As-cast TiAl Alloy Research Articles

Enhancement of mechanical properties and oxidation resistance of TiAl alloy with addition of Nb and Mo alloying elements

In this study, small button-shaped ingots were prepared by a non-self-consuming vacuum arc melting method. The effect of the content of Nb and Mo elements on the microstructure, mechanical properties and oxidation resistance of TiAl alloys were investigated. The as-cast TiAl alloy presented laminar microstructure. When only adding Nb element, a small amount of β phase was distributed in the form of dots at the grain boundaries of α2/γ lamellar structure. When adding both Mo and Nb, the precipitated β phase started to precipitate in lines inside the α2/γ lamellar structure. The microhardness of the alloy increased with the increasing Nb and Mo content. The compressive strength of the alloys increased firstly with the increasing Nb content and decreased when the content of Nb exceeded 7 wt%. However, with the addition of Mo, the compressive strength of the alloys decreased drastically. The optimal compressive strength of the alloy, i.e. 2250 MPa, was obtained with 5 wt% Nb and 5 wt% Mo addition. The oxidation resistance of the alloys were enhanced with the addition of Nb and Mo elements and the TiAl–5Nb–5Mo alloy also showed the best oxidation resistance at 800 °C.

Twinned dendrites in TiAl alloys

A novel dendrite splitting phenomenon in as-cast TiAl alloys was observed to show microstructural morphologies as well as crystallographic characteristics similar to those of twinned dendrites. For a β-solidified TiAl alloy, the solidified dendrite is splitted in the center by a {111} plane, the parts on both sides of which follow a particular orientation relationship that resembles twinned dendrites in Al alloys. In other words, the dendrite splitting yields twinned dendrite. The formation mechanism of twinned dendrites is related to the atom attachment behaviors at the liquid-solid interface during solidification. For the TiAl alloy with the primary phase being γ, straight twinned {111} plane layers with the thickness of a few to dozens of microns form longitudinally in the center of the columnar dendrites. The same orientation relationships appear in both β- and γ-solidified TiAl alloys, although their primary phases are different. This paper provides new details involved in the solidification of TiAl alloys.

Reinforcement of TiAl alloy by evenly distributed micron-/nano-Y2O3 particles

TiAl alloy has attracted widespread attention as an alternative material to aero-engine turbine blades. Because the obvious brittleness of TiAl alloy, TiAl alloy reinforced with nano-sized reinforcement is a feasible pathway to improve its mechanical properties due to the nano-sized reinforcement does not easily cause stress concentration and crack initiation while strengthening the alloy matrix. However, since the high specific surface energy and van der waals attraction of nanoparticles, they tend to agglomerate to form unevenly distributed micron-sized particles in alloy melt, which makes it difficult to fabricate TiAl alloys reinforced with nano-sized reinforcements by low-cost casting method. In this work, a TiAl alloy reinforced with evenly distributed micron−/nano-Y2O3 was successfully fabricated by casting method and subsequent heat treatment based on an innovative strategy of stepwise dispersion combined with solid solution-precipitation. Micron-Y2O3 in as-cast TiAl alloy “inherits” the uniform distribution state of Y-rich particles in Al-based master composite; the subsequent heat treatment and furnace cooling process make the nano-Y2O3 uniformly precipitated in the TiAl matrix. Nano-Y2O3 particles (∼ 50 nm) were uniformly distributed in both α2/γ lamellae and lamellar colony boundaries. The ultimate compressive strength and fracture strain of TiAl alloy reinforced with micron−/nano-Y2O3 particles are 1970 MPa and 29.5%, respectively.

Correlation between the strain rate sensitivity exponent m and microstructure evolution during hot deformation of an as-cast TiAl alloy

The hot-deformation behavior of an as-cast TiAl alloy with a near-lamellar microstructure was investigated at a deformation temperature within the range of 1020–1200 °C and a strain rate of 0.001–1.0 s−1. A visual three-dimensional diagram of the strain rate sensitivity exponent m was developed based on hot compression data to reflect the correlation between the process parameters and microstructural evolution characteristics. The results showed that the m-value increased with an increase in temperature and a decrease in the strain rate. As the temperature increased, the growth of the m-value at a higher strain rate was faster than the growth of the m-value at a lower strain rate. Moreover, microstructure analysis at different m-values indicated a correlation between the m-value and microstructural evolution characteristics during hot deformation. The mechanism of microstructure evolution during hot deformation can be divided into three evolution modes according to different strain rate sensitivity exponent m-values. At a lower m-value, numerous γ/α2 lamellar colonies appeared similar characteristics to the initial microstructure. At a moderate m-value, broken lamellae in the microstructure formed several chain-like fragments with similar orientations. At a higher m-value, several formed equiaxed α2 and γ phases satisfied the Blackburn orientation relationship in the microstructure.

Microstructure refinement and improvement of mechanical properties of Ti46Al8Nb0.9B alloys by electromagnetic cold crucible continuous solidification

Microalloying with boron is effective and economical for refining microstructures of as-cast TiAl alloy. However, microstructural refinement is easily affected by the alloy composition and cooling rate, leading to inhomogeneous grain size in different areas of the ingot. In this study, the Ti46Al8Nb0.9B alloys are continuously solidified by the electromagnetic cold crucible, aiming to obtain homogeneous and fine microstructure. After electromagnetic cold crucible continuous solidification (EMCS), the obtained ingot shows finer and more homogeneous microstructure with lamellar colony size of about 50 μm. The results of tensile test show that the EMCS ingot exhibited improved properties with yield strength (σ0.2) of 548 MPa and plastic elongation of 0.6%.

The effect of nano-Y2O3 addition on tensile properties and creep behavior of as-cast TiAl alloy

The effect of nano-Y2O3 addition on tensile properties and creep behavior of Ti–45Al–6Nb-2.5V alloy prepared by induction skull melting (ISM) has been studied in this paper. The results reveal that 0.15 at% Y2O3 increases both ultimate tensile strength and elongation of the alloy, as 666 MPa and 0.56% at room temperature, 747 MPa and 14.1% at 800 °C, 597 MPa and 22.5% at 900 °C. And the alloy with nano-Y2O3 addition displays a better creep resistance and longer creep life at 800 °C. Additionally, the fracture feature shows that the main failure mechanism of TiAl alloy at 800 °C during creep is initiation, growth and coalescence of voids and cracks in grain boundary. Particularly, Y2O3 could impede dislocation movement and defer initiation and propagation of cracks to improve creep resistance.

A novel continuous and controllable method for fabrication of as-cast TiAl alloy

A novel continuous and controllable combustion synthesis process for fabrication of TiAl alloy from TiO2, Al, KClO3 and CaO was developed in this paper. The effects of different material ratios on the composition of alloy and the separation effect of alloy and slag were studied. Based on the mass action concentration model, the phase equilibrium composition of Ti-O-Al system at 1800 K was also investigated. The as-cast TiAl alloys were analyzed by X-ray diffractometer, scanning electron microscope, inductively coupled plasma emission spectrometer and Oxygen/Nitrogen/Hydrogen analyzer. According to the comparison between theoretical calculation and experimental results, the variation trends for the alloy composition from experiment are well consistent with calculation results along the increase of Al addition amounts in raw materials. The analyzed composition of alloy for Ti, Al and O are 56.53, 40.99 and 1.09 (wt. %), respectively. This present method hopefully supplied an efficient way for preparation of Ti- or Al-based alloys.

A Two-Stage Physical-Based Model for Predicting Flow Stress of As-cast TiAl Alloy Under Hot Deformation Conditions

The hot deformation behavior of Ti-30Al-4.2Mn-4.5Nb-0.2B alloy was investigated using the isothermal compression experiment at temperatures of 1020-1200 °C and strain rates of 0.001-1 s−1. The flow stress was sensitive to the deformation parameters like temperature and strain rate, which decreases with the increase in temperature and decrease in strain rates. Based on the true stress-true strain data, a two-stage physical-based model was proposed to describe the flow stress curve of as-cast TiAl alloy during hot deformation process. For establishing the model, at first, the flow curves of dynamic recovery (DRV) were modeled by employing stress-dislocation relation and adjusting dislocation annihilation coefficient Ω. Then, the flow curves of dynamic recrystallization (DRX) were modeled by considering the dynamic softening behavior into Avrami equation. Finally, the flow curves in the entire deformation stages could be described by embedding the predicted data of DRV model (i.e., flow stress before the critical strain) into the predicted data by DRX model (i.e., flow stress after the critical strain). The critical strain for initiation of DRX was determined by the double-differentiation method. To evaluate the applicability and effectiveness of DRX kinetics equation, the DRX curves were calculated and were consistent with the microstructure observation. Comparison between the experimental and predicted data shows that the proposed physical-based model can well forecast the flow stress under a wide working domain.

Improvement of microstructure, mechanical properties and hot workability of a TiAl-Nb-Mo alloy through hot extrusion

γ-TiAl alloys are excellent candidate materials for high-temperature applications, although poor hot workability limits their widespread application. In the present study, the microstructure, mechanical properties and hot workability of TiAl-Nb-Mo alloy in the as-cast and as-extruded conditions were investigated. The as-cast TiAl alloy consisted of γ/α2 lamellae, γ and β phase distributed at colony boundaries and on the lamellar interfaces. The tiny particles of ω0 phase were formed, transformed from β phase during HIP treatment, which dissolved after hot extrusion. The TiAl alloy exhibited improved mechanical properties and excellent hot workability after hot extrusion because of grain refinement, the introduction of β phase and the disappearance of ω0 phase. The anomalous yield stress phenomenon occurring in as-cast material disappeared in the as-extruded condition, which was ascribed to grain refinement that reduced the resistance of grain boundaries to intragranular slip at elevated temperature. The findings demonstrate the benefits of introducing small amounts of β phase to improve hot workability of TiAl-Nb-Mo alloys, particularly when followed by heat treatment to eliminate the β phase and increase the mechanical properties.

Hot Deformation Behavior and Microstructural Evolution Characteristics of Ti-44Al-5V-1Cr Alloy Containing (γ + α2 + B2) Phases

The hot deformation behavior and microstructural evolution of Ti-44Al-5V-1Cr alloy were investigated by hot compression tests at temperatures of 1000–1250 °C and strain rates of 0.001–1 s−1. It was indicated that the dependence of peak stress on deformation temperature and strain rate could be accurately described by a hyperbolic sine type equation. The activation energy, Q, was estimated to be 632 kJ/mol. The hot processing map was developed at different strains on the basis of dynamic materials modeling and the Murty criteria. As a result, the instability zones occurred in the regions of low temperature (<1050 °C) and a high strain rate (>0.1 s−1). The flow soft mechanism of the instability regions is stress relaxation caused by localization deformation at lamellar boundaries. Dynamic recrystallization is the mainly refining and spheroidizing mechanism of lamellar microstructures. The optimum hot working condition of as-cast TiAl alloy occurs in the temperature range of 1175–1225 °C and the strain rate range 0.05–0.1 s−1. The large-size TiAl alloy rectangular bars with crack-free appearance were successfully prepared by hot extrusion. After annealing, the fine and uniform microstructure with excellent deformation ability was obtained.

Dynamic recrystallization behavior of an as-cast TiAl alloy during hot compression

High temperature compressive deformation behaviors of as-cast Ti–43Al–4Nb–1.4W–0.6B alloy were investigated at temperatures ranging from 1050°C to 1200°C, and strain rates from 0.001s−1 to 1s−1. Electron back scattered diffraction technique, scanning electron microscopy and transmission electron microscopy were employed to investigate the microstructural evolutions and nucleation mechanisms of the dynamic recrystallization. The results indicated that the true stress–true strain curves show a dynamic flow softening behavior. The dependence of the peak stress on the deformation temperature and the strain rate can well be expressed by a hyperbolic-sine type equation. The activation energy decreases with increasing the strain. The size of the dynamically recrystallized β grains decreases with increasing the value of the Zener–Hollomon parameter (Z). When the flow stress reaches a steady state, the size of β grains almost remains constant with increasing the deformation strain. The continuous dynamic recrystallization plays a dominant role in the deformation. In order to characterize the evolution of dynamic recrystallization volume fraction, the dynamic recrystallization kinetics was studied by Avrami-type equation. Besides, the role of β phase and the softening mechanism during the hot deformation was also discussed in details.

Microstructural evolution of Ti–47Al–2Cr–2Nb–0.8B alloy prepared by semi-solid process

The refinement of large boride ribbons in the as-cast TiAl alloy is the essential issue for aerospace industry application, which is difficult to avoid by classical casting techniques. The present paper seeks to explore the possibility of the semi-solid process in manufacturing Ti–47Al–2Cr–2Nb–0.8B (at%) alloy. An important result is that, except forming a nondendritic globular structure, the semi-solid process also plays a crucial role in refining large borides for the TiAl alloys with boron. In the current alloy, the long ribbon borides can be successfully converted to fine, equiaxed particles with an average size of 2–5 μm. Furthermore, subsequent proper heat treatment is necessary to control and achieve a fine, nearly fully lamellar microstructure. The key microstructure-refinement mechanism is due to the fine borides obtained during semi-solid process, which act as the effective nucleation centers for the formation of interdendritic fine γ grains, providing the pinning effect on prior α-grain boundaries. These results prove that semi-solid process can be an ideal candidate for the microstructure refinement in TiAl alloys.

The Effects of Cr and Mo on the Microstructure and Mechanical Properties of As-Cast TiAl Alloys

The effect of the alloying elements of Cr and Mo on the microstructure and mechanical properties of as-cast TiAl alloys produced by a locally made arc-melting furnace was studied. X-ray diffraction (XRD) was used to identify the phases present in the samples. The microstructure of the TiAl samples was characterized using scanning electron microscopy combined with energy dispersive spectroscopy (EDS). Compression tests were carried out at room temperature using an Instron servohydraulic testing machine. The results show that the Ti-48Al alloy exhibited a duplex microstructure, whereas with the addition of Cr a nearly lamellar microstructure was observed in Ti-48Al-2Cr and with the addition of both Cr and Mo also in Ti-48Al-2Cr-2Mo. The hardness values increased slightly as compared to the Ti-48Al alloy with the addition of the alloying elements. The presence of Cr in Ti-48Al-2Cr resulted in a slight increase in compressive fracture strain. The as-cast Ti-48Al-2Cr-2Mo alloy produced a higher yield strength and fracture strain in compression as compared to the other as-cast TiAl alloys. On the fracture surfaces of the as-cast TiAl alloys, mixed brittle transgranular and interlamellar fracture modes were predominantly observed.

Grain refinement by means of phase transformation and recrystallization induced by electropulsing

The specimens cut from as-cast TiAl alloy and cold-rolled TA15 sheet were treated by high density electropulsing. The optical microscope (OM) and transmission electron microscope (TEM) were used to examine the changes of the microstructure before and after the electropulsing. It is found that fine and homogeneous microstructures with the grain size of 30–50 μm can be obtained from the as-cast TiAl based alloy with the initial lamellae of about 1 000μm. The electropulsing treatment can induce the completed recrystallization in cold-rolled TA15 sheet. After electropulsing treatment, the microstructure of TA15 sheet transformed from primary α laths grain to smaller α equiaxed grain. The research results show that the electropulsing treatment is an effective method to refine grain of as-cast TiAl alloy and cold-rolled titanium alloys sheet. Because the conventional thermo-mechanical processing such as forging or extrusion or rolling as well as heating are not required, the refining process by electropulsing is simpler than conventional ones.

Properties and CA4GE-Engine Test of TiAl-Based Alloy Valves by Permanent Mold Casting

TiAl-based alloy valves were manufactured by combining charges compressed /vacuum arc melting (VA)/ induction skull melting (ISM) procedure with permanent mold centrifugal casting method. Microstructures, compositions and mechanical properties of as-cast and hot isostatical pressed (HIPed) valves are detected. Results show that the permanent mold centrifugal casting process obviously refines the size of grain in TiAl alloy and the tensile strength of as-cast and HIPed valves are 550MPa and 580MPa at 20°C, 370MPa and 470MPa at 815°C, respectively. As-cast specimens show ~0% elongation at 20°C and 1~2% at 815°C, while HIPed ones show an elongation of 1~2% at room temperature and about 10% at 815°C. Furthermore, a 200-hour test was carried out with CA4GE-engine, which demonstrated the possibility of as-cast TiAl alloy valves for the substitution of present steel ones.

Effects of Preparation Processing on Mechanical Properties of TiAl Intermetallic

Mechanical properties of the TiAl alloy produced by centrifugal spray deposition (CSD), compared to that produced by ingot metallurgy (IM), were investigated at different temperatures from 293 to 973K. The result shows that the ultimate strength, yield strength and plasticity of the CSD TiAl alloys, with excellent compression properties and plasticity, are higher than those of as-cast TiAl alloys at room temperature as well as at high temperature. There exists a critical temperature of 873K in the relationship between strength and temperature, in which strength increases with increasing temperature above 873K. The effects of CSD on mechanical properties of the TiAl alloy are discussed, and the higher strength with moderate ductility achieved is because of the finer lamellar structure got in the CSD processing, and this structure is also believed to be beneficial to ductility.

Effect of trace tungsten on the microstructure of TiAl alloys containing Nb and B

New TiAl alloys, containing 45 at.% Al, 7 at.% Nb, x at.% W, and 0.15 at.% B ( x = 0, 0.2, 0.4, and 0.7) were prepared by arc melting and drop casting consequently. Using optical microscopy, scanning electron microscopy (SEM), and electron superprobe technologies, the effects of tungsten on the microstructural evolution of the TiAl alloys, including the colony size and lamellar spacing, were analyzed. It was found that cellular structures and dendrites were formed in the as-cast TiAl alloys, and heavy metals, such as niobium and tungsten, tend to segregate strongly at the interface of the cellular structures and dendrites. Trace tungsten can effectively impede the grain growth and narrow the interlamellar spacing. 0.4 at.% tungsten is more effective in refining the microstructure of the TiAl alloys.

Effects of Forming Processing on Mechanical Properties of Ti-48Al-2Mn-2Nb Intermetallics

Ti-48Al-2Mn-2Nb alloy was produced by “centrifugal spray deposition” (CSD), and then hot isostatic pressing (HIP) was employed to remove the porosity formed by CSD. The effects of CSD and HIP processing on the mechanical properties and microstructure of the TiAl alloy were investigated. The results show that the CSD and HIP processing can both improve the strength, plasticity of the TiAl alloy, and the tensile elongation values of the CSD or HIP samples are around 3%, which are better than those of as-cast TiAl alloys in room temperature. Especially, they show more excellent compressive properties at ambient temperature with a compressive ratio of 33.8% and compressive strength of 2210MPa for the CSD samples, and a compressive ratio of 37.8% and compressive strength of 2348MPa for the HIP samples. The CSD processing also improves the fracture toughness of TiAl alloy, which is much higher than that of the HIP processing, while the HIP processing seems to be beneficial the ductility and plasticity as having a duplex structure. The effects of CSD and HIP processing on microstructure and properties of TiAl alloys are discussed to understand the deformation and fracture process of the alloy.

Microstructures and mechanical properties of as-cast TiAl alloys with higher C additions

TiAl alloys with higher C additions were fabricated using a XDTM method. The results investigated by XRD, OM and SEM indicated that in Ti-34Al-0.5C alloy, single phase TiC particles and particles having a “core” structure in which the TiC phase was coated by Ti3AlC phase were formed and they uniformly distributed at the grain boundaries. However, when C content is more than 0.5%, the particles with a plate-shape were single phase Ti2AlC and TiC phase coated by Ti2AlC phase. These results suggested that in TiAl alloys with higher C additions, the primary is TiC, and the Ti3AlC and Ti2AlC result from peritectic reaction, L + TiC. The results measured by MPM show that with the increasing of C content, the microhardness both TiC, Ti3AlC and Ti2AlC is higher than that of the matrix Ti3Al and TiAl. However, the elastic modulus of particle phases and matrix has little variation and no change tendency can be found with increasing C content.

Grain refining of TiAl alloy by rapid deformation

Grain refining of TiAl intermetallics by rapid deformation was investigated. The as-cast TiAl alloy has very large grains and serious composition segregation. Most of the large grains of TiAl alloy can be refined after the first rapid deformation and heat treatment. However, a few of the coarse grains of TiAl still remained. When the second rapid deformation and heat treatment was conducted, all of the remaining coarse TiAl grains can be refined. As the grains were refined, the properties of TiAl alloy were significantly improved.