The characterization of shape memory effect for low elastic modulus biomedical β-type titanium alloy

This work investigates the textures of biomedical TiNbTaZr alloy rolled by 99% cold reduction ratios in thickness. The relationship between textures and superelasticity of the specimens treated at 873 K and 1223 K for 1.2 ks is studied. The microstructure of tensile specimen is investigated by transmission electron microscopy. Textures of cold-rolled and heat-treated specimens are studied. During unloading, the anisotropy of superelastic strain and pure elastic strain in the heat-treated specimens is observed. Superelastic strain along rolling direction and transverse direction is larger than those along 45° from rolling direction while pure elastic strain shows the highest value along 45° from rolling direction in the specimen treated at 873 K. For the specimen treated at 1223 K, higher pure elastic strain is obtained along rolling direction. The maximum recovered strain around 2.11% is obtained along rolling direction.     

Improvement in fatigue strength while keeping low Young's modulus of a β-type titanium alloy through yttrium oxide dispersion

Improvement in fatigue strength in spite of maintaining low Young's modulus was achieved in Ti-29Nb-13Ta-4.6Zr (TNTZ) by hard-particles dispersion. A certain amount of Y₂O₃ additions was added into TNTZ. TNTZ with 0.05-1.00mass%Y consists of a β-phase with a small amount of Y₂O₃. Young's moduli of TNTZ with 0.05-1.00mass%Y are maintained low, and are almost similar to that of TNTZ without Y₂O₃. The tensile strength of TNTZ with 0.05-1.00mass%Y is slightly improved and the elongation does not deteriorate by Y₂O₃ additions. However, the 0.2% proof stress decreases with the increase in Y concentration. Although tensile properties are not changed drastically, the fatigue strength is significantly improved by Y₂O₃ additions. The dispersion of Y₂O₃ particle increases the resistance to fatigue initiation. However, Y₂O₃ with too large diameter at the surface of the specimen works harmfully as the fatigue initiation site. The Y₂O₃ diameter and volume fraction increase with the increase in Y concentration. As a result, the fatigue limit of the alloys with 0.05-1.00mass%Y firstly increases and then decreases with the increase in Y concentration. TNTZ with 0.1mass% Y exhibits the best combination of higher fatigue strength and low Young's modulus.     

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.     

Corrosion behaviour of β-Ti20Mo alloy in artificial saliva

To evaluate the potential of β-Ti20Mo alloy as a dental material, we tested its corrosion behaviour in artificial saliva in comparison to that of cp-Ti. Open-circuit potential (E_OC), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used as electrochemical methods to characterize the corrosion behaviour of Ti20Mo alloy and cp-Ti, respectively. Corrosion current and passive current densities obtained from the polarization curves showed low values indicating a typical passive behaviour for Ti20Mo alloy. The EIS technique enabled us to study the nature of the passive film formed on the binary Ti20Mo alloy at various imposed potentials. The Bode phase spectra obtained for Ti20Mo alloy in artificial saliva exhibited two-time constants at higher potential (0.5 V, 1.0 V), indicating a two-layer structure. According to our experimental measurements, Ti20Mo alloy appears to possess superior corrosion resistance to that of cp-Ti in artificial saliva.     

Povidone–iodine as a corrosion inhibitor towards a low modulus beta Ti-45Nb implant alloy in a simulated body fluid

Povidone-iodine and various bactericidal agents used in dental procedures may affect the corrosion response of an implant/prosthesis in the oral environment. The effect of various concentrations of povidone–iodine (PI) on the corrosion behavior of a low modulus beta titanium alloy, Ti-45Nb, has been investigated in normal saline solution. The open circuit potential, electrochemical impedance spectroscopy and potentiodynamic polarization measurements have been used to assess the electrochemical response of the alloy surface on PI addition so as to effectively predict the prosthetic treatment outcome. As the concentration of PI is increased, the corrosion rate decreases, suggested by decreased R_p values. Povidone–iodine acts as an anodic inhibitor by adsorbing on the anodic sites of the alloy. Addition of PI to a simulated body fluid such as normal saline leads to a decrease in corrosion rate of Ti-45Nb alloy.     

Biocompatibility evaluation of a novel hydroxyapatite-polymer coating for medical implants (in vitro tests)

Nanocomposites consisting of hydroxyapatite (HA) and a sodium maleate copolymer (maleic polyelectrolyte), synthesized by hydrothermal method and deposited on titanium substrates by Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique were tested for the biological properties. Coating bioanalysis was carried out by triple staining of actin, microtubules and nuclei followed by immunofluorescence microscopy. Within 24 h cells that occupied the biomaterial surface displayed the morphology and cytoskeleton pattern similar to the controls. Cells grown on nanocomposite coated surfaces had a higher proliferation rate than their counterparts grown on Ti coated with HA alone, indicating that maleic polyelectrolyte improved surface bio-adhesive characteristics. The capacity to induce cell attachment, spreading and proliferation demonstrated the potential of Ti coated with HA-polymer nanocomposites to be used as scaffolds in dental or orthopedic implantology.     

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.     

Osseointegration behavior of novel Ti–Nb–Zr–Ta–Si alloy for dental implants: an in vivo study

This study aimed to evaluate the effects of Ti–Nb–Zr–Ta–Si alloy implants on mineral apposition rate and new BIC contact in rabbits. Twelve Ti–Nb–Zr–Ta–Si alloy implants were fabricated and placed into the right femur sites in six rabbits, and commercially pure titanium implants were used as controls in the left femur. Tetracycline and alizarin red were administered 3 weeks and 1 week before euthanization, respectively. At 4 weeks and 8 weeks after implantation, animals were euthanized, respectively. Surface characterization and implant-bone contact surface analysis were performed by using a scanning electron microscope and an energy dispersive X-ray detector. Mineral apposition rate was evaluated using a confocal laser scanning microscope. Toluidine blue staining was performed on undecalcified sections for histology and histomorphology evaluation. Scanning electron microscope and histomorphology observation revealed a direct contact between implants and bone of all groups. After a healing period of 4 weeks, Ti–Nb–Zr–Ta–Si alloy implants showed significantly higher mineral apposition rate compared to commercially pure titanium implants (P?<?0.05), whereas there was no significant difference between Ti–Nb–Zr–Ta–Si alloy implants and commercially pure titanium implants (P?>?0.05) at 8 weeks. No significant difference of bone-to-implant contact was observed between Ti–Nb–Zr–Ta–Si alloy implants and commercially pure titanium implants implants after a healing period of 4 weeks and 8 weeks. This study showed that Ti–Nb–Zr–Ta–Si alloy implants could establish a close direct contact comparedto commercially pure titanium implants implants, improved mineral matrix apposition rate, and may someday be an alternative as a material for dental implants.     

Corrosion behavior of Ti60 alloy under continuous NaCl solution spraying at 600 ℃

The corrosion behavior of Ti60 alloy was investigated under continuous NaCl solution spraying at 600 ℃.Results indicate that the corrosion rate of the Ti60 alloy is lower than that obtained with the solid NaCl deposit film in H₂O + O₂. The outer corrosion products are compact layers mainly containing Na₂TiO₃ and nanocrystalline TiO₂. The inner layer is mainly composed of Ti₂O, Ti O and SnO₂. Ti₂O has a lamell...     

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.     

Fretting–fatigue behavior of steel wires in low cycle fatigue

The effect of strain amplitude on fretting–fatigue behavior of steel wires in low cycle fatigue was investigated using a fretting–fatigue test rig which was capable of applying a constant normal contact load. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus displacement amplitude. The variations of the normalized tangential force with increasing cycle numbers and fretting–fatigue lives at different strain amplitudes were explored. The morphologies of fretting contact scars after fretting–fatigue tests were observed by scanning electron microscopy and optical microscopy to examine the failure mechanisms of steel wires. The acoustic emission technique was used to characterize the fretting–fatigue damage in the fretting–fatigue test. The results show that the fretting regimes are all located in mixed fretting regimes at different strain amplitudes. The increase in strain amplitude increases the normalized tangential force and decreases the fretting fatigue life. The abrasive wear, adhesive wear and fatigue wear are main wear mechanisms for all fretting–fatigue tests at different strain amplitudes. The accumulative total acoustic emission events during fretting–fatigue until fracture of the tensile steel wire decrease with increasing strain amplitude. An increase of the strain amplitude results in the accelerated crack nucleation and propagation and thereby the decreased life.     

Mechanical behavior of Au–In intermetallics for low temperature solder diffusion bonding

In this study, gold (Au)–indium (In) intermetallic compounds (IMCs) formation for low temperature solder bonding was investigated by imbedding a gold wire into the annealing indium solder. According to available research on liquid–solid reaction of gold and indium, experiments were only conducted at an annealing temperature in the range of 200–300 °C. To investigate the feasibility of forming the Au–In IMCs at lower temperature, a low annealing temperature of 160 °C was applied in this study, which is just above the melting point of indium of 156 °C. AuIn₂ precipitates were confirmed to be predominately formed in the IMCs by X-ray diffraction. Different annealing times of 10, 40, and 120 min were applied to study the stabilization time of IMC AuIn₂. With thermal considerations, AuIn₂ was confirmed to form with a low annealing temperature of 160 °C, and a short annealing time of 10 min. In addition, the microstructure of the cross-sections in the interfacial region of the gold wire and indium solder was investigated by scanning electron microscopy. The mechanical behavior of gold, indium, and their IMCs with different annealing times were studied by nanoindentation. Mechanical properties including reduced modulus and hardness were extracted after taking into account of the pile-up effect. Increased reduced modulus and hardness were observed with increasing annealing times, due to the strengthening of the atomic bonding in the compounds. The reduced modulus and hardness measured from nanoindentation indicate a significant strengthening of the indium solder by the AuIn₂ nanoparticles.     

Strength enhancement in a metastable β-titanium alloy: Ti-15Mo

The tensile properties of a metastable alloy, Ti-15Mo*, in a number of different aged conditions have been correlated with the microstructures observed by TEM. Ageing at a temperature, which is approximately the upper limit of phase stability, allowed the and phases to coexist, the a phase apparently having been nucleated on the particles. An extremely fine distribution of the phase was achieved in this way, resulting in increased strength, limited ductility and good toughness. This method of precipitate refinement is then compared with the other treatments which have been applied to metastable -titanium alloys. Changes in strength due to variations in heating rate to the ageing temperature, to simulate the effect of section size, are also reported and discussed in terms of the refinement in a precipitate size. It is shown that these refinements in precipitate size are reflected in higher values of the ratio 0.2% proof stress/Young's modulus.     

Corrosion behavior of Al–60 vol.% SiCp composites in NaCl solution

In this study, corrosion behavior of pure Al and Al–4 wt.% Mg alloy matrix composites, comprising 60 vol.% SiC particles, has been investigated. Composites were produced by pressure infiltration technique at 750 °C. The corrosion tests were carried out in 3.5 wt.% NaCl environment up to 28 days. The weight loss of the composites increased with increasing duration time up to 3–5 days then remained constant. Scanning electron microscopy (SEM) analysis showed that Al–4 wt.% Mg alloyed matrix composite exhibited higher corrosion resistance than pure Al matrix composite although potentiodynamic polarisation measurements showed higher i_corr values of Al–4 wt.% Mg alloyed matrix composites than pure Al matrix composites. Experimental results revealed that precipitation of Mg₂Si as a result of reaction between Al–Mg alloy and SiC particle has a beneficial effect on corrosion resistance of Al–4Mg alloy matrix composites due to interruption of the continuity of the matrix channels within the pressure infiltrated composites.     

Effects of a high-gradient magnetic field on the migratory behavior of primary crystal silicon in hypereutectic Al–Si alloy

Abstract

The migration of primary Si grains during the solidification of Al–18 wt%Si alloy under a high-gradient magnetic field has been investigated experimentally. It was found that under a gradient magnetic field, the primary Si grains migrated toward one end of the specimen, forming a Si-rich layer, and the thickness of the Si-rich layer increased with increasing magnetic flux density. No movement of Si grains was apparent under a magnetic field below 2.3 T. For magnetic fields above 6.6 T, however, the thickness of the Si-rich layer was almost constant. It was shown that the static field also played a role in impeding the movement of the grains. The primary Si grains were refined in the Si layer, even though the primary silicon grains were very dense. The effect of the magnetic flux density on the migratory behavior is discussed.