Α-titanium Alloys Research Articles

Influence of heat treatment on the structure and mechanical properties of pseudo α-titanium alloy in a welded joint

The object of this study was the weld material of a new Ti-6.1Al-6.0Zr-6.3Sn-3.2Mo-1.1Nb-1.2Si alloy, which was obtained by electron beam welding. Electron beam welding (EBW) provides rapid melting and cooling with crystallization under significant temperature gradients. For the pseudo α-titanium alloy, this leads to the appearance of significant residual stresses in the joint and determines the specificity of the dispersion decay of the β-phase. Residual stresses are reduced by preliminary heating, removed by heat treatment. Such processing exerts a critical influence on the formation of the structure and phase composition of the weld and the zone of thermal influence of the electron beam; it can form macro defects, undesirable phases, and structural states. The conditions of heat treatment have been determined to bring the welded joint from complex alloyed pseudo α-titanium alloy to the required level of mechanical characteristics. The structure, phase morphology, elemental composition, and mechanical characteristics of welded joints without additional heat treatment have been compared, with preliminary local heating of 400 °C, with additional post-weld local annealing at ТТ=750 °С, tT=4 minutes and sequential annealing in the furnace at ТТ=850 °C, tT=60 minutes. It has been established that a full cycle of heat treatment of a welded joint provides the highest characteristics of strength and plasticity, but local heat treatment also makes it possible to obtain a defect-free joint with satisfactory characteristics for less responsible products. It is shown how heat treatment of pseudo α-titanium alloy makes it possible to get rid of unwanted phase formations (Zr,Ti)5Si3Al and transform them into (Zr,Ti,Nb,Mo)3SiAl. The results are promising for use in the production of aircraft engine parts

Overview of Surface Modification Techniques for Titanium Alloys in Modern Material Science: A Comprehensive Analysis

Titanium alloys are acclaimed for their remarkable biocompatibility, high specific strength, excellent corrosion resistance, and stable performance in high and low temperatures. These characteristics render them invaluable in a multitude of sectors, including biomedicine, shipbuilding, aerospace, and daily life. According to the different phases, the alloys can be broadly categorized into α-titanium and β-titanium, and these alloys demonstrate unique properties shaped by their respective phases. The hexagonal close-packed structure of α-titanium alloys is notably associated with superior high-temperature creep resistance but limited plasticity. Conversely, the body-centered cubic structure of β-titanium alloys contributes to enhanced slip and greater plasticity. To optimize these alloys for specific industrial applications, alloy strengthening is often necessary to meet diverse environmental and operational demands. The impact of various processing techniques on the microstructure and metal characteristics of titanium alloys is reviewed and discussed in this research. This article systematically analyzes the effects of machining, shot peening, and surface heat treatment methods, including surface quenching, carburizing, and nitriding, on the structure and characteristics of titanium alloys. This research is arranged and categorized into three categories based on the methods of processing and treatment: general heat treatment, thermochemical treatment, and machining. The results of a large number of studies show that surface treatment can significantly improve the hardness and friction mechanical properties of titanium alloys. At present, a single treatment method is often insufficient. Therefore, composite treatment methods combining multiple treatment techniques are expected to be more widely used in the future. The authors provide an overview of titanium alloy modification methods in recent years with the aim of assisting and promoting further research in the very important and promising direction of multi-technology composite treatment.

Interface microstructure and mechanical response of similar and dissimilar joints of (γ + α2) titanium aluminide and α-titanium alloy produced by diffusion bonding

Titanium aluminides are one of the potential materials for aerospace applications because of their high specific strength and good resistance to creep and oxidation at elevated temperatures. However, it is quite challenging to obtain similar and dissimilar joints with these materials by fusion welding due to their high crack sensitivity, limited ductility, and low thermal conductivity. In this study, joints of (γ + α2)-titanium aluminide (TiAl) to itself and with α-Ti alloy were produced by solid-state diffusion bonding performed under vacuum in the temperature range of 750–950 °C and pressure of 10–20 MPa for 10–30 min. The resultant interface microchemistry and microstructures were examined in-detail by scanning electron microscope in conjunction with energy dispersive spectroscopy and electron backscattered diffraction. Instrumented indentation and shear testing were employed to examine hardness variation across the interface and to evaluate the bond strength of the joints, respectively. The TiAl/TiAl joint possessed a single thin interface layer of α2 (Ti3Al), whereas two interphase layers comprised, viz. α + α2 (layer 1) and α2 (layer 2) were formed at interface of the α-Ti/TiAl joint. The minimum bonding temperatures of 900 °C and 800 °C were identified for producing metallurgically sound joints of TiAl/TiAl and α-Ti/TiAl, respectively, under an applied pressure of 10 MPa for 30 min. The bond strength of ∼400 MPa, which is ∼90% shear strength of TiAl base alloy, was achieved for TiAl/TiAl joints produced at 900 °C and above. The α-Ti/TiAl joint produced at 950 °C showed a maximum bond strength of ∼504 MPa. The intimate contact of surfaces through the complete collapse of their asperities and formation of phases, similar to that present in TiAl base alloy, at the interface led to enhanced joint strengths. The fracture surface of the shear tested specimens displayed a transition in failure mode from brittle to transgranular failure of the interface along with TiAl base alloy with an increase in bonding temperature.

Superior tensile properties and unique damage-fracture characteristics of Ti-2.5Cu α-titanium alloy at liquid-nitrogen temperature

α-titanium alloys are extensively applied under cryogenic environment and thereby their cryogenic mechanical properties and service failures deserve to be deeply studied. Here, tensile behaviors and damage-fracture characteristics of Ti-2.5Cu α-titanium alloy were systematically investigated at liquid-nitrogen temperature (LT) plus room temperature (RT) for comparison. It is found that the LT samples possess superior strength and ductility concurrently. Both yield strength (σy) and ultimate tensile strength (σb) improve more than 70%, whilst total elongation at fracture (A) adds more than 33% in comparison with the RT counterparts. Meanwhile, the LT samples are stretched mainly by uniform strain without noticeable necking. Fracture analysis indicates that being different from typical cup-and-cone fracture surface in the RT samples, the ‘radial’ fracture surface appears in the LT samples. Such specific fracture surface is constructed by two characteristic morphologies of crack-initiation zone at sample surface and the ensuing crack-propagation zone. Furthermore, it is seen that a large number of micro-voids are dispersed underneath fracture surface in the RT samples, whereas the number of micro-voids is sharply reduced at LT. Nevertheless, Vickers hardness at the position is always much higher in the LT samples. These novel mechanical properties and damage-fracture characteristics in the LT samples are closely associated with the prevalent deformation twinning in microstructure. Deformation twinning itself and plentiful twins-induced dynamic Hall-Petch effect steadily accommodate plastic deformation, contributing to the simultaneous enhancement of strength and ductility. This process also results in high stress state in samples, increasing the sensitivity of surface cracking. Hence, the ‘radial’ fracture morphology is finally formed. These findings enrich our fundamental understanding on deformation-damage-fracture process of α-titanium alloys, and also provide implicit guidance for the design of titanium alloys with high performance.

Effects of He-ion irradiation on microstructures of low activation Ti-Ta-V alloy from atomic simulations and irradiation experiments

Most conventional titanium alloys contain nuclides with long half-lives, which is not conductive to material recovery after decommissioning. To recover components from post-service reactors safely and effectively, it is necessary to use materials with low activation and rapid induction of radioactive decay. In this work, a low-activation α-titanium alloy Ti-5Ta-1.6 V was prepared and irradiated with 80 keV helium ions to 6.8 dpa at room temperature. The mechanical properties and irradiation swelling properties of Ti-5Ta-1.6 V alloy were mainly investigated. The results showed that compared with the commercially pure titanium alloy (CP-Ti), the mechanical properties of Ti-5Ta-1.6 V were substantially improved. After ion irradiation, the irradiation swelling rate of Ti-5Ta-1.6 V (in mass ratio) was found to be significantly lower than that of CP-Ti by atomic force microscopy (AFM) technology. Molecular dynamic (MD) simulations were used to study the irradiation behavior and cascade collisions process, and the Ti-5Ta-1.6 V produced a small number of stable defects with cluster size and number density, which is consistent with the results of irradiation experiments. The results indicate that Ti-5Ta-1.6 V is expected to be a promising new titanium alloy for nuclear applications, which provides a new idea for the selection and optimization of low-activation and radiation-resistant nuclear materials.

Effect of process induction on densification behavior, microstructure evolution and mechanical properties of Ti-5Al-2.5Sn ELI fabricated by laser powder bed melting

In this paper, Ti-5Al-2.5Sn ELI α titanium alloy was prepared by laser powder bed fusion. The effects of heat treatment and HIP process on the densification behavior, microstructure evolution, low-temperature mechanical properties, and fracture behavior of Ti-5Al-2.5Sn ELI titanium alloy were analyzed. The experimental results show that the density of Ti-5Al-2.5Sn ELI titanium alloy is improved after HIP treatment. Complete recrystallization occurred in the deposited microstructure, and the recrystallized α grains coarsened with the increase in holding time. After heat treatment, the tensile strength decreases gradually, while the elongation increases with the increase of holding time. Compared with 300 K, the ultimate tensile strength and yield strength of the sample at 77 K were improved, but the elongation decreased. Ti-5Al-2.5Sn ELI titanium alloy annealed at 850 °C/4 h exhibits mixed ductile-brittle fracture morphology at 77 K and obtains ultrahigh strength and ductility. At the same time, Ti-5Al-2.5Sn ELI titanium alloy exhibits unit slip deformation behavior at 300 K, while at 77 K, it mainly exhibits the coupling of dislocation slip and twin deformation behavior. These findings lay a foundation for the rapid preparation of high-strength α-titanium alloy products with good ductility.

Limits of powder metallurgy to fabricate porous Ti35Nb7Zr5Ta samples for cortical bone replacements

The use of β-Titanium alloys to fabricate metal implants with Young's modulus that resembles bone tissues is presented as an alternative to commercially pure titanium or α-Titanium alloys, although it is still necessary to introduce proper implant porosity to reach the Young's modulus of cortical bones. In this work, porous samples were fabricated by loose sintering (0 MPa) and compared to samples manufactured at 1000 MPa, both sintered under the same conditions. Raw powders and sintered samples of the β-Titanium alloy, Ti35Nb7Zr5Ta, were characterized in detail in terms of both physicochemical and microstructural properties. Moreover, the tribo-mechanical behavior of sintered samples was evaluated by performing ultrasound technique, instrumented micro-indentation (P-h curves), and scratch tests. The bio-functional behavior was studied by impedance spectroscopy and contact angle measurements. The results allowed the evaluation of the limits of conventional powder metallurgy (percentage of porosity, size, and morphology of pores), as well as the influence of the porosity and chemical composition to achieve a better biomechanical and bio-functional behavior that would guarantee bone requirements. The Ti35Nb7Zr5Ta alloy showed relatively high electrical impedance values compared to commercially pure titanium, indicating an improved bio-corrosion behavior. Furthermore, wettability measurements indicated that porous disks fabricated by loose sintering exhibit higher hydrophilicity, often associated with a better antibacterial response.

Martensite colony engineering: A novel solution to realize the high ductility in full martensitic 3D-printed Ti alloys

Generally, additively manufactured (AM) full martensitic titanium alloys (hexagonal-close-packed α’ phase) exhibits high yield strength (>1GPa) but poor elongation to failure (<8% usually). However, this only α’-phase microstructure is not intrinsically brittle and exhibits high ductility under certain process conditions. Therefore, it is crucial to unravel the mechanism of ductility fluctuation for process control. Here, we found the less martensite-colony microstructures (LMCM) could effectively distribute stress to variant interfaces and avoid strain localization. Therefore, it results in a 194% ductility enhancement in the AM-produced α’-Ti-6Al-4 V (Ti64) compared with the rich martensite-colony microstructure (MCM). At the same time, we also successfully improved the strength-ductility dilemma for Ti64 alloy (yield strength, Ys0.2 = 1044 ± 10 MPa, elongation at break, EL = 15 ± 1.5%). We attribute the LMCM to refined prior-β grain and a weak variant selection during βBCC to α’HCP by increasing hatch spacing. What’s more, to rule out the possible influence of the β phase and demonstrate the generality of our conclusion, we further apply it on α-titanium alloy (Ti-6.5Al-2Zr-1Mo-1 V, TA15 alloy) and obtain the same results.

Influence of the Composition of α-Titanium Alloys on Thermal Conductivity

Influence of the Composition of α-Titanium Alloys on Thermal Conductivity

Influence of the composition of α-titanium alloys on thermal conductivity

Among titanium alloys, modern α- and pseudo-α-alloys occupy a special place due to the unique combination of their mechanical properties, corrosion resistance, low density and high specific strength, which determines their effectiveness in various industries. Analysis of structural materials used for heat exchange equipment of nuclear power plants showed that the increase in the efficiency and compactness of tube systems made of a-titanium alloys is constrained by their thermal conductivity characteristic, which does not exceed 89 W/(m·K) at a temperature of 20°C. An exception is the VT1-0 grade alloy, the scope of which is limited to a maximum operating temperature of no more than 250°C. The paper considers the results of studies of a new titanium alloy of the Ti-Zr-Al-O composition with increased thermal conductivity for pipe systems of power equipment.

ThermoCalc Modelling of the Effect of Chemical Composition on the Phase Transformations Temperatures of Ti-0.4Al Alloy

The effect of chemical composition of Ti-0.4 Al α-titanium alloy (PT-1M alloy) on the phase transformation temperatures up to melting temperature has been investigated using ThermoCalc software. The possibility of appearance of Ti2N nitride, Tix(CN)-type carbonitride, and Ti2Fe-type intermetallic in the alloy structure in equilibrium state has been proved by thermodynamic calculations. It has been proposed the so-called aluminum equivalent (Aleq) for PT-1M alloy to analyze the effect of chemical composition on the phase transformation temperatures change. The following equivalents have been suggested: for transformation, for transformation, and for transformation. It has been shown the linear correlation between the characteristic temperatures of phase transformations in the investigated alloy and the proposed aluminum equivalents.

Formation of a Microcrystalline Structure in a VT5-1 Alloy during Uniaxial Compression at 800 and 900°C

The influence of the state of stress on the dynamic recrystallization characteristics and the deformation behavior of the VT5-1 α-titanium alloy during compression at temperatures of 800 and 900°C in the strain rate range 4.0 × 10–5–6.0 × 10–3 s–1 is studied. The effects of the temperature, the strain rate, and the hydrostatic component of the state of stress on the fine structure, the texture, the grain size, and the grain misorientation distribution are found. The grain size and the intensity of dynamic recrystallization during compression at 900°C are shown to become strongly dependent on the strain rate. An increase in the hydrostatic component of the stress tensor decreases the average grain size and the volume fraction of recrystallized grains.

Tensile properties of α-titanium alloys at elevated temperatures

Hot-deep drawing is an innovative processing technology to produce complex shaped sheet metal components with constant wall thickness from high-strength lightweight materials. For some aerospace and automotive applications oxidation resistance at medium to high temperatures is an important aspect. In terms of this titanium α-alloys are often used due to their balanced relation of strength and oxidation resistance. In the presented study the stress-strain characteristics of several α-titanium alloys were determined at ambient and elevated temperatures by means of hot tensile tests. Besides the commercially pure Titanium alloy ASTM-Grade 4, two novel α-titanium alloys were investigated. Regarding the hot forming properties a comparison with α-β Ti-6Al-4V alloy was conducted. The hot tensile tests were carried out by means of a particular forming dilatometer type “Gleeble 3500” at 400, 500, 600, 650, 700 and 800 °C. The test showed favorable peak plasticity for all α-alloys at the temperature range between 600 and 650 °C in contrast to lower or higher temperatures. All samples were metallographically characterized.Key words: titanium α-alloys, hot tensile properties, elevated temperatures, Gleeble 3500.

Determination of orientation relationships between FCC-hydride and HCP-titanium and their correlation with hydrides distribution

Since the crystallographic orientation relationships (ORs) play an important role in the hydrides precipitation as well as in the deformation behavior of α-titanium alloys containing hydrides, the present work aimed at exploring thoroughly the ORs and their interface planes between the fcc-hydride and the α-titanium using the stereographic projections of the corresponding OR plane and OR direction and a trace analysis. In addition to the general reported two ORs in the α-titanium, a new OR closely related to the texture component of the rolled Ti-sheet is also determined in the present work. The overall identified ORs and their interface planes are accounted for by edge-to-edge matching model. Furthermore, a quantitative analysis using the high-angular resolution SEM-EBSD highlighted the dependency of hydride distribution on the ORs and the parent grain orientation.

Influence of Cutting Speed on Interaction Type and Temperature during Turning of α-Titanium Alloy

The subject of the research is the influence of contact interaction at the face surface on average cutting temperature at the face surface.

Dynamic strain-rate effect on uniaxial tension deformation of Ti5Al2.5Sn α-titanium alloy at various temperatures

ABSTRACTThe uniaxial tensile experiments for Ti5Al2.5Sn alloy were performed at strain rates ranging from 10−3–10+3 s−1 and test temperatures of 153–873 K. Experimentally measured stress-strain responses indicate the yield strength exhibits positive strain-rate dependency, while the yield strength increases as the test temperature is decreased. To understand the thermomechanical coupling of dynamic plastic deformation, a specially developed single-tensile-pulse loading technique was used, and the isothermal stress-strain curves for the rates of 180 and 450 s−1 were obtained at temperatures of 203, 298 and 573 K. The plastic strain hardening measurements obtained here are essentially athermal and largely independent of strain rate, consistent with titanium and its alloys being bcc-structure-like in mechanical behaviour. Based on the experimentally obtained plastic deformation features of the alloy, the physically based Voyiadjis-Abed constitutive relationship was modified to model the dynamic tensile deformation of the Ti5Al2.5Sn alloy at low and high temperatures.

Grain size characterization by laser-based ultrasonics based on the centroid frequency shift method

To solve such problems as the sensitivity of ultrasonic attenuation method for grain size evaluation to geometric spreading, reflection and transmission effects, and the accuracy of ultrasonic echo amplitude measurements, this paper addresses the application of the centroid frequency shift method for characterization of mean grain size in polycrystalline materials. A general model was presented for the correlation of the mean grain size to the downshift in the spectral centroid frequency of a Gaussian pulse during propagation in a medium with a nonlinear frequency dependence of attenuation. The model was further extended to a particular case of laser-ultrasonic inspection, and an explicit formula for the mean grain size was derived in terms of the centroid frequency ratio of two successive echoes under a quadratic frequency assumption. Experimental inspection for mean grain size was then conducted in α-titanium alloys by a laser-based ultrasonic method. Microstructures with different mean grain sizes of 26–49 μm were obtained by annealing at 800°C for different holding times. The centroid frequency shift reveals a linear dependence on the mean grain size. Referring to the metallography analysis, the nondestructive evaluation errors over all the test samples are smaller than 5.4%. The provided results help support the application of the centroid frequency shift method for the grain size evaluation in polycrystalline materials, and this method is more insensitive to effects of wave diffraction, reflection, and transmission in comparison with some other nondestructive methods.

Spark plasma sintering for high-rate diffusion welding of a UFG titanium alloy PT3V

This is the first ever paper to address the prospects of using Spark Plasma Sintering (SPS) for high-rate diffusion welding of a high-strength ultrafine-grained (UFG) α-titanium alloy Ti–5A–2V. The effect of growing diffusion welding intensity in UFG titanium alloys is also described. Welds of a UFG α-titanium alloy Ti–5Al–2V obtained with SPS are characterized by high density, strength, and corrosion resistance. The rate of weld sealing in UFG alloys is shown to be in nonmonotonic (maximum) dependence on the heating rate.

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.

Effect of non-Schmid stresses on 〈a〉-type screw dislocation core structure and mobility in titanium

Density functional theory calculations of 〈a〉-type screw dislocation core structures in hcp titanium under a series of applied stresses with zero critical resolved shear stress were performed to investigate the origin of non-Schmid law slip behavior in α-titanium alloys. It was found that compression applied on the [11¯00] direction perpendicular to the Burgers vector leads to more compact pyramidally-oriented cores, while tension applied on the same axis causes an extension of these cores along the prismatic plane. Nudged elastic band calculations of the barrier to slip of the dislocations on the prismatic plane show that extension along this plane leads to reduced slip barriers. This suggests that non-Schmid stresses alter the critical resolved shear stress by inducing changes in the equilibrium core structure.