TiAl Alloys Research Articles

Effect of high strain rates on the mechanical behavior and structure formation of the titanium alloy Ti–6Al–4V

Effect of high strain rates on the mechanical behavior and structure formation of the titanium alloy Ti–6Al–4V

Enhancing microstructure, nanomechanical and tribological properties of TiAl alloy processed by spark plasma sintering with Si3N4 ceramic particulates addition

TiAl matrix composites reinforced with varying weight fractions of Si3N4 ceramic particles were successfully fabricated by the spark plasma sintering method. The microstructure, nanomechanical and tribological properties of the sintered composites were investigated. The microstructural characterization revealed the evolution of a quasi-continuous and continuous network structure consisting of minor fractions of in-situ formed Ti2AlN, unreacted Si3N4 ceramic particles and dominant Ti5Si3 intermetallic phases within the TiAl matrix at Si3N4 content above 1.5 wt%. The in-situ precipitated phases enhanced the nanomechanical and tribological properties of the composites. The 7Si3N4/TiAl composite displayed the best nanomechanical properties, including nanohardness, elastic modulus, and H/Er ratio among the sintered samples. The specific wear rate of the composites decreases with increasing reinforcement content. 7Si3N4/TiAl composite exhibited the lowest specific wear rate of 0.38 ± 0.55 10-4 mm3/Nm, representing a 95.6% improvement in wear resistance compared to the unreinforced pure TiAl alloy. The improved wear performance of the composites was attributed to their load-bearing capacity and wear resistance of the hard, in-situ Ti2AlN, Ti5Si3 and unreacted Si3N4 particles in the TiAl matrix. The composites displayed a transition from adhesive wear to predominantly abrasive wear where the hard Si3N4 particles prevented direct metal-to-metal contact and facilitated the formation of a protective tribolayer, resulting in enhanced wear resistance. Hence, the developed Si3N4/TiAl composites are suitable for various structural and tribological applications.

Oxidation behavior and mechanical properties of a directionally solidified high Nb TiAl based alloy between 800 °C and 900 °C

The high temperature oxidation behavior and mechanical property response of a directionally solidified Ti-46Al-7Nb-0.4W-0.6Cr-0.1B alloy were investigated. The ultimate tensile strength of the alloy at 800 °C, 850 °C and 900 °C are 647 MPa, 590 MPa and 508 MPa, respectively, and the tensile fracture mode changed from brittle cleavage fracture to micro-void accumulation ductile fracture. At lower temperatures Al2O3 forms first, growing along the γ lamellae to form oxide bands aligned with the lamellar orientation. The oxidation mass gain of the alloy after 900 °C/100 h isothermal oxidation is only 0.91 mg/cm2. The oxidation kinetics results show the microalloyed high Nb TiAl alloy has excellent oxidation resistance, which is due to the formation of a TiO2 layer containing Nb, Cr and W, the AlNb2 phase and an Al/Cr rich transition layer above the directionally solidified lamellar matrix.

In-situ synchrotron high energy X-ray diffraction study on the deformation mechanisms of D019-α2 phase during high-temperature compression in a TiAl alloy

The α2 phase with an ordered hexagonal structure has a strong mechanical anisotropy. The limited plasticity interacting with crystallographic texture formation undoubtedly complicate the deformation mechanisms of the α2 phase. Taking advantage of in-situ synchrotron high energy X-ray diffraction (HEXRD), this study unveils the origin of the α2 texture and its correlation with plastic deformation. Upon loading at 800 °C, the dislocation glide in the γ phase initiates at a stress of 370 MPa and that in the α2 phase at a stress of 670 MPa. Nevertheless, the texture evolution of the α2 phase sets in in parallel with that of the γ phase, and these textures are preliminarily established at a stress of 650 MPa. A main <11 2‾ 0> fiber texture with the c axis of the α2 unit cell aligned vertically to the compression axis has been unexpectedly evolved. This texture forms primarily via rigid body rotation and is correlated with glide and twin activation in the neighboring γ phase. The preferential orientation primarily favors double prismatic glide in α2 and in addition triggers the tensile twin activation. The grain rotation caused by the tensile twin slightly affects the α2 transverse texture evolution. The results unveil a complex interaction between the α2 phase deformability, texture evolution and the combined elastic-plastic deformation of the phases γ and α2 in TiAl alloys.

Laser welding of dissimilar TiAl/Ti6-Al-4V materials: Effects of rapid in-situ laser pre-heating on microstructure and mechanical properties of the welding joint

TiAl alloy has broad application prospects in the aerospace field. In practical working environments, connecting with other materials is considered an important means of fully utilizing their excellent properties. This study employed a laser in-situ pre-scanning method to preheat the weld seam, achieving the connection between TiAl alloy and Ti-6Al-4V. On this basis, the microstructural evolution of the fusion zone (FZ), fusion line (FL), and heat-affected zone (HAZ) of the welded joint was analyzed in detail. By comparing with traditional welding processes, the effects of laser in-situ preheating on the microstructure and properties of the welded joints were studied. The results showed that the FZ area consists of α2 and martensitic α-Ti, while the FL on the TiAl side features a transition layer of β/B2 phases, providing plasticity and toughness to the welded joint. The in-situ scanning method reduced the cracks generated during welding, resulting in a more uniform distribution of the transition layer. The obtained FZ exhibited higher microhardness. The room temperature tensile strength of the welded joint reached 650 MPa. As the temperature increased, at 400 °C, the tensile strength and elongation reached 571 MPa and 3 %, respectively, significantly higher than the 450 MPa and 1.5 % achieved by conventional welding at the same temperature. When the temperature further increased to 500 °C and 600 °C, Ti-6Al-4V experienced significant softening, becoming the weak point of the welded joint.

The newly discovered competitive growth behavior of γ and α2 phases

This study reports a new phenomenon of competitive growth between TiAl alloy structures. By annealing the β-solidified Ti-44Al-8Nb-0.1B alloy for a short time, a new phenomenon of repeated competitive growth between α2 and γ phases is found. Finally, the (α2 + γ) lamellar structure engulfs other structures and grows, transforming into a full lamellar structure.

Exploring the brittle-to-ductile transition and microstructural responses of [formula omitted]−TiAl alloy with a crystal plasticity model incorporating dislocation and twinning

γ−TiAl alloy, with its high specific strength and creep resistance, is ideal for aerospace engines and gas turbines, but its brittleness poses significant manufacturing and processing challenges. To address these issues, this study employs a crystal plasticity finite element method incorporating dislocation and twinning to analyze the brittle-to-ductile transition behavior of γ−TiAl alloy at different temperatures. Additionally, the Bayesian optimization methods are employed to efficiently and accurately obtain parameters related to numerical calculations of crystal plasticity. The results indicate that at room temperature, the high activation resistance of the slip systems in the α2 phase leads to limited slip activity, resulting in poor plasticity. However, at 750 °C and 850 °C, the strength of the slip systems decreases significantly, allowing more α2 phase lamellae in the γ-TiAl alloy to undergo greater plastic deformation. This enhancement in the plastic deformation capacity of the α2phase lamellae reduce the overall deformation incompatibility in the TiAl alloy, thereby improving the overall ductile of the γ-TiAl alloy.

Establishment of Multi-Physics coupling model and analysis on thermal stress and crack risk in directional growth of TiAl alloys under electromagnetic confinement

The electromagnetic confinement directional solidification (EMCDS) technique is an optimal method for preparing large-size and non-contamination directional TiAl alloy crystals. Despite its advantages, the high temperature gradient inherent to this process induces thermal stress within the ingot, which increases the risk of cracking. To address this challenge, an innovative Integrated Multi-Physics Coupling Model was established to map and study the thermal stress field during EMCDS in this study. It synchronized the computation of the electromagnetic field, temperature field, solute field, flow field, and stress field during the crystal growth of TiAl alloy, and its high accuracy was proved by the micro-indentation experiment. Our analysis reveals that transverse temperature differences are crucial in inducing thermal stresses, and identifies that hot cracks and cold cracks are prone to occur respectively at the area of radial 23R along the sample (X = 23R) and on the sample surface, which aligns with experimental observations impressively. This model can more accurately and efficiently optimize the process parameters of the EMCDS process to avoid cracks and promote its industrial application.

Microstructural evolution and dynamic recrystallization mechanisms of additively manufactured TiAl alloy with heterogeneous microstructure during hot compression

Microstructural evolution and dynamic recrystallization (DRX) mechanisms of a Ti−48Al−2Cr−2Nb (at.%) alloy prepared by selective electron beam melting (SEBM) during hot deformation at 1150 °C and 0.1 s−1 were investigated by hot compression tests, optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD) and transmission electron microscope (TEM). The results show that the initial microstructure of the as-SEBMed alloy exhibits layers of coarse γ grains and fine γ+α2+(α2/γ) lamellar mixture grains alternately along the building direction. During the early stage of hot deformation, deformation twins tend to form within the coarse grains, facilitating subsequent deformation, and a small number of DRX grains appear in the fine-grained regions. With the increase of strain, extensive DRX grains are formed through different DRX mechanisms in both coarse and fine-grained regions, involving discontinuous dynamic recrystallization mechanism (DDRX) in the fine-grained regions and a coexistence of DDRX and continuous dynamic recrystallization (CDRX) in the coarse- grained regions.

In-Situ Regulation to Tailor Additive Manufacturing of TiAl Alloy with Homogeneous Fine Fully Lamellar Colony

In-Situ Regulation to Tailor Additive Manufacturing of TiAl Alloy with Homogeneous Fine Fully Lamellar Colony

A novel Ta-contained TiAl alloy with excellent high temperature performance designed for powder hot isostatic pressing

TiAl alloys offer strong potential for replacing conventional nickel-base alloys in structural applications at high temperatures, which requires good high temperature performance. This study analyzes the creep performance and thermal exposure characteristics for a powder hot isostatic pressing (P-HIP) Ta-contained TiAl alloy. The microstructure characteristics, creep performance, and thermal exposure properties of the nearly lamellar (NL) alloy with a size of about 145 μm were investigated. The alloy remains 0.72 % fracture strain at room temperature after exposure at 750 ℃ for 1000 h. The fitting results of creep curves show that the creep stress index n is 15.82 at 750 ℃, indicating the power law creep mechanism. By reducing the interfacial energy, Ta can facilitate the formation of metastable structures and achieve refinement. The enrichment of Ta element at the lamellar edge, both observed after thermal exposure and creep, inhibits the growth of lamellae and hinders the expansion of cavities due to its low diffusion rate, improving the thermal stabilities and creep performance. The P-HIP Ta-contained TiAl alloy shows exciting high temperature performance.

Effect of alloying composition on solidification microstructure and mechanical properties of TNM-based TiAl alloys

It is quite sensitive for TiAl alloy to composition, and the solidification microstructure and mechanical properties may be changed by compositional minor-adjustment. In this paper, two TNM-based alloys with different solidification modes (TNM-45Al alloy and TNM-0.3Si-0.7C alloy) were obtained basing on TNM alloy via two different compositional minor-adjustment routes. Compared with TNM alloy, the effects of compositional minor-adjustment routes on localized segregation, solidification microstructure and compression properties at room temperature were investigated. It is found that the solidification mode of TNM-45Al alloy is still β solidification, compared with TNM alloy, localized segregation is evidently weakened, lamellar colonies are coarsened and solidification texture is strengthened, and the yield strength, compressive strength and fracture strain at room temperature are increased by 14.04 %, 30.21 % and 28.97 %, respectively. And the solidification mode of TNM-0.3Si-0.7C alloy is transformed into peritectic solidification, with regional polarization of localized segregation, rapid coarsening of lamellar colonies and severe strengthening of solidification texture, the yield strength increased by 16.51 % at room temperature, and the compressive strength and fracture strain decreased by 32.01 % and 71.31 %, respectively. It is indicated that the coarsening of lamellar colonies and the intensification of solidification texture of TNM-45Al alloy can be inhibited by the precipitation of TiB, but those of TNM-0.3Si-0.7C alloy can not be restrained by the TiB/Ti5Si3 symbiotic structure found for the first time and the precipitation of TiB. In addition, the change of localized segregation behavior is the main factor for the strengthening and toughening of TNM-45Al alloy, and the solid solution of Si/C atoms is the main factor leading to the embrittlement of TNM-0.3Si-0.7C alloy.

Microstructure evolution and mechanical properties of bioinspired interpenetrating Ti2AlNb/TiAl matrix composite with a crossed-lamellar structure

TiAl alloys with low density, high creep resistance and high temperature performance are considered as candidate materials to replace nickel-based superalloys in the range of 700∼800 °C. However, the intrinsic brittleness of TiAl alloys has always been the biggest bottleneck restricting their development. In this paper, a bioinspired interpenetrating Ti2AlNb/TiAl composite with crossed-lamellar structure was prepared by combining selective laser melting (SLM) and vacuum hot press sintering (HPS) under the condition of 1150 °C/1 h/45 MPa, to improve the strength and toughness of the composite. Meanwhile, the metallurgical defects and microstructure of Ti2AlNb reinforcement skeleton printed under different volume energy densities (VEDs) were investigated, as well as the evolution of the microstructure at the interface region of the composite was systematically studied. What's more, we studied the mechanical properties of the composite including nanoindentation test, room temperature tensile and bending tests. The results show that the VED is 88.89 J/mm3, an almost completely dense reinforcement skeleton (∼99.8 %) is obtained. The interface region can be divided into four different reaction layers, namely LⅠ, LⅡ, LⅢ and LⅣ, due to the diffusion of elements. LⅠ is mainly composed of Othick/thin lath-like phase and O short rod-like phase. LⅡ is mainly composed of B2/β phase, acicular α2 phase and nanoscale ω-Ti3NbAl2 phase. The LⅢ mainly consists of B2/β phase. The LⅣ is composed of α2 phase. The deformability of each phase in the composite: B2/β phase > O phase >γ phase >α2 phase >ω phase. The tensile strength and fracture toughness of bioinspired interpenetrating Ti2AlNb/TiAl matrix composite are increased by 24.0 % and 89.0 %, respectively, compared with TiAl alloy, which is mainly contributed to the strong interfacial bonding between matrix and reinforcement as well as the synergistic effect of Ti2AlNb reinforcement with high strength and toughness.

Quantitative analysis on microstructure characteristic of pre-strained β-solidified TiAl alloy during post-heat treatment

The β/βo phase in β-solidified TiAl alloys plays a contradictory role in improving processability at high temperatures but deteriorating performance at service temperature due to its ordering transformation. After exerting its positive role during thermomechanical processing, it must be eliminated or reduced to a minimum through post-heat treatment. In this study, the microstructural evolution of the pre-strained Ti-43.24Al-8.42Nb-0.20W-0.21B-0.24Y alloy on post-heat treatment is investigated, and a quantitative relationship between βo phase content and the pre-strain is established. Due to the difference in driving force of phase transformation, with decreasing pre-strain the microstructure displays varied characteristics from a mixed structure comprising (α2+γ) lamellar colonies, γ blocks, and βo phase, to a nearly-lamellar structure after post-heat treatment at 1270 °C/4h/FC. Only if the pre-strain is less than 0.78, a refined nearly-lamellar structure can be achieved. This work provides important theoretical guidance for practical forging processing of β-solidified TiAl alloys.

Droplet transition behavior and compositional homogenization of TiAl alloys fabricated via dual-wire electron beam-directed energy deposition

The dual-wire electron beam-directed energy deposition (EB-DED) process presents considerable advantages for fabricating titanium aluminide (TiAl) alloys via in situ reactions between Ti and Al. However, variations in the thermophysical properties of pure Ti, pure Al, and TiAl alloys pose challenges in achieving consistent droplet transition behavior and uniform chemical composition. This study explored the effects of wire feeding modes on the forming quality of TiAl alloys and droplet transition behaviors and discussed compositional homogenization and elemental evaporation in TiAl alloy components. Both single-side and double-side feeding modes were utilized, and the electron beam directly melted the Ti wire in the double-side mode, thereby achieving superior surface morphology. Random disturbances and suboptimal wire feeding angles led to the wires deviating from the electron beam center and then being inserted into or passing through the molten pool, thereby degrading the forming quality. By positioning the Al wire tip beneath the Ti wire tip, a common droplet emerged as the Ti droplets melted the Al wire. The droplet transition behavior was regulated by the distance between the wire tips and component surfaces, achieving a liquid bridge transition at distances ranging from 0.75 to 5.82 mm. Although the droplet composition was generally uniform owing to elemental evaporation, the fluctuations among the droplets exceeded those observed in the as-deposited components. The extensive molten pool and keyhole effect enhanced compositional homogenization within the TiAl alloy components. During the deposition process, the evaporation rate of Al surpassed that of Ti due to its higher saturation vapor pressure, consequently yielding a lower actual Al content. The loss rate of Al initially decreased and then increased as the calculated Al content increased, which was influenced by the Al content in the molten pool and temperature variations. This research advances the fundamental understanding of TiAl alloy fabrication via in situ reactions and contributes to the development of the EB-DED process.

Advanced TiAl Based Alloys: From Polycrystals to Polysynthetic Twinned Single Crystals

AbstractTiAl alloys stand out for low density, high specific strength, and excellent creep resistance, making them promising for high‐temperature aerospace applications. However, traditional TiAl alloys suffer from poor room‐temperature ductility and low service temperature that limit their critical applications in aerospace structures. To address these issues, research has focused on improving the mechanical properties of TiAl alloys through alloying and microstructural design. After decades of effort, the evolution of TiAl alloys has progressed from polycrystalline TiAl to high‐performance polysynthetic twinned (PST) TiAl single crystals. The well‐aligned PST TiAl single crystals enriched with Nb enable an excellent combination of strength and ductility, significantly outperforming polycrystalline TiAl alloys. This review summarizes recent progress on TiAl alloys, particularly focusing on newly developed PST single crystals. First, the development history of TiAl alloys is overviewed; then their crystal structures, phase diagrams, and typical microstructures are systematically discussed, along with the design strategies based on alloying elements. Additionally, recent advances in TiAl columnar crystals, which are between polycrystals and single crystals, are reviewed. Subsequently, the mechanical anisotropy, preparation methods, and superior mechanical properties of the PST single crystals are analyzed in detail. The final remark highlights the future development and application prospects of TiAl alloys.

Process optimization and tribological behavior of Ti-48Al-2Cr-2Nb prepared by selective electron beam melting

Selective electron beam melting (SEBM) technology was utilized to fabricate TiAl alloys which possess low density and excellent high-temperature performance, to achieve lightweight design of automotive engines. The forming process of Ti-48Al-2Cr-2Nb was systematically optimized, and found that the microstructure of TiAl formed by SEBM consisted of a layer of alternating γ-band structure and duplex-like structure. As the energy density increases, grain coarsening, proportion increasing of high angle grain boundaries, and anisotropy strengthening occur successively. The forming quality of TiAl reaches its peak at an energy density of 22.5 J/mm3, at which point its mechanical properties and wear resistance also perform the best. Subsequently, in-depth friction experiments at high and room temperatures were conducted, and it was found that at room temperature, as the load increased, the friction coefficient of the samples decreased and the wear rate increased. In addition, the friction coefficient decreases from 0.54 to 0.44, while the wear rate increases from 19.90×10−5 mm3/(N·m) to 25.97×10−5 mm3/(N·m). At this point, abrasive and adhesive wear were the main wear mechanism. Under high temperature conditions, the wear mechanism of the substrate is mainly abrasive wear, adhesive wear, and oxidative wear. And the friction coefficient decreases from 0.69 to 0.49. At lower temperatures, the detached debris was filled and solidified on the wear marks under the action of the friction ring, forming a hardened layer called “enamel”, which can alleviate the negative effects of wear. Under the influence of extreme high temperature environment, the unworn surface forms a light blue oxide film due to rapid oxidation, known as the “burning blue” phenomenon. Along with the high-temperature oxidation phenomenon confirmed by EDS, the oxide layer is more prone to peeling off and forming debris. In addition, melting softening occurs inside the annular wear scar, which increases the amount of deformation under external force, ultimately leading to a significant increase in wear.

Microstructure, Mechanical, and Tribological Properties of Nb-Doped TiAl Alloys Fabricated via Laser Metal Deposition.

TiAl alloys possess excellent properties, such as low density, high specific strength, high elastic modulus, and high-temperature creep resistance, which allows their use to replace Ni-based superalloys in some high-temperature applications. In this work, the traditional TiAl alloy Ti-48Al-2Nb-2Cr (Ti4822) was alloyed with additional Nb and fabricated using laser metal deposition (LMD), and the impacts of this additional Nb on the microstructure and mechanical and tribological properties of the as-fabricated alloys were investigated. The resulting alloys mainly consisted of the γ phase, trace β0 and α2 phases. Nb was well distributed throughout the alloys, while Cr segregation resulted in the residual β0 phase. Increasing the amount of Nb content increased the amount of the γ phase and reduced the amount of the β0 phase. The alloy Ti4822-2Nb exhibited a room-temperature (RT) fracture strength under a tensile of 568 ± 7.8 MPa, which was nearly 100 MPa higher than that of the Ti4822-1Nb alloy. A further increase in Nb to an additional 4 at.% Nb had little effect on the fracture strength. Both the friction coefficient and the wear rate increased with the increasing Nb content. The wear mechanisms for all samples were abrasive wear with local plastic deformation and oxidative wear, resulting in the formation of metal oxide particles.

Microstructure and mechanical properties of multi-phase TiAl alloy matrix composites consolidated via field-assisted sintering technique

AbstractIn this study, fully densified Si3N4/TiAl composites were fabricated using the field-assisted sintering technique (FAST). Microstructural analysis showed the evolution of a continuous network structure consisting of minor fractions of in-situ formed Ti2AlN, unreacted Si3N4 ceramic particles and dominant Ti5Si3 intermetallic phases within the TiAl matrix at Si3N4 content above 1.5 wt%. The hardness of the developed composites increases with increasing Si3N4 content, with 7Si3N4/TiAl composite exhibiting the highest hardness of approximately 487 HV1.0, which was about 57% higher than that of the sintered pure TiAl alloy. Among the sintered samples, 1.5Si3N4/TiAl composite displayed the highest flexural strength of 832.65 ± 12.88 MPa (34.3% higher than pure TiAl matrix) with a deflection of 0.14 mm. In contrast, the lowest flexural strength and deflection of 535.44 ± 21.14 MPa and 0.09 mm were obtained in composite reinforced with 7 wt% Si3N4 ceramic content. The fractured surface of the sintered samples displayed predominantly cleavage fracture mode.

Effect of Cr on microstructure and high temperature oxidation resistance of Ti-Al-Si composite coatings prepared by two-step hot-dipping + pre-oxidation method on Ti65 alloy

Titanium alloys have a thermal barrier temperature of 600℃, however, the preparation of Ti-Al-Si coating is one of the effective methods to improve the high temperature oxidation resistance of titanium alloys. Due to the preferential combination of Si and Ti, it is difficult to form a Ti-Al alloy phase layer with excellent oxidation resistance at high temperature. In order to solve this problem, the two-step hot-dipping + pre-oxidation method was creatively used to prepare Ti-Al-Si composite coatings, and a small amount of Cr was added to improve the high temperature oxidation resistance of the coatings. The phase structure of the coating was analyzed by XRD. The microstructure of the coating was characterized by SEM. EDS was used to analyze the content and distribution of elements in the coating. The results shown that the Cr-added Ti-Al-Si composite coatings were in the sequence of Ti3Al layer, TiAl layer, TiAl2 + Ti5Si3 layer, Ti(Al,Si,Cr)3 layer, L-(Al,Si) layer, and Al2O3 layer from the substrate. Cr segregated on the Ti(Al,Si,Cr)3 phase boundary, and promoted the selective oxidation of Al to form Al2O3. The dense and continuous Ti-Al binary intermetallic layer can prevent cracks from spreading to the substrate, which further improves the high temperature oxidation resistance.