Effect of preliminary strain hardening on the flow stress of titanium and a titanium alloy during shock compression

The effect of preliminary strain hardening of VT1-0 titanium and a Ti-6 wt % Al-4 wt % V alloy on their mechanical properties under quasi-static and high-rate (τ;10⁵ s^?1) loading is studied. Preliminary hardening is accomplished using equal-channel angular pressing (which results in a significant decrease in the grain size and a twofold increase in the quasi-static yield strength) and shock waves. High-rate deformation is attained via shock-wave loading of samples. The experimental results show that structural defects weaken the dependence of the yield strength on the strain rate. The difference in the rate dependences can be so high that the effect of these defects on the flow stress can change sign when going from quasi-static to high-rate loading.     

Formation of titanium carbide layer by laser alloying with a light-transmitting resin

The weight reduction of mechanical components is becoming increasingly important, especially in the transportation industry, as fuel efficiency continues to improve. Titanium and titanium alloys are recognized for their outstanding potential as lightweight materials with high specific strength. Yet they also have poor tribological properties that preclude their use for sliding parts. Improved tribological properties of titanium would expand the application of titanium into different fields.Laser alloying is an effective process for improving surface properties such as wear resistance. The process has numerous advantages over conventional surface modification techniques. Many researchers have reported the usefulness of laser alloying as a technique to improve the wear resistance of titanium. The process has an important flaw, however, as defects such as cracks or voids tend to appear in the laser-alloyed zone.Our group performed a novel laser-alloying process using a light-transmitting resin as a source for the carbon element. We laser alloyed a surface layer of pure titanium pre-coated with polymethyl methacrylate (PMMA) and investigated the microstructure and wear properties. A laser-alloyed zone was formed by a reaction between the molten titanium and thermal decomposition products of PMMA at the interface between the substrate and PMMA. The cracks could be eliminated from the laser-alloyed zone by optimizing the laser alloying conditions. The surface of the laser-alloyed zone was covered with a titanium carbide layer and exhibited a superior sliding property and wear resistance against WC-Co.     

Optical characteristics of titanium with a natural surface layer

We discuss the solutions of the inverse problem of multiangle ellipsometry for titanium with a natural surface layer, namely, the determination of the thickness of the near-surface layer and optical characteristics by use of which one can interpret the titanium structure. It turned out that the studied samples of pure polycrystalline titanium have a rather thick inhomogeneous layer of polycrystalline titanium with oxygen and, on this layer, there is a surface layer similar to titanium oxide. The data on the optical characteristics of pure polycrystalline titanium agree well with those determined earlier by other optical methods.     

Field emission properties of electrophoretic deposition carbon nanotubes film

The field emission properties of electrophoretic deposition(EPD) carbon nanotubes (CNTs) film have been improved by depositing CNTs onto the titanium (Ti)-coated Si substrate, followed by vacuum annealing at 900 °C for 2 h, and the enhanced emission mechanism has been studied using X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. Field emission measurements showed that the threshold electric field was decreased and the emission current stability was improved compared to that of EPD CNTs film on bare Si substrate. XRD and Raman spectroscopy investigations revealed that vacuum annealing treatment not only decreased the structural defects of CNTs but made a titanium carbide interfacial layer formed between CNTs and substrate. The field emission enhancement could be attributed to the improved graphitization of CNTs and the improved contact properties between CNTs and substrate including electrical conductivity and adhesive strength due to the formed conductive titanium carbide.     

The role of curvature of the crystal structure in the formation of micropores and crack development under fatigue fracture of commercial titanium

The multiscale mechanism of fatigue fracture of titanium with the surface layer hydrogenated under alternating bending at room temperature is studied. It is shown that the generation of the fatigue fracture occurs in the surface layer subjected to plastic deformation in conjunction with an elastically loaded substrate. The latter causes the appearance of a strong curvature of the material and the appearance of micropores in these areas along with any fatigue cracks. The emergence of the local curvature of the crystal structure plays a central role in the origin and the development of the fatigue fracture as the structural phase decomposition of the material under cyclic loading.     

Surface hardening of titanium by pulsed Nd:YAG laser irradiation at 1064- and 532-nm wavelengths in nitrogen atmosphere

The surface hardness of titanium modified by laser irradiation at different wavelengths in nitrogen atmosphere was investigated. Further, surface characteristics such as morphology, chemical state, and chemical composition in the depth direction were also studied. The size and depth of the craters observed in the laser-irradiated spots increased monotonically with an increase in the laser power. Furthermore, the crater formed by the 532-nm laser was deeper than that formed by the 1064-nm laser for the same laser power. Laser power beyond a certain threshold value was required to obtain a titanium nitride layer. When the laser power exceeds the threshold value, a titanium nitride layer of a few tens of nanometers in thickness was formed on the substrate, whereas a titanium oxide layer containing small amounts of nitrogen was formed when the laser power is below the threshold value. Thus, it was shown that laser irradiation using appropriate laser parameters can successfully harden a titanium substrate, and the actual hardness of the titanium nitride layer, measured by nanoindentation, was approximately five times that of an untreated titanium surface.     

Propagation of Cellular Modes of Titanium-Powder Layer Combustion in Air Channels,Allowing for the Effect of Natural Convection of Gas

In this paper, we have studied the effect of impurity and inert gases on the formation and propagation of cellular-combustion regimes for their inhomogeneous distribution above the surface of the reacting metal layer. The gas-dynamic aspects of the formation and steady propagation of inhomogeneous wave structures in the combustion of a titanium powder layer in through and semi-closed inclined air canals and in channels with uneven loading are considered. The gas composition heterogeneity over the reaction zone and the gas stratification, i.e., the stratification of a gas mixture of different densiies above the reacting layer, are shown to lead to the formation of inclined non-uniform and cellular fronts under conditions of a lack of active gas in the reaction zone and the loss of stability of the planar front.     

Fretting wear behavior of laser-nitrided Ti–5Al–5Mo–5V–1Cr–1Fe alloy fabricated by laser melting deposition

Fretting wear behavior of laser-nitrided titanium alloy (Ti–5Al–5Mo–5V–1Cr–1Fe) fabricated by laser melting deposition (LMD) has been investigated to explore surface engineering for protection against wear damage of laser melting deposited titanium alloy. The morphology and volume of the wear scars of unmodified and laser-nitrided LMD Ti–5Al–5Mo–5V–1Cr–1Fe tested at different frequencies, 10 and 50 Hz, were studied using non-contact three-dimensional surface profilometer and scanning electron microscope. Friction coefficients measured at different frequencies or loading forces were compared for unmodified and laser-nitrided LMD specimens. Experimental results show that laser-nitrided LMD specimens have shown fretting resistance superior to unmodified LMD specimens due to the presence of hard TiN dendrites in the laser-nitrided layer. W-shaped wear scar caused by local rotation of fretting ball at the two ends of the scar was observed. Given a constant loading force of 50 N, unmodified and laser-nitrided LMD specimens exhibited similar friction coefficients and their friction coefficients increased with test frequency. The friction coefficients of both specimens increased with the reduction of normal load, which corresponds to the trend in Hertzian contact model.     

Structural characteristics of 2024 aluminum alloy plasma-based ion implanted with nitrogen then titanium

As an interlayer in the gradient layers such as AlN/Ti/TiN/DLC prepared by plasma-based ion implantation (PBII) on 2024 aluminum alloy, titanium layer plays an important role in enhancing adhesion, reducing thermal stress, limiting the crack propagation, etc. A series of dual-layers prepared by PBII with nitrogen then titanium at various sputtering currents of titanium target on 2024 aluminum alloy have been reported in this paper. The composition distributions and the chemical states are analyzed using X-ray photoelectron spectroscopy (XPS). The structures are studied with grazing X-ray diffraction (GXRD). The results show that PBII with titanium strongly depends on the sputtering current. It is found that there exists a critical sputtering current corresponding only to a titanium-implanted layer containing TiAl₃. When the sputtering current exceeds the critical value, a titanium-deposited layer rich in -Ti is formed on a titanium-implanted layer. By controlling the sputtering current an appropriate titanium interlayer can be prepared to meet the requirement of forming a proper gradient layer.     

Effect of nano-titanium hydride on formation of multi-nanoporous TiO2 film on Ti

The effect of titanium hydride on the formation of nanoporous TiO₂ on Ti during anodization has been investigated by X-ray photoelectron spectroscopy, grazing incident X-ray diffraction, transmission electron microscopy and scanning electron microscopy. Titanium hydride (TiH₂) was formed after cathodization, profoundly impacting the formation of nanoporous TiO₂ on Ti by anodization. Oxide layer and nanocrystal structure were observed after anodization with cathodic pretreatments. A multi-nanoporous TiO₂ layer was formed on the titanium. The titanium hydride is a nanostructure. The nanostructure is directly changed to nanoporous TiO₂ by a dissolution reaction during anodization. The nanoporous layer is difficult to form without cathodization. The nanostructural TiH₂ is important in forming a nanoporous TiO₂ layer. Anodization treatment with cathodic pretreatment not only yields a titanium surface with a multi-nanostructure, but also transforms the titanium surface into a nanostructured titanium oxide surface.     

Skin electric explosion in double-layer conductors with a low-conductivity deposited layer

The experiments on explosion of cylindrical conductors aimed at comparison of plasma formation during skin explosion of homogeneous and double-layer conductors with an external layer with a lower conductivity are carried out on a high-current MIG generator (current amplitude up to 2.5 MA and current rise time 100 ns). The generator is loaded with cylindrical copper conductors with a diameter of 3 mm on the cathode part of which a titanium layer of thickness 20, 50, and 80 μm is deposited in vacuum. This type of loading makes it possible to compare the behaviors of the homogeneous and double-layer conductors in identical conditions. It is shown that using the double-layer structure of the conductor with an external layer of thickness 20–80 μm with a lower conductivity, which is obtained by vacuum arc deposition, higher values of magnetic induction (as compared to homogeneous conductor) can be attained on its surface prior to plasma formation and spread.     

Effect of pulsed laser parameters on in-situ TiC synthesis in laser surface treatment

Commercial titanium sheets pre-coated with 300-μm thick graphite layer were treated by employing a pulsed Nd:YAG laser in order to enhance surface properties such as wear and erosion resistance. Laser in-situ alloying method produced a composite layer by melting the titanium substrate and dissolution of graphite in the melt pool. Correlations between pulsed laser parameters, microstructure and microhardness of the synthesized composite coatings were investigated. Effects of pulse duration and overlapping factor on the microstructure and hardness of the alloyed layer were deduced from Vickers micro-indentation tests, XRD, SEM and metallographic analyses of cross sections of the generated layer. Results show that the composite cladding layer was constituted with TiC intermetallic phase between the titanium matrix in particle and dendrite forms. The dendritic morphology of composite layer was changed to cellular grain structure by increasing laser pulse duration and irradiated energy. High values of the measured hardness indicate that deposited titanium carbide increases in the conditions with more pulse duration and low process speed. This occurs due to more dissolution of carbon into liquid Ti by heat input increasing and positive influence of the Marangoni flow in the melted zone.     

Microstructure and oxidation of hot-dip aluminized titanium at high temperature

High-temperature diffusion of a hot-dip aluminized titanium is conducted to study microstructure changes and oxidation behavior of the aluminized titanium. After aluminizing, the titanium substrate is covered by a black layer in which tiny block-shaped TiAl₃ particles are scattered in aluminum matrix. Based on the diffusion experiment results, the thickness of the aluminum diffusion layer at 800 °C increases with diffusion time. However, the aluminum diffusion layer at 900 °C grows and reaches its maximum thickness in 6 h, and then the thickness of the aluminum diffusion layer is reduced with prolonged diffusion time. An inversion of the diffusion layer thickness versus time appears for the aluminized titanium treated at 1000 °C, and the thickness of the diffusion layer keeps declining with diffusion time. The phases present in the outer and middle sublayers are titanium-rich TiAl₃ and equilibrium TiAl₃, respectively. However, the phase in inner sublayer changes from titanium-rich TiAl₃ to TiAl₂ and TiAl as diffusion temperature and time increase. Through energy-dispersive X-ray and X-ray diffraction analysis, the oxides formed in the oxidation process are Al₂O₃ and Al₂TiO₅. Although the oxide scale formed on the surface of the aluminized titanium has an insufficient stability and integrity, the thermal oxidation resistance of the aluminized titanium is still improved by over 5 times compared with that of the pure titanium.     

The structure and properties of titanium oxide nanoheterogeneous films doped with molybdenum oxide

Thin films of titanium oxide nanoparticles doped with molybdenum oxide were synthesized by vacuum cocondensation of titanium atom and molybdenum oxide molecule flows onto a substrate cooled with liquid nitrogen. Atomic-force, scanning tunnel, and scanning electron microscopy were used to study the structure and composition of the films. It was shown that doping caused the formation of a large number of point defects in the titanium oxide lattice and an increase in its specific conductivity. Point defects were three-dimensional F centers and surface hydrate complexes. Tests of films as photoanodes in a photoelectrochemical cell showed that doping of titanium oxide with molybdenum oxide offered promise for increasing the effectiveness of sources for the decentralized production of hydrogen by electrochemical methods.     

Formation and study of electrospark coatings based on titanium aluminides

Coatings containing Ti-Al intermetallics are fabricated by the electrospark deposition of titanium on aluminum and aluminum on titanium. The microstructure and composition of the grown coatings is studied by scanning electron microscopy, X-ray diffraction, and S-ray microanalysis. It is found that the surface layer formed in argon mostly contains the α-TiAl₃ intermetallic independent of the duration and frequency of discharge pulses. The γ-TiAl and α₂-Ti₃Al phases can be obtained by aluminum deposition on titanium followed by the subsequent deposition of a second titanium layer. Aluminum oxide and titanium nitride are additionally formed during the deposition of electrospark coatings in air.     

Defect structure and thermomechanical stability of nano- and microcrystalline titanium obtained by different methods of intense plastic deformation

Mechanical stability under prolonged loading and thermostability under annealing have been studied for nano- and microcrystalline titanium obtained by different methods of intense plastic deformation. The effect of nanoporosity and the fraction of high angle boundaries formed due to intense plastic deformation has been revealed and analyzed. It has been established that, depending on the loading or the annealing temperature, thermomechanical stability of titanium can be affected, apart from the above structural characteristics, by either twin grain boundaries or titanium-carbide disperse particles.     

Growth of negative filamentary crystals during spark cutting of silicon carbide

Experiments are described which show that under pulsed thermal loading conditions, a damaged layer is formed in SiC which inherits the typical erosion defects (craters, chips, microcracks). Zh. Tekh. Fiz. 68, 133–135 (July 1998)     

Spectra of optical characteristics of titanium with a binary surface layer

The solution of the inverse problem of spectral ellipsometry is obtained for titanium with a surface layer of titanium oxide that was formed as a result of thermal (450°C) oxidation of the metal substrate. Using the Drude-Zener and Cauchy dispersion models, it is shown that a rather thick (46 nm) inhomogeneous binary surface layer including about 23% of titanium and 77% of titanium oxide is present on the studied samples of pure polycrystalline titanium. A natural weakly absorbing surface layer with a thickness of 13 nm is on this layer.     

Characterization of self-assembled decyl bis phosphonate-Collagen layers on titanium by QCM-D and osteoblast-compatibility

Quartz crystal microbalance dissipation (QCM-D) was used to monitor the layer by layer (LBL) self-assemble process of decyl bis phosphonate (DBP) and Collagen on titanium. The mass and viscoelastic properties of self-assembled layers were obtained using QCM-D. The stability of DBP and Collagen layer on titanium was tested to be very good. Osteoblasts cell culture was performed on LBL modified samples and that after BSA adsorption. The morphology of cells was observed by a scanning electron microscope (SEM). The total metabolic activity and differentiation of osteoblasts were evaluated by a metabolic assay and alkaline phosphatase (ALP) activity, respectively. These tests showed that osteoblasts have better activity, proliferation, and differentiation on Collagen terminated samples and BSA adsorbed samples.These results, along with the good biomineralization and protein adsorption abilities of Ti/DBP/Collagen surface (tested in our previous work), suggest titanium modified by this layer by layer technique has the potential application for medical implants.     

Strain Localization in Titanium with a Modified Surface Layer

Plastic flow localization in commercially pure titanium (VT1-0 according to the Russian classification) with the surface modified by low-energy high current electron beams has been numerically studied. The structure and mechanical properties of the modified surface layer and titanium substrate correspond to the experimentally observed ones and are taken into account explicitly as initial data of a dynamic boundary value problem. The tension of titanium structures with a modified surface layer is simulated by the finite difference method in a plane strain formulation. The dependence of the plastic strain localization parameters on the mechanical properties of structural elements in the titanium substrate has been determined.