TiAl Intermetallic Alloy Research Articles

Research on production of microgroove arrays on TiAl intermetallic alloy by micro milling

TiAl intermetallic alloy is a superior lightweight material with excellent performance, and its micro features or components have promising applications in manufacturing fields. In this work, an investigation of micro milling of microgroove arrays on TiAl alloy was studied. The effects of cutting parameters including the feed per tooth (fz) and depth of cut ( ap) on the milling force, surface morphology, surface roughness and burr size of the micro-slots were investigated. The wear types and mechanisms of the micro end mills were also studied. The results showed that the optimal cutting parameters were fz = 3.5 μm/z and ap = 4 μm, which provided the best surface roughness Sa of 152 nm, top-burr heights on up- and down-milling sides of 19 and 19.5 μm respectively. Using the optimized cutting parameters, microgrooves with a width of 500 μm, aspect ratio of 5 and array period of 10 were fabricated successfully. Surface topography, dimensional accuracy and perpendicularity of the microgroove arrays were characterized. Results demonstrated that an increase in the microgroove number, microgroove edge chipping as well as burrs of the entrance and exit increased gradually. The dimensional error between the obtained width and the designed width of the microgroove was less than 2%, and the slope of the sidewalls reached more than 89°. The wear mechanisms of the micro end mills were abrasive wear and adhesive wear, and wear types were coating delamination, cutting edge chipping, and tool nose breakage.

Translating strain to stress: a single-layer Bi-LSTM approach to predicting stress-strain curves in alloys during hot deformation

This study introduces a novel deep learning network that integrates a single-layer bidirectional long short-term memory (Bi-LSTM) network with a coding layer to analyze the hot deformation behavior of various alloys. The single-layer Bi-LSTM model adeptly predicts experimental stress–strain curves obtained under different deformation temperatures and strain rates, demonstrating superior effectiveness and excellent performance in modeling hot deformation behaviors of the FGH98 nickel-based alloy and TiAl intermetallic alloy. The present model achieves the coefficient of determination of 0.9051 for FGH98 and 0.9307 for TiAl alloys, whereas the corresponding values of 0.8105 and 0.8356 are obtained by the conventional strain-compensated Sellars constitutive equation (SCS model). Additionally, the mean absolute percentage error of the single-layer Bi-LSTM model are 11.37% for FGH98 and 7.16% for TiAl alloys, while the SCS model gains the corresponding error of 15.29% and 17.01%. These results show that the present model has enhances the predictive accuracy exceeding 10% for both FGH98 and TiAl alloys over the SCS model. Consequently, the proposed single-layer Bi-LSTM model provides substantial potential for optimizing manufacturing processes and improving material properties.

Жароміцні інтерметалідні сплави та особливості їх легування

The work considers the prerequisites for the development of creep-resistant and heat-resistant alloys, based on intermetallics, taking into account the optimal combination of their physical and mechanical properties, as well as structural characteristics. It is shown that there is a sufficient number of intermetallics, which have a density lower than iron-based alloys, which, in combination with high melting temperatures and heat resistance, makes them promising materials for aerospace application, in particular, for gas turbine engine parts manufacturing. The most suitable creep-resistant and heat-resistant intermetallics can be called aluminides and silicides of titanium, nickel and molybdenum. For these compounds, it is possible to combine high values of specific strength with alloying-favorable types of crystal lattice. One of the most important and common creep-resistant alloys based on intermetallics is Ni3Al with FCC lattice. For this material, complex optimized alloying is the main way to increase creep resistance. The heat resistance of such alloys is significantly increased by applying ceramic coatings. Titanium aluminides Ti3Al and TiAl mainly have only a low density among the advantages. Their fragility at room temperatures and tendency to superplasticity at high temperatures significantly limits their application. The impact of disadvantages may be reduced by applying thermomechanical processing of such materials, which aims to change their structure. Alloying titanium aluminides with a large amount of niobium was chosen as a solution that significantly reduced marked week sides of Ti-Al intermetallic alloys. As a result, this led to the creation of a new alloy based on intermetallic Ti2AlNb with a high set of operational characteristics. Mo-Si and Mo-Si-B can be considered as one of the most perfect systems for creating heat-resistant intermetallic alloys. They combine an acceptable density, high creep and heat resistance, which can be additionally increased by reinforcing with ceramic particles. Keywords: intermetallic-based alloys, creep resistance, alloying, Ni-Al, Ti-Al, Mo-Si, Mo-Si-B.

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.

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.

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.

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.

Tailoring the Microstructure of Laser-Additive-Manufactured Titanium Aluminide Alloys via In Situ Alloying and Parameter Variation

Titanium aluminide (TiAl) alloys have emerged as promising materials for high-temperature applications due to their unique combination of high-temperature strength, low density, and excellent oxidation resistance. However, the fabrication of TiAl alloys using conventional methods is challenging due to their high melting points and limited ductility. Selective laser melting (SLM), an additive manufacturing technique, offers a viable solution for producing TiAl alloys with intricate geometries and the potential for tailoring their microstructure. This study investigates the effect of in situ copper alloying and multiple laser scans on the microstructure and mechanical properties of TiAl-based alloys fabricated using SLM. The results demonstrate that copper alloying enhances the formation of the α2-Ti3Al phase, refines the microstructure, and improves the mechanical properties of TiAl alloys. Multiple laser scans allow for the creation of distinct microstructural regions within a single component, enabling the tailoring of properties that are suitable for specific operating conditions. The findings provide valuable insights into the fabrication and optimization of TiAl intermetallic alloys with diverse applications.

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.

Oxidation of TiAl alloy by oxygen grain boundary diffusion

Ti–Al intermetallic alloys have a number of advantages for the manufacture of aviation and car details. The use of these alloys is limited by their oxidation at high temperatures, which is associated with structural features and the presence of micro and nanosized grains. In such materials, grain boundary diffusion of oxygen can be active, which contributes to the acceleration of oxidation upon heating. In this paper, we present a two-dimensional model for research the diffusion which is accompanied by oxidation. The structure of TiAl is specified as symmetrically alternating grains with a clear separation of boundaries and triple junctions between them. Diffusion and chemical properties of grains and boundaries are different. The problem is formulated in a two-dimensional formulation and solved numerically. The influence of changes in the composition over time on the penetration of oxygen is taken into account. For the convenience of numerical research and analysis of the results, dimensionless variables are used. To solve the diffusion equation, an implicit difference scheme of splitting in coordinates is used. The equations of chemical kinetics are solved using an algorithm similar to the implicit Euler method using an iterative procedure. The results demonstrate the influence of different variants of kinetic regimes on the concentration distribution and integral concentrations of reagents and oxides.

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.

EFFECT OF HEAT TREATMENT ON THE STRUCTURE AND PROPERTIES OF TITANIUM ALUMINIDE ALLOY TI-AL-V-NB-CR-GD PRODUCED BY SELECTIVE ELECTRON BEAM MELTING

This study examines the influence of hot isostatic pressing and heat treatment on the microstructure and mechanical properties of samples manufactured by selective electron beam melting (SEBM) of metal powder composition (MPC fraction 40-100 pm) of a new six-component intermetallic beta-solidifying TiAl alloy Ti-44.5Al-2V-1Nb-2Cr-0.1Gd, at % (Ti-31.0Al-2.5V-2.5Nb-2.5Cr-0.4Gd, wt %). It is shown that SEBM with a high line energy input (El = 285 J/m) produces a fine-grained microstructure in the as-built material with a grain size of 5-14 pm and residual porosity of less than 0.5 vol %. An increase in the electron beam current (I) from 9.5 to 19.0 mA causes Al evaporation, and as a result the fraction of large columnar grains (d = 30-100 pm in width, h = 150-400 pm in height) formed mainly in Al-depleted regions (layers) increases. Heat treatment of the as-built SEBM samples by two-stage annealing in the (a + y) and (a2 + у + в) phase fields or by thermal cycling in the (a + y) phase field leads to complete or partial fragmentation of columnar grains. For the samples produced at lower values of I, a combined post-processing treatment by hot isostatic pressing in the а phase field and two-stage annealing completely eliminates residual porosity and transforms the columnar structure into a fine-grained one with a grain size of less than 150 pm. As a result, the achieved short-term mechanical characteristics at 20 °С (аВ = 525 ± 5 MPa, 5 = 1.1%) and 750 °С (аВ = 405 ± 10 MPa, 5 = 3.8%) are comparable to those of the studied TiAl alloy in the as-cast state.

Формирование биметаллического материала Ti–Al методом проволочного электронно-лучевого аддитивного производства

Currently, there is a request from aerospace and aircraft for the construction materials with sufficiently high mechanical strength, thermal creep, corrosion and oxidation resistance. The conventional alloys used for these purposes are too heavy. At the same time, alternative light materials such as Ti–Al-based alloys have many flaws, when they are produced by conventional methods. This work considers the possibility to produce the Ti–Al-based alloys by the method of a wire-feed electron-beam additive manufacturing (EBAM). We study the chemical and phase compositions, microstructure and microhardness of a bimetallic Ti–Al alloy, obtained by this method. It is found the formation of five characteristic regions between titanium and aluminum parts of the bimetallic billet. The mixing zone consists of TiAl and TiAl3 intermetallics, that is confirmed by the investigation of microstructure, chemical and phase compositions. According to XRD (X-ray diffraction) and EDS (energy-dispersive X-ray spectroscopy) analyses, it can be assumed that TiAl intermetallic prevails over TiAl3 one. The average microhardness of the mixing zone equals to 450 HV (≈4.4 GPa). This zone has developed dendritic microstructure, and even distribution of the phases without link to dendritic and inter-dendritic zones. The cracks appearing in this area are filled with the material of the upper layers, so the whole material is poreless and defect-free. Thus, the results of this work have shown a fundamental possibility to produce the intermetallic Ti–Al alloys with the use of the EBAM.

Sliding Wear Behavior of Intermetallic Ti-45Al-2Nb-2Mn-(at%)-0.8vol%TiB2 Processed by Centrifugal Casting and Hot Isostatic Pressure: Influence of Microstructure.

Intermetallic alloys such as titanium aluminides (TiAl) are potential materials for aerospace applications at elevated temperatures. TiAl intermetallics have low weight and improved efficiency under aggressive environments. However, there is limited information about wear behavior of these alloys and their microstructure. The present work aims to study the influence of the microstructure in the tribological behavior of TiAl intermetallic alloy (45Al-2Mn-2Nb(at%)-0.8 vol%TiB2). Wear tests were performed on samples manufactured by centrifugal casting (CC) and hot isostatic pressure (HIP). Reciprocating sliding wear test was carried out for TiAl, it was combined with different loads and frequencies. Wear tracks were analyzed through opto-digital microscopy and electron microscopy (SEM). The results obtained reveal that CC intermetallics present the lowest volume wear lost, approximately 20% less than HIP intermetallics. This good behavior could be related to the high hardness material, associated with the main microstructure where CC intermetallic has nearly lamellar microstructure and HIP intermetallics present duplex microstructure.

Effect of Sn addition on the mechanical properties and high-temperature oxidation resistance of intermetallic TiAl alloys by first principles study and experimental investigation

Tin (Sn) can be used as an effective TiAl powder sintering aid, but it is unclear whether Sn will deteriorate its properties as a micro-alloyed element in TiAl intermetallic. This work focuses on the influence of Sn on the mechanical properties and oxidation resistance of TiAl alloys. The effect of Sn on the mechanical properties of TiAl alloys is investigated by experiments and first-principles calculations. Sn atoms preferentially occupy the Al sites in γ-TiAl alloys and increase the lattice volume, which is consistent with the XRD pattern. The elastic constants of TiAl alloys with different Sn contents are also systematically presented. It is found that 1.85 at.% Sn addition improves the ductility of TiAl, but a higher Sn content reduces the ductility. This result is also verified by room temperature compression test. The density of states is also used to prove the ductility improvement of TiAl alloys with a certain amount of Sn. Last, the diffusion barrier of O and Al/Ti–O bond strength were calculated to evaluate the oxidation resistance of TiAl–Sn alloys. The addition of Sn markedly increases the O diffusion barrier and Al–O bond force constant, indicating that Sn can improve the oxidation resistance of TiAl alloy.

The formation of Si-aluminide coating formed by plasma spraying and subsequent diffusion annealing on Ti-Al-7Nb intermetallic alloy

In the article, the kinetic growth phenomena of aluminide coating formed by plasma spraying pure Al-Si powder and subsequent diffusion annealing on TiAl intermetallic alloy in inert atmosphere were investigated. The Al-Si powder was thermal sprayed (APS) on TiAl7Nb intermetallic alloy and annealed in Ar atmosphere during 5, 15, 30, 60, 240 and 480 min. The kinetic growth of the coating was observed using the scanning electron microscopy method (SEM), and chemical composition was analysed using the EDS method. The Kirkendall Effects pores formation, as well as titanium silicides on the grain boundary of TiAl3, was found. The oxidation resistance of the developed coating might be analysed in further work. The developed coating might be used for the production of protective aluminide coatings on TiAl intermetallic alloys. The description of aluminide coating formation in a new technological process.

Size-Dependent Fracture Characteristics of Intermetallic Alloys

BackgroundLightweight alloys such as intermetallic titanium aluminide (TiAl) alloys are poised to be a potential candidate for replacing heavier nickel based super alloys in an aero engine. However, before an industry wide implementation is possible, it is indispensable to develop physically accurate computational material models which account for essential deformation and fracture mechanisms. This assists the virtual prototyping required for the new product development using TiAl components.ObjectiveThe objective of this work is to determine the effect of size of tested specimens on their fracture energy and provide a physically motivated scaling law.MethodsIn this work, the quasi-brittle behavior of TiAl alloys is experimentally and numerically investigated. A total number of 29 geometrically identical TiAl specimens of three different sizes are tested in a three-point bending setup. Since the final abrupt failure of each specimen is preceded by plasticity, a theoretical and numerical framework which accounts for both elastic and plastic work densities is applied in simulations.ResultsThe fracture energy density for each tested size is calculated numerically which is found to be lower for larger volumes, thereby, confirming the size effect in intermetallic TiAl alloys. A novel size effect law is proposed which is based on two physically motivated coefficients.ConclusionsThe work concludes with the quantitative knowledge of the size-dependent fracture energy of intermetallic alloys and an empirical scaling law to predict the same. Excellent predictive capability of the proposed law is successfully established with data of various quasi-brittle materials from literature.

Microstructural modulation of TiAl alloys for controlling ultra-precision machinability

TiAl intermetallic alloys have attracted considerable attention in aerospace applications over the last few decades owing to their low density and superior mechanical properties at high temperatures. However, these alloys are also known as difficult-to-machine materials that hinder efficient manufacturing. This study presents a systematic investigation on the machinability of TiAl alloys with three types of microstructures obtained by different heat treatment parameters. These were classified as near-gamma (NG), duplex (DP), and fully lamellar (FL). The machinability was evaluated based on the cutting forces and machined surface roughness. The material with α2/γ lamellar structures (FL) exhibited the lowest cutting force (3.21 N). However, produced a rougher surface (80 nm Ra) as compared to the NG microstructure (4.69 N and 47 nm Ra). Electron backscattering diffraction (EBSD) evaluation of the primary deformation zone in the cutting chips revealed that insufficient heat energy was converted from plastic deformation for recrystallization or β+γ→α2 phase transformation to occur. This indicated that the deformation mechanisms were significantly dependent upon the plasticity. The NG microstructure demonstrated a higher degree of plasticity in the primary deformation zone, which was attributed to the combined effect of super-dislocation decomposition, ordinary dislocation slip, and refined mechanical twins with preferred orientation along the <112¯> {111} crystallographic orientation. Conversely, the FL microstructure exhibited brittleness during chip formation due to the weak bonding force between hexagonal α2 and tetragonal γ phases that led to preferential micro-cracking along each interface. Reducing the crystal orientation is conductive for improving machined surface quality. The notion of enhanced brittleness to explain the reduction in cutting forces due to the dissipation of energy through fracture was supported with numerical simulations. Microscopic evaluation was used to understand the deformation differences of the equiaxed γ grain and α2/γ lamellar microstructure during micro-cutting. Additionally, enhanced the understanding of the deformation mechanism of these multi-phase alloys.

Designing of Novel Microstructure in 2nd and 3rd Generation Advanced Intermetallic Tial Alloys: Effect of Alloy Addition and Phase Transformation on Mechanical Properties

Designing of Novel Microstructure in 2nd and 3rd Generation Advanced Intermetallic Tial Alloys: Effect of Alloy Addition and Phase Transformation on Mechanical Properties