-Al2O3 phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr3O and (Al0.948Cr0.052)2O3 phases. The PEO ceramic coatings formed in NaAlO2 electrolytes with 2 g L−1 and 4 g L−1 Na2CrO4 additive show better oxidation resistance than those PEO coatings formed in a NaAlO2 basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO2 electrolyte with 4 g L−1 Na2CrO4 additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO2 basic electrolyte without any Na2CrO4 additive.

Oxidation behavior of Al/Cr coating on Ti2AlNb alloy at 900 °C

In this paper, the Al/Cr coating was fabricated on the surface of Ti2AlNb alloy via rf magnetron sputtering and double glow treatment to enhance oxidation resistance. The protective coating with an outer layer of Al and inner layer of Cr has great bonding strength due to the in-diffusion of Cr and the inter-diffusion between Al and Cr to form Al-Cr alloyed layer which has great hardness. Acoustic emission curve which was detected via WS-2005 scratch tester indicates the bonding strength between Al/Cr coating and substrate is great. Morphology of Ti2AlNb alloy with Al/Cr coating after scratch test shows that the scratch is smooth without disbanding, and the depth and breadth of scratch are changed uniformly. The mass change was reduced after oxidation test due to the Al/Cr protective coating. Isothermal oxidation test at 900 °C was researched. Results indicate that Al/Cr coating provided oxidation resistance of Ti2AlNb alloy with prolonged air exposure at 900 °C. Al2O3 was detected by XRD patterns and SEM images, and was formed on the surface of Ti2AlNb alloy to protect substrate during oxidation test. A certain content of Cr is beneficial for the formation of Al2O3. Besides, Cr2O3 was produced under Al2O3 by outward diffusion of Cr to protect substrate sequentially, no cracks were discovered on Al/Cr protective coating. The process of Ti outward diffusion into surface was suppressive due to integration of Cr-Ti and Al-Ti intermetallics. A steady, adherent and continuous coated layer of Al/Cr on Ti2AlNb alloy increases oxidation resistance.

Microstructural evolution and mechanical properties of vacuum brazed Ti2AlNb alloy and Ti60 alloy with Cu75 P t filler metal

Cu75 P t filler metal was firstly used to vacuum braze Ti2AlNb alloy to Ti60 alloy and the sound joints were obtained in this work. The solidus temperature of Cu75 P t filler was 1127 °C. The typical interfacial microstructure of Ti60/Cu75 P t/Ti2AlNb joint brazed at 1180 °C for 10 min was Ti60 substrate/β-Ti + α-Ti + Ti3Pt/Ti3Pt/B2 + α2/Ti2AlNb substrate. The effects of brazing temperature on the interfacial microstructure and mechanical properties of the Ti60/Cu75 P t/Ti2AlNb brazed joints were investigated. The increasing of brazing temperature accelerated the diffusion of liquid filler into substrate, resulted in the decrease of Ti3Pt brittle phase and the grain growth of substrates. The shear strength of the joints brazed at 1200 °C for 10 min reached the maximum of 93 MPa. The fracture surface of the joint was characterized as Ti3Pt brittle phase and it cracked in β-Ti when brazing temperature reached 1210 °C.

SELECTIVE LASER MELTING OF INTERMETALLIC TITANIUM ALLOY

The in-situ synthesis of the Ti2AlNb-based intermetallic alloy using selective laser melting of powder materials was studied. The object of research is the Ti–22Al–25Nb alloy (at.%), the main phase of which is the Ti2AlNb intermetallic compound with an ordered orthorhombic lattice (O phase). The Ti–22Al–25Nb alloy has good mechanical properties at room and elevated temperatures, low specific weight, and is considered as a promising material for aerospace industry applications. Experiments used a mechanical mixture of pure titanium, aluminum and niobium powders in a ratio required for Ti–22Al–25Nb alloy synthesis. Selective laser melting as an additive technology is the most promising way for additive layer manufacturing of parts. This technology allows manufacturing complex-shaped items based on CAD model data. Selective laser melting was used to make compact samples for investigations. Their microstructure, density, phase composition and microhardness were studied. In addition, the effect of heat treatment homogenization at 1250 °C for 2,5 h and then aging at 900 °C for 24 h on the microstructure and chemical homogeneity of samples were studied. It was shown that the compact material obtained by selective laser melting contains unmelted niobium particles. Homogenization annealing makes it possible to dissolve these particles completely in the alloy. As a result, the material microstructure consists of B2 phase grains of different sizes and needle-like precipitates of the orthorhombic phase.

Fabrication of a high strength and ductility Ti‒22Al‒25Nb alloy from high energy ball-milled powder by spark plasma sintering

A high strength and ductility Ti‒22Al‒25Nb alloy was fabricated from high energy ball-milled powder by spark plasma sintering. The pre-alloyed powder with a normal composition (at %) of 53 Ti, 22 Al and 25 Nb was firstly ball-milled for different milling times, and subsequently consolidated by spark plasma sintering. The effects of ball-milling time on the microstructure and mechanical properties of the ball-milled powder and sintered compact were investigated. The Ti‒22Al‒25Nb alloy sintered by SPS at 950 °C for 10 min under a pressure of 50 MPa from the ball-milled powder showed the microstructure with a large amount of ultrafine/nano O-phase. The best balance of tensile properties was exhibited in the Ti‒22Al‒25Nb alloy sintered from the ball-milled powder for 20 h; the yield strength, the tensile strength and the elongation to failure were 1092 MPa, 1105 MPa and 9.4% at room temperature respectively, exceeding other coarse grain Ti‒22Al‒25Nb alloys fabricated by other powder metallurgy methods in the comprehensive tensile property. The precipitation of secondary phase, the grain refinement and the change of residual dislocation density in the sintered alloy directly affected the tensile properties of the sintered alloy.

Effect of Cr on microstructure and oxidation behavior of TiAl-based alloy with high Nb

Three novel multi-microalloying TiAl-based alloys containing high Nb were designed and fabricated. Thermogravimetric method was applied to investigate the influence of Cr on the oxidation behavior of high Nb-TiAl alloy at 1,073 K for 200 h in laboratory air. The 2at.% and 4at.% Cr were added into the alloy, (respectively named 2Cr and 4Cr compared to the Cr-free ternary alloy, 0Cr alloy). The alloys’ microstructure and composition as well as the composition distribution of the oxidation scale were analyzed by means of Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and X-Ray Diffractometry (XRD). The results show that the addition of Cr decreases the grain size of the Nb-TiAl alloy and leads to a transformation from a fully lamellar structure to a nearly fully lamellar structure. When oxidized at 1,073 K for 200 h, the oxidized mass gain of the alloy increases with an increase in Cr addition amount in the first 100 h and decreases in the last 100 h. With the increase of Cr content, the oxidation surface turns compact but uneven in morphology, which may affect the oxidation resistance of the alloy by increasing the peeling off risk of the oxidation layer at friction conditions.