Intermetallic TiAl Research Articles

Microstructure evolution and strengthening mechanism of pulsed current diffusion bonding TiAl/Ti2AlNb joints with in-situ rapid hot deformation

With the development of aero-engine toward high thrust-to-weight ratio, improving the strength of TiAl/Ti2AlNb joint has attracted considerable attention, as it is the key to manufacturing TiAl-Ti2AlNb composite turbine that combines high temperature resistance and weight reduction. However, the TiAl/Ti2AlNb diffusion bonded joint that is currently receiving the most attention is limited in strength by brittle interface structure and interfacial residual stresses, which cannot meet application requirements. In this study, a novel strategy of employing strong pulsed current to promote diffusion bonding of TiAl/Ti2AlNb joints and in-situ rapid hot deformation treatment was applied to improve this situation. This result shows that TiAl and Ti2AlNb alloys can be efficiently joined by pulsed current diffusion bonding (PCDB) with parameters of 950 ℃/10 min/10 MPa, the shear strength of the joint was 258.3 MPa. The subsequent in-situ rapid hot deformation treatment significantly strengthened the TiAl/Ti2AlNb joint, resulting in a shear strength of 443 MPa, which was 171.1 % of the pre-deformation strength. The formation of nanograined microstructure and precipitates acicular O phases after hot deformation-induced phase transformation of the diffusion bonding layers (DBLs) were the main strengthening mechanisms for the deformed joint. The large number of subgrain boundaries and the high dislocation density formed in the DBLs further increase the strength in this region. This study verified an effective of pulsed current diffusion bonding TiAl/Ti2AlNb joint with in-situ rapid hot deformation strengthening, providing a new strategy for obtaining high-strength dissimilar Ti-Al intermetallics joints.

Electronic structures, elastic and anisotropic properties of TiAl3 intermetallics doped with Nb

The elastic and anisotropic properties of TiAl3 intermetallics doped with Nb are investigated systematically based on the electronic structures calculation. It shows that the Nb atoms are preferentially to substitute Ti sites forming a stable crystal structure. The DOS of Nb–doped TiAl3 suggests that there is a pseudogap in (Ti18–x + Nbx)Al54. It reinforces atomic covalent bonds while the metallic bond in Ti18(Al54–x + Nbx) is increased. The predicted elastic properties of Nb–doped TiAl3 implies that its strength can be reinforced when the Ti sites substituted by Nb whereas the ductility of the doped intermetallics can be ameliorated when Al sites substituted by Nb. The anisotropy of Young's modulus and shear modulus indicate the uniform deformation ability of (Ti18–x + Nbx)Al54 is better than that of Ti18(Al54–x + Nbx). Poisson's ratio of Ti18Al54 and Ti18(Al51+Nb3) suggest an auxetic feature may be existed when a tensile stress is applied on a specific direction.

Enhancing strength and ductility in high Nb-containing TiAl alloy additively manufactured via directed energy deposition

Intermetallic TiAl alloys have found applications in the aerospace and automotive fields. Nevertheless, to satisfy the increasingly complex environmental requirements, these lightweight alloys must be improved particularly in strength and ductility. Additive manufacturing (AM) exhibits a rapid solidification rate and may strengthen intermetallic TiAl alloys. In this study, an additively manufactured (AMed) Ti-48Al-8 Nb alloy with fine formability was successfully fabricated by optimizing the power used during directed energy deposition. The preferential crystal plane for the obtained alloy is {111}γ due to its lowest surface energy, and the presence of the thermal gradient during the AM process results in the alloy growing along the building direction. The TiAl alloy AMed at 500 W delivers an excellent tensile strength of 880 MPa that is 1.71 times higher than that of its as-cast counterpart, as well as elongation of 0.7% at room temperature. The superior properties of this alloy are due to the formation of numerous deformation twins during tensile deformation at room temperature that enables twin intersections to produce high-density nano-twins while contributing to stress release. Moreover, the AMed TiAl alloy exhibits outstanding high-temperature strength retention performance.

Laser butt welding of AA7075 aluminium alloy and Ti6Al4V titanium alloy using a Cu interlayer

Laser welding of AA7075 and Ti6Al4V, two very different aerospace alloys, was performed with a copper (Cu) interlayer to avoid the brittle Ti–Al intermetallics. The joint was formed owing to the diffusion of Cu into the AA7075 alloy and limited diffusion in Ti6Al4V alloy, followed by eutectic formation at the AA7075/Cu interface. Microstructural and energy dispersive spectroscopy (EDS) analyses of the joint area revealed the presence of eutectic phases at grain boundaries inside the AA7075 fusion zone (FZ). An interfacial Cu3Ti2 phase formed at the solid-state Ti6Al4V/Cu interface. A robust and sound joint was achieved through the effective utilisation of Cu as an interlayer.

Indentation size effect in 2nd and 3rd generation advanced intermetallic TiAl alloys: theoretical and experimental estimation of dislocation density

Indentation size effect in 2nd and 3rd generation advanced intermetallic TiAl alloys: theoretical and experimental estimation of dislocation density

Designing of novel microstructure and its impact on the improved service temperature mechanical performance of 2nd and 3rd generation advanced intermetallic TiAl alloys

Advanced lightweight TiAl based intermetallic systems, capable of surviving elevated temperatures, have been a topic of interest, considering their ever-increasing demands in aerospace industries. The optimization of solidification and processing methods stands as a pivotal factor in microstructural engineering for TiAl systems, aiming to attain desired mechanical properties. The present study focuses on two distinct alloy systems, a commercial grade 2nd generation, i.e., Ti–48Al–2Cr–2Nb (at. %), and a newly developed 3rd generation, i.e., Ti–45Al–2Cr-7.5Nb-0.2B (at. %) alloys. Interestingly, the variation in the solidification path, triggered by higher Nb content along with a minor amount of B, accounts for the substantial microstructural refinement for the 3rd generation TiAl alloy. Nevertheless, prominent segregation of Nb and Cr to the β0 phase is observed in both the as-cast microstructure, which restricts the applicability of the alloys. Remarkably, a novel bilamellar-biglobular (BLBG) microstructure consisting of γ and α2 phases in both lamellar and globular morphologies is achieved through heat treatment for both categories of alloy. Elevated temperature tensile testing depicts an exceptional strength-ductility combination for 3rd generation BLBG microstructure. Excellent twin-twin activity and twin-induced plasticity effect are observed to govern the deformation mechanism. Overall, the path of solidification, phase distribution, and its effect on the deformation mechanism are thoroughly analyzed, which is of particular interest from a design and application point of view.

Results of X-ray studies of titanium powder obtained for additive machines from OT4 alloy metal waste in alcohol

The results of experiments aimed at conducting X-ray studies of powders obtained by electrodispersing titanium alloy OT4 metal waste in alcohol are presented. It is noted that the particles of the electroerosion powder consist of titanium and aluminum, which form phases of pure titanium α-Ti, titanium oxide TiO and intermetallic Ti3Al. The conducted studies have shown that by the method of electroerosive dispersion of metal waste of the OT4 alloy, it is possible to obtain spherical titanium powders for additive machines of the required fractional composition from secondary raw materials in small batches with minimal energy consumption and minimal damage to the environment.

Effect of microstructure and temperature on the bulk and small-scale deformation behavior of 2nd and 3rd generation advanced intermetallic TiAl alloys

Advanced intermetallic TiAl alloys have attracted considerable attention as a high-temperature structural material in aerospace and automobile sectors owing to their low density, elevated temperature strength, superior creep, and oxidation resistance. The present study is focused on assessing the effect of alloy composition, heat treatment and microstructural variation on the bulk and small-scale deformation behavior of commercial grade 2nd generation and newly developed 3rd generation TiAl alloys at 25 °C (298 K), 300 °C (573 K), and 500 °C (773 K). Twinning is noted to be the dominant deformation mechanism irrespective of the temperature. The highest yield strength and hardness are noted for the as-cast 3rd generation TiAl alloy owing to the finer microstructure and compositional variation. Heat treatment however modifies the TiAl microstructure significantly resulting to unique bilamellar-biglobular (BLBG) microstructure, devoid of the brittle β0 phase. Consequently, nano twins- and micro twins-induced enhanced deformability is noted for such microstructure, as compared to the as-cast counterparts. Interestingly, an anomalous increase in strength with an increase in temperature is observed for the studied alloys owing to the presence of high-density twins and Kear-Wilsdorf locks. Most importantly, a temperature dependant hardness anomaly is noted in the TiAl alloys that opens new avenues for further research in nanoscale deformation. This piece of research potentially strengthens the understanding of the role of constituent phases and temperature in the deformation behavior of complex TiAl alloys. Twin-induced deformation mechanism noted for the novel BLBG microstructure, can also be valid for different generations of TiAl alloys to achieve enhanced deformability that paves the way for the development of next-generation material.

Influence of nitrogen vacancies on the decomposition route and age hardening of wurtzite Ti1−xAlxNy thin films

The wurtzite phase of TiAlN has been known to form in industrial grade coatings with high Al content; yet, a significant knowledge gap exists regarding its behavior at high temperatures and the impact of defects on its properties. Specifically, its response to high temperatures and the implications of defects on its characteristics are poorly understood. Here, the high-temperature decomposition of nitrogen-deficient epitaxial wurtzite Ti1−xAlxNy (x = 0.79–0.98, y = 0.82–0.86) films prepared by reactive magnetron sputtering was investigated using x-ray diffractometry and high-resolution scanning transmission electron microscopy. The results show that wurtzite Ti1−xAlxNy decomposes by forming intermediary MAX phases, which then segregate into pure c-TiN and w-AlN phases after high-temperature annealing and intermetallic TiAl nanoprecipitates. The semicoherent interfaces between the wurtzite phase and the precipitates cause age hardening of approximately 4−6 GPa, which remains even after annealing at 1200 °C. These findings provide insight into how nitrogen vacancies can influence the decomposition and mechanical properties of wurtzite TiAlN.

Carburizing of Ti–6Al–4V alloy: Structure, growth mechanism and wear performance

Ti–6Al–4V alloy was pack carburized at temperatures of 950–1100 °C within a holding time range of 1–10 h. The structures and constituent phases of carburized layers were analyzed by optical microscope, scanning electron microscope (SEM) and X-ray diffraction (XRD). The structure of carburized layer was strongly influenced by carburizing temperature, it mainly consisted of a transition zone sub-layer of α-Ti (C) solid solution at 950 °C, and a compound (Ti-carbides + Ti-intermetallics) sub-layer as well as a transition zone sub-layer at temperatures of 1000–1100 °C. The growth of compound sub-layer was controlled by the growth of Ti–Al intermetallics and chemical reaction between C and Ti–Al intermetallics, and the growth of compound sub-layer required a gestation period depending on carburizing temperature. The growth kinetics of compound sub-layers followed a parabolic law beyond gestation period, from which the diffusion activation energy was calculated to be 192.597 kJ/mol. Carburizing of Ti6Al–4V alloy leads to a reduction of over 96.16 % in wear rate, and the wear resistance is enhanced with increasing carburizing temperature.

Indicatory surface of anelastic-elastic properties of Ti alloys

The simultaneous influence of hydrogen H and ultrasound deformation on internal friction Q −1 and dynamic elastic modulus E of intermetallic Ti3Al alloy after cutting and polishing were studied. The relaxation maximum of internal friction Q −1 M1 at temperature Т М1 ≈ 398 К, conditioned by the mechanism caused by reorientation interstitial atoms in dumbbell configurations H-H was discovered. Internal friction maximum Q −1 M2 in intermetallic Ti3Al at temperature T M2 ≈ 439 K was discovered with an activation energy H 2 = 0.86±0.1 eV. 2D and 3D atomic force microscopy microstructure images of Ti (VT8) alloy after mechanical and thermal treatment are presented. Strengthening of Ti alloys is related to the cooperation of dislocations with point defects.

Regulating the interfacial microstructure in TiAl/Ti2AlNb vacuum diffusion bonded joints for superior mechanical performance

Dissimilar material joining between TiAl and Ti2AlNb intermetallic alloys is the key to integrating the attractive properties of these lightweight structural materials in one hybrid component. However, some intractable problems, such as brittle reaction layers, residual thermal stress at the interface, and poor plasticity at ambient temperature, hamper its application. To overcome these deficiencies, diffusion bonding of intermetallic TiAl alloy to Ti2AlNb alloy using a series of newly designed Ti100-xMox (x = 2.56 ∼ 14.26 at. %) interlayers was investigated. By regulating the Mo content in the interlayer, a fine α-Ti layer and a gradient α-Ti + β-Ti composite layer were regulated in the reaction zone when the Mo content was 8.09 at. %. Such elaborately tailored interfacial microstructure led to superior mechanical performance for which the plasticity markedly exceeded the TiAl substrate while maintaining a considerable tensile strength (402 MPa). The improvement of mechanical performance of bonded joints could be attributed to the competition and collaboration between initiation of cracking in the TiAl/Ti91.91Mo8.09 interface and stress-induced martensitic transformation in the interlayer. Tensile testing showed that the bonded joints finally ruptured at the brittle α2 reaction layer. The results in this paper provide important insights into fabricating high-quality TiAl-based hybrid joints by diffusion bonding.

Revealing cellular reaction mechanisms and high-temperature structure stability in β-stabilized TiAl alloy

This study reveals that the cellular reaction (CR) is dominated by grain boundary (GB) migration mechanism controlled by element diffusion, during which the α2 → γ + βo phase transformation occurs. CR transforms supersaturated α2/γ lamellar system from metastable to equilibrium state via microstructure coarsening. Moreover, elements enrichment of Ti and Nb, Mo Ti, Nb and Mo occurs during CR, triggering the α2 → γ + βo phase transformation. Increasing the aging temperature accelerates the CR rate and extending the soaking time transforms more volume of α2/γ lamellae into pearlitic-like microstructure (PM). The PM exhibits almost negligible coarsening with the increase in aging temperature and extension of soaking time in the temperature between 950 and 1100 °C, indicating excellent thermal stability far beyond the service temperatures of TiAl intermetallics. Therefore, PM is a worthy pursuit in microstructural design for high-temperature application of TiAl alloy.

Electron beam welding of dissimilar Titanium to aluminium: Interface microstructure and mechanical properties

Welding of dissimilar metals, titanium–aluminium having wide differences in mechanical and thermal properties was highly challenging. In the present investigation, the electron beam welding method was used to join titanium alloy with pure aluminium. The results show that at the optimum parameter levels such as beam current of 6 mA, welding speed of 8 mm/s, and beam offset of 70:30 (Ti:Al) toward titanium, a defect-free joint with the higher mechanical strength of 120 MPa was achieved compared to the other combination of parameters. The percentage contribution of significant parameters was welding speed of 62%, and beam current of 25%. The scanning electron microscope was used to study the Ti–Al interface microstructure in detail. A significant increase in the micro-hardness in the weld zone was due to a change in titanium microstructure from equiaxed α-phase to acicular α and the formation of Ti-Al intermetallics in the fusion zone. X-ray diffraction analysis was used to analyze the phases present in the fusion zone. It was confirmed that there were mainly Al-Ti compounds like Al2Ti, Al3Ti, AlTi, AlTi2, Al11Ti5. These phases are harmful and contributed to higher hardness and brittleness in the fusion zone. At optimum conditions, a defect-free joint without intermetallics was obtained without an increase in hardness in the fusion zone.

Effect of Al2O3 fiber on twin intersections-induced dynamic recrystallization in fine-grained TiAl matrix composite

Dynamic recrystallization (DRX) is of great significance for the thermomechanical processing and microstructural regulation of TiAl intermetallics. However, the underlying DRX mechanism remains poorly understood. In this study, an Avrami kinetics model for DRX was established, which was capable of predicting the DRX fraction accurately. In addition, the effect of Al2O3 short fiber on the DRX mechanisms of TiAl matrix composite during the isothermal compression was investigated for the first time. The results showed that other than inhibiting DRX by particles in the TiAl matrix composites, the addition of Al2O3 short fiber accelerated a novel DRX process, which was induced by twinning and twin intersections (TDRX). Thus, this composite exhibited a higher DRX rate than that of the as-cast TiAl monolithic alloy. The origin of the twin intersection and TDRX for the composite was revealed. The stress concentration near the Al2O3 fiber was above the critical shear stress for twinning and thus was favorable for the formation of twinning and twin intersections. The high stored strain energy at the regions of twins and twin intersections provided the driving force for TDRX. TDRX accelerated the grain refinement in the TiAl matrix near the Al2O3 fiber. The present findings would provide a new perspective on DRX mechanisms, and provide the scientific guidance for optimizing the microstructures of TiAl matrix composites.

Densification, microstructure, and mechanical properties of sintered TiAl-NbN composites

Unlocking the full potential of intermetallic TiAl alloys in aerospace and automotive engines demands critical attention, as their limitations, particularly due to the room temperature brittle nature, inadequate oxidation and strength beyond 800 °C. Hence, the need to improve the mechanical properties is crucial to expanding their versatile applications. Herein, the effect of Niobium nitride (NbN) addition on the densification, microstructure, and mechanical characteristics of Titanium aluminide (TiAl) was studied. TiAl powders with various concentrations of NbN at 0, 2, 4, 6, 8, and 10 wt.% were synthesized through the spark plasma sintering technique. Each blended composite powder was measured according to stoichiometry into a 30 mm internal diameter graphite die and sintered at a temperature of 1150 °C, a pressure of 50 MPa, a heating rate of 100 °C/min, and a dwell time of 10 mins. The densification phenomenon, phase present, microstructure, and mechanical properties were investigated via Archimedes’ principle, X-ray diffraction (XRD), and Vickers hardness tester, respectively. Results shows that the addition of NbN to TiAl at constant sintering parameters enhanced the mechanical properties of TiAl. The microhardness of the composite material increases as the NbN addition increases, with maximum hardness of 467 HV, and ultimate tensile strength (UTS) up to 1458.9 MPa, attained at 10 wt.% NbN addition. Maximum porosity content of an impressive 1.6% was recorded at the addition of 10 wt.% NbN, which shows exceptional efficiency of spark plasma sintering technique. The results on densification and mechanical properties are presented and discussed with respect to the material composition and processing condition of the sintered materials.

Habit planes of climbing and gliding dislocations in TiAl determined in three dimensions by electron tomography

In this study, the habit planes of dislocations moving by glide and climb at 850–900 °C have been determined in three dimensions (3D) by electron tomography, using the TiAl intermetallics as a model system. An almost complete digital 3D reconstruction approach has been implemented, based on algorithms developed in life sciences, with which the habit planes orientations have been obtained with ±5° accuracy. It could thus be demonstrated, notably, that climbing a[001] pure edge dislocations loops, previously identified in literature, actually constitute Bardeen-Herring sources. Moreover, mixed-climb in {011) planes has been properly evidenced, as well as coexistence of gliding and climbing dislocation segments in the same region of the crystal. Consequently, improving the material's strength at 850–900 °C would require controlling both the glide and climb mechanisms by means of suitable solutes.

Influence of Al2O3 reinforcements and Ti-Al intermetallics on corrosion and tribocorrosion behavior of titanium

Ti-Al2O3 composites have demonstrated favorable characteristics for use in load-bearing biomedical implant applications; however, the influence of Al2O3 reinforcement particles and Ti-Al intermetallics on the electrochemical and tribo-electrochemical responses of Ti are not well-understood. This study explored the corrosion and tribocorrosion characteristics of powder metallurgy-manufactured Ti-Al2O3 composites in a simple physiological saline solution at body temperature. Electrochemical analysis was performed by electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization and tribo-electrochemical mechanisms were explored under open circuit potential (OCP) against a 10 mm diameter alumina ball in a ball-on-plate tribometer with reciprocating configuration. Results revealed that the corrosion behavior of Ti was adversely affected by the development of a heterogeneous oxide film on the Ti matrix and the Ti-Al intermetallic phases formed by the interaction of Ti and Al2O3 particles. However, there was a drastic improvement in tribocorrosion behavior, evidenced by decreased corrosion tendency under sliding and a marked reduction in wear volume, primarily as a result of the decreased wear damage resulting from the load-bearing reinforcements.

The Influence of Cu on Combustion Synthesis Mechanism of Ti–Al Intermetallic Compounds Produced From Ti and Al Elemental Powders

The Influence of Cu on Combustion Synthesis Mechanism of Ti–Al Intermetallic Compounds Produced From Ti and Al Elemental Powders

Strengthening in gradient TiAl alloys

Gradient structure is emerging as an effective strategy to fabricate metals with remarkable mechanical performance, but have not been verified in intermetallic compounds for high-temperature applications. Through experiments and atomic simulations, we show that a typical intermetallic TiAl alloy with gradient structure has a significant strengthening effect both at room temperature and high temperatures. The room-temperature compressive strength of TiAl alloys with gradient grain obtained by additive manufacturing is 2.57 GPa, which is ∼2.7 times as strong as that with equiaxed grain. The strengthening effect is attributed to more sessile dislocations in gradient structure caused by the intersections of multiple slip systems in gradient grain. More importantly, the strengthening effect is still effective at high temperatures and the compressive strength is 1.28 GPa at 750 °C. The simulation results show that this strengthening effect is due to the increased Hirth dislocation at high temperatures. This study expands the applications of TiAl alloys for load-bearing structures and provides a new strategy for improving the strength of intermetallic compounds at both room temperature and high temperatures.