Ti2AlNb Alloy Research Articles

Dissimilar Laser Welding of a NiTi Shape Memory Alloy to Ti2AlNb

NiTi-based shape memory alloys and the Ti2AlNb alloy have gained increasing importance in the aerospace field. The joining of these two materials can further increment the importance and usage of these relevant engineering materials and expand their potential applications. However, when joining NiTi-based shape memory alloys to Ti-based alloys, the formation of brittle Ti-rich intermetallic compounds often occurs, significantly limiting their functionality and use. Dissimilar joints between a NiTi shape memory alloy and Ti2AlNb alloy were obtained using a 0.1 mm thick Niobium (Nb) interlayer via laser welding. By process optimization, sound joints were obtained. The microstructure evolution was assessed by means of electron microscopy, whereas the mechanical strength of the joints was evaluated using lap shear tensile testing. The best performing joints were seen to fracture at maximum loads above 1230 N, thus allowing us to consider this dissimilar pair for structural applications.

Combustion behavior and mechanisms of Ti2AlNb compared to an α + β Ti alloy

The combustion behavior and mechanism of Ti2AlNb alloy are revealed by Promoted Ignition Combustion (PIC) tests and compared with Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) alloy. Although the ignition temperatures of two alloys show little difference, however, the burning velocity of Ti2AlNb is faster than TC11. Such difference is attributed to different locations of Nb and Mo elements enrichment, and decomposition of O phase in heat affected zone. The location of alloying elements enrichment, and phase transition in heat affected zone should be considered as key factors influencing the burn resistance of Ti2AlNb and TC11 alloys.

Dissimilar diffusion bonding behaviour of Ti2AlNb alloy and TiBw/Ti64 composites using the Ti interlayer

The Ti2AlNb alloy and TiBw/Ti64 composites were successfully bonded using a Ti interlayer at 900°C–1050°C for 20–80 min. The interface of the joints showed a layered microstructure. Moreover, the width of the diffusion zone gradually increased from 30 to 58 µm owing to sufficient atomic diffusion and phase transition at higher bonding temperatures with longer bonding times. The elemental concentration gradients caused by atomic diffusion promoted the precipitation of the O phase (Zone I) and the growth of the lamellar α phase (Zone II). A well-bonded joint was obtained and showed the highest shear strength of 460.9 MPa. Microcracks emerged in the diffusion zone and propagated into the TiBw/Ti64 substrate during the shear test, and the fracture mechanism inside the composites was intergranular ductile fracture mode.

Microstructure evolution and fracture mechanism of a TiAl–Nb alloy during high-temperature tensile testing

A TiAl–Nb alloy was prepared by casting, and its tensile properties and microstructure evolution at different temperatures were investigated. The results show that the brittle-ductile transition temperature (BDTT) of the alloy is 880–920 °C. At low temperatures, deformation mainly occurs in the γ lamellae and coarsening γ phase between the lamellar colonies. As the temperature increase in the transitory stage, a small number of dislocations may pass through the interface from the coarsening γ phase into the γ lamellae. During the brittle stage, the dislocation and dislocation array bow out from the phase interface, and twins are emitted from the phase interface, which may contribute to the accommodation of stress concentration. During the ductile stage, the γ lamellae coarsen due to the thinner and smaller α2 lamellae dissolving and phase boundary transfer. The dislocations interact with the dislocation walls to form subboundaries in the γ lamellae and transform into equiaxed crystals by dynamic recrystallization (DRX), which increases the plasticity of the alloy. With the transition from the brittle model to the ductile model, the fracture mode of the alloy changes from the mixed fracture mode of transgranular fracture and translaminar fracture to intergranular fracture.

Microstructure evolution and mechanical properties of high Nb–TiAl alloy/GH4169 joints brazed using CuTiZrNi amorphous filler alloy

A CuTiZrNi amorphous filler metal was fabricated and utilized for brazing high Nb–TiAl alloys and GH4169 superalloys. The microstructure and mechanical properties of the high Nb–TiAl/GH4169 joints brazed from 900 °C to 1020 °C temperature range were studied. Results showed that the high Nb–TiAl/GH4169 joints consisted of three zones. The interfacial microstructure of the joints was high Nb–TiAl alloy/Ti3Al + Al3(Ni, Cu)Ti2 + Al(Ni, Cu)2Ti/AlCu2(Ti, Zr) + (Ti, Zr)(Ni, Cu) + Ti2(Ni, Cu)/Cr-rich (Cr, Ni, Fe)ss + Ni-rich (Ni, Cr, Fe)ss/GH4169 alloy. The element diffusion between the substrates and the filler alloy was intensified as the brazing temperature increased, and the interfacial reaction layer thickened. At 960 °C, the joint exhibited a maximum shear strength which was approximately 241.9 MPa. At higher brazing temperatures, the excessive AlCu2(Ti, Zr) phase and coarse Ti2(Ni, Cu) deteriorated the joint properties due to their inherent brittleness.

The interface structure and its impact on the mechanical behavior of TiAl/Ti2AlNb laminated composites

In this work, the Ti2AlNb/TiAl laminated composite is successfully fabricated by hot pressing combined with the foil-foil metallurgy. The microstructure and the orientation relationship of the phases in the interface region, the distribution of strain and hardness along the interface, and the stress releasing mechanisms by the plastic deformation accompanied with two types of dislocation sources near the interface are detailly discussed. The composite presents an obvious yield phenomenon in contrast with the TiAl alloy and shows an improved tensile strength at room temperature. Further, an enhanced strength-ductility synergy at high temperature has been achieved. The synergetic deformation ability between the interface and the base alloys, deformation behaviors, and the effects of the laminated structure on crack distribution and morphology are also carefully investigated. The excellent mechanical performances of the composite are attributed to the outstanding metallurgical interface bonding, the introducing of ductile Ti2AlNb alloy into the laminated composite and the well-coordinated deformation ability of the laminated structure. The presented findings provide a new structural design strategy for property optimization of TiAl alloys.

Yttrium for the selective laser melting of Ti-45Al-8Nb intermetallic: Powder surface structure, laser absorptivity, and printability

High Nb-TiAl alloys are important materials to realize critical, light-weight parts of high-temperature applications. It has been challenging, however, to realize their laser-based additive manufacturing (AM) due to the materials’ high crack sensitivity. To mitigate the cracking problem particularly relating to oxygen, this study is designed to investigate the impact of introduced rare earth element (Y) on the microstructure, surface chemistry, and laser absorptivity of gas-atomized Ti-45Al-8Nb powder, and consequently on the printability of the alloy regarding its selective laser melting (SLM). It is observed that the Y addition significantly improves the SLM printability of the alloy and realizes samples that are free of macrocracks. The change in the surface structure of the powder is regarded as a critical factor contributing to improved printability. The corresponding chemical state and layer thickness of the oxide film covering the powder are determined by X-ray photoelectron spectroscopy (XPS) depth profile and transmission electron microscopy (TEM). It is further found that the surface structure of the powder leads to a higher laser absorption. As suggested by the study, modification of powder chemistry and powder surface structure by rare earth elements can be an effective means to improve the SLM formability of crack-sensitive materials.

Micro-mechanism and mechanical properties of solid-powder hot isostatic pressing diffusion-bonded Ti2AlNb alloy

Achieving the near-net shaping of brittle and difficult-to-machine materials is still challenging. Thus, we explore a method to prepare Ti-22Al-25Nb alloy by solid-powder hot isostatic pressing (HIP) diffusion bonding. The grain size, microstructure, interface features and mechanical properties of the fabricated alloy were systematically investigated. The results show that the solid-powder interface realizes a complete metallurgical bonding, and the grain size, composition and microstructure in transition zone is formed on the side of preform through recrystallization. There is a huge difference for the grain size between the powder forming zone and the preforming zone. As a result, the fabricated sample for solid-powder transition zone exhibits an excellent mechanical properties, with a tensile strength of 940 MPa, elongation of 2.9% and torsional strength of 815 MPa, respectively. In response to the torsional force, the crack starts from the preforming zone, and the crack deflection and branching occurs in the transition zone, thereby preventing the crack from propagating to the powder forming zone. The torsional strength of the solid powder HIP diffusion bonding zone is basically the same as that of the preformed zone. This study proposes a new solution for fabricating brittle and difficult-to-process materials and is of great significance in the development of the overall near-net shaping technology for complex components of such material.

Vacuum diffusion bonding of Ti2AlNb alloy and TC4 alloy

The Ti2AlNb alloy was joined with TC4 alloy by vacuum diffusion bonding. The relationship between bonding parameters, and joint microstructure and shear strength was investigated. The results indicated that the diffusion of Al, Ti, Nb and V elements across bonding interface led to the formation of three reaction layers: B2/β layer and α2 layer on the TC4 side, and α2+B2/β layer on the Ti2AlNb side. The bonding temperature determined the atomic activity, thus controlling the growth of reaction layers and influencing the shear strength of the joint. When the Ti2AlNb alloy and TC4 alloy were bonded at 950 °C for 30 min under 10 MPa, the shear strength of the joint reached the maximum of 467 MPa. The analysis on the fracture morphology showed that the fracture occurred within the B2/β layer and the fracture model was ductile rupture. Meanwhile, the formation mechanism of the Ti2AlNb/TC4 joint was discussed in depth.

Effect of Surface Integrity on Hot Fatigue Life of Ti2AlNb Intermetallic Alloy.

The effect of surface integrity on the hot fatigue performance of Ti2AlNb alloy was investigated. A turning process was used to prepare the standard specimens for hot fatigue tests. The surface integrity characterization and axial fatigue tests were performed. The results show that the influence of surface roughness on the hot fatigue performance of the Ti2AlNb alloy is a secondary factor. The compressive residual stress and enhanced microhardness in the surface layer has a significant effect on the hot fatigue life and they are dominant in the hot fatigue behavior of the Ti2AlNb alloy. Through the investigation on the characteristics of the fatigue fractures, the fatigue propagation process was significantly suppressed because of the strong residual compressive stress and microhardness distribution on the surface layer of the Ti2AlNb specimen.

Zn-Doped CaP-Based Coatings on Ti–6Al–4V and Ti–6Al–7Nb Alloys Prepared by Magnetron Sputtering: Controllable Biodegradation, Bacteriostatic, and Osteogenic Activities

New TiNb-based alloys, such as Ti–6Al–7Nb, are currently being studied around the world as an alternative to other Ti alloys, e.g., instead of Ti–6Al–4V. We conducted a pilot study where thin (approximately 1.2 micron) CaP coatings containing low doses of Zn2+ (0.4–0.8 wt.%) were prepared by the radio frequency magnetron sputtering (RFMS) of Zn-hydroxyapatite (HA) target on Ti–6Al–4V and Ti–6Al–7Nb substrates and investigated their physicochemical properties, in vitro solubility, cytotoxicity, and antibacterial and osteogenic activities. The thickness of the obtained coatings was approximately 1.2–1.3 microns. Zn substitution did not result in roughness or structural or surface changes in the amorphous CaP coatings. The distributions of Ca, P, and Zn were homogeneous across the film thickness as shown by the EDX mapping of these elements. Zn doping of CaP coatings on both types of Ti-based alloys statistically influenced the results of the scratch-test. However, obtained values are satisfactory to use Zn-CaP coatings on biomedical implants. Increased Zn2+ release vs. tapered output of Ca and phosphate ions occurred during 5 weeks of an in vitro immersion test in 0.9% NaCl solution. Ti–6Al–7Nb alloy, unlike Ti–6Al–4V, promoted more linear biodegradation of CaP coatings in vitro. As a result, CaP-based surfaces on Ti–6Al–7Nb, compared with on Ti–6Al–4V alloy, augmented the total areas of Alizarin red staining in a 21-day culture of human adipose-derived mesenchymal stem cells in a statistically significant manner. Moreover, Zn–CaP coatings statistically reduced leukemic Jurkat T cell survival within 48 h of in vitro culture. Along with the higher solubility of the Zn–CaP surface, a greater reduction (4- to 5.5-fold) in Staphylococcus aureus growth was observed in vitro when 7-day extracts of the coatings were added into the microbial culture. Hence, Zn–CaP-coated Ti–6Al–7Nb alloy with controllable biodegradation as prepared by RFMS is a prospective material suitable for bone applications in cases where there is a risk of bacterial contamination with severe consequences, for example, in leukemic patients. Further research is needed to closely investigate the mechanical features and pathways of their solubility and antimicrobial, antitumor, and osteogenic activities.

Microstructure and mechanical properties of Al2O3 ceramic and Ti2AlNb alloy joints brazed with Al2O3 particles reinforced Ag–Cu filler metal

This study focuses on the brazing of Al2O3 ceramic to Ti2AlNb alloy using composite filler metal (AgCu28 modified with micron-scale Al2O3 particles). The effect of Al2O3 particles concentration on the microstructure and mechanical properties of Al2O3/Ti2AlNb joint were investigated. Ti3(Cu, Al)3O has been identified as the main product of the reactions at the alumina–brazing alloy interface. The addition of Al2O3 particles significantly reduced the mismatch of coefficients of thermal expansion between Al2O3 ceramic and Ti2AlNb alloy substrates, relieving the residual stresses of the joint. Nevertheless, a high concentration of Al2O3 particles led to excessive Ti3Cu3O phase exhibiting high elasticity modulus surrounded Al2O3 particles which impaired the plastic deformation ability of the brazing beam, thus, decreased the joint strength. The maximum shear strength of 110.49 MPa was achieved when the joint was brazed at 880 °C for 10 min using AgCu28 + 2 wt % Al2O3 particles filler metal. The fracture morphology revealed that the cracks initiated in the brazing seam, and mainly indicating the propagation of cracks occurred in the Al2O3 ceramic substrate.

Microstructure and Mechanical Properties of Vacuum Diffusion Bonded Ti2AlNb/Ti/TC4 Joint

In this paper, the Ti2AlNb alloy was bonded to TC4 alloy using the vacuum diffusion bonding method with a Ti interlayer. The interfacial microstructure of the Ti2AlNb/Ti/TC4 joint was characterized. The relationship between the bonding parameters and the microstructure and mechanical property of the joints was explored. Results indicated that the interdiffusion of Nb and Al elements between the interlayer and substrates promoted the formation of the lamellar α + β dual-phase structure in the joint. The bonding parameters determined the diffusion distance of Nb and Al elements, thus controlling the characteristics of the lamellar α + β dual-phase structure. When the Ti2AlNb alloy and TC4 alloy were bonded at 950 °C for 30 min under a pressure of 10 MPa, the elemental diffusion in the bonding couple was sufficient and the joint possessed the maximum shear strength of 549 MPa.

Surface Properties of Graphene Functionalized TiO2/nHA Hybrid Coatings Made on Ti6Al7Nb Alloys via Plasma Electrolytic Oxidation (PEO).

Nano-hydroxyapatite (nHA)-matrix coatings containing graphene nanosheets (GNS)-nHA were coated on Ti6Al7Nb alloys by plasma electrolytic oxidation (PEO) treatment for the improvement of their surface properties. Crystallographic properties, functional groups, and elemental analysis of coatings were characterized by XRD, ATR–FTIR, and EDS analysis. Surface morphological changes of the coated surfaces were investigated by AFM and SEM. The electrochemical corrosion behavior of the coatings was examined by using the potentiodynamic scanning (PDS) tests under in-vitro conditions in simulated body fluid (SBF). The results showed that the GNS was successfully deposited in ceramic matrix coatings on Ti6Al7Nb alloys. Also, the microstructural observations revealed that the coatings have a porous and rough structure. The XRD and ATR–FTIR quantitative analysis have proved the appearance of HA and GNS in the coating layers. An increase in the coating thickness, surface hardness, and anatase/rutile transformation rate was determined, while the GNS ratio in the coating layers was increased. The microhardness of the nHA coating reinforced with 1.5 wt% GNS was measured at 862 HV, which was significantly higher than that of GNS-free (only nHA) coating (584 HV). The best in-vitro resistance to corrosion in SBF was observed in the nHA/1.5GNS wt% coating.

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

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

Ti6Al7Nb-based TiB-reinforced composites by selective laser melting

The alloy Ti6Al7Nb with the in situ reinforcement of TiB has been processed by selective laser melting (SLM). Premixed Ti6Al7Nb-xTiB2 powders were used for the powder bed. The microstructure of the Ti6Al7Nb alloy was characterized by martensite (αʹ), while the addition of TiB2 led to a bimodal microstructure with refined features as a result of in situ formation of fine TiB phase. A prismatic ││BD fiber texture was evident in all samples, though the texture formation was found to be affected by the change in the phase transformation pathway due to the formation of TiB. Higher hardness and yield strength (YS) in the composites as compared to the base material are attained. The examination of corrosion response revealed a lower corrosion current (Icorr) value for all the specimens in simulated body fluid (SBF) in comparison to Ti6Al4V alloy. The improved mechanical behavior and better corrosion resistance indicate the potential of SLM-processed Ti6Al7Nb/TiB composite as a suitable replacement for Ti6Al4V alloys for bio-implants.

Optimizing the Integrity of Linear Friction Welded Ti2AlNb Alloys

The knowledge of process parameters–weld integrity-aging treatments–tensile property relationship is of great concern for linear friction welded (LFWed) Ti2AlNb-based alloy and requires a systematic characterization. Thus, the Ti2AlNb-based alloy was LFWed under various process parameters and then subjected to different aging treatments. Twelve welding conditions were used to evaluate the weld integrity, showing that impurities and cracks at weld interface can be eliminated under strong welding parameters and the feed rate has the greatest influence on the weld integrity among all process parameters. Relationships among aging temperatures, microstructure evolution, and mechanical properties were investigated. After aging treatment, acicular O phase has precipitated in B2 grains both in the weld zone and thermo-mechanical affected zone (TMAZ). The size of precipitated O phase increases along with the increase of temperature, and the α2 + O mixtures have finally decomposed into the aggregated acicular O phase. The microhardness and tensile strength of the joints have been enhanced due to the precipitation hardening of O phase and refined grain strengthening after aging treatments.

Adhesion between Biocomposites and Different Metallic Structures Additive Manufactured

This study was concerned with the adhesion of resin cement to metal surfaces obtained by selective laser melting process (SLM), and how it could be improved the bond strength at the biocomposite-metal junction. The SLM substrates were manufactured out of pure titanium (Ti), Ti6Al7Nb, and CoCr alloys. The metallic surfaces were covered with 5 types of biocomposites: 2 commercially resin-modified glass-ionomer cements (GC Fuji Plus and KETAC CEM) and 3 types of in-house developed materials. These biocomposites were mechanical characterized under compression and bending trials. The biocomposites-metal adhesion was settled both on as built metallic surfaces and after they were sandblasted with alumina. All the sandblasted SLM surfaces presented higher adhesion strength in comparison with the untreated specimens. The CoCr specimens show the highest bonding value. Additionally, the morphological aspects of joining interfaces were investigated using a scanning electron microscope (SEM). The mechanical properties and metal adhesion of these biocomposites were influenced by the liquid powder ratio. It is essential to apply a surface treatment on SLM substrate to achieve a stronger bond. Also, the chemical composition of biocomposite is a major factor which may improve the adhesion of it on different metallic substrates.

Laser deposition of graded γ-TiAl/Ti2AlNb alloys: Microstructure and nanomechanical characterization of the transition zone

Additive manufacturing (AM) is a transformative technology to the aerospace industry. As one of the AM techniques, laser metal deposition (LMD) enables the fabrication of engine blades and a disk, as a single component, known as blisk. In this study, we use the LMD technique to fabricate a γ-TiAl/Ti2AlNb graded metallic alloy by depositing γ-TiAl powder on a Ti2AlNb alloy substrate. High-resolution scanning electron microscope (SEM) and high-speed nanoindentation are employed to characterize the microstructure and mechanical properties of the transition zone from the Ti2AlNb substrate (disk) to the γ-TiAl alloy (blade). The results show that the transition zone includes three layers (I, II and III) with gradient compositions and phases: (I) mainly β/B2 matrix with randomly distributed α2 and γ phases, (II) (α2 + γ) lamella with γ and β/B2 phases, and (III) similar microstructure with Layer II but finer γ and β/B2 phases. The results of nanoindentation mapping show good correlations between the mechanical properties (nanohardness and elastic modulus) and microstructure in the transition zone. Attributing to the rule of mixtures, the nanohardness and elastic modulus gradually increase from the substrate Ti2AlNb to Layer I, and gradually decrease from Layer I to γ-TiAl. This work demonstrates that the microstructure and phase analysis in combination with high-speed nanoindentation offers a new opportunity to study graded materials made using LMD.

Hydrothermal synthesis of nanocrystalline hydroxyapatite\u2013graphene nanosheet on Ti-6Al-7Nb: mechanical and in vitro corrosion performance

The hybrid coatings containing the graphene nano-sheet (GNS) and nano-hydroxyapatite (nHA) phases have been successfully synthesized on Ti6Al7Nb alloys by a one-step hydrothermal method. The hydrothermal reaction was carried out for 24 h at 200 °C. The GNS ratio has been altered as 1, 3, 5 and 7 wt.% in the coatings and, the results have compared with non- GNS containing coatings. The effect of the GNS ratio on the microstructure, hardness, and in vitro corrosion responses has been investigated in detail. The characterizations of the coatings were carried out by SEM, EDS, AFM, XRD and, FTIR. The corrosion behavior of the hybrid coatings was compared in Kokubo’s solution at 37 °C by using potentiodynamic polarization tests. The results showed that the hydroxyapatite phases were deposed on the graphene layers with nano-size nucleation with its Ca/P stoichiometric ratio. The best hydrophilicity (~52°) property has been obtained in nHA/3GNS coatings. In addition, the corrosion rates of coatings increased in the following order: nHA/3GNS < nHA/1GNS < nHA/7GNS < nHA/5GNS < only nHA.