Intermetallic TiAl Research Articles

Role of Ti3AlC2 MAX phase on characteristics of in-situ synthesized TiAl intermetallics. Part IV: mechanical properties

In this study, the 4th part of a series of publications on the sintering and characterization of TiAl-Ti3AlC2 composite materials, the mechanical properties were measured and discussed. For this purpose, different contents of synthesized Ti3AlC2 reinforcement (10, 15, 20, 25, and 30 wt%) were added to metallic Ti and Al powders, then ball-milled and manufactured by spark plasma sintering (SPS) for 420 s at 900 ºC under 40 MPa. Flexural strength, fracture toughness and Vickers hardness were measured by 3-point technique, SENB method, and indentation technique, respectively. Increasing the Ti3AlC2 content resulted in improvement of the mechanical properties, so that TiAl-25 wt% Ti3AlC2 composite showed the best flexural strength and Vickers hardness (270 MPa and 4.11 GPa, respectively). Increasing amount of Ti3AlC additive had no significant effect on fracture toughness. Densification improvement, in-situ formation of Ti2AlC, and limitation of grain growth were recognized as the reasons of mechanical properties enhancement. In contrast, further addition of Ti3AlC2 (30 wt%) decreased the mechanical properties due to the reduction of density and formation of more Ti2AlC agglomerates in grain boundaries.

Fabrication and joining of NiAl and TiAl intermetallics by additive sintering

To manufacture hard-to-deform intermetallics and their components at high temperatures and for a short-time, a novel additive sintering method based on solid-state powder densification is proposed, which aims to achieve the fabrication and joining of intermetallics. In this study, the sintering and joining of NiAl/NiAl and NiAl/TiAl alloys were conducted using this method. The results show that good bonding was achieved between similar intermetallics of NiAl/NiAl. Nevertheless, due to the formation of brittle phases, defects were observed in the bonding interface of the NiAl/TiAl alloy. An interlayer of V was employed to optimize the microstructure of the interface, which can prevent the formation of the brittle phase and improve the property of the interface.

Crystallographic Origin of Phase Transformation and Lamellar Orientation Control for TiAl-Based Alloys

TiAl intermetallics are typical metallic materials involving complex solid-state phase transformations, with crystal orientations that are difficult to control due to multi-transformation variants. However, lamellar orientation control is crucial to the development of a polysynthetic twinned single crystal structure in TiAl-based alloys for jet engines or other high-temperature systems. In this study, β-solidifying TiAl alloys were used to study the relationships between the lamellar structure and the phase transformation process under directional solidification (referred to as the directional phase transformation, DPT). It was found that the β → α phase transition affects the lamellar orientations and that the subsequent process of α → α2 + γ leads to the final formation of the polysynthetic lamellar structure. Detailed analyses based on crystallography show that the β/α phase interface is responsible for the different oriented lamellar structures with the 0° or 45° orientation. With a lower interfacial energy, the 0° oriented α phase nucleates more easily but grows much more slowly than the 45° oriented α phases during DPT, which makes it feasible to control the lamellar orientations for TiAl-based alloys. The crystallographic origin for the control of lamellar orientations was then studied and confirmed by using EBSD in a β-solidifying Ti–Al–Nb alloy.

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.

Effect of magnesium in breaking the correlation between reaction speed and intensity in the fabrication of TiAl3 intermetallics

When TiAl3 intermetallics were fabricated by the reactive synthesis of titanium and aluminum powder, the correlation between the speed and intensity of the reaction was stable. The magnesium-addition method with the addition of trace magnesium broke this correlation for the first time. Magnesium controlled the reaction in three stages in different ways: the initial melting and reaction area, the dynamic equilibrium formed by the diffusion rate consistent with the liquidus, and the high flowability eutectic liquid phase. The diffusion state of magnesium in each stage and the eutectic liquid phase formed with aluminum were the critical factors.

One-step sintering of porous TiAl3 intermetallics using TiH2 as raw material in thermal explosion reaction

Porous TiAl3 intermetallics with high porosity of 56.0% were synthesized by thermal explosion (TE) reaction using TiH2 as raw material, from TiH2-75 at.%Al elemental powders. Results shown that an obvious thermal explosion reaction occurs (1225 ℃) when one-step sintering is carried out without the pre-heat treatment. After TE, TiAl3 was the dominate phase in the sintered products. Porous TiAl3 intermetallics exhibited an open pore structures and uniform pore size distribution, which ensures the application of porous TiAl3 materials in the fields of solid–liquid filtration and separation.

The effect of B and Ti–Al intermetallics additions on the microstructure and mechanical properties of hot-pressed B4C

In this work, we studied the microstructure and mechanical properties of hot-pressed B4C, B4C + 5 wt% TiAl, and B4C + B + 5 wt% TiAl. The X-ray diffraction and Raman spectroscopy results showed that the addition of Ti–Al alone did not led to Al doping in B4C, and it did not reduce the degree of amorphization of B4C generated under high shear stresses. In contrast, the co-addition of Ti–Al and B resulted in both B and Al solid solutions in the B4C and effectively mitigated the amorphization of B4C. Electron probe microanalysis (EPMA) and energy dispersive X-ray spectroscopy (EDS) analyses suggested that the co-addition of Ti–Al and B promoted the interdiffusion of lightweight elements such as B, C, Al, Mg, and Si at high temperatures, resulting in a dense body with uniformly distributed second phase. The transmission electron microscopy (TEM) observations of the B4C + B + 5 wt% TiAl sample showed that a nano–core–shell structure with aggregated dislocations in the shells which was generated at TiB2 grains due to the segregation and dissolution-precipitation between TiB2 and B-rich boron carbide. The short-range ordered substitution of B and the disordered doping of Al in B4C led to lattice distortion and expansion, resulting in the formation of high-density defects, including stacking faults and nano-twins, improving the mechanical properties for B4C. The B4C + B + 5 wt% TiAl sample exhibits a combination of fascinating properties, including low density (2.52 g/cm3), high Vickers hardness (35.53 ± 0.83 GPa) and moderate fracture toughness (4.37 ± 0.35 MPa m1/2).

Achieving Superior High-Temperature Strength and Oxidation Resistance of TiAl Nanocomposite through In Situ Semicoherent MAX Phase Precipitation.

Increasing the service temperature of TiAl intermetallics is the main challenge for the development of next-generation aircraft. Dispersion-strengthening, an effective means to further improve the high-temperature performance of metals, fails to implement in TiAl intermetallics due to difficulties in interface optimization. Here, we successively fabricate a TiAl naocomposite with fully lamellar microstructures and homogeneously dispersed Ti2AlC nanoprecipitates via spark plasma sintering. The composite consisted of semicoherent interfaces among γ-TiAl/Ti2AlC precipitates/α2-Ti3Al, in addition to continuous polysynthetic nanotwins. Strong pinning effects as well as strain-induced nanoscale TiCr2 precipitation uplift the operation temperature of TiAl nanocomposites by more than 50 °C. Furthermore, we experimentally proved that semicoherent interfaces among in situ Ti2AlC precipitates and its surrounding matrix serve as oxygen diffusion barrier during isothermal oxidization and significantly drop down the mass gain of TiAl nanocomposites during operation, making the present nanocomposite a highly potential candidate for use as light-weight structural materials in automotive and aerospace industries.

Structure and mechanical characteristics of a layered composite material based on TiB/TiAl/Ti

In this work, a TiB/TiAl/α-Ti layered composite material has been obtained by unrestricted SHS compression using titanium, boron, and aluminum as the initial components. It has been found that combustion and subsequent high-temperature shear deformation (thermal and mechanical conditions of unrestricted SHS compression) results in composite ceramic (TiB/Ti) and intermetallic (TiAl, Ti3Al) macrolayers to be oriented along the direction of material flow and perpendicularly to the applied load. It is shown that the synthesis of the material proceeds according to the solid/liquid-phase mechanism. The mechanical characteristics of the obtained layered composite material have been studied, namely hardness, microhardness, and fracture toughness. The mechanism of crack propagation has been established and the dependence of the values of the stress intensity factor on the orientation of microvolumes in the material has been determined.

In-situ Al3Ti particle reinforcement for stiff aluminum die castings

The intermetallic phase Al3Ti is a promising candidate to increase the stiffness of aluminum casting alloys due to its high Young’s modulus. Reinforcement particles are formed by the in-situ reaction between solid titanium particles and the liquid aluminum melt. A high-shear mixer is used to peel off the reaction layer on the surface of the Ti particles and distribute the Al3Ti particles homogeneously in the melt. This work investigates the dissolution velocity of different size fractions of Ti powder in three different aluminum casting alloys. The results were used to fabricate specimens with different volume fractions of Al3Ti particles by high pressure die casting. Microstructure characterization was performed to investigate the distribution of the Al3Ti particles within the castings. The mechanical properties were determined by tensile tests and resonant frequency analysis.

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

Role of Ti3AlC2 MAX phase on characteristics of in-situ synthesized TiAl intermetallics. Part II: Phase evolution

In this research, the 2nd part of a series of papers on the processing and characterization of TiAl–Ti3AlC2 composites, the phase evolution during the manufacturing process was investigated by X-ray diffraction (XRD) analysis and Rietveld refinement method. Metallic Ti and Al powders with different amounts of previously-synthesized Ti3AlC2 additives (10, 15, 20, 25 and 30 wt%) were ball-milled and densified by spark plasma sintering (SPS) under 40 MPa for 7 min at 900 °C. Before the sintering process, XRD test verified that the powder mixtures contained metallic Ti and Al as well as Ti3AlC2 and TiC (lateral phase synthesized with Ti3AlC2) phases. In the sintered composites, the in-situ synthesis of TiAl and Ti3Al intermetallics as well as the presence of Ti3AlC2 and the formation and Ti2AlC MAX phases were disclosed. The weight percentage of each phase in the final composition of the samples and the crystallite size of different phases were calculated by the Rietveld refinement method based on the XRD patterns. The size of Ti3AlC2 crystallites in sintered samples was compared with the crystallite size of synthesized Ti3AlC2 powder.

Fabrication of hybrid nanocrystalline Al–Ti alloys by mechanical bonding through high-pressure torsion

This study demonstrates an approach of utilizing high-pressure torsion (HPT) to fabricate a novel hybrid material by the direct bonding of Al and Ti disks at room temperature under a compressive pressure of 6.0 GPa and with increasing numbers of HPT turns up to 50. Detailed structural observations revealed the formation of a multi-layered nanostructure in the edge regions of the disks with a grain size of ∼30 nm. X-ray diffraction (XRD) and selected area electron diffraction (SAED) confirmed the presence of three intermetallic compounds, AlTi, Al3Ti and Ti3Al, in the layered structures. Processing by HPT led to the formation of a hybrid nanocomposite with exceptional hardness (over 300 Hv) in the edge regions of the disks. Special emphasis was placed on understanding the evolution of hardness in the hybrid material. The investigation demonstrates a significant opportunity for using HPT processing to deepen the knowledge on diffusion bonding and mechanical joining technologies as well as for fabricating new and valuable hybrid nanomaterials.

Laser lap welding of TC4 titanium alloy to 6082 aluminum alloy using a CoNiCuNb0.5 V1.5 high entropy alloy filler

To avoid the formation of brittle Ti-Al intermetallics, a new welding procedure for Ti-Al alloy joints was proposed. The feasibility of CoNiCuNb0.5V1.5 high-entropy alloy filler metal for laser lap welding of TC4 titanium alloy to 6082 aluminum alloy is investigated in this paper. By adjusting the welding parameters, a small amount of base material on the titanium side was melted. The high entropy of mixing and kinetic sluggish diffusion effects of high-entropy alloy (HEA) are used to facilitate the formation of solid solution structure, obstruct the mixing of titanium atoms and aluminum atoms and thus inhibit the formation of Ti-Al IMCs. The weld seam without Ti-Al IMCs was obtained. At the high-entropy alloy-Al alloy interface, the joint fractured with a maximum tensile strength of 190 MPa.

Protective properties of diffused chrome-calorizing coatings with TiN and Ti2AlN barrier layers on VT6 alloy

The results of the research of phase and chemical composition, structure and micro hardness of diffusion coatings, obtained by chrome-calorizing of VT6 structural titanium alloy, are presented. Chrome-calorizing was carried out at a temperature of 1050 °C for 4 h in powder mixtures of chromium, aluminum, inert additive and activator. As a result of saturation, multilayered coatings with the participation of nitrides TiN, Ti2AlN and intermetallic Al3 (Ti, Cr), Al2Ti, AlTi3 are formed. The micro hardness of TiN, Ti2AlN surface layers is 16.0–14.0 GPa, which increases the wear resistance of the VT6 alloy by 3.7 times. Moreover, it has been established that the presence of Ti2AlN and Al3 (Ti, Cr) phases increase the heat resistance of the VT6 alloy in the air atmosphere to a temperature of 700 °C.

Исследование режимов изнашивания композиционного материала на основе антифрикционного сплава АОМ20-1 упрочненного частицами Ti

Сomposite material samples were obtained by the method of reaction casting by mixing titanium particles to obtain intermetallic phases Al3Ti. Dry sliding wear tests were carried out using a fixed sleeve (steel 45) against a rotating disk (sample) at sliding speeds from 0.25 to 0.75 m/s and loads from 0.5 to 3.5 MPa.There were constructed maps of wear rate, which determine the friction modes during testing. There were shown boundaries and conditions of changing wear modes.

Reaction Behavior of Porous TiAl3 Intermetallics Fabricated by Thermal Explosion with Different Particle Sizes.

Porous TiAl3 intermetallics were prepared by the thermal explosion (TE) and space holder method with different particle sizes of Ti and Al powders, and their reaction behaviors were investigated. The results showed that with the increase in the particle size of the Ti and Al powders, the interfacial contact between the particles decreased, resulting in low interfacial energy and reaction activity, making the process difficult to initiate. Meanwhile, the heat flow rose from 358.37 J/g to 730.17 J/g and 566.74 J/g due to the extension of the solid–liquid diffusion time. The TiAl3 structures obviously expanded, and the formation of connected pore channels promoted the porosity. Only when the Ti and Al particle sizes were both small did the solid–solid diffusion significantly appear. At the same time, the TE reaction weakened, so the product particles had no time to fully grow. This indicates that the particle size of the raw materials controlled the TE reaction process by changing the solid–liquid diffusion reaction time and the degree of solid-phase diffusion.

Low-temperature superplasticity of β-stabilized Ti-43Al-9V-Y alloy sheet with bimodal γ-grain-size distribution

The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100 °C was systematically investigated in the temperature range of 750−900 °C under an initial strain rate of 10−4 s−1. A bimodal γ grain-distribution microstructure of TiAl alloy sheet, with abundant nano-scale or sub-micron γ laths embedded inside β matrix, exhibits an impressive superplastic behaviour. This inhomogeneous microstructure shows low-temperature superplasticity with a strain-rate sensitivity exponent of m = 0.27 at 800 °C, which is the lowest temperature of superplastic deformation for TiAl alloys attained so far. The maximum elongation reaches ~360% at 900 °C with an initial strain rate of 2.0 × 10−4 s−1. To elucidate the softening mechanism of the disordered β phase during superplastic deformation, the changes of phase composition were investigated up to 1000 °C using in situ high-temperature X-ray diffraction (XRD) in this study. The results indicate that β phase does not undergo the transformation from an ordered L20 structure to a disordered A2 structure and cannot coordinate superplastic deformation as a lubricant. Based on the microstructural evolution and occurrence of both γ and β dynamic recrystallization (DR) after tensile tests as characterized with electron backscatter diffraction (EBSD), the superplastic deformation mechanism can be explained by the combination of DR and grain boundary slipping (GBS). In the early stage of superplastic deformation, DR is an important coordination mechanism as associated with the reduced cavitation and dislocation density with increasing tensile temperature. Sufficient DR can relieve stress concentration arising from dislocation piling-up at grain boundaries through the fragmentation from the original coarse structures into the fine equiaxed ones due to recrystallization, which further effectively suppresses apparent grain growth during superplastic deformation. At the late stage of superplastic deformation, these equiaxed grains make GBS prevalent, which can effectively avoid intergranular cracking and is conducive to the further improvement in elongation. This study advances the understanding of the superplastic deformation mechanism of intermetallic TiAl alloy.

Effects of TiAl Alloy as a Binder on Cubic Boron Nitride Composites.

Owing to their extreme hardness, cubic boron nitride (cBN) composites are widely used in cutting applications. The performance of cBN composites is closely related to the characteristics of the binder. Therefore, novel binders must be developed to improve the performance of cBN composites. In the present work, TiAl intermetallics were used as binders to fabricate cBN composites by employing a high-temperature and high-pressure sintering method. The phase transformation, sintering reaction mechanism, thermal stability, and mechanical properties of the resultant cBN composites were investigated. It was found that during the sintering process, Ti atoms preferentially reacted with boron nitride particles, whereas Al atoms enriched and transformed into TiAl3 phases and formed cBN/AlN, AlB2/TiN, and TiB2/TiAl3-layered structures eventually. The composites maintained good oxidation resistance at 1200 °C. A decrease in the particle size of the TiAl binder improved the uniformity of particle size distribution and increased the flexural strength of the composites.

Phase transformation and deformation behavior of a TiAl–Nb composite under quasi-static and dynamic loadings

To improve the high-temperature deformability of TiAl intermetallics, a TiAl–Nb intermetallic matrix composite was prepared by powder metallurgy to try to introduce the deformable β phase. The microstructure and deformation behavior of the TiAl–Nb intermetallic matrix composite under quasi-static and dynamic conditions were systematically investigated. It is found that the microstructure of the composite is composed of a near-gamma TiAl matrix and Nb-rich regions. In the Nb-rich regions a coexistence of βo, ωo and γ phases was identified where ωo and γ phases were supposed to directly precipitate in the βo matrix. The ultimate tensile strength of the composite exhibits different temperature-dependent behaviors under quasi-static and dynamic loading conditions. It increases with temperatures up to 650 °C and then declines with temperature under quasi-static loading, while it does not decrease from 650 to 850 °C under the dynamic loading. In the deformed microstructures, the Nb-rich regions at all conditions are still composed of βo, ωo and γ phases, indicating that the solvus temperature of the ωo phase could be above 850 °C. Additionally, the TiAl matrix appears to not significantly contribute to the plastic deformation of the composite, where the propensity of dislocations and mechanical twins do not significantly change when varying loading conditions. However, the γ particles formed in Nb-rich regions contain substantial mechanical twins and dislocations under both quasi-static and dynamic loading conditions. Thus, the γ particles directly transformed from the βo phase exhibit better deformability than the γ phase in the matrix. This could be probably explained by a decreased stacking fault energy for the newly formed γ phase due to a decrease in Al fraction and a significant increase in Nb fraction. Meanwhile, twin intersections occurred in the γ particles, and the induced stress concentration in the intersection regions was relieved by lattice distortion and detwinning processes. This twin intersection behavior could be beneficial for improving the composite strength without sacrificing much ductility.