Evidence of FCC titanium hydride formation in β titanium alloy: An X-ray diffraction study

This study has shown a complete phase transformation from the bcc β phase Ti-30 wt% Mo alloy to the fcc δ phase titanium hydride by thermal hydrogenation treatment. This fcc δ hydride has a lattice parameter of 0.4444 (± 0.0001) nm and acomposition of TiH_1.96 and it is very brittle. It appears that the types of hydride formed depend on the phase regime where thermal hydrogenation takes place, i.e., the fcc δ hydride forms from the β phase regime, the fct (c/a > 1) γ hydride forms from the α phase regime, and a mixture of both hydrides forms from the α + β phase regime.     

In situ formation of titanium carbide in titanium powder compacts by gas–solid reaction

Porous titanium compacts of fine and coarse sponge titanium powders were reacted with methane gas to produce Ti–TiC in situ composites. The kinetics of titanium carbide formation during the reaction were studied in relation to powder size, reaction temperature and time, and methane flow rate. The titanium carbide was initially formed as a layer around each titanium powder and the rate of formation was found to be diffusion-controlled. Titanium hydride was also formed during the reaction but was easily removed by post-vacuum annealing. A significant reduction of chlorine content in the compact also resulted during the processing. High temperature vacuum sintering could densify the reacted compacts to over 95% theoretical density and, at the same time, alter the layered titanium carbide phase into rounded particles. It was possible to produce fully dense titanium base composites containing up to 30 vol% by the present gas–solid reaction-based processing.     

Electrical conductivity and creep of polycrystalline α-Al2O3 doped with titanium or iron plus titanium

Creep and conductivity were measured for polycrystalline -Al₂O₃ doped with titanium or iron plus titanium. Both series of samples show a transition from acceptor domination to donor domination at the point where the concentrations of titanium and acceptors are approximately equal. Parameters in the expression for the creep rate S ⁿd-^PP^r _{O 2} exp (-QlkT) (whereS is the applied stress,d the grain size,Q an activation energy andn, p, r are constants characterizing the creep mechanism) indicate limitation by bulk diffusion in the acceptor-dominated samples and in the donor-dominated samples at low [Ti]. In the latter samples the ratelimiting species isO ⁱ at highP _{O 2}, VA ^Al at lowP _{O 2}, with involvement of e at mediumP _{O 2}. At higher titanium concentrations the creep rate is limited by generation/recombination of defects at grain boundaries. Iron appears to increase the rate of these grain-boundary reactions. When second-phase particles of Al₂TiO₅ are present, is decreased but ionic conductivity is increased.     

Mechanical synthesis of nanostructured titanium–nickel alloys

The methods of mechanically assisted synthesis and mechanical alloying are used to obtain nanostructured TiNi shape-memory alloys. A stoichiometric mixture of Ti and Ni powders was subjected to intense mechanical treatment in a planetary ball mill. It was shown that after 30 h milling, the synthesis of the product with a mean particle size of 20–30 nm proceeded at 550 °C. XRD data show mainly the presence of TiNi, Ti₂Ni and TiNi₃ phases. Prolongation of the milling process up to 40 h leads to direct synthesis of a product with a similar phase composition. SEM and TEM analyses are used to study morphological changes of reagent and product particles in the course of mechanical treatment and after the synthesis of products. The mechanochemical synthesis routes are compared with the traditional method of thermal synthesis. The advantages of the mechanochemical methods of synthesis of nanostructured products and obtaining Ti–Ni powders with a high sinterability are also discussed. It was shown that after cold pressing and sintering at 800 °C, compacts containing even distributed pores with a mean size of 1 μm were obtained. TiNi bodies with similar structural peculiarities are suitable for the purposes of implantology.     

Fabrication and properties of titanium–hydroxyapatite nanocomposites

Titanium–hydroxyapatite nanocomposites with different HA contents (3, 10, 20 vol%) were produced by the combination of mechanical alloying (MA) and powder metallurgical process. The structure, mechanical and corrosion properties of these materials were investigated. Microhardness test showed that the obtained material exhibits Vickers microhardness as high as 1030 and 1500 HV_0.2, which is more than 4–6 times higher than that of a conventional microcrystalline titanium. Titanium nanocomposite with 10 vol% of HA was more corrosion resistant (i_C = 1.19 × 10⁻⁷ A cm⁻², E_C = −0.41 V vs. SCE) than microcrystalline titanium (i_C = 1.31 × 10⁻⁵ A cm⁻², E_C = −0.36 V vs. SCE). Additionally, the electrochemical treatment in phosphoric acid electrolyte results in porous surface, attractive for tissue fixing and growth. Mechanical alloying and powder metallurgy process for the fabrication of titanium–ceramic nanocomposites with a unique microstructure are developed.     

Room temperature age-hardening in a β titanium alloy

A metastable titanium alloy containing 10 wt % Zr and 12 wt % V has been found to undergo a substantial age-hardening reaction at temperatures as low as 20° C. The reaction involves continued growth of athermal-phase particles produced during water quenching from the-phase field. The morphology of the as-quenched is retained, implying the absence of long-range diffusion during ageing: this is consistent with the low value of the activation energy measured (93 kJ kg mol^–1). It is suggested that the growth is caused by unpinning of/ interfaces as a result of the short-range motion of interstitials present in the alloy. The age-hardening produces a severe loss in tensile ductility and inhibition of stress-induced martensite formation.     

Corrosion behavior of β titanium alloys for biomedical applications

The corrosion behavior of biocompatible β titanium alloys Ti–13Mo–7Zr–3Fe (TMZF) and Ti–35Nb–7Zr–5Ta (TiOsteum) was investigated in 0.9% NaCl and 5 M HCl solutions. Extra-low-interstitial Ti–6Al–4V, which is also a candidate material for biomedical applications, was studied for comparison. The as-received TiOsteum and TMZF alloys exhibited single-phase β and α + β microstructures, respectively, so the latter was also investigated in the solutionized and quenched condition. In 0.9% NaCl solution, all three alloys exhibited spontaneous passivity and very low corrosion rates. Ti–6Al–4V and the as-received TMZF exhibited active-passive transitions in 5 M HCl whereas TiOsteum and TMZF in the metastable β condition showed spontaneous passivity. Potentiodynamic polarization tests, weight loss and immersion tests revealed that TiOsteum exhibited the best corrosion resistance in 5 M HCl. Analysis of surfaces of the corroded specimens indicated that the α/β phase boundaries were preferential sites for corrosion in Ti–6Al–4V while the β phase was preferentially attacked in the two-phase TMZF. The performance of the alloys in corrosive environment was discussed in terms of the volume fraction of the constituent phases and partitioning of alloying elements between these phases.     

On the massive transformation in γ-based titanium aluminides

-based titanium aluminides are candidate materials for several high temperature structural applications. The orientation relationship, substructure and the interfacial structure of the massive product: matrix interface has been examined by transmission electron microscopy (TEM). The interfacial structure consisted of faceted, macroscopically planar interfaces, which were found to consist of regular arrays of ledges, no interfacial dislocations were observed. The substructure revealed a high density of planar defects such as twins, stacking faults and antiphase domain boundaries, such defects were also found in the vicinity of the interface. In another alloy, which had very few defects within the massive product, the expected orientation relationship between the product and parent phases was established. The low value of ledge height to interplanar spacing can explain the fast growth kinetics. The implications of these results on the understanding of massive transformations are discussed.     

Hybrid titanium–CFRP laminates for high-performance bolted joints

This paper presents an experimental and numerical investigation of the mechanical response of bolted joints manufactured using new hybrid composite laminates based on the substitution of CFRP plies with titanium plies. The local hybridization of the material is proposed to increase the efficiency of the bolted joints in CFRP structures. Two modeling strategies, based on non-linear finite element methods, are proposed for the analysis of the bolt-bearing and transition regions of the hybrid laminates. The bolt-bearing region is simulated using a three-dimensional finite element model that predicts ply failure mechanisms, whereas the free-edge of the transition region is simulated using plane stress and cohesive elements. The numerical and experimental results indicate that the use of hybrid composites can drastically increase the strength of CFRP bolted joints and therefore increase the efficiency of this type of connection. In addition, the numerical models proposed are able to predict the failure mechanisms and the strength of hybrid composite laminates with a good accuracy.     

In vitro behavior of human osteoblast-like cells (SaOS2) cultured on surface modified titanium and titanium–zirconium alloy

In this study, titanium (Ti) and titanium–zirconium (TiZr) alloy samples fabricated through powder metallurgy were surface modified by alkali-heat treatment and calcium (Ca)-ion-deposition. The alteration of the surface morphology and the chemistry of the Ti and TiZr after surface modification were examined. The bioactivity of the Ti and TiZr alloys after the surface modification was demonstrated. Subsequently, the cytocompatibility of the surface modified Ti and TiZr was evaluated via in vitro cell culture using human osteoblast-like cells (SaOS2). The cellular attachment, adhesion and proliferation after cell culture for 14 days were characterized by scanning electron microscopy (SEM) and MTT assay. The relationship between surface morphology and chemical composition of the surface modified Ti and TiZr and cellular responses was investigated. Results indicated that the surface-modified Ti and TiZr alloys exhibited excellent in vitro cytocompatibility together with satisfactory bioactivity. Since osteoblast adhesion and proliferation are essential prerequisites for a successful implant in vivo, these results provide evidence that Ti and TiZr alloys after appropriate surface modification are promising biomaterials for hard tissue replacement.     

Mechanical properties of nickel–titanium foams for reconstructive orthopaedics

Porous NiTi materials were obtained in order to allow the ingrowth of living tissue, thus increasing the mechanical anchorage of implants. This promotes long-term fixation without the need for bone grafting. This study focuses mainly on the determination of mechanical properties: compression, bending and compressive fatigue. The foams obtained had an appropriate range of pore sizes and interconnectivity, which enabled a morphology similar to that of bone to be achieved. Their mechanical properties proved to be highly compatible with those of bone (an elastic modulus similar to that of cancellous bone), and the compressive fatigue limit was 7.5 MPa at 10⁷ cycles with samples exhibiting significant plastic deformation. SEM examination showed cracking at strut junctions 45° to the axis of the applied load. The results indicate that the material provides structural support while bone ingrowth is taking place.     

High-temperature strength and fracture toughness in γ-phase titanium aluminides

High-temperature strengths and fracture toughnesses of -phase titanium aluminides were estimated at room and elevated temperatures. The effects of chromium on these mechanical properties were investigated. It was found that addition of chromium substantially improved the room- and high-temperature strength and toughness of the binary titanium aluminides. The transition temperature at which the strength drops was found to increase due to the addition of chromium. The fracture behaviour of binary and chromium-alloyed titanium aluminides were investigated. The fracture mechanism was affected by the addition of chromium. Ductile tearing was observed for the ternary material at 800 °C, and this was delayed for the binary material.     

The effect of titanium additions onM s of β-CuZn

The effect of titanium additions to binary -CuZn alloys was investigated: the concentration range of at high temperatures (860° C), solid solution hardening of this phase, and the change in martensite temperature,M _s. Titanium in solution produces considerable solid solution hardening, both by replacing copper or zinc. Only replacement of zinc leads to a constant or increasingM _s, while replacing copper decreases it. Ageing of the -phase causes strong hardening and a decrease inM _s. The results have been interpreted considering the role of thermodynamic and mechanical properties in determiningM _s.     

High creep exponents in a nearly-lamellar γ-based titanium aluminide intermetallic

Secondary creep data are reported for an extruded nearly-lamellar Ti-48Al-1.5Cr-alloy tested in a temperature range of 700 to 900°C. Within this temperature regime, this alloy exhibits a two-stage creep deformation behavior, with relatively high (approximately 8–12) creep exponents occurring in the high stress/high temperature regime. The high exponents in this regime are explained by dynamic recrystallization phenomena observed ₂ + in the nearly-lamellar microstructure.     

Electrophoretic deposition of carbon nanotubes–hydroxyapatite nanocomposites on titanium substrate

Carbon nanotubes–hydroxyapatite (CNTs–HA) composites were synthesized, using an in situ chemical method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). HA particles were uniformly absorbed on the CNTs, with strong interfacial bonding. The CNTs–HA composites behaved like single composites when deposited on a titanium substrate by electrophoretic deposition (EPD). EPD was carried out at 10, 20 and 40 V, for 0.5 to 8 min at each voltage. Coating efficiency and weight increased with increasing deposition time, while the slope of the curves decreased, indicating a decrease in deposition rate. The CNTs–HA coating morphology was analyzed with scanning electron microscopy (SEM). The results revealed that decreasing the voltage used for deposition coatings could reduce cracking frequency. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies showed that the deposition coatings protected the titanium substrate from corroding in simulated body fluid (SBF). In addition, in vitro cellular responses to the CNTs–HA coatings were assessed to investigate the proliferation and morphology of osteoblast cell line.