Ti2AlNb-based Alloy Research Articles

Enhancing the Mechanical Properties Induced by Ta Microalloying in TIG-Welded Ti2AlNb-Based Intermetallic Alloy

Enhancing the Mechanical Properties Induced by Ta Microalloying in TIG-Welded Ti2AlNb-Based Intermetallic Alloy

Grain Size Effects on the Rheological Behavior of Ti2AlNb-Based Alloy by Transverse Ultrasonic Vibration-Assisted Compression

Grain Size Effects on the Rheological Behavior of Ti2AlNb-Based Alloy by Transverse Ultrasonic Vibration-Assisted Compression

Effect of Er2O3 and Y2O3 on microstructure and mechanical properties of Ti2AlNb alloy

Rare earth oxides can significantly refine the microstructure of the cast alloys and improve their mechanical properties. In this study, Er2O3 and Y2O3 particles were chosen to add into the Ti2AlNb-based alloy with vacuum non-consumable arc melting method. For the Er2O3 particles, it has obvious dissolution and re-precipitation characteristics during melting. While for the Y2O3 particles, they are more stable, and aggregated and grew up during melting. These two kinds of Er2O3 or Y2O3 both have obvious refinement effects on the B2 grains due to their heterogeneous nucleation. Specially, adding Er2O3 particles can promote the decomposition of the B2 and α2 phases and the increases of the O-phase content. But adding the Y2O3 particles can inhibits the B2 decomposition and just only promotes the α2 decomposition into the O-phase. Additionally, the TAC-Er2O3 ingot shows the worst mechanical properties at room-temperature due to its large size and grain boundary segregation of the Er2O3 reinforcements. Conversely, the TAC-Y2O3 alloy exhibits excellent strength and ductility whatever at room- and high- temperatures owing to its fine grain strengthening and effective strengthening of the second precipitated Y2O3 phases.

Microstructure and mechanical properties of continuous drive friction welded Ti2AlNb alloy under different rotational rates

Ti2AlNb-based alloy was joined in a continuous drive friction welding machine under different rotational rates (500, 1000 and 1500 r/min). The microstructure and mechanical properties of the joints were investigated. It is shown that the weld zone (WZ) is fully composed of recrystallized B2 phase, and the grain size decreases with increasing rotational rate. The thermo-mechanically affected zone (TMAZ) suffers severe deformation during welding, due to which most of original precipitation phase is dissolved and streamlines are present. In the heat affected zone (HAZ), only the fine O phase is dissolved. The as-welded joint produced using 1000 r/min has the best mechanical properties, whose strength and elongation are both close to those of the base metal, while the as-welded joint obtained using 500 r/min exhibits the worst mechanical properties. Post-weld annealing treatment annihilates the deformation microstructure and fine O phase precipitates in the joints, consequently improving the mechanical properties significantly. Decomposed α2 phase is a weakness for the mechanical performance of the joint since microcracks are apt to form in it in the tensile test.

The effect of post heat treatment on microstructural evolution and mechanical properties of Ti2AlNb intermetallic alloy prepared by point-forging and laser-deposition

Ti2AlNb-based intermetallic alloy is considered to be the latest class of light-weight structural material severing in the range of 600∼750 °C. Hybrid interlayer point-forging with laser-deposition (PF-LD) provides a new idea to produce near-net-shape Ti2AlNb part with promising mechanical performance. The present work mainly investigated the effect of post heat-treatment on microstructural evolution and tensile mechanical properties of PF-LDed Ti2AlNb alloy. The experimental results demonstrated that solution-treated at 940 °C–1020 °C followed by aging at 840 °C was effective to regulate phase morphology and tensile mechanical properties. With the consecutive increase of solution temperature, more fine-lamellar O-phase was reprecipitated within parent B2/β matrix during secondary aging treatment. The ultimate tensile strength was enhanced significantly from 1055.5 MPa to 1148.1 MPa with the help of precipitation hardening effect. In the meantime, stronger stress accumulation introduced by fine-lamella also exerted a detrimental impact on tensile strain capability, and the elongation after failure decreased from 10.9 % to 7.4 % with increasing solution temperature. Considering above results, this study provides a basic research for exploring the heat treatment of PF-LDed Ti2AlNb-alloy, and it is feasible to obtain high-strength and high-ductility properties by tailoring phase morphology.

Structure and mechanical properties of Ti2AlNb-based alloy welded joints using keyhole plasma arc welding with subsequent heat treatment

Using keyhole plasma arc welding, welded joints of a Ti2AlNb-based alloy, VTI-4, were obtained, and their structure and mechanical properties were studied. It has been established that the dynamic effect of a keyhole arc had a positive effect on the quality of the welded joint; namely, lack of penetration, porosity, and microcracks were eliminated. The welded joint consisted of a fusion zone (FZ), a heat-affected zone (HAZ), and a base metal (BM). Depending on the phase composition and morphology of the obtained phases, the HAZ can be divided into four zones: HAZ1 with large β-phase grains near the melting line, HAZ2 with large β-phase grains + α2, HAZ3 with more fragmented β-phase grains retaining more α2-phase, and HAZ4 with the phase composition β + α2 + O. Subsequent heat treatment (HT: quenching at 920 °C for 2 h, cooling in air, followed by aging at 800 °C for 6 h, cooling in air) preserved the zone structure of the weld but led to the formation of the O-phase within β-grains. The microhardness of the weld in the zone corresponds to 360±15 HV0.2, but after HT, it increased to 382±20 HV0.2. The strength properties of the welded joint after HT were above 90 % of the base metal (σucs = 1120 MPa, σ0.2 = 1090 MPa), while elongation to failure is close to the initial condition (δ = 2.1 %).

Simultaneously enhancing strength-ductility synergy in the Ti2AlNb diffusion bonding joint via heterogeneous high-entropy interface design

Brittle defects, such as continuous intermetallic compounds and straight bonding line composed of grain or phase boundaries, often form at the interface of dissimilar metals joints, resulting in the premature cracking at the interface. Therefore, bonding interfaces with excellent strength and ductility are attractive for application in welding structural components. However, increasing strength usually comes at the expense of ductility, which is known as strength-ductility trade-off. In this work, we explored a novel interface design strategy by introducing multi-scale grains into the bonding interface of the Ti2AlNb-based alloy using a heterogeneous refractory high entropy alloy. The interface was composed of the multi-sized grains, which successfully eliminated the brittle defects. Through this approach, the strength and ductility of the joint synergistically improved to 877 MPa and 11.54 %, respectively. Both strength and ductility exceeded 98 % of the mechanical performances of the base metal. The exceptional strength-ductility combination was benefiting from cooperative strain hardening mechanisms, which increased the interfacial hardening capability via in situ refinement of free path of dislocations motion. This hetero-interface strategy could expand the design ideas for the bonding interface of intermetallic compound-based alloys, and provides a new approach to synergistically increase the mechanical performance.

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

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

Variant selection of orthorhombic phase precipitation and its effect on colony side plates formation in Ti–22Al–25Nb alloy

Heat treatments with controlled cooling rates have been used to investigate orthorhombic (O) phase precipitation and its variant selection (VS) in Ti–22Al–25Nb alloy, a latest Ti2AlNb-based intermetallic alloy with excellent comprehensive properties. This study uncovers the misorientation types of different O variants mainly belong to two types with a sharing angle of 90° and distinct rotated axis of <012> or <100>. A notable lamellar structure, colony side plates (CSPs), frequently develop on both sides of adjoining B2 grains, which always have a detrimental effect on the final properties. This structure significantly influenced by VS, variants having (001) planes that are paralleled to the common {110} plane of adjacent B2 grains are selected in colony plates on both sides. If the <111> axis of the two B2 grains coincidentally close as well, the colony plates may appear to penetrate the GB. Moreover, the CSPs structure could form on condition of much slow cooling rate after holding in rather high temperature. These findings could provide insights on how to control processing conditions and tailor the lamellar structure to obtain the desired properties.

Structure and mechanical properties of welded joints from alloy based on VTI-4 orthorhombic titanium aluminide produced by pulse laser welding

Ti2AlNb-based alloys are promising materials for operation at high temperatures in aerospace industry. Meanwhile, the existing difficulties of weldability restrict opportunities of their application. This work is devoted to studies of welded joints from Ti2AlNb-based VTI-4 alloy, obtained using pulsed laser welding (PLW). The optimum PLW modes have been determined providing uniform faultless joint. The features of formation of external defects, internal pores, cracks and non-uniform penetration depth were detected depending on welding conditions. The main PLW parameters influencing on formation of welded joint are voltage and duration of laser pulse. It was demonstrated that at insufficient medium and high peak powers sawtooth seam roots and internal pores can be formed. However, at higher rates of energy input thermal hydraulic processes in welding bathe are violated, accompanied by metal splashing (spattering), heterogeneity of pulse imposition is observed. This leads to formation of cracks, higher porosity, heterogeneity of melting zone, and as a consequence, poor mechanical properties. Microstructure analysis of the welded joints obtained by means of PLW has demonstrated that the melting area is comprised of long dendritic grains of β phase, and the heat affected zone from two regions of β + α2phases and β + α2+ O phases. Herewith, the achieved joint strength equals to ~80 % of the base metal produced using the optimum PLW mode.

Effect of post-heat treatment on Ti2AlNb-based alloy fabricated by twin-wire alternating dual-electron beam additive manufacturing technology

Thanks to its excellent high-temperature performance and moderate density, Ti2AlNb-based alloy is considered to be a new generation of high-temperature structural material in the aerospace field. However, its application is restricted currently due to the limitations of traditional processing methods. Recently, our group first successfully prepared this alloy with an unconventional approach named wire-based in-situ additive manufacturing (AM) technology, and great mechanical performance has been obtained. The unbalanced thermal process of AM easily causes inconsistent or undesired microstructures, as well as mechanical properties. Therefore, in this study, we further carried out post-heat treatment research on the as-printed sample so as to optimize its mechanical performance. Results show that the α2-phase will precipitate from the as-deposited samples (B2/β+O) after solution treatment(ST), while all precipitates were dissolved to the B2/β-matrix when the solution temperature was 1100 °C. The aging treatment(AT) promoted a great number of O-phases precipitated and led to an increase in its proportion. With the increase in aging temperature, the tensile strength decreased (995 to 821 MPa) gradually coupling the increase of fracture strain (1.65% to 2.12%), while the aging duration time did not show an obvious effect on its performance. In addition, after proper heat treatment, the high temperature (650 °C) tensile strength of the samples was as high as 818 and 792 MPa. This research not only promotes the development of Ti2AlNb-based alloy fabricated through in-situ AM, but also facilitates its further application in the aerospace field.

Mechanisms of the preferential cleavage of Ti2AlNb-based alloy: The influence of grain size and local plasticity

An in-depth understanding on the preferential cleavage of the matrix B2 phase, with an ordered body-centered cubic (BCC) structure, is pivotal to improve the processability of Ti2AlNb-based alloys. This research employed in-situ micro tensile tests, multiscale microstructure characterizations, and crystal plasticity simulations to study the grain-size dependent cleavage of the alloy with a full B2 phase. The material was intentionally prepared with two grain sizes differing by order of magnitude, i.e., the millimeter-level coarse grain (CG) and the micron-level fine grain (FG). The experimental results showed that the cleavage of CGs always occurred on the {100} planes, even when the loading direction was much less favorable for the cleavage of these planes, whereas the cleavage of FGs took place on both {100} and {110} planes. The investigation demonstrated that the plastic deformation takes a critical role in selecting cleavage planes. Full-field crystal plasticity simulations demonstrated that, for the CGs, the plastic deformation associated with cleavage on {100} planes is less than that associated with cleavage on {110} planes, indicating a less plastic deformation-induced blunting effect with cleavage on {100} planes. For the FG material, however, the existence of plentiful grain boundaries (GBs) leads to significantly heterogeneous plastic deformation, local stress distribution deviating from the imposed global stress, and hot spots of stress and strain concentration at GB triple junctions. These local plasticity heterogeneities enable the possibility for cleavage on both the {100} and {110} planes.

Fracture behavior of B2 phase matrix of Ti2AlNb-based alloy with microcracks of different orientations

The influence mechanism of crack orientation on B2 phase crack propagation behavior under cyclic loading is studied by using the meta-atom molecular dynamics (MAMD) method, and the simulation results are compared and verified based on the theory of linear elastic fracture mechanics. According to the different propagation speed of crack in B2 phase matrix, the crack propagation process can be divided into three stages: elastic stage, rapid propagation stage and slow propagation stage. The toughness or brittleness of the crack mainly depends on the competition between the cleavage fracture of the atomic bond and the plastic slip at the crack tip. The crack propagation behavior is obviously orientation dependent. Orientation I and orientation II expand in the form of ductile fracture. Orientation III shows obvious brittleness and expands in the form of cleavage fracture. Orientation IV (Orientation I-IV represent the four angles between the crack plane and the crystal orientation) shows both ductile fracture and brittle cleavage fracture. The defects and stress concentration at the crack tip provide a driving force for martensitic nucleation, and the hcp phase produced by martensitic transformation effectively increases the toughness of the crack. The simulation results of molecular dynamics are basically consistent with the theoretical prediction of linear elastic fracture mechanics. This suggests that the variation of the orientation could tune the crack tip plasticity and toughening in B2 phase matrix.

Effect of Volume Fraction of Reinforcement on Microstructure and Mechanical Properties of In Situ (Ti, Nb)B/Ti2AlNb Composites with Tailored Three-Dimensional Network Architecture.

The mechanical properties of (Ti, Nb)B/Ti2AlNb composites were expected to improve further by utilizing spark plasma sintering (SPS) and inducing the novel three-dimensional network architecture. In this study, (Ti, Nb)B/Ti2AlNb composites with the novel architecture were successfully fabricated by ball milling the LaB6 and Ti2AlNb mixed powders and subsequent SPS consolidation. The influence of the (Ti, Nb)B content on the microstructure and mechanical properties of the composites was revealed by using the scanning electron microscope (SEM), transmission electron microscopy (TEM) and electronic universal testing machine. The microstructural characterization demonstrated that the boride crystallized into a B27 structure and the α2-precipitated amount increased with the (Ti, Nb)B increasing. When the (Ti, Nb)B content reached 4.9 vol%, both the α2 and reinforcement exhibited a continuous distribution along the prior particle boundaries (PPBs). The tensile test displayed that the tensile strength of the composites presented an increasing trend with the increasing (Ti, Nb)B content followed by a decreasing trend. The composite with a 3.2 vol% reinforcement had the optimal mechanical properties; the yield strengths of the composite at 25 and 650 °C were 998.3 and 774.9 MPa, showing an 11.8% and 9.2% improvement when compared with the Ti2AlNb-based alloy. Overall, (Ti, Nb)B possessed an excellent strengthening effect and inhibited the strength weakening of the PPBs area at high temperatures; the reinforcement content mainly affected the mechanical properties of the (Ti, Nb)B/Ti2AlNb composites by altering the α2-precipitated amount and the morphology of (Ti, Nb)B in the PPBs area. Both the continuous precipitation of the brittle α2 phase and the agglomeration of the (Ti, Nb)B reinforcement dramatically deteriorated the mechanical properties.

Refinement mechanism of centimeter-grade coarse grains in as-cast Ti2AlNb-based alloy during multi-directional forging

Centimeter-grade coarse grains commonly exist in heavy ingots of Ti2AlNb-based alloy and severely deteriorate their workability. To investigate the refinement of coarse grains, multi-directional isothermal forging (MDIF) was carried out on samples cut from an ingot weighing about three tons. The results showed that the alloy can be refined by MDIF after 4 passes with 40% reduction per compression at 1200℃, while it cannot be refined even after 10 passes with 30% reduction per compression at 1200℃ or with 40% reduction per compression at 1050℃. The investigation showed that, as an intermetallics with low strain hardening rate, the material is very difficult to be refined by recrystallization if only increasing the total strain. The special refinement mechanism, i.e., grain thinning and shearing (GTS) induced recrystallization, is proposed. The reduction per compression is crucial to refine the coarse grains of this alloy. Coarse grains should be firstly squashed and thinned by large reduction, and then bent and bowed to form shear bands in the following perpendicular compressions. Coarse grains are truncated by repeated shearing, and the truncation increases interfaces within the coarse grains and contributes to the local misorientation accumulation, which in turn promotes the completion of recrystallization.

Grain refinement of Ti2AlNb-based alloy through canned multi-directional forging

Ti2AlNb-based intermetallic alloy has a great potential application in the manufacturing of aeronautical structures. However, the casted ingots of this alloy often consist of very coarse grains even as large as in centimetre level. The refinement of the coarse grain by hot deformation is essential for prompting this material to the engineering applications. In this work, multi-directional isothermal forging (MDIF) was conducted on Ti-20Al-24Nb at 1200°C and 1050°C with 2, 4, 6, 8 and 10 cycles, and a comparative study was emphasized on the grain refinement by two different reductions (30% and 40%) in each compression. The microstructure evolution has been systematically investigated. Results show that dynamic recrystallization (DRX) occurred preferentially near grain boundary, and very large strains were required to refine B2 grains. For 40% reduction per compression, the recrystallization volume fraction after four cycles was more than 95%, and the grain size was evenly refined from centimetre level to ~250μm. However, for 30% reduction per compression, the original coarse grains still exist after 10 cycles, and the recrystallization is far from complete, although the total strain is much greater than the process by using 40% reduction but with fewer cycles. The results demonstrate that the grain refinement of this material depends mainly upon the height reduction per compression and the deformation temperature and the magnitude of deformation or strain is not the critical factor to determine the recrystallization.

Research progress of microstructure transformation and kinetics in Ti2AlNb-based alloy

Research progress of microstructure transformation and kinetics in Ti2AlNb-based alloy

Interface Characterization of Bimetallic Ti-6Al-4V/Ti2AlNb Structures Prepared by Selective Laser Melting.

Additive Manufacturing (AM) of multimaterial components is a promising way of fabricating parts with improved functional properties. It allows for the combination of materials with different properties into a single component. The Ti2AlNb-based intermetallic alloy provides high temperature strength, while the Ti-6Al-4V (Ti64) alloy has good fracture toughness, ductility, and a relatively low cost. A combination of these alloys into a single component can be used to produce advanced multimaterial parts. In this work, Ti2AlNb/Ti-6Al-4V bimetallic structures were fabricated from pre-alloyed powders using the Selective Laser Melting (SLM) process. The effects of high-temperature substrate preheating, post-processing by annealing, and hot isostatic pressing on defect formation, the microstructural evolution of the interface area, and the mechanical properties of the bimetallic samples were investigated. High-temperature substrate preheating during the SLM process was necessary to prevent reheat cracking of the Ti2AlNb part, while annealing and hot isostatic pressing post-processing improved the chemical and microstructural homogeneity of the transition zone and enhanced the tensile properties of the bimetallic structure.

The effect of 0.02–0.37 strain on the precipitation law of O phase colonies in Ti2AlNb-based alloy during aging

O phase is the main strengthening phase of the lightweight high-temperature structural material—Ti2AlNb-based alloy, its morphology, size and distribution have significant impact on the mechanical properties of the alloy. This paper obtained different deformation areas with strains within the range of 0.02–0.37 in Ti2AlNb-based alloy sample through tensile deformation and precisely characterized strain values with DIC technique. The results indicate that while the strain was larger than 0.04, O phases would precipitate at the B2 grain boundary in the form of phase colony, whereas others precipitated in B2 grains appeared as fine granule. Phase colony structure, with its size reaching to dozens of micrometers, was composed of lamellar O phase. Within the strain range of 0.08–0.24, the precipitation amount of the O phase colonies was correlated linearly with strain level. It increased with the accumulating of strain. Within the strain range of 0.24–0.37, the amount of the O phase colonies no longer increased and stabilized at 15%. The mechanism of strain promoting precipitation of O phase colonies at the B2 grain boundary is shown as follow. Deformation made B2 matrix store a lot of geometrically necessary dislocations (GND), which led atoms in deformation area arrange in disorder and thus provided sufficient nucleate sites for O phase, promoting the concentrated nucleate of the O phase in this area and formed phase colony structure. The GND density at the B2 grain boundary was noticeable higher than that inside grain, thus O phase colonies primarily formed at grain boundary.

Oxidative Corrosion Mechanism of Ti2AlNb-Based Alloys during Alternate High Temperature-Salt Spray Exposure

This study investigates the corrosion damage mechanisms of Ti2AlNb-based alloys under high temperature, salt spray and coupled high temperature-salt spray conditions. This alloy was analysed in detail from macroscopic to microscopic by means of microscale detection (XRD, SEM and EDS). The results indicated that Ti2AlNb-based alloy surface oxide layer is dense and complete, and the thickness is only 3 µm after oxidation at 650 °C for 400 h. Compared to the original sample, the production of the passivation film resulted in almost no damage to Ti2AlNb-based alloy after 50 cycles of salt spray testing at room temperature. The tests showed that Ti2AlNb alloy shows good erosion resistance at 650 °C and in salt spray. However, this alloy had an oxide layer thickness of up to 30 µm and obvious corrosion pits on the surface after 50 cycles of corrosion under alternating high temperature-salt spray conditions. The Cl2 produced by the mixed salt eutectic reaction acted as a catalytic carrier to accelerate the volatilisation of the chloride inside the oxide layer and the re-oxidation of the substrate. In addition, the growth of unprotected corrosion products (Na2TiO3, NaNbO3 and AlNbO4) altered the internal structure of the oxide layer, destroying the surface densification and causing severe damage to the alloy surface.