TiAl Research Articles

Microstructure evolution of Ti17 titanium with ultrahigh strength and moderate plasticity fabricated by laser powder bed fusion

This study utilized laser powder bed fusion (LPBF) to fabricate Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti17) specimens, exploring their ultrahigh strength microstructural strengthening mechanisms and examining heat treatment effects on α lamellae and related mechanical property trends. In the as-deposited LPBF-Ti17, α lamellae features extremely fine dimensions, with an average lamellar thickness of 45 nm, and comprises approximately 52.2% of total volume. This finely dispersed α lamellar microstructure provides numerous α/β phase interfaces, enabling LPBF-Ti17 mechanical properties to reach ultra-high strengths close to 1500 MPa. During heat treatment, the Ostwald ripening mechanism primarily drives growth in α-phase lamellar thickness. The rate of α lamellar thickness growth reaches its peak between 650 °C and 700 °C. The main dislocation distribution of LPBF-Ti17 lies at α lamellae edges, adjacent to β phase, and dislocation density diminishes with rising heat treatment temperature. Concurrently, the Termination migration mechanism causes α lamellae ends to become progressively rounded from sharp features with increasing heat treatment temperature. The increase in α lamellar thickness and reduction in dislocation density collectively contribute to the effective enhancement of Ti17 plasticity.

Development process and future trends of chemical refining agents' influence on grain refinement in aluminum alloys

Aluminum is the world's largest production of non-ferrous metals, with light weight, high specific strength, excellent electrical and thermal conductivity and other characteristics. With the continuous development of aerospace, precision electronic instruments, photovoltaic semiconductors and other emerging strategic industries, the demand for high-performance materials of aluminum alloys has become increasingly strong. This paper reviews the research progress of aluminum alloy melt refinement treatment, summarizes the development history of grain refinement, and evaluates the advantages and disadvantages of various types of refining agents, such as Al–Ti–B, Al–Ti–C, Al–Ti–B/C–Re, etc. The article points out the shortcomings of the current research on the melt refinement treatment of aluminum alloys, and looks forward to the future research direction.

Phase formation and nanohardness of Ti–48Al–2Cr–2Nb-1.5C alloy by melt spinning: The effect of cooling rates

The effect of wheel speed on the phase formation and nanohardness of Ti–48Al–2Cr–2Nb-1.5C alloy by melt spinning was investigated. The results illustrate that with increasing wheel speed, the solubility of C atoms in the matrix increases, which reduces the volume fraction of Ti2AlC precipitates from 7.8 vol% to 2.5 vol%. The length-diameter ratio of precipitated Ti2AlC decreases from 32.2 to 2.2 with increasing cooling rate. Different wheel speeds didn't change the matrix (γ phase), while the primary phase formation was affected by it during solidification. When the wheel speed is 16 m/s, the α phase can directly nucleate in the liquid phase, leading to an increase in the volume fraction of α2 after RS. Meanwhile, owing to the decrease in lamellar spacing, the increase in the solid solution, and the nanoscale Ti2AlC dispersed in the lamellae, the nanohardness of the lamellae increases with increasing cooling rate. It increases from 4.92 ± 0.12 GPa at conventional solidification to 7.69 ± 0.21 GPa at a wheel speed of 24 m/s, which is an increase of approximately 56.1%.

Microstructure evolution and strengthening mechanisms of a high-performance TiN-reinforced Al–Mn–Mg–Sc–Zr alloy processed by laser powder bed fusion

In this work, a high-strength crack-free TiN/Al–Mn–Mg–Sc–Zr composite was fabricated by laser powder bed fusion (L-PBF). A large amount of uniformly distributed L12-Al3(Ti, Sc, Zr) nanoparticles were formed during the L-PBF process due to the partial melting and decomposition of TiN nanoparticles under a high temperature. These L12–Al3(Ti, Sc, Zr) nanoparticles exhibited a highly coherent lattice relationship with the Al matrix. All the prepared TiN/Al–Mn–Mg–Sc–Zr composite samples exhibit ultrafine grain microstructure. In addition, the as-built composite containing 1.5 wt% TiN shows an excellent tensile property with a yield strength of over 580 MPa and an elongation of over 8 %, which were much higher than those of wrought 7xxx alloys. The effects of various strengthening mechanisms were quantitatively estimated and the high strength of the alloy was mainly attributed to the refined microstructure, solid solution strengthening, and precipitation strengthening contributed by L12–Al3(Ti, Sc, Zr) nanoparticles.

Designing of novel microstructure and its impact on the improved service temperature mechanical performance of 2nd and 3rd generation advanced intermetallic TiAl alloys

Advanced lightweight TiAl based intermetallic systems, capable of surviving elevated temperatures, have been a topic of interest, considering their ever-increasing demands in aerospace industries. The optimization of solidification and processing methods stands as a pivotal factor in microstructural engineering for TiAl systems, aiming to attain desired mechanical properties. The present study focuses on two distinct alloy systems, a commercial grade 2nd generation, i.e., Ti–48Al–2Cr–2Nb (at. %), and a newly developed 3rd generation, i.e., Ti–45Al–2Cr-7.5Nb-0.2B (at. %) alloys. Interestingly, the variation in the solidification path, triggered by higher Nb content along with a minor amount of B, accounts for the substantial microstructural refinement for the 3rd generation TiAl alloy. Nevertheless, prominent segregation of Nb and Cr to the β0 phase is observed in both the as-cast microstructure, which restricts the applicability of the alloys. Remarkably, a novel bilamellar-biglobular (BLBG) microstructure consisting of γ and α2 phases in both lamellar and globular morphologies is achieved through heat treatment for both categories of alloy. Elevated temperature tensile testing depicts an exceptional strength-ductility combination for 3rd generation BLBG microstructure. Excellent twin-twin activity and twin-induced plasticity effect are observed to govern the deformation mechanism. Overall, the path of solidification, phase distribution, and its effect on the deformation mechanism are thoroughly analyzed, which is of particular interest from a design and application point of view.

Hot working behaviour of Ti–8Al–1Mo–1V alloy through the hot compression test

Hot deformation behaviour of Ti–8Al–1Mo–1V alloy (Ti-811) was investigated through the hot compression tests at the temperatures of 950–1075 °C and at strain rates of 0.001–1 s−1. The flow curves of the Ti-811 alloy were successfully modelled by means of the sinh equation and constitutive equations were proposed for isothermal hot compression of Ti-811 alloy. According to the date of flow curves, the values were calculated for the strain rate sensitivity, stress exponent and activation energy. The amounts of the strain rate sensitivity, stress exponent, and activation energy were calculated as 0.19, 3.95 and 730 kJ mol−1 for α + β region and 0.21, 3.6 and 194 kJ mol−1 for β region, respectively.

Regulated Phase Separation in Al-Ti-Cu-Co Alloys through Spark Plasma Sintering Process.

With the goal of developing lightweight Al-Ti-containing multicomponent alloys with excellent mechanical strength, an Al-Ti-Cu-Co alloy with a phase-separated microstructure was prepared. The granulometry of metal particles was reduced using planetary ball milling. The particle size of the metal powders decreased as the ball milling time increased from 5, 7, to 15 h (i.e., 6.6 ± 6.4, 5.1 ± 4.3, and 3.2 ± 2.1 μm, respectively). The reduction in particle size and the dispersion of metal powders promoted enhanced diffusion during the spark plasma sintering process. This led to the micro-phase separation of the (Cu, Co)2AlTi (L21) phase, and the formation of a Cu-rich phase with embedded nanoscale Ti-rich (B2) precipitates. The Al-Ti-Cu-Co alloys prepared using powder metallurgy through the spark plasma sintering exhibited different hardnesses of 684, 710, and 791 HV, respectively, while maintaining a relatively low density of 5.8-5.9 g/cm3 (<6 g/cm3). The mechanical properties were improved due to a decrease in particle size achieved through increased ball milling time, leading to a finer grain size. The L21 phase, consisting of (Cu, Co)2AlTi, is the site of basic hardness performance, and the Cu-rich phase is the mechanical buffer layer between the L21 and B2 phases. The finer network structure of the Cu-rich phase also suppresses brittle fracture.

Phase transformation and equation of state in Ti–45Al alloy under high pressure

The phase transformations and pressure-volume dependencies of the Ti–45Al alloy with respect to pressure have been investigated by means of in-situ observation using multi anvil-type high-pressure devices and synchrotron radiation. Under hydrostatic compression from 0 to 10.1 GPa, about 2.3 vol % of γ transforms continuously to α2. Lattice parameters as well as volume fractions of these two phases have been determined as functions of pressure. Bulk moduli estimated using Birch-Murnaghan's equation of state are 146.2 GPa for the γ phase, 136.7 GPa for the α2 phase, and 145.6 GPa for their two-phase mixture of Ti–45Al alloy. First-principles have also applied to investigate bulk moduli of two single phases, and the deviation between calculations and measurements is discussed and attributed to mainly phase transformation. The present study provides useful insights into thermodynamics of α2 and γ phases under high pressure.

Electrochemical and Tribological Performance of Ti–Al with xNb Addition Synthesized via Laser In situ Alloying

Additive manufacturing is a growing technique of producing 3D parts directly using metal powders or wires melted with a high-powered intensity beam or laser. It is still a challenging process as to how laser processing parameters such as gas flow rate and powder flow rate can profitably be adopted to significantly produce Ti–Al-based materials from elemental powders to synthesize alloys that are defect-free and have good mechanical properties. The density of titanium aluminide (Ti–Al) intermetallic alloys makes it gain lots of interests due to its potential ability to substitute nickel-based superalloys in gas turbine engines. This work aims to investigate the effects of Niobium (Nb) additions on Ti–Al–xNb ternary alloys created via the use of 3D printing technology, specifically looking at microstructural evolution, microhardness, electrochemical behavior, and tribological properties. Ti–Al–Nb alloy was synthesized at scan speed of 26 in/min and laser power of 450 W. The structural morphology of the alloys produced was investigated using scanning electron microscopy equipped with energy dispersive spectroscopy and the electrochemical studies of the in situ alloyed Ti–Al–xNb were studied using potentiodynamic techniques. Using an Emco microhardness tester, the microhardness characteristics of the produced TiAl–xNb alloys were examined. From the results obtained, it was observed that the microstructure showed not much substantial cracking or crack initiation. The micrographs are evident of refined microstructure associated to increase in Nb feed rate with α-Ti3Al, γ-TiAl and precipitates of β-TiAl phases as the distinctively identified in the microstructure. The highest recorded microhardness value of 679.1 HV0.5 was achieved at Nb feed of 0.5 rpm and gas carrier of 2 L/min. The fabricated Ti–Al–Nb alloys showed good corrosion resistance behavior in HCl and appreciable wear characteristics with coefficient of friction of 0.412, 0.401, and 0.414 µ at B1, B3, and B5, respectively.

The Enhancement Effect of Carbides on the Printability and Mechanical Properties of a Ni–Fe–Cr–Al–Ti Alloy Processed by Electron Beam Freeform Fabrication

The Enhancement Effect of Carbides on the Printability and Mechanical Properties of a Ni–Fe–Cr–Al–Ti Alloy Processed by Electron Beam Freeform Fabrication

Economic analysis of eco-friendly lubrication strategies for the machining of Ti48Al2Cr2Nb aluminide

Titanium aluminides have been identified as potential materials in the aeronautical sector. However, the high processing costs and their low machinability limit their application. This research evaluates the effectiveness of minimum quantity lubrication technique (MQL) and cryogenic lubrication on the turning process of Ti48Al2Cr2Nb aluminide from an economic and industrial productivity point of view, with the aim of determining the optimum cutting conditions to reduce manufacturing costs and maximize the production rate. To this end, a preliminary experimental analysis will be carried out, determining the tool life as a function of the cutting speed for each lubrication condition (dry, MQL and LN2). The results show that, although both MQL and LN2 conditions allow the tool wear to be reduced, the MQL technique stands out for its higher efficiency and for the possibility of reaching a higher cutting speed. In addition, the high cost of liquid nitrogen implies a 47% additional cost. Consequently, this research finds out that the turning process should be carried out in the cutting speed range of 70–76 m/min under MQL conditions to obtain the highest profit rate.

Computational Modeling of Directed Energy Deposition of Titanium Aluminide Plate Geometries

In this work, transient thermomechanical analysis of the laser-based Directed Energy Deposition (DED) process was carried out to predict melt pool dimensions, thermal cycles and residual stress during Titanium Aluminide (TiAl) plate deposition. The predicted melt pool length, width, and depth increased as the laser power was increased from 250 W to 350 W at a constant travel speed of 15 mm/s. The melt pool length increased from 0.425 mm to 0.55 mm, while the depth increased from 0.1 to 0.29 mm, and the width increased from 0.412 to 0.5 mm. The plate's edges (tracks 1 and 6) were found to experience a slightly higher temperature than the middle tracks, which may contribute to higher thermal gradients at the edges. The highest Principal stress of 140 MPa was found in track 1. The magnitude of Principal stresses diminished as track deposition progressed from track 2 to 6 due to the variation of the thermal gradient along the tracks.

Macroscopic Deflections of Fatigue Crack in Direct Energy Deposited Ti–5Al–5Mo–5V–1Cr–1Fe

Macroscopic Deflections of Fatigue Crack in Direct Energy Deposited Ti–5Al–5Mo–5V–1Cr–1Fe

Tracing of Deoxidation Products in Ti‐Stabilized Interstitial Free Steels by La and Ce on an Industrial and Laboratory Scale

The continuous casting of Al‐killed Ti‐stabilized interstitial free steels is often affected by clogging. By today, the mechanism behind this phenomenon is not entirely clarified. The active tracing method, which involves the direct addition of rare‐earth elements (REEs), enables tracking of nonmetallic inclusions (NMIs) over process time. Due to the high oxygen affinity of these elements, preexisting NMIs are partially reduced and marked by this tracer. Using scanning electron microscopy with energy‐dispersive spectroscopy, it is possible to differentiate such labeled NMIs from particles formed at later stages in the process by the absence of these tracing elements. Active tracing is used in industrial and laboratory settings to trace preexisting alumina NMIs with La or Ce. In the investigation, it is revealed that the number of small NMIs increases after the addition of REEs, and subsequently, the size increases again after FeTi is alloyed. In both settings, traced and untraced Al–Ti oxides are found. The separation tendency of traced NMIs is studied over time by analyzing the composition of the slags. Furthermore, the impact of deoxidation products on the formation of the clogging layer within the submerged entry nozzle is investigated, indicating that these NMIs contribute to its formation.

Multimodal and multiscale strengthening mechanisms in Al-Ni-Zr-Ti-Mn alloy processed by laser powder bed fusion additive manufacturing

The unique thermokinetics of laser-powder bed fusion additive manufacturing (L-PBFAM) has been exploited for development of a novel high-strength Al-Ni-Ti-Zr-Mn alloy. The addition of 0.5 wt% Mn leads to extraordinary improvement in ultimate tensile strength (502 MPa) and work hardening due to the activation of two Mn-induced strengthening mechanisms. First, by a bimodal particle strengthening effect due to Al31Mn6Ni12 nano-quasi-crystal particles rejected in inter-dendritic spaces and fibrous Al3Ni eutectic dendritic channels, which predominately contributes to the strength improvement, and second by solid solution strengthening from remaining Mn entrapped in Al. These mechanisms supplement the particle strengthening effect imparted by coherent and incoherent Al3(Ti,Zr) co-precipitates present at melt pool boundaries, dislocation strengthening due to solidification induced strain, and Hall-Petch and backstress strengthening effect due to heterogenous grain size distribution occurring at various length scales. The solidification dynamics and hierarchical heat distribution that are associated with L-PBFAM resulted in complex spatial variations in these strengthening phenomena and were investigated via a high-throughput multiscale structure–property correlation that involved thermokinetic simulation, X-ray diffraction, high-resolution nanoindentation mapping, and site-specific transmission electron microscopy of the alloy.

Boosting the grain refinement of commercial Al alloys by compound addition of Sc

For decades, Al–Ti–B has been widely used for refining commercial Al alloys. However, the addition of Al–Ti–B master alloys to recycled Al alloys suffers from Si-poisoning and cannot achieve desired refinement and thus final strength-ductility. In this study, a novel grain refiner made of Al–Ti–B-Sc has been developed and its application in refining recycled Al–Si–Cu alloys has been quantified using X-ray computed tomography (XCT). It has been found that the new Al–5Ti–1B-0.8Sc master alloy exceed the performance of Al–5Ti–1B in three aspects: (1) formation of Al3(Sc,Ti) heterogeneous nucleation particles other than Al3Ti; (2) dispersing TiB2 by forming TiB2/Al3(Sc,Ti) clusters and increasing Al melt viscosity; (3) reducing the specific surface area of Al3Ti to improve its high-temperature stability. Surprisingly, the compound addition of Sc has been found to further reduce the grain and Si particle size due to accelerated kinetics of α-Al nucleation and diminished Si poisoning. Therefore, this study proposes not only a new master alloy but also a new grain refinement method, which can effectively increase the tensile strength and elongation of recycled Al alloys up to 433.2 ± 10.5 MPa and 3.0 ± 0.1 %, respectively.

Effect of applied load on tribological behaviour of Ti–48Al–2Cr–2Nb alloy under oil-lubricated condition

With their low weights and excellent mechanical properties, TiAl alloys are expected to be applied in pistons to achieve energy savings and emissions reductions in internal combustion engines. The tribological behaviours of the Ti–48Al–2Cr–2Nb alloy against the QT600-3 material at different applied loads under oil-lubricated conditions were investigated in this study. The results showed that as the load increased, the friction coefficient and wear-scar volume significantly decreased. The wear surface of the TiAl alloy was analysed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The wear mechanism of the TiAl alloy under oil lubrication conditions was mainly the combined effect of adhesive wear and oxidation wear, different from the wear mechanism under dry-friction conditions. Furthermore, by observing the cross-sectional morphology of the TiAl alloy after focused ion beam, it was found that the QT600-3 wear debris had compacted and accumulated on the surface of the TiAl alloy, preventing further contact between the TiAl alloy and the QT600-3. This study has the potential to accelerate the application of the Ti–48Al–2Cr–2Nb alloy in lightweight pistons.

Interatomic Interaction at the Al–TiC Interface

The interaction of a titanium carbide nanoparticle with aluminum (100), (110), and (111) substrates is investigated within the density functional theory. The nanoparticle–substrate interaction energies are determined; the electron density distribution and the electron localization function between aluminum, titanium, and carbon atoms are analyzed. It has been established that the atoms in the upper layers of the aluminum (100) and (110) substrates are significantly displaced relative to their initial positions as a result of the interaction with the nanoparticle, whereas a minor displacement of atoms is typical for the (111) substrate. The interaction between aluminum and carbon atoms at the Al–TiC interface is due to the formation of covalent Al–C chemical bonds. The aluminum atoms forming carbide bonds do not form chemical bonds with titanium atoms. The aluminum atoms that are adjacent to the titanium atoms and are not involved in the formation of carbide bonds form metallic Al–Ti bonds.

In-situ synthesized a dual-scale Ti2AlC reinforced TiAl composites with superior mechanical properties

To enhance the mechanical properties and service temperature of TiAl alloy, the dual-scale Ti2AlC particles reinforced TiAl composites were successfully designed and fabricated by spark plasma sintering (SPS), utilizing pre-alloyed Ti–48Al–2Nb–2Cr powder and multi-walled carbon nanotubes. The TiAl composites exhibited a fully lamellar structure, with micro-laminated Ti2AlC particles formed at grain boundaries and nano-laminated Ti2AlC particles precipitated at the interfaces of α2 and γ lamellae by adding 0.5 wt% multi-walled carbon nanotubes. The TiAl composites exhibited excellent mechanical properties at room temperature and elevated temperatures. The ultrahigh tensile strength of TiAl composite was 599.6 MPa at 800 °C which was improved by 28.3 % compared with TiAl matrix, while the fracture strain remained 4.2 % without sacrifice. These superior mechanical properties were mainly attributed to the refinement strengthening, solid solution strengthening, and the formation of dual-scale Ti2AlC particles. Moreover, the dual-scale Ti2AlC particles reinforced α2 and γ lamellar interfaces and lamellar colony boundaries resulting in improvement of strength and toughness simultaneously. The TiAl composite was reinforced by dual-scale Ti2AlC particles, which expected to further break the property limitation of TiAl alloy and expanded its application at high temperature.

Effect of thermomechanical processing on microstructure evolution and mechanical properties of metastable β Ti–5Al–5V–5Mo–3Cr alloy

Two distinct thermomechanical processing (TMP) schedules are applied to as-cast Ti-5553 alloy pancakes, to optimize its microstructure and mechanical performance. While TMP-I involves 40 % thickness reduction via forging, TMP-II employs rolling with 20 % thickness reduction. Typical bimodal microstructures with nearly similar sizes, distributions, and morphologies for the primary αp grains and colonies of secondary αs and β are noted to generate for both TMP-I and TMP-II. Certainly, rolling for only half the extent of that used for forging, appears ideal and economical for obtaining the targeted microstructure. Interestingly, similar and improved localized hardness values with respect to as-cast alloy, are obtained for bimodal microstructure, generated through both the TMP schedules. However, indentation size effect is noted to be nominally lower for the rolled alloy, thereby proving its suitability for large-scale application. Global behavior assessed through tensile tests also shows that bimodal microstructures obtained through forging or rolling leads to higher strengths, toughness and ductility, as compared to as-cast counterparts. This is primarily owing to non-uniform strain distribution in the as-cast alloy, resulting to crack initiation and failure, as revealed through digital image correlation-based study. Overall, TMP employing rolling with reduced deformation in particular appears beneficial for the mechanical property improvement of the alloy.