Initial Ti Research Articles

In situ TEM observations on the structural evolution of a nanocrystalline W-Ti alloy at elevated temperatures

The thermal stability and nanoscale structural evolution at elevated temperatures of a sputter deposited W-Ti alloy thin film were studied by a combination of ex situ and in situ techniques. XRD, FIB, SEM-EDX and STEM-EDX were used to characterise the film annealed ex situ in vacuum at 1373 K for 48 h. In situ TEM heating experiments were conducted at various temperatures up to 923 K to capture transitional phenomena occurring in the alloy upon heating and cooling. At a microscopic level, the alloy annealed at 1373 K for 48 h transformed from a single-phase β-(WTi) solid solution into a two-phase alloy consisting of Ti-rich grains in equilibrium with Ti-depleted β-(WTi) solid solution grains. In situ TEM observations revealed initial Ti segregations along columnar grain boundaries at T ∼ 423–573 K, followed by Ti-rich clusters formation in the grains interior at T ∼ 573–773 K. The microstructure observed at 923 K remained stable upon cooling to room temperature and consisted of Ti-rich segregations along the columnar grain boundaries and of alternate Ti-rich and Ti-depleted nanoscale domains in the grains interior, which formed a stable dual-phase nanocrystalline structure.

A novel method for the fabrication of Al-matrix nanocomposites reinforced by mono-dispersed TiAl3 intermetallic via a three-step process of cold-roll bonding, heat-treatment and accumulative roll bonding

A novel method to fabricate aluminum metal-matrix composites reinforced by mono-dispersed TiAl3 intermetallic particles, by cold-roll bonding, heat treatment, followed by accumulative roll bonding (ARB), is demonstrated. Roll bonding of 1100-Al sheets inserted with Ti powder was carried out at room temperature with 50% thickness reduction. Complete conversion of the initial Ti particles into TiAl3 intermetallic was achieved by a post-rolling annealing treatment at 590 °C for 2 h, which results in the formation of coarse TiAl3 particles along the interface of the original 1100-Al sheets. Subsequent ARB process was applied to break up and disperse the TiAl3 phase, and after 5 cycles of ARB, an aluminum-matrix composite with mono-dispersed TiAl3 particles of submicron sizes was achieved. The Al matrix also undergoes dynamic recrystallization during the ARB process to result in a fine-grained microstructure (size less than 500 nm). The hardness, tensile strength and elongation of the Al/TiAl3 composite were found to increase with the number of ARB cycles. The tensile strength of the Al/TiAl3 composite after the 5th cycle reaches 400 MPa, which is significantly higher than the 300 MPa of monolithic 1100-Al processed in the same way. The tensile elongation of the Al/TiAl3 composite after 5 cycles of ARB exhibits a good value of ∼8%, with a ductile fracture mode. This is the first successful attempt to achieve Al-matrix nanocomposites reinforced by mono-dispersed TiAl3 intermetallic based on an ARB method.

Continuous-wave modulation of a femtosecond oscillator using coherent molecules.

We describe a new method to broaden the frequency spectrum of a femtosecond oscillator in the continuous-wave (CW) domain. The method relies on modulating the femtosecond laser using four-wave mixing inside a Raman-based optical modulator. We prepare the modulator by placing deuterium molecules inside a high-finesse cavity and driving their fundamental vibrational transition using intense pump and Stokes lasers that are locked to the cavity modes. With the molecules prepared, any laser within the optical region of the spectrum can pass through the system and be modulated in a single pass. This constitutes a CW optical modulator at a frequency of 90THz with a steady-state single-pass efficiency of ∼10-6 and transient (10μs-time-scale) single-pass efficiency of ∼10-4. Using our modulator, we broaden the initial Ti:sapphire spectrum centered at 800nm and produce upshifted and downshifted sidebands centered at wavelengths of 650nm and 1.04μm, respectively.

Simultaneously improving mechanical properties and corrosion resistance of pure Ti by continuous ECAP plus short-duration annealing

This work investigated the effect of short-duration annealing on mechanical properties and corrosion behavior of bulk ultrafine-grained (UFG) pure Ti (grade 2) produced by high-efficiency equal-channel angular pressing (ECAP), serving the fast developing of marine industries. The initial coarse-grained (CG) Ti achieved higher strength after continuous ECAP at 420 °C for 4 passes, while additional short-duration annealing at 300 °C for 15 min of UFG Ti further led to extraordinary improvement of strength, which was associated with texture strengthening. The yield strength was increased from 362 MPa in as-received state to 561 MPa in post-ECAP annealed condition, and ultimate tensile strength from 450 MPa to 663 MPa. The ECAP-processed UFG Ti had better corrosion resistance in artificial seawater than the initial CG Ti. Short-duration annealing further improved the corrosion resistance of UFG Ti, which was related to stronger basal texture. The corrosion current density decreased from 68.37 nA cm−2 for CG Ti in as-received condition to 41.77 nA cm−2 for UFG Ti processed by ECAP plus short-duration annealing. The fraction of TiO2 within the surface oxide film for the post-ECAP annealed sample was increased. The bulk UFG Ti after post-ECAP short-duration annealing revealed an excellent combination of high strength, good ductility and superior corrosion resistance for marine applications.

Quantitative characterization of TiO2 nanoparticle release from textiles by conventional and single particle ICP-MS

TiO2 is ubiquitously present in a wide range of everyday items, both as an intentionally incorporated additive and naturally occurring constituent. It can be found in a wide range of consumer products, including personal care products, food contact materials, and textiles. Normal use of these products may lead to consumer and/or environmental exposure to TiO2, possibly in form of nanoparticles. The aim of this study is to perform a leaching test and apply state-of-the-art methods to investigate nano-TiO2 and total Ti release from five types of commercially available conventional textiles: table placemats, wet wipes, microfiber cloths, and two types of baby bodysuits, with Ti contents ranging from 2.63 to 1448 μg/g. Released particle analysis was performed using conventional and single particle inductively coupled plasma mass spectrometry (ICP-MS and spICP-MS), in conjunction with transmission electron microscopy (TEM), to measure total and particulate TiO2 release by mass and particle number, as well as size distribution. Less than 1% of the initial Ti content was released over 24 h of leaching, with the highest releases reaching 3.13 μg/g. The fraction of nano-TiO2 released varied among fabric types and represented 0–80% of total TiO2 release. Particle mode sizes were 50–75 nm, and TEM imaging revealed particles in sizes of 80–200 nm. This study highlights the importance of using a multi-method approach to obtain quantitative release data that is able to provide an indication regarding particle number, size distribution, and mass concentration, all of which can help in understanding the fate and exposure of nanoparticles.

Metal injection moulding of non-spherical titanium powders: Processing, microstructure and mechanical properties

The critical processing parameters for metal injection moulding (MIM) of titanium using inexpensive non-spherical hydride-dehydride (HDH) powders with an average particle size of 45μm were established through a series of laboratory scale injection moulding investigations. A slow heating rate of 1.0K/min during thermal de-binding and sintering resulted in samples with negligible or limited distortion and nearly uniform shrinkage in all directions. Sintering at 1250°C for 120min was identified to be a suitable condition for MIM of HDH Ti, which produced samples with a relative density of 96.5%, uniform shrinkage of 13.9% along the length direction and 14.7% along the thickness direction. The as-sintered samples achieved a tensile strength of 395MPa and elongation of 12.5%. The promising tensile elongation indicates that MIM of HDH titanium powder can cope with the relatively high oxygen level in the initial inexpensive HDH Ti powder as well as the unavoidable further increase in oxygen level during the MIM cycle.

Influence of initial Ti particle size on microstructure and fracture toughness of reactive plasma sprayed TiCN coatings

TiCN coatings were fabricated by reactive plasma spraying (RPS) using Ti-graphite feedstocks with different initial Ti particle size. The microstructure and fracture toughness of the coatings were investigated. The results show that the coatings consist of TiCN, Ti2O and amorphous C and CNx phases. TiCN coatings present some partial-melted particles, un-melted particles, pores, voids and cracks. As the increase of the Ti particle size, the number of un-melted particles increases and the morphology changes from relatively flat to uneven. Compared with the coating sprayed with Ti particle size of 14μm or 48μm, the coating with Ti of 28μm has a higher strength-toughness. This is due to the relative more amounts of TiCN solid solution and close-grained continuous structure.

Interface effect on titanium distribution during Ti-doped sapphire crystals grown by the Kyropoulos method

Large ingot Ti-doped sapphire crystals were successfully grown by Kyropoulos method. Optical characterization of 10 cm diameter crystals shows non-uniform radial distribution of titanium. The measurements of Ti3+ ion distribution in several slices cut perpendicular to the growth direction show that the concentration is higher at the periphery of the crystal as compared to the central part of the ingot. Numerical modeling is applied to investigate heat transfer, melt convection and species transport during the Kyropoulos growth process. The transient simulation shows an unsteady convection generated by the strong interaction between the flow and the thermal field. The distribution of Ti in the melt is nearly uniform due to the intense convective mixing. The radial distribution of titanium depends mainly on the shape of the crystal-melt interface. The measured U-shaped concentration profile is explained by accounting the conical shape of the interface. High curvatures of the growth interface observed in experiments are explained by the increased thermal conductivity of the sapphire crystal, which is participating to the radiative heat transfer. The interface curvature depends on the absorption coefficient, which is higher for Ti-doped sapphire crystals than for undoped crystals. Therefore, the initial Ti concentration in the melt should be decreased in order to reduce the absorption coefficient and the interface curvature. The comparison of numerically computed titanium distribution in the crystals to experimental measurements shows a good agreement.

Evolution of microstructure and mechanical properties of Al-5 wt% Ti composite fabricated by P/M and hot extrusion: Effect of heat treatment

An Al-5wt% Ti composite was fabricated via powder metallurgy and hot extrusion from initial pure Al and Ti powders. The effect of subsequent heat treatment at 600°C for different time intervals on microstructure and mechanical properties was investigated. XRD and EDS analyses showed the in-situ formation of Al3Ti intermetallic. SEM micrographs demonstrated the formation of core-shell structured particles in Al matrix for composites heat treated up to moderate heating times (4h). Generation of voids and pores in the heat treated composites was considered as a result of the Kirkendall effect. Mechanical properties of composites were studied by hardness and tensile testing methods at room temperature. By increasing heating times up to 6h, hardness and tensile strength of the composites increased due to the rise in Al3Ti volume fraction. Heat treating more than 6h resulted in reduction of both hardness and strength of the composite. Ductility of Al-5wt% Ti composites declined with increasing thermal exposure duration. SEM imaging was used to study fracture surface of tensile test specimens. It was revealed that failure mechanism of composites changed from particle fracture in moderate heating times, to particle debonding and particle pull-out in long heat treatment time intervals.

Mechanical and biological behavior of ultrafine-grained Ti alloy aneurysm clip processed using high-pressure torsion

Severe plastic deformation (SPD) has recently been advanced as the main process for fabricating bulk ultrafine grained or nanocrystalline metallic materials, which present much higher strength and better bio-compatibility than coarse-grained counterparts. Medical devices, such as aneurysm clips and dental implants, require high mechanical and biological performance (e.g., stiffness, yield strength, fatigue resistance, and bio-compatibility). These requirements match well the characteristics of SPD-processed materials. Typical aneurysm clips are made of a commercial Ti-6Al-4V alloy, which has higher yield strength than Ti. In this work, Ti and Ti-6Al-4V workpieces were processed by high-pressure torsion (HPT) to enhance their mechanical properties. Tensile tests and hardness tests were performed to evaluate their mechanical properties, and their microstructure was investigated. The hardness and yield stress of the HPT-processed Ti are comparable to those of the initial Ti-6Al-4V due to significantly refined microstructure. Finite element analyses for evaluating the opening performance of a specific geometry of the YASARGIL aneurysm clip were carried out using mechanical properties of the initial and HPT-processed Ti and Ti-6Al-4V. These results indicate that SPD-processed Ti could be a good candidate to substitute for Ti-6Al-4V in aneurysm clips.

Transverse eV ion heating by random electric field fluctuations in the plasmasphere

Charged particle acceleration in the Earth inner magnetosphere is believed to be mainly due to the local resonant wave-particle interaction or particle transport processes. However, the Van Allen Probes have recently provided interesting evidence of a relatively slow transverse heating of eV ions at distances about 2–3 Earth radii during quiet times. Waves that are able to resonantly interact with such very cold ions are generally rare in this region of space, called the plasmasphere. Thus, non-resonant wave-particle interactions are expected to play an important role in the observed ion heating. We demonstrate that stochastic heating by random transverse electric field fluctuations of whistler (and possibly electromagnetic ion cyclotron) waves could explain this weak and slow transverse heating of H+ and O+ ions in the inner magnetosphere. The essential element of the proposed model of ion heating is the presence of trains of random whistler (hiss) wave packets, with significant amplitude modulations produced by strong wave damping, rapid wave growth, or a superposition of wave packets of different frequencies, phases, and amplitudes. Such characteristics correspond to measured characteristics of hiss waves in this region. Using test particle simulations with typical wave and plasma parameters, we demonstrate that the corresponding stochastic transverse ion heating reaches 0.07–0.2 eV/h for protons and 0.007–0.015 eV/h for O+ ions. This global temperature increase of the Maxwellian ion population from an initial Ti∼0.3 eV could potentially explain the observations.

Effect of Superheat and Oxide Inclusions on the Fluidity of A356 Alloy

The effect of melt superheat and oxide inclusions on the fluidity of a commercial A356 alloy has been investigated. Fluidity measurements have been performed by means of Archimedean spiral in sand moulds. The specific testing method and the experimental apparatus show a good reproducibility. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes and the amount of defects occurring at the tip of the spirals. The results reveal that oxide films increase the variability in the fluidity results obtained at the same apparent experimental conditions. A long permanence in the holding furnace and the introduction of some turbulence during sampling increase the oxide formation and entrapment in the molten bath, thus decreasing the repeatability of the fluidity results. The fluidity increases linearly with superheat and it extrapolates to zero at the temperature corresponding to a fraction solid of about 23%. The initial Ti content in the alloy produces an independent crystallization during freezing of the fluidity spirals.

Hard x-ray photoemission study of oxidation states of Ti underlayer in Fe/Ti film system

The Fe/Ti catalyst system that was recently found to be effective for the growth of dense carbon nanotube (CNT) forests was studied using hard x-ray photoemission spectroscopy (HAXPES). It was previously found that the Ti support layer was partially oxidized at room temperature (RT) and absorbed oxygen from the Fe overlayer at higher temperatures, which gave rise to the dense CNT forest growth. The aim of the present study was to elucidate the reason for the initial oxidation of the Ti layer at RT, which remained unclear from the results of the previous study. The control of the initial Ti oxidation is important because it could affect the reduction and activation of the Fe layer at higher temperatures. Depth-dependent HAXPES measurements using different x-ray incidence angles revealed that the degree of oxidation of the 1-nm-thick Ti layer in a sample that had been aged at RT for approximately three months varied depending on the depth. This suggests that oxidation of the Ti layer proceeded after the Fe/Ti film fabrication. Then, the effects of aging on the Ti oxidation states were examined. The Ti 2p3/2 spectrum measured for another sample that had been aged for approximately nine months showed a higher degree of oxidation of the Ti layer than that measured for the sample aged for three months. These results indicate that the Ti layers absorbed oxygen despite the existence of a 2-nm-thick Fe overlayer.

3D hierarchical macro/mesoporous TiO2 with nanoporous or nanotubular structures and their core/shell composites achieved by anodization

TiO2 is one of the most extensively investigated materials owing to its unique properties and several promising applications. The properties of TiO2 are highly dependent on its microstructure and feature size. Herein, we report a strategy for the synthesis of hierarchical porous TiO2 with two distinguishable scales of ligament sizes, which consists of both macro-pores and meso-pores. A metallic melt dealloying process with CuTi alloy was followed to form a porous Ti precursor. Subsequently, this precursor was used as a substrate for anodization to form TiO2. By controlling the anodization potential, time and initial porous Ti ligament size, nanotubular nanotubes or nanoporous TiO2 were distributed uniformly on the ligaments of porous Ti. Both the sizes of the macropore (400 nm to 2 μm) and mesopore (10 to 30 nm) can be tuned. A core–shell structured TiO2/Ti composite with a nanoporous or nanotubular structure in the shell and un-anodized porous Ti in the core can be achieved by partial anodization. The hierarchical structure with high surface area is essential for mass transport (large pore) and functionality (small pore), which makes it a potential candidate for various applications such as in sensors and catalysts.

EFFECT OF GRANULATION, MECHANICAL ACTIVATION, THERMOVACUUM TREATMENT AND AMBIENT GAS PRESSURE ON TI–0,5C SYSTEM SYNTHESIS REGULARITIES

This paper studies dependences of combustion velocities and changes in sample relative lengths from ambient pressure and thermovacuum degassing (TVD) time for the Ti + 0,5C mixture. It was shown that Ti + 0,5C mixture TVD significantly (doubly) increase combustion velocity as well as sample shrinkage. Sample shrinkage occurs under the influence of surface tension due the sufficient quantity of a liquid phase in combustion products that ensure their low viscosity. It was not possible before to obtain pressed samples from the mechanically activated (MA) Ti + C mixture because of their low strength, and therefore to investigate their combustion regularities. To solve this problem, the authors used preliminary granulation of the initial Ti + 0,5C mixture before MA. As a result, dependences of combustion velocity and post-combustion changes in sample lengths from MA time were obtained for the first time ever for the Ti + 0,5C preliminary granulated mixture. It was found that mechanical activation decreases mixture combustion velocity and significantly (triply) increases sample elongation after combustion. Combustion velocity after MA mixture TVD reaches combustion velocity of initial mixtures at the same TVD time. The samples of condensed reaction products from the MA mixture retained a slight elongation (within 8 %) after TVD. X-ray phase analysis revealed no reaction products formed during MA. Combustion products of all mixtures (initial, activated mixture and activated granules) contain the Ti2C phase and Ti traces. X-ray analysis of the initial and MA mixtures shows peaks intensity broadening and reduction of peak intensity to the background intensity ratio after MA thus indicating a higher defect rate in the crystal structure of the components of mixtures. This effect intensifies with an increase in activation time. The study results are interpreted based on the conduction – convection model of reaction wave propagation.

The effects of thickness of original Ti foils on the microstructures and mechanical properties of Ti2Ni/TiNi laminated composites

Ti2Ni/TiNi laminated composites with different TiNi volume fractions were fabricated by adjusting the thickness of initial Ti foils. The microstructure evolution was discussed. The compression properties and fracture toughness in different directions were systematically investigated. The results show that the formation of Ti2Ni/TiNi laminated composites experiences a complex diffusion process between Ti and Ni atoms. TiNi layers in the composites possess a complex monoclinic B19′-type crystal structure and Ti2Ni participates in the TiNi layers have a great effect on the ductility of the TiNi layers. The compression tests demonstrate that the compression strength and strain increase with the increase of the thickness of TiNi layers in both tested directions. The tests also reveal that whether the double yielding phenomenon can be obviously observed in compression curves is determined not only by martensitic re-orientation of TiNi layers, but also by the load distribution on different layers. The fracture toughness of the composites increases with increase of the thickness of TiNi layers in both orientations: in the divider orientation, it is related to the strength and ductility of TiNi layers; and in the arrester orientation, it is improved primarily through the mechanisms of crack bridging and deflection by TiNi layers.

A design spectrum model for flexible soil sites in regions of low-to-moderate seismicity

Design spectrum (DS) models in major codes of practice for structural design of buildings typically stipulate empirical site factors for each of the five, or six, site classes. Although the phenomenon of resonant like amplification behaviour of the structure caused by multiple wave reflections is well known, the potentials for such periodic amplification behaviour are not explicitly considered in code models. This is partly because of expert opinion that such effects are very “localised” in the frequency domain and can be suppressed readily by damping. However, investigations into the risk of collapse of non-ductile, and irregular structural systems, common in regions of low-to-moderate seismicity, revealed the extensive influence of periodic base excitations on flexible soil sites (with initial small-strain natural period Ti >0.5s). In this paper, an alternative DS model which addresses the important phenomenon of soil resonance without the need of computational site response analysis of the subsurface model of the site is introduced.

Effect of initial Ti powders size on the microstructures and mechanical properties of Al3Ti/2024 Al composites prepared by ultrasonic assisted in-situ casting

In this work, high-intensity ultrasonic vibration was employed in in-situ casting of aluminum matrix composites. In-situ Al3Ti/2024 Al composites with ideal microstructures and mechanical properties were successfully fabricated. The effect of Ti powder sizes on in-situ reaction, microstructures and mechanical properties of Al3Ti/Al composites were studied. The results show that the morphologies of Al3Ti particles and α-Al matrix are dominated by the original Ti powders. When the added Ti powders are too large (e.g. 60 μm and 40 μm) or too small (e.g. 10 μm), some agglomerated Al3Ti particles or blocks existed in the α-Al matrix. However, when the average size of added Ti powders is 20 μm, fine Al3Ti particles were formed and evenly distributed in the α-Al matrix. The refining mechanism of α-Al matrix was also discussed. The composites are significantly strengthened due to ultrasonic vibration and evenly distributed Al3Ti particles. The yield strength, ultimate tensile strength and elongation to fracture are 361 MPa, 449 MPa and 3.16%, respectively. The thermoplastic formability of in-situ Al3Ti/2024Al composites is similar with 2024 alloy.

Microstructure and mechanical properties of Al2O3/TiAl joints brazed with B powders reinforced Ag-Cu-Ti based composite fillers

In this manuscript, Al2O3 ceramic and TiAl alloy were brazed with Ag-Cu-Ti based composite fillers containing B powder additive. The effects of B content, Ti content, and the brazing temperature on the microstructure and mechanical properties of the brazed joints were investigated. The typical interfacial microstructure of the brazed joint was: Al2O3/Ti3(Cu, Al)3O/Ag(s.s)+TiB+Ti(Cu, Al)+fine AlCu2Ti/blocky AlCu2Ti+AlCuTi/TiAl. TiB whiskers synthesized in situ by the reaction of the B additive with Ti, served as nucleation sites for the formation of fine Ti(Cu, Al) and AlCu2Ti grains and improved the bonding properties. With increasing B content, more fine-grains were formed in the brazing seam and the thickness of the Ti3(Cu, Al)3O reaction layer adjacent to the Al2O3 ceramic decreased. Brazing parameters also influenced the joint microstructure. Growth of Ti3(Cu, Al)3O layer and AlCu2Ti formed with increase in brazing temperature. Moreover, the formation of dispersed fine-grains in the brazing seam depended largely on the initial Ti content in the composite filler. With increase in initial Ti content, the TiB-reinforced area was broadened while the blocky AlCu2Ti content decreased gradually. The maximum shear strength of Al2O3/TiAl joint brazed with B-reinforced composite filler was 148MPa, which was 246% higher than that brazed without B addition. The joint shear strength was determined by the combined influences of the thermal expansion mismatch between the joined materials, the thickness of the Ti3(Cu, Al)3O layer, and the presence of defects in the joints.

The use of titanium aluminides to form electric-spark coatings

Titanium aluminides (TiAl3, TiAl, Ti3Al) fabricated by powder metallurgy were used as alloying electrodes for the formation of electric-spark coatings. Intermetallic coatings were deposited on steel substrates in argon or nitrogen. The microstructure and composition of fabricated coatings were investigated by scanning electron microscopy, X-ray structural analysis, and electron probe microanalysis. It is established that initial Ti–Al intermetallic phases are present in fabricated coatings; however, the ratio between Ti and Al concentrations is shifted to aluminum compared with the stoichiometric one. When depositing titanium aluminide in the nitrogen medium, titanium nitride is additionally formed in surface layers. Thermal and tribotechnical tests showed that the Ti3Al coating deposited in nitrogen possesses high wear resistance and heat resistance.