Influence of noble metals alloying additions on the corrosion behaviour of titanium in a fluoride-containing environment

Titanium alloys exhibit excellent corrosion resistance in most aqueous media due to the formation of a stable oxide film, and some of these alloys (particularly Ti-6Al-7Nb) have been chosen for surgical and odontological implants for their resistance and biocompatibility. Treatment with fluorides (F⁻) is known to be the main method for preventing plaque formation and dental caries. Toothpastes, mouthwashes, and prophylactic gels can contain from 200 to 20,000 ppm F⁻ and can affect the corrosion behaviour of titanium alloy devices present in the oral cavity. In this work, the electrochemical corrosion behaviour of Ti-1M alloys (M = Ag, Au, Pd, Pt) was assessed in artificial saliva of pH = 3.0 containing 910 ppm F⁻ (0.05 M NaF) through open circuit potential, E_OC, and electrochemical impedance spectroscopy (EIS) measurements. The corrosion behaviour of the Ti-6Al-7Nb commercial alloy was also evaluated for comparison. E _OC measurements show an active behaviour for all the titanium alloys in fluoridated acidified saliva due to the presence of significant concentrations of HF and HF₂ ⁻ species that dissolve the spontaneous air-formed oxide film giving rise to surface activation. However, an increase in stability of the passive oxide layer and consequently a decrease in surface activation is observed for the Ti-1M alloys. This behaviour is confirmed by EIS measurements. In fact, the Ti-6Al-7Nb alloy exhibits lower impedance values as compared with Ti-1M alloys, the highest values being measured for the Ti-1Au alloy. The experimental results show that the corrosion resistance of the studied Ti-1M alloys is similar to or better than that of Ti-6Al-7Nb alloy currently used as biomaterial, suggesting their potential for dental applications.     

A study on the hot corrosion behavior of Ti-6Al-4V alloy

The titanium alloys are potential materials for high temperature applications in turbine components due to their very high temperature strength and lightweight properties. However, hot corrosion is a life-limiting factor when Ti alloys are exposed to different chemical environments at high temperature. In the present paper, hot corrosion behavior of Ti-6Al-4V (Ti-31) alloy in different salt environments viz. air, Na₂SO₄-60% V₂O₅ and Na₂SO₄-50% NaCl at 750 °C was studied. The parabolic rate constants were calculated for different environments from the thermo-gravimetric data obtained for the samples and they show that corrosion rate is minimum in air when compared to chemical environment. The scale formed on the samples upon hot corrosion was characterized by using X-ray diffraction (XRD), SEM, and EDAX analysis to understand the degradation mechanisms.     

Potentiodynamic behaviour of Ti alloys in physiological solution containing lubricant

Ti based alloys are finding wide spread usage in various biomedical applications including joint replacement. The corrosion behaviour of these alloys in simulated body fluid conditions in the presence of lubricant is not reported so far. Thus, present work is undertaken to understand the influence of the lubricant on potentiodynamic behaviour of three types of Ti alloy, namely commercially pure (C.P.) Ti having α structure, Ti-6Al-4Fe having α + β structure and Ti-13Ta-29Nb-4.6Zr having β structure in Ringer's solution. The results show that lubrication does not alter the corrosion rates of single-phase alloys, which have low corrosion rates. However, it significantly reduces the corrosion rates of alloy Ti-6Al-4Fe having α + ß structure.     

Anodic oxidation and stress corrosion cracking (SCC) of titanium alloys

Uniform and reproducible oxide films were formed on alloy Ti-6Al-6V-2.5Sn by anodic oxidation in aqueous 0.5% H₃BO₃, at 10 mA cm⁻² and voltages up to 110V; dielectric break down occurred above 120V. A parallelism was found between the effect of environmental factors on stress corrosion cracking (SCC), as reported in the literature and the anodic behaviour, as observed by ourselves: factors that increased susceptibility to SCC (increase in temperature or viscosity, alloying, introductions of CI⁻ or methanol, lowering of pH) reduced passivity under anodic polarization, while factors that inhibited SCC (thicker oxide films, high pH, phosphate ions) increased passivity. The passivity was associated both with the presence of the anodic oxide and with a transitory effect of the electric field across the oxide and the oxide-electrolyte interface.     

Superplastic deformation characteristics of two microduplex titanium alloys

A comparison of the superplastic deformation behaviour of Ti-6Al-4V (wt%) between 760 and 940‡ C and Ti-6Al-2Sn-4Zr-2Mo between 820 and 970‡ C has been carried out on sheet materials possessing similar as-received microstructures. High tensile elongations were obtained with maximum values being recorded at 880‡ C for Ti-6Al-4V (Ti-6/4) and at 940‡ C for Ti-6Al-2Sn-4Zr-2Mo (Ti-6/2/4/2), under which conditions both alloys possessed aΒ phase proportion of approximately 0.40. For a given deformation temperature the Ti-6/4 alloy had a slightly lower flow stress than the Ti-6/2/4/2, and this was attributed to the lowerΒ phase proportion in the latter alloy. However, at the respective optimum deformation temperatures the Ti-6/2/4/2 alloy had the lower flow stress. The results show that suitably processed Ti-6/2/4/2 alloy is capable of withstanding substantial superplastic strains at relatively low flow stresses, although the optimum deformation temperature is higher for this alloy than for Ti-6/4 material possessing a similar microstructure.     

Effects of TiO2/Al ratio on microstructures and mechanical properties of TiAl base composites

Effects of TiO₂/Al ratio on the microstructures and mechanical properties of in situ Al₂O₃/TiAl based composites were investigated. The results indicate that the as-sintered products consist of grains of nearly lamellar α₂ + γ structure with a dispersion of randomly oriented Al₂O₃ particles. A 43.9Ti-38.6Al-17.5TiO₂-nNb₂O₅ system was compared to 57.46Ti-36.78Al-5.76TiO₂-nNb₂O₅ system. The lamellar spacing of the products increases and the α₂ phase volume decreases with decreasing TiO₂/Al ratio. For each system, as the volume of α₂ phase increases, the average lamellar spacing decreases. Strength increases with an increasing TiO₂/Al ratio due to the amount of α₂ phase. Al₂O₃ phase increases with increasing TiO₂/Al ratio. Toughness increases with decreasing TiO₂/Al ratio. When the Nb₂O₅ content is smaller than 6 wt.%, the lamellar spacing plays an important role in toughness than the Al₂O₃ content. When the Nb₂O₅ content is larger than 6 wt.%, the Al₂O₃ content exhibits significantly increases the values of toughness than lamellar spacing.     

Nanoscale characterization of anodic oxide films on Ti-6Al-4V alloy

The paper analyses, at nanoscale levels, the chemical composition and mechanical properties of the anodic oxide films formed on Ti-6Al-4V alloy by galvanostatic polarization at maximum final voltages of 12-100 V. For the investigations Auger Electron Spectroscopy, Photoelectron Spectroscopy and nanoindentation measurements have been used. The results have shown that anodizing the Ti-6Al-4V alloy produces an oxide film whose thickness depends on the final voltage. The chemical composition is not significantly dependent on the thickness, the film consists of TiO₂ and Al₂O₃. However, the best insulating properties of the films, determined from the growth parameter nm/V, are achieved with a final voltage between 30 and 65 V. Nanohardness and Young's modulus measurements have shown that the anodic films formed by different voltages exhibit similar mechanical properties which is consistent with the results of the surface analysis.     

The microstructural observation and wettability study of brazing Ti-6Al-4V and 304 stainless steel using three braze alloys

Both Ti-6Al-4V and 304 stainless steels (304SS) are good engineering alloys and widely used in industry due to their excellent mechanical properties as well as corrosion resistance. Well-developed joining process can not only promote the application of these alloys, but also can provide designers versatile choices of alloys. Brazing is one of the most popular methods in joining dissimilar alloys. In this study, three-selected silver base filler alloys, including Braze 580, BAg-8 and Ticusil®, are used in vacuum brazing of 304SS and Ti-6Al-4V. Based upon dynamic sessile drop test, Braze 580 has the lowest brazing temperature of 840°C, in contrast to 870°C for BAg-8 and 900°C for Ticusil® braze alloy. No phase separation is observed for all brazes on 304SS substrate. However, phase separation is observed for all specimens brazed above 860°C on Ti-6Al-4V substrate. The continuous reaction layer between Braze 580 and 304SS is mainly comprised of Ti, Fe and Cu. The thickness of reaction layer at Braze 580/Ti-6Al-4V interface is much larger than that at Braze 580/304SS interface. Meanwhile, a continuous Cu-Sn-Ti ternary intermetallic compound is found at the Braze 580/Ti-6Al-4V interface. Both Ticusil® and BAg-8 brazed joint have similar interfacial microstructures. Different from the Braze 580 specimen, there is a thick Cu-Ti-Fe reaction layer in both BAg-8/304SS and Ticusil®/304SS interfaces. The formation of Cu-Ti-Fe interfacial layer can prohibit wetting of BAg-8 and Ticusil® molten brazes on 304SS substrate. Meanwhile, continuous Ti₂Cu and TiCu layers are observed in Ti-6Al-4V/BAg-8 and Ti-6Al-4V/Ticusil® interfaces.     

Phase,microstructural characterization and dielectric properties of Ca-substituted Sr<Subscript>5</Subscript>Nb<Subscript>4</Subscript>TiO<Subscript>17</Subscript> ceramics

The effect of Ca substitution for Sr on the phase, microstructure and microwave dielectric properties of the Sr_5−x Ca _x Nb₄TiO₁₇ composition series was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), an LCR meter, and vector network analyzer. Below 1450 °C, Sr_5−x Ca _x Nb₄TiO₁₇ (x = 1, 2, 3, or 4) compositions formed single-phase Sr₄CaNb₄TiO₁₇, Sr₃Ca₂Nb₄TiO₁₇, Sr₂Ca₃Nb₄TiO₁₇, and SrCa₄Nb₄TiO₁₇ ceramics, respectively. At x = 0 and 5, Sr₅Nb₄TiO₁₇ and Ca₅Nb₄TiO₁₇ formed, but along with Sr₂Nb₂O₇ (at x = 0) and CaNbO₃ and CaNb₂O₆ (at x = 5) secondary phases. Above 1450 °C, all the compositions formed two-phase ceramics. At low frequencies, a phase transition was observed in the composition Sr₅Nb₄TiO₁₇. The substitution of Ca for Sr enabled processing of highly dense Sr₂Ca₃Nb₄TiO₁₇, with ε_r ~ 53.4, τ_f ~ −6.5 ppm/°C and Q _u  × f _o  ~ 1166 GHz. Further investigations are required to improve the quality factor of these ceramics for possible microwave applications.     

Properties of Va metal-B films prepared by r.f.-sputtering

Nb_100−x B _x alloy films were prepared by the r.f.-sputtering method in the chemical composition range of 30 ≦x ≦ 76. Nb_100−x B _x (30 ≦x ≦ 54) films consisted of the amorphous state, and NbB₂ crystal phase was observed on Nb_100−xB_x (67 ≦x ≦ 76) films. A remarkable preferred orientation with the (001) plane of NbB₂ in parallel to the film surface was observed on Nb₃₃B₆₇ film. d.c. electrical conductivity of Nb_100−xB_x (30 ≦x ≦ 76) films decreased with increasing content of boron in the range from 7.3×10³ to 7.6 ×10²Ω⁻¹cm⁻¹. Micro-Vickers hardness of Nb_100−x B _x (30 ≦x ≦ 76) films exhibited the values of 1070 to 2060 kg mm².     

Densification behavior, microstructure, and electrical properties of sol–gel-derived niobium-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics

Superfine powders (~100 nm) with compositions of (1 − x)(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ + xNb⁵⁺ (BNTBT6 + xNb) with x from 0 to 0.02 were synthesized by an improved citrate sol–gel method using Nb₂O₅ as Nb⁵⁺ source. The sintering behavior, microstructure, and various electrical properties of BNTBT6 + xNb ceramics were investigated. The results indicated that the addition of a small amount of Nb⁵⁺ has no remarkable effect on the crystal structure and grain morphology, but the electrical properties of the ceramics are obviously influenced. The BNTBT6 + 0.005Nb compositions exhibit enhanced densification behavior and improved electrical properties of a piezoelectric constant d ₃₃ ~ 205 pC/N and a planar electromechanical coupling factor k _p ~ 33%.     

Fabrication of TiC/TiB/Ti3AlC2 phases reinforced coatings on Ti-6Al-4V substrate

Intermetallic matrix composite coatings reinforced by TiC, TiB₂, and Ti₃AlC₂ were fabricated by laser cladding the mixed power Ti, Al, and B₄C on the Ti-6Al-4V alloy. X-ray diffraction, scanning electron microscope, and energy dispersive spectroscopy were chosen to investigate the structures and morphologies of the coatings. Results showed that the coatings mainly consisted of the reinforcements of TiC, TiB₂, and Ti₃AlC₂ and the matrix of Ti₃Al, TiAl, TiAl₃, and α-Ti. The hardness and wear-resisting property of the prepared specimens of Ti-45Al-10B₄C and Ti-45Al-20B₄C were studied contrastively. It was found that the coating was metallurgical bonded to the Ti-6Al-4V substrate. The micro-hardness and dry sliding wear-resisting properties of the specimen of Ti-45Al-20B₄C were enhanced further. And the micro-hardness of Ti-45Al-20B₄C was from 900 HV_0.2 to 1225 HV_0.2. The wear-resisting property of Ti-45Al-20B₄C was four times as large as that of the Ti-6Al-4V alloys.     

Surface characteristics of a porous-surfaced Ti-6Al-4V implant fabricated by electro-discharge-compaction

Electro-discharge-compaction (EDC) is a unique method for producing porous-surfaced metallic implants. The objective of the present studies was to examine the surface characteristics of the Ti-6Al-4V implants formed by EDC. Porous-surfaced Ti-6Al-4V implants were produced by employing EDC using 480 F capacitance and 1.5 kJ input energy. X-ray photoelectron spectroscopy was used to study the surface characteristics of the implant materials. C, O, and Ti were the main constituents, with smaller amounts of Al and V. EDC Ti-6Al-4V also contained N. Titanium was present mainly in the forms of mixed oxides and small amounts of nitride and carbide were observed. Al was present in the form of aluminum oxide, while V in the implant surface did not contribute to the formation of the surface oxide film. The surface of conventionally prepared Ti-6Al-4V primarily consists of TiO₂, whereas, the surface of the EDC-fabricated Ti-6Al-4V consists of complex Ti and Al oxides as well as small amounts of titanium carbide and nitride components. However, preliminary studies indicated that the implant was biocompatible and supports rapid osseointegration.     

Corrosion behavior of biomedical Ti–24Nb–4Zr–8Sn alloy in different simulated body solutions

Corrosion behavior of a multifunctional biomedical titanium alloy Ti–24Nb–4Zr–8Sn (wt.%) in 0.9% NaCl, Hank's solution and artificial saliva at 37 °C was investigated using open circuit potential, impedance spectroscopy and potentiodynamic polarization techniques, and some results were compared with pure titanium and Ti–6Al–4V alloy. The results showed that the alloy exhibited good corrosion resistance due to the formation of a protective passive film consisting mainly of TiO₂ and Nb₂O₅, and a little of ZrO₂ and SnO₂. Ca ions were detected in the passive film as the alloy immersed in Hank′s and artificial saliva solutions and they have negative effect on corrosion resistance. The EIS results indicated that either a duplex film with an inner barrier layer and an outer porous layer or a single passive layer was formed on the surface, and they all transformed into stable bilayer structure as the immersion time increased up to 24 h. The polarization curves demonstrated that the alloy had a wider passive region than pure titanium and Ti–6Al–4V alloy and its corrosion current density (less than 0.1 μA/cm²) is comparable to that of pure titanium.     

Corrosion behavior in SBF for titania coatings on Mg–Ca alloy

TiO₂ coating was obtained by sol–gel method to improve the corrosion resistance of Mg–Ca alloy in human body environment. The corrosion behavior of Mg–1.0 Ca alloy with TiO₂ coating was investigated by electrochemical tests and immersion tests in simulated body fluid (SBF). Bare Mg–1.0 Ca alloy suffered serious attack after immersed in simulated body fluid only for 48 h. While for the Mg–1.0 Ca alloy with TiO₂ coating, the surface almost maintained intact with only several collapses after immersed in SBF for 168 h. The electrochemical test results showed that the free corrosion current (i _corr) of Mg–1.0 Ca alloy substrate was 3.3275e−2A/cm², while the i _corr of TiO₂ coating was only 1.58549e−5A/cm². Therefore, TiO₂ coating significantly improved the corrosion resistance of Mg–1.0 Ca alloy in SBF. This enhances the potential of Mg–Ca alloy used as biodegradable orthopedic material.     

Blistering in alloy Ti-6Al-4V from H<Superscript>+</Superscript> ion implantation

The effect of H⁺ ion implantation on surface morphology of the titanium alloy, Ti-6Al-4V, was studied, following H⁺ ion implantation of 150 keV and 250 keV energy to fluence of 2·6 × 10¹⁸ cm⁻² and 2·5 × 10¹⁹ cm⁻², respectively at ambient temperature. No detectable change was observed in surface features of either of the above specimen immediately after the implantation. However, vein like features (VLF) were observed to appear on the surface of the sample, implanted at 150 keV to a fluence of 2·6 × 10¹⁸ cm⁻², following natural ageing at room temperature for 150 days. Subsequent annealing of the above naturally aged sample, at 423 K for 150 min under vacuum (10⁻³ torr), led to development of a macroblister.     

Mechanism of surface modification of a porous-coated Ti-6Al-4V implant fabricated by electrical resistance sintering

A porous-coated Ti-6Al-4V implant was fabricated by electrical resistance sintering, using 480 F capacitance and 1.5 kJ input energy. X-ray photoelectron spectroscopy (XPS) was used to study the surface characteristics of the implant material before and after sintering. There were substantial differences in the content of O and N between as-received atomized Ti-6Al-4V powders and the sintered prototype implant, which indicates that electrical resistance sintering alters the surface composition of Ti-6Al-4V. Whereas the surface of atomized Ti-6Al-4V powders was primarily TiO₂, the surface of the implant consisted of a complex of titanium oxides as well as small amounts of titanium carbide and nitride. It is proposed that the electrical resistance sintering process consists of five stages: stage I – electronic breakdown of oxide film and heat accumulation at the metal-oxide interface; stage II – physical breakdown of oxide film; stage III – neck formation and neck growth; stage IV – oxidation, nitriding, and carburizing; and stage V – heat dissipation. The fourth stage, during which the alloy repassivates, is responsible for the altered surface composition of the implant.     

High temperature oxidation of Ti-48Al-8Cr-2Ag alloy with sputtered coating at 1000°C in air

Magnetron-sputter deposition was used to produce a Ti-48Al-8Cr-2Ag (at.%) coating on a cast alloy substrate with the same composition. The oxidation behaviour of the cast Ti-48Al-8Cr-2Ag alloy and its sputtered coating was investigated in air at 1000°C. The resulting scale structures were analyzed in great detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The nanocrystalline coating showed higher oxidation rate than the cast alloy, because an outer TiO₂ layer formed over a finegrain Al₂O₃ scale on the coating. The oxidation mechanism was discussed.     

Effects of Nb doping on microstructure and properties of Bi4Ti3?xNbxO12 thin films prepared by magnetron sputtering

Bi₄Ti_3−xNb_xO₁₂ ferroelectric thin films were fabricated on p-Si substrates by magnetron sputtering. The effects of Nb doping on microstructure and properties of Bi₄Ti_3−xNb_xO₁₂ films were investigated. Bi₄Ti_3−xNb_xO₁₂ films had the same structure as Bi₄Ti₃O₁₂ with smaller and more uniform grains. The dielectric and ferroelectric properties of Bi₄Ti_3−xNb_xO₁₂ films were improved by Nb doping. Bi₄Ti_3−xNb_xO₁₂ films have better dielectric and ferroelectric properties with P _r = 16.5 μC/cm², E _C < 100 kV/cm, ε _r > 290, low dielectric loss (<0.9%) and clockwise C–V curves with a memory window of 0.9 V when x = 0.03–0.045, while an excessive Nb doping would lead to bad dielectric and ferroelectric properties.     

Effect of thermal treatments on tensile strength of commercially cast pure titanium and Ti-6Al-4V alloys

Heating titanium structures is assumed to relieve tensions induced by the casting process as well as possibly optimizing some mechanical properties. The aim of this investigation was to evaluate the effect of thermal treatments on tensile strength of commercially pure titanium (CP Ti) and Ti-6Al-4V alloy. Thirty dumbbell rods, with diameters of 3.0 mm at the central segment and lengths of 42 mm, were cast for each metal using the Rematitan System. CP Ti and Ti-6Al-4V specimens were randomly divided into three groups of ten: a control group that received no thermal treatment and two test groups. One (T1) was heated at 750 ^∘C for 2 h and the other (T2) was annealed at 955 ^∘C for 1 h and aged at 620 ^∘C for 2 h. Tensile strength was measured with a universal testing machine (MTS model 810). Tensile strength means and standard deviations were statistically compared using a Kruskal-Wallis test at a α = 0.05 significance level. No statistically significant differences in tensile strength were observed among CP Ti groups. For the Ti-6Al-4V alloy, the control and T1 groups revealed statistically higher tensile strengths when compared to the T2 group, with no significant difference between the control and T1 groups.