Single-phase TiAl Research Articles

Microstructure characteristics of a γ single-phase Ti-55Al-7.5Nb alloy fabricated via additive manufacturing

Developing a new ordered single-phase TiAl alloy, with no solid-state transformation operated at a long-term high temperature, is important in the aerospace industry. Here, the preparation of newly Ti-55Al-7.5Nb alloy via direct laser deposition (DLD) is proposed and studied. The oriented grain growth in Ti-55Al-7.5Nb alloy is successfully achieved by using laser additive manufacturing (AM) without interlayer pause during the laser scaning back and forth. AM features a rapid cooling rate while Ti-55Al-7.5Nb alloy owns a narrow L → α and α → γ phase transformation temperature window, and then enabling the TiAl liquid phase directly transform into a γ single-phase, which is confirmed by the combination of experiments and thermodynamic calculations. The heat treatment in the AM thermal cycles induces a thermal gradient, promoting the new recrystallized grains growing base on the preferred growth of [111]γ along the heat flow during the DLD solidification stage. The AM Ti-55Al-7.5Nb alloy exhibits continuous γ single-phase columnar grains, meanwhile, accompanied by the twins growing along the building direction.

Temperature-dependent deformation processes in two-phase TiAl + Ti3Al nano-polycrystalline alloys

Although deformation processes in single-phase nano-polycrystalline alloys at different temperatures are well described, the deformation mechanism in two-phase nano-polycrystalline alloys at different temperatures is still unclear. Here, the deformation behaviour of the two-phase TiAl+Ti3Al nano-polycrystalline alloys is investigated at different temperatures by molecular dynamic simulation. For a comprehensive understanding of the processes, mechanical properties of the single-phase TiAl and Ti3Al nano-polycrystalline alloys are explored as well. The results of numerical simulations indicate that temperature has a strong influence on the deformation mechanism of the alloys. When temperature is below 800 K, the critical grain size (~8.3 nm) becomes a dominant factor controlling the deformation process in the single-phase TiAl. In the two-phase TiAl+Ti3Al nano-polycrystalline alloys, the motion of dislocations in the TiAl phase with the grain size smaller than the critical size dominates the deformation process. On the other hand, no obvious phenomena linked with the critical size occur in the Ti3Al phase. Once the strain exceeds 18.0%, the dislocation emission is observed in the Ti3Al grains. At high temperatures (≥800 K), in the two-phase nano-polycrystalline alloy, the deformation mechanism changes from the plastic deformation to the boundary slip and recrystallization.

Structure Formation during High-Temperature Synthesis in an Activated Ti + Al Powder Mixture

This paper presents a detailed experimental study of phase formation processes in a mechanically activated Ti + Al powder mixture. High-temperature synthesis has been performed in thermal explosion mode using induction heating of the mixture. We present the first evidence that, during a continuous transition from rapid heating to high-temperature annealing, the composition of the synthesis products depends on the secondary structuring time. Early stages of annealing involve structural relaxation processes, which make the phase composition more uniform and lead to the formation of an essentially single-phase TiAl compound. In later stages, the system undergoes a transition to thermodynamic equilibrium, which is accompanied by the formation of compounds that are in equilibrium at the annealing temperature.

Formation of TiAl–Ti 2AlC in situ composites by combustion synthesis

Preparation of TiAl–Ti 2AlC in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS) with compressed samples from the mixture of elemental powders. When compared with SHS formation of monolithic TiAl, the addition of carbon particles to the Ti–Al powder mixture enhances the sustainability of the reaction. It was found that no prior heating was required for the samples prepared to produce the composites containing more than 20 mol% Ti 2AlC, in contrast to the need of preheating at 200 °C for single-phase TiAl formation. This is attributed to the fact that formation of Ti 2AlC is more exothermic than that of TiAl. As a result, the combustion temperature and combustion wave velocity increase with the content of Ti 2AlC formed in the TiAl–Ti 2AlC composite, and approach the values associated with formation of single-phase Ti 2AlC when considerable amounts of Ti 2AlC are yielded. The XRD analysis of the end products confirms formation of TiAl–Ti 2AlC in situ composites. Moreover, simultaneous formation of Ti 2AlC promotes the phase evolution of the aluminide compounds. That is, the secondary aluminide phase, Ti 3Al, was no longer detected in the TiAl–matrix composites containing Ti 2AlC of 30 mol% or above.

EFFECT OF MILLING PARAMETERS ON MICROSTRUCTURE OF Ti/Al COMPOSITE POWDER PRODUCED BY DISCUS MILLING

The motivation of this research is to develop a process for producing Ti/Al composite powders that enable synthesis of a single-phase TiAl intermetallic bulk material, which is then used as a target for physical vapour deposition (PVD) coating. This study reports the effects of milling time, amount of process control agent (PCA) added as well as the powder-to-medium ratio on the microstructure of the Ti/Al composite powder. Optical microscopy and scanning electron microscopy (SEM) were used to evaluate the level of mixing and the resulting powder particle size. X-ray diffraction analysis and differential thermal analysis were used for determining the phase constituents and the solid state reaction temperature of the as-milled powders.

Oxide precipitation in V-doped TiAl-based alloys

Oxide precipitation in V-doped TiAl-based alloys was studied by transmission electron microscopy (TEM). The experimental results showed that the interstitial oxygen in single-phase TiAl alloy precipitates in the form of oxide. A Ti–Al–O ternary oxide was observed and the structure of the precipitated oxide was determined. However, in two-phase TiAl/Ti 3Al alloy no oxide was observed. The reasons responsible for the fact that oxide precipitates in TiAl-based alloys were discussed.

Influence of the temperature on the plastic deformation in TiAl

A new alloy, Ti-48.6Al-1.9Cr-1.9Nb-1B, with a near-equiaxed γ microstructure and with a lamellar microstructure is investigated by compression tests between 20 and 800 °C and transmission electron microscopy (TEM). The yield stress exhibits no anomaly as a function of the temperature, while an anomaly, related to strain hardening, is found at 400 °C in the hardening rate and the activation volume for both the lamellar and nonlamellar structure. Above 700 °C, a change in the deformation mechanism occurs and the material becomes remarkably softer. The TEM micrographs highlight the importance of ordinary dislocation motion for both structures at all temperatures. The comparison with previously reported TEM observations on single-phase TiAl alloys shows definitely that the density of ordinary dislocations is higher in the investigated two-phase TiAl alloy deformed at room temperature. Also, the presence of the lamellar interfaces drastically changes the mechanical properties of the alloy and the deformation mechanism. In contrast to the nonlamellar samples, superdislocations are rare, and twinning is very frequent in the lamellar structure.

Physical constants, deformation twinning, and microcracking of titanium aluminides

Physical properties that are relevant to mechanical behavior of single-phase TiAl and Ti3Al and two-phase TiAl/Ti3Al alloys are summarized. By using planar-fault energies and temperature-dependent elastic constants, dislocation dissociation reactions applicable to twin formation in TiAl are analyzed, and a pole mechanism based on a jogged [110]/2 ordinary dislocation is proposed to explain the available experimental data on deformation twinning in γ-TiAl single crystals. The strong plastic anisotropy reported in TiAl polysynthetically twinned (PST) crystals is attributed in part to the localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. The experimental findings reported on cleavage habit planes of PST crystals are discussed in terms of the calculated ideal work of adhesion and possible extrinsic factors.

Hydrogen uptake in titanium aluminides in high pressure hydrogen

Abstract The weight gains of Ti–25Al, Ti–45Al and Ti–53Al alloys, with typical single-phase Ti 3 Al, two-phase Ti 3 Al/TiAl (fully lamellar) and single-phase TiAl microstructures, respectively, were measured at temperatures up to 923 K in high pressure hydrogen, up to 10 MPa. The total hydrogen uptakes during heating to 923 K at constant hydrogen pressures and during increasing the hydrogen pressure to 10 MPa at constant temperatures increased with increasing amounts of the Ti 3 Al in the alloys. The Ti–25Al alloy cracked and then spontaneously disintegrated at high-hydrogen pressures. A ternary (Ti–Al–H) hydride then formed, whose crystal structure is the same as that of the γ hydride (f.c.c.), known in the titanium–hydrogen binary system. No hydride could be detected in the Ti–45Al and Ti–53Al alloys. Most of the hydrogen taken up in the Ti–45Al and Ti–53Al alloys during heating and during pressure increase was released during cooling to room temperature or during pressure decrease to 0 MPa.

In situ high-voltage electron microscope deformation study of a two-phase (α 2 + γ) Ti-Al alloy

In situ straining experiments in a high-voltage electron microscope have been performed on two-phase Ti-47at.%Al-2at.Cr-0.2at.%Si at room temperature and at 900 K, Two microstructures were investigated which were produced by thermomechanical treatments: a so-called near-gamma and a nearly lamellar microstructure. The processes controlling the motion of individual dislocations in the γ phase are very similar in both materials. At room temperature, ordinary dislocations and superdislocations show a jerky motion which is impeded by localized obstacles and jogs. At high temperature, the dislocations are smoothly bent or arranged in preferred orientations and move in a viscous way. This mode of motions as well as the nonplanar arrangement of dislocations point to the action of climb as an essential process during the high-temperature deformation of Ti-Al. These observations are very similar to those on Al-rich single phase TiAl investigated earlier. The considerably higher flow stress of the two-phase alloys is an effect of their particular microstructures, i.e. of grain boundaries and lamella interfaces.

Interstitial solubility in γ and α2 phases of TiAl-based alloys

Measurements of interstitial concentrations were performed on ( α 2 + γ) two-phase TiAl alloys and on γ single-phase TiAl alloys. This study allowed the determination of the maximum solubility of oxygen and carbon in the γ phase. The influence on this solubility of various parameters such as temperature, α 2 and γ phase stoichiometry and the alloying addition of a third element will be discussed. This paper also presents an explanation for the higher solubility of oxygen in α 2 phase compared to γ phase. From our results it is shown unambiguously that the better ductility of ( α 2 + γ) two-phase alloys can no longer fit with the widespread idea of a scavenging effect of the α 2 phase which would lower in that way the interstitial level in γ phase.

Recent progress in our understanding of deformation and fracture of two-phase and single-phase TiAl alloys

Extensive work has been carried out on the microstructural characteristics and property/microstructure relationships of two-phase TiAl alloys in the last decade. Based on these achievements, various engineering two-phase TiAl alloys have been developed and industries are seeking to accomplish their introduction for various structural components. This article reviews recent progress in understanding the deformation and fracture not only of such two-phase TiAl alloys but also single-phase TiAl. The single-phase TiAl is very brittle and exhibits mechanical properties which are considerably different from those of the TiAl phase in the two-phase alloys. However, clarifying the difference in mechanical properties between two-phase and single-phase TiAl alloys and its reasons is indispensable to deepen our understanding of the deformation mechanisms of the TiAl phase.

Composition dependence of atomic mobility in single phase γ TiAl intermetallic compounds

Residual electrical resistivity measurements were used for characterizing γ-TiAl intermetallic compounds and for investigating the kinetics of long-range order (LRO) relaxation. In annealed and slowly cooled materials, a linear relationship between residual resistivity and aluminium content was observed in the composition range from 50 to 56.1 at.% Al. The isothermal relaxation of LRO after small temperature changes was investigated in γ-TiAl compounds (with 49.8, 50.7, 52.6 and 54.1 at.% Al). The approach to equilibrium could be satisfactorily represented by a sum of two exponential relaxation processes. The slower process was assigned to LRO relaxation. From the investigation of the temperature dependence of relaxation times, an activation enthalpy of 2.97±0.15 eV was determined for the stoichiometric alloy. This value is close to the activation enthalpy for titanium self-diffusion in TiAl. For Al contents higher than 50.7 at.%, the LRO changes were found to be superimposed on a supplementary phenomenon, which produces a continuous resistivity decrease. A TEM investigation showed that this drift was associated with the precipitation of Ti3Al5, which is a superstructure of the L10 structure.Des mesures de résistivité électrique résiduelle ont été utilisées pour caractériser le composé intermétallique TiAl γ et pour étudier les cinétiques de relaxation de l'ordre à grande distance (OGD). Dans les matériaux recuits et refroidis lentement, une relation linéaire entre la résistivité résiduelle et la teneur en aluminium a été observée dans un domaine de composition de 50 à 56,1 at.% Al. La relaxation isotherme de l'OGD, consécutive à un faible changement de température, a été étudiée dans les composés TiAl γ (à 49,8, 50,7, 52,6 et 54,1 at.% Al). La cinétique d'approche de l'équilibre peut être représentée de façon satisfaisante par une somme de deux exponentielles, qui correspondent chacune à un processus de relaxation. Le processus lent a été associé à la relaxation de l'OGD. A partir des variations des temps de relaxation avec la température, une enthalpie d'activation de 2,97±0,15 eV a été déterminée pour l'alliage stoechiométrique. Cette valeur est proche de l'enthalpie d'activation pour l'autodiffusion du titane dans TiAl. Pour des teneurs en aluminium supérieures à 50,7 at.%, un phénoméne supplémentaire se superpose aux variations de l'OGD, qui se traduit par une diminution continue de la résistivité. Une étude au MET a montré que cette dérive était associée à la précipitation de la phase Ti3Al5, qui est une surstructure de la structure L10 de TiAl γ.

Deformation mechanisms of near-stoichiometric single phase TiAl single crystals: A combined experimental and atomistic modeling study

The purpose of this paper is to present the results of a combined experimental and theoretical investigation of the reason why the room temperature deformation of PST (polysynthetically twinned) crystals of TiAl occurs primarily by twinning while deformation of single phase TiAl occurs by dislocation slip. It is not clear at present whether this difference in deformation behavior is due to differences in microstructure or differences in composition. Since the stress needed to produce twinning in PST crystals is substantially less than the stress required to produce dislocation slip in the single phase material, it would appear that the twinning stress is dramatically increased by increasing the aluminum content. This suggest is indeed supported by the results of atomistic studies of dislocation dissociations and core structures in the L1{sub 0} TiAl presented in this paper. However, since it is impossible to grow single crystals of single phase material having less than 54 at% Al, this finding cannot be easily checked experimentally For this purpose the authors have developed a novel sample preparation technique, Employing solid state diffusion to reduce the Al content, which allows them to produce layers of 51 at% Al single phase TiAl sandwiched between Ti{sub 3}Al andmore » TiAl with a surplus of Al. These samples were then tested in compression and it was, indeed, found that the single phase TiAl with low aluminum content deforms by twinning, in the same way as the PST crystals with a similar aluminum content.« less

The enhanced work hardening rates of the constituent TiAl and Ti 3Al phases in a lamellar microstructure

The minimum creep rate of a two-phase TiAl/Ti 3 Al alloy with a lamellar microstructure is significantly lower than the minimum creep rates of single-phase TiAl and Ti 3Al alloys tested in compression at the same temperature and applied stress. It has been proposed previously that the arrangement and increased density of geometrically necessary dislocations in a plastically inhomogeneous lamellar microstructure enhance the work hardening rate and decrease the minimum creep rate compared with the constituent single-phase alloys. In support of this proposition, a strong correlation has been found between the increase in dislocation densities and decrease in minimum creep rates of the TiAl and Ti 3Al phases within the lamellar microstructure compared with the constituent single-phase alloys.

Modeling creep deformation of a two-phase TiAI/Ti3Al alloy with a lamellar microstructure

A two-phase TiAl/Ti3Al alloy with a lamellar microstructure has been previously shown to exhibit a lower minimum creep rate than the minimum creep rates of the constituent TiAl and Ti3Al single-phase alloys. Fiducial-line experiments described in the present article demonstrate that the creep rates of the constituent phases within the two-phase TiAl/Ti3Al lamellar alloy tested in compression are more than an order of magnitude lower than the creep rates of single-phase TiAl and Ti3Al alloys tested in compression at the same stress and temperature. Additionally, the fiducial-line experiments show that no interfacial sliding of the phases in the TiAl/Ti3Al lamellar alloy occurs during creep. The lower creep rate of the lamellar alloy is attributed to enhanced hardening of the constituent phases within the lamellar microstructure. A composite-strength model has been formulated to predict the creep rate of the lamellar alloy, taking into account the lower creep rates of the constituent phases within the lamellar micro-structure. Application of the model yields a very good correlation between predicted and experimentally observed minimum creep rates over moderate stress and temperature ranges.

Atom-probe determination of interstitial element concentration in two-phase and single-phase TiAl-based alloys

Interstitial elements (O, C, N) seem to have a key role with respect to mechanical properties of TiAl-based alloys. This paper presents an atom-probe study of the distribution of interstitial elements in the α 2 and γ phases of two-phase TiAl alloys. These elements are found to preferentially segregate in the α 2 phase. This effect is particularly important for oxygen. Measurements of oxygen concentration in the γ phase of two-phase and of single-phase alloys were performed. This allows the maximum solubility of oxygen in the γ phase to be determined. The result shows that the poor low-temperature ductility of single-phase TiAl alloys cannot be simply due to the presence of oxygen in the γ phase.

Growth and Facet Orientations in Single-Phase TiAl Single Crystals

AbstractSingle crystals of single-phase TiAl alloy (Ti-56 at. % Al) have been grown at lOmm/h solidification rate in an ASGAL FZ-SS35W Optical Floating Zone Furnace. The orientations of the resulting single crystals have been determined using the Laue X-ray diffraction, and Electron Backscattering Pattern (EBSP) methods. A correlation between orientations of the crystal growth and grown-in facets has been established.

An Atomistic Study of Dislocations Controlling the Deformation Behavior of TiAl

AbstractAtomistic studies of dislocation cores in a model TiAl compound (L10) carried out using Finnis-Sinclair type many body potentials are presented. Two different core configurations, one planar and the other non-planar, were found for both 1/2〈110] ordinary dislocations and 〈101] superdislocations while a core containing a microtwin was found in the case of the l/2[112] dislocation. The impact of these core structures upon the plastic behavior or PST and single-phase TiAl crystals is discussed.

Large-scale deformation of a single-phase TiAl alloy at elevated temperatures

Forging trials of the TiAl-based alloy Ti52Al have been carried out over a range of temperatures and strain rates. Values of activation energy and strain rate sensitivity have been obtained from stress-strain data, and microstructures from forged specimens have also been evaluated. This has shown that partial recrystallization has occurred.