Binary TiAl Alloys Research Articles

Effect of Al content on the fracture toughness and deformation behavior of Ti-Al binary alloys

The fracture toughness and deformation behavior of Ti-xAl (x = 0, 2, 4 wt%) binary alloys were investigated. The non-thickness-dependent fracture toughness was measured based on the J-integral multiple-sample approach. Increased Al content induced reduced fracture toughness from 265 to 148 MPa·m1/2 of Ti-xAl alloys. The fracture toughness was discussed in this article by the mutual influence of the elastic component (Jel) and the plastic component (Jpl). The solid solution strengthening effect of Al was identified as the primary factor contributing to the increase in Jel. Additionally, the evolution mechanism of Jpl with varying Al content was elucidated by slip modes, twin activation, and dislocation configurations at the crack tip. Prismatic slip was found to be the predominant slip mode in Ti-xAl alloys. With the increase in Al content, the elevated critical resolved shear stress associated with prismatic slip hindered the alleviation of stress concentration, resulting in a decrease in Jpl. The increase in content of Al in the Ti-xAl alloys inhibited the formation of {11 2¯ 2} twins. Compared to Ti-0Al and Ti-2Al alloys, a significant increase in <c> dislocations was observed in Ti-4Al alloy. The additional <c> dislocations impeded the movement of subsequent <a> dislocations, which potentially resulted in a reduction in Jpl. Furthermore, a crack tip stress analysis method based on twin variant activation was proposed. In Ti-0Al and Ti-2Al alloys, twins activated by internal stress were more pronounced compared to the Ti-4Al alloy. The internal stress may be composed of back stresses, which are generated by twinning activation.

Effect of solutes segregation in a binary TiAl alloy: A first-principles calculation method

Comprehending the interfacial structures and energetics of interfaces between γ-TiAl and α2-Ti3Al phase is critical to understanding the stabilities of the binary-phase TiAl-Ti3Al alloy. Here a comprehensive first-principles investigation is reported on the structures and energetic properties of γ-TiAl(111)/α2-Ti3Al(0001) interfaces with 19 segregated transition metals considered. The results indicate that all considered atoms have a tendency to segregate at the interface. The interfacial strength enhancement occurs after V, Cr, Mn, Fe, Nb, Mo, Tc, Ru segregated in TiAl, Ag segregated in Ti3Al, and almost all solute atoms can improve interface stability. Compared with the pure interface, the bonding between the segregated atoms and the surrounding atoms is stronger at the interface after V or Nb segregation, while the bonding between Cd and the surrounding atoms is weaker. Our calculations provide the necessary interfacial structures and energetics for predicting the stabilities of binary TiAl alloy.

Stabilization of Ti5Al11 at room temperature in ternary Ti-Al-Me (Me = Au, Pd, Mn, Pt) systems

Ti5Al11 is known as a high-temperature phase in binary Ti-Al alloys. However, its existence at low temperatures was previously observed in ternary Ti-Al-based systems alloyed with some transition metals. In this study, we systematically evaluated Ti-Al-Me ternary systems (Me = Au, Pd, Mn, or Pt) to determine the influence of transition elements on low-temperature stabilization of Ti5Al11 phase. The temperature ranges in which Ti5Al11 existed in Ti-Al-Me systems were experimentally found using in situ synchrotron X-ray diffraction (SXRD). It was established that addition of Mn and Pt retains Ti5Al11 at room temperature. The obtained data were compared with predictions of density functional theory (DFT). The total energy, volume, and bond length are especially significantly reduced by addition of Mn and Pt. Ti5Al11 compound containing both of these elements is less prone to saturation with Ti upon preserving the lattice tetragonality and suppressing Ti5Al11 → TiAl transformation. These factors finally contribute to the retention of this phase at room temperature.

A molecular dynamics simulation-based laser melting behavior analysis for Ti–Al binary alloy

Due to the lack of theoretical basis for the determination of process parameters, there are problems such as incomplete melting and excessive melting in the molten pool for laser processing, which ultimately cause the increase of defects such as air hole and crack. The melting behavior of Ti–Al binary alloy during laser processing is studied on the atomic scale by molecular dynamics simulation. The embedded atom method potential is adopted to establish the melting model of Ti–Al binary alloy. According to the changes of temperature, atomic structure and mean square displacement, the influence of different process parameters on the melting behavior of Ti–Al binary alloy is discussed. The results show that a short temperature drops from initial melting to complete melting. Mean square displacement increases rapidly with the increase of laser time after completely melting. The atoms start to move violently and the atomic structure is completely transformed into other structure. The heating and cooling rate increases with the increase of power. The laser power has little effect on the overall distribution of atoms after completely melting and only affects the variation rate of various physical quantities in melting process. The research results provide a deeper practical and theoretical evidence for the determination and optimization of the 3D printing and laser heat treatment.

Crystallographic ordering of Al and Sn in α-Ti

Increasing attention is being paid to α2 Ti3(Al,Sn) precipitation from the α phase of titanium alloys owing to its effect on slip band formation, localisation and the implications for fatigue performance in jet engine titanium. However, the early stages of α2 precipitation have historically been difficult to observe by electron microscopy, neutron diffraction or atom probe analysis. Here, small angle X-ray scattering is used to reexamine the phase boundary in binary Ti-Al and Ti-Sn alloys with around 500 ppmw O. It is found that the phase boundaries in the literature are approximately correct, at 6.2 wt.% Al and 16.9 wt.% Sn, and that this favours the use of Al as a solid solution strengthener over Sn for ambient temperature applications. However, once O content and phase partitioning in α+β alloys are taken into account, this implies that Aleq limits for future alloy design of critical rotating parts should be lowered substantially.

Impact Resistance of Commercially Applied TiAl Alloys and Simple-Composition TiAl Alloys at Various Temperatures

For currently used TiAl alloys, the impact resistance is a critically important property that determines their suitability for use, especially in settings when continuous use under harsh conditions is necessary. However, there are almost no examples of the investigation of the impact resistance of these alloys at realistic temperatures. Therefore, in this study, the impact resistance from room temperature to 1000 °C of various cast and forged TiAl alloys proposed to date or still in commercial use, as well as simple composition TiAl alloys and Inconel 713C—a commonly used material—were evaluated using the Charpy impact test, which is the simplest and most realistic way to evaluate industrial impact resistance. It was found that the TiAl alloys underwent brittle fracturing, even at high temperatures, and had significantly lower impact resistances than Inconel 713C. In addition, the impact resistances of all commercial TiAl alloys were inferior to those of the binary alloys, and those of the TiAl4822 and TNM alloy were not significantly different. Crucially, it was found that ternary alloys containing Cr or V had much better impact resistance than the commercial and binary TiAl alloys.

Accurate interatomic potential for the nucleation in liquid Ti-Al binary alloy developed by deep neural network learning method

For the traditional empirical potentials, the accuracy of chemical bonding in a wide temperature range is regrettable, and thereby they are not an appropriate choice to explore the nucleation process of crystals. As a developed superalloy in aerospace, the liquid structure and nucleation mechanism of Ti-Al alloy desire to be investigated to comprehend the solidification process deeply. This work develops an accurate deep neural network (DNN) potential for Ti-Al binary system by employing a machine-learning method. Then, the local structure and the nucleation process of liquid Ti-48 %Al and Ti-52 %Al (atomic percentage) alloys from the normal to the undercooled state are performed by molecular dynamics simulation with the DNN potential. It is found that the formation of the locally ordered structures depends on the Ti-Al atom pair during the solidification. The Honeycutt-Andersen indices of 1431,1541 and 1551 are dominant, and the liquid structure is mainly characterized by perfect and distorted icosahedral clusters. By Voronoi Polyhedron (VP) analysis, Ti-centered VPs favor the formation of body-centered-cubic (BCC) structure, while Al-centered VPs have the main contribution to the icosahedral structure. Besides, liquid Ti-52 %Al alloy prefers to form face-centered-cube (FCC) nuclei than BCC nuclei in a high undercooling, which is closely related to the ability to capture Ti-Al bonds. The favor of the nucleation of the γ phase causes the nuclei with non-equilibrium composition, comprised of Ti-rich BCC and Al-rich FCC coordinated atoms, respectively. Novelty, the FCC nuclei in liquid Ti-Al alloys are independently generated without the precursor of BCC coordinated atoms. High undercooling can transform the nucleation mechanism from two steps to one step.

Preparation of Ti-Al-Si Gradient Coating Based on Silicon Concentration Gradient and Added-Ce

Titanium and titanium alloys have excellent physical properties and process properties and are widely used in the aviation industry, but their high-temperature oxidation resistance is poor, and there is a thermal barrier temperature of 600 °C, which limits their application as high-temperature components. The Self-generated Gradient Hot-dipping Infiltration (SGHDI) method is used to prepare the Ti-Al-Si gradient coating based on the silicon concentration with a compact Ti(Al,Si)3 phase layer, which can effectively improve the high-temperature oxidation resistance of the titanium alloy. Adding cerium can effectively inhibit the generation of the τ2: Ti(AlxSi1−x)2 phase within a certain hot infiltration time so as to form a continuous dense Al2O3 layer to further improve the oxidation resistance of the coating. Studies have found that multiple Ti-Al binary alloy phase layers are formed during the high-temperature oxidation process, which has the effect of isolating oxygen and crack growth, and effectively improving the high-temperature resistance of the coating oxidation performance.

Selection of Additive Elements Focusing on Impact Resistance in Practical TiAl Cast Alloys

Despite the widespread use of TiAl alloys in many fields, their poor impact resistance has recently emerged again as a major issue. Therefore, in this study, the practical effects of additive elements were examined by considering impact resistance as the most important property of practical TiAl cast alloys. The impact resistance was evaluated via the Charpy impact test, which has been rarely applied to TiAl alloys, revealing that only V, Cr, Mn, and B slightly improved the impact resistance, compared to that of the Ti–Al binary alloy, both in the as-cast and heat-treated states. In contrast, Nb, Mo, W, Fe, Ni, Si, C, and N decreased the impact resistance. If a slight decrease in impact resistance is permitted, then only Si and C can improve the creep strength, and Nb and W can improve the anti-oxidation resistance. These results confirm the excellence of practical TiAl cast alloys developed a considerable time ago, containing these elements in addition to Cr, Mn, and B.

Composition-induced microcrack defect formation in the twin-wire plasma arc additive manufacturing of binary TiAl alloy: An X-ray computed tomography-based investigation

Composition-induced microcrack defect formation in the twin-wire plasma arc additive manufacturing of binary TiAl alloy: An X-ray computed tomography-based investigation

Effects of Cu Addition on Mechanical Behaviour, Microstructural Evolution and Anti-Corrosion Performance of TiAl-Based Intermetallic Alloy under Different Strain Rates.

TiAl-based intermetallic alloys are prepared with Cu concentrations of 3–5 at.% (atomic ratio). The mechanical properties and microstructural characteristics of the alloys are investigated under static and dynamic loading conditions using a material testing system (MTS) and split-Hopkinson Pressure Bar (SHPB), respectively. The electrochemical properties of the various alloys are then tested in Ringer’s solution. It is shown that the level of Cu addition significantly affects both the flow stress and the ductility of the samples. For Cu contents of 3 and 4 at.%, respectively, the flow stress and strain rate sensitivity increase at higher strain rates. Furthermore, for a constant strain rate, a Cu content of 4 at.% leads to an increased fracture strain. However, for the sample with the highest Cu addition of 5 at.%, the flow stress and fracture strain both decrease. The X-ray diffraction (XRD) patterns and optical microscopy (OM) images reveal that the lower ductility is due to the formation of a greater quantity of γ phase in the binary TiAl alloy system. Among all the specimens, that with a Cu addition of 4 at.% has the best anti-corrosion performance. Overall, the results indicate that the favourable properties of the TiAlCu4 sample stem mainly from the low γ phase content of the microstructure and the high α2 phase content.

Effects of Al content and α2 precipitation on the fatigue crack growth behaviors of binary Ti–Al alloys

The influence of Al content and α2 precipitation on the fatigue crack growth (FCG) behaviors of binary Ti - xAl (x = 2, 4, 6, 8 wt %) alloys was investigated. Ti–Al binary alloys with different Al contents were solution-treated and followed by aging treatment to obtain Widmanstatten microstructure and promote α2 precipitation at higher Al content. Results show that the increase of Al content dramatically reduces the FCG rate. The enhanced resistance to FCG propagation is related to the increase of the critical resolved shear stress (CRSS) for dislocation slipping that acts as crack initiation sites, basing on solution strengthening with the addition of Al. Furthermore, roughness-induced crack closure with increasing the Al content also contributes to the decrease of FCG rate. With the increment of the Al content, the early fatigue crack propagation stage, in which the crack propagation significantly affected by microstructural factors, will be enlarged due to the decrease of crack tip plastic zone, leading to crack deflection and the reduction of FCG rate. In Ti–8Al alloy, the presence of α2 precipitates and the increment of its size and volume fraction after aging treatment will result in significant increase of FCG rate. Moreover, the activation of large-scale slipping in relatively larger colonies promotes the crack to propagate along the large-scale slipping that induces smoother crack path and higher FCG rate.

Microstructure and elevated temperature tensile property of Ti–46Al–7Nb-(W,Cr,B) alloy compared with binary and ternary TiAl alloy

The newly developed high-Nb TiAl with a small amount of W, Cr, and B addition prepared by electromagnetic cold crucible has been investigated by comparing with the microstructure and elevated temperature tensile properties of binary and ternary high-Nb TiAl alloy. Results indicate that microstructures of the three alloys are fully lamellar (FL), near fully lamellar (NFL) and near fully lamellar alloy with TiB (NFL + TiB). The dispersed TiB particles in the alloy have the B27 structure and are highly enriched with Nb and W but depleted with Al by the characterization of transmission electron microscopy. The elevated temperature tensile test results show that the high-Nb TiAl alloy with NFL + TiB microstructure exhibits excellent mechanical properties compared with the other two alloys, and the tensile strength and elongation at 800 °C are 648 MPa and 5.73%, respectively. Fracture morphology shows the alloy has a mixed mode of trans-lamellar and inter-lamellar fracture. As the dispersed second phase particles, rod-like TiB can act as the obstacle to crack propagation and dislocation movement. The excellent mechanical properties of the alloy at elevated temperatures are attributed to the combined effects of dislocation block and nanotwins formation during deformation within the γ lamellae. Moreover, the probable generation and propagation mechanism of dislocations and twins in the γ lamellae are further discussed.

Microstructural and elastic properties of stable aluminium-rich TiAl and TiAl2 formed phase Intermetallics

We studied aluminium-rich Ti-Al (Ti32Al68 and Ti40Al60) binary alloys that were composed of TiAl and TiAl2 lamellar microstructures. The law of mixtures was employed in calculating the theoretical Young’s moduli. The lattice parameters of the alloys showed that both were tetragonal crystals. In the computational study, we made use of our modified method for the stress–strain calculation of elastic constants. The alloys at the respective chemical compositions were modelled by creating titanium (Ti) supercells, which were then doped by replacing some of the Ti atoms with aluminium atoms. The values of elastic moduli were verified by the ab initio calculation in this work, which showed a perfect agreement. The Pugh’s ratio showed that both the alloys are ductile.

Atomic-Scale Mechanism Investigation of Mass Transfer in Laser Fabrication Process of Ti-Al Alloy via Molecular Dynamics Simulation

This article deals with a Ti-Al alloy system. Molecular dynamics simulation was used to simulate and explore the mass transfer behavior during the laser fabrication process at atomic scale. The research goal is to investigate the mass transfer mechanism at atomic scale and the movement of solute atoms during the laser fabrication process. The mean square displacement (MSD), radial distribution function (RDF), atomic number density, and atomic displacement vector were calculated to characterize it. The results show that the TiAl alloy is completely melted when heated up to 2400 K, and increasing the temperature past 2400 K has little effect on mass transfer. As the heating time increases, the diffusion coefficient gradually decreases, the diffusion weakens, and the mass transfer process gradually stabilizes. In Ti-Al binary alloys, the diffusion coefficients of different solute atoms are related to the atomic fraction. During the melting process, the alloy particle system has a greater diffusion coefficient than the elemental particle system.

Characterization of α2 Precipitates in Ti–6Al and Ti–8Al Binary Alloys: A Comparative Investigation

Transmission electron microscopy (TEM) and atom probe tomography (APT) techniques were used to investigate the nano-scale ordered α2 (Ti3Al) precipitates in Ti–Al binary alloys. Ti–6Al and Ti–8Al binary alloys were solution treated and aged to obtain Widmanstatten microstructure and promote α2 precipitates. The TEM results displayed strong short-range ordering of α2 precipitates in Ti–8Al alloy, while no evidence of the superlattice reflections of α2 in Ti–6Al alloy. The results acquired from APT showed the α2 clusters and atoms distribution at the interface between the matrix and α2 precipitates. The size and morphology of α2 particles in Ti–8Al alloy, respectively, obtained by TEM and APT are closely consistent. Meanwhile, the APT results displayed tiny size clusters in Ti–6Al alloy, which supposed to give evidence of the initial ordering process of α2 precipitates in the absence of correlative results from TEM.

Effects of alloying and processing on ultrasonic fatigue behavior in binary Ti-Al alloys

Three model binary Ti-Al alloys with single α phase were investigated in this study to develop a fundamental understanding of high cycle and very high cycle fatigue behavior in α-titanium alloys. Ultrasonic fatigue instrumentation with a cyclic frequency of nominally 20 kHz was used for fully reversed loading fatigue testing and combined with customized optical microscopy to study small crack growth from artificial cracks initiated at micro-notches produced by focused ion beam. Fractography showed the formation of low ΔK facets with different features near the crack initiation sites. Micro-beach marks were observed to form on some facets which could be correlated with the spatial angle between facet normal and loading direction. Aluminum content and processing (forging/rolling history) were found to have no significant effect on the small crack growth behavior of the studied alloys. Different crack growth models were compared and discussed to estimate the fatigue performance, especially for cross-rolled Ti-4Al alloy with void-type defects.

Microstructural Properties of Heat-Treated LENS In Situ Additively Manufactured Titanium Aluminide

This study reports on the microstructure, phase identification and hardness property of the LENS-manufactured binary Ti-Al alloy which was achieved via laser in situ alloying. The in situ alloying method, using laser beam as an energy source, indicated that it is possible to achieve a binary gamma phase microstructure from elemental powders of Ti and Al. The as-produced sample, however, was characterized of having different microstructure at the top, middle and bottom regions. The middle and top layers had similar hardness which was lower than that of the bottom region. The as heat-treated sample was characterized of lamellar grain microstructure at the top and just grains at the bottom and the middle. The observed grains were different in size, phase and hardness. This study indicated that in addition to the pores and the precipitation of α2 phase or aluminum segregation on the grain boundary veins intermetallics lead to severe cracking. Overall, the hardness values of the as-produced and heat-treated samples were similar, and due to extensive cracking, it was inferred that both samples will lack ductility.

Numerical and experimental study on peritectic transition in TiAl-Nb alloys

The solidification pathway and peritectic transition characteristic related to the solute concentration in TiAl-Nb system are calculated using the Thermo-Calc software. The results show that there are four typical solidification modes in the TiAl-Nb system, which are single β, hypo-peritectic, hyper-peritectic and single α solidification. The mole fraction of peritectic growth αp can be used to describe the undergoing extent of the peritectic tranformation (EPT) directly for the alloys with the peritectic transition, and βp/βmax can also be used to describe the EPT for the hypo-peritectic and Lp/(1 − βmax) for the hyper-peritectic. The mole fraction of αp shows a parabolic shape variation on a plot of solute Al at constant Nb content. The composition of the complete peritectic transformation (PT) is around 46.5 at.% Al for the TiAl binary alloy, and it gradually shifts to the direction of high Nb content with increase of solute Al. A temperature gap (△Tp) corresponding to the peritectic transition exists in Nb containing TiAl alloys and the △Tp shows a parabolic shape on a plot of solute Al at constant Nb content. The experimental results are generally in accordance with the theoretical rules.

Effect of Zr on microstructure and mechanical properties of binary TiAl alloys

Abstract Ti43Al and Ti47Al alloys with different contents of zirconium were prepared by non-consumable vacuum arc melting furnace. The microstructure and mechanical properties were investigated. The results showed that Zr had no obvious effect on microstructure morphology of Ti43Al, while that of Ti47Al was modified from dendrites into equiaxed grains. The addition of Zr could refine the grains. Zr promoted the formation of γ phase significantly and the solubility values of Zr in γ phase were 12.0% and 5.0% (molar fraction) in Ti43Al and Ti47Al, respectively. Zr-rich γ phase mainly formed through β→γ in Ti43Al−xZr (molar fraction, %) and β→α→γ in Ti47Al−xZr (molar fraction, %). Fine-grain strengthening and solution strengthening were beneficial to improving the compressive strength while severe micro-segregation was detrimental to compressive properties. Large solubility of Zr was bad for ductility of alloys as well. The maximum compressive strengths of Ti43Al−xZr and Ti47Al−xZr were 1684.82 MPa (x=5.0%) and 2158.03 MPa (x=0.5%), respectively. The compressive strain fluctuated slightly in Ti43Al−xZr and reached the maximum value of 35.24% (x=0.5%) in Ti47Al−xZr. Both alloys showed brittle fracture.