Corrosion and corrosion-fatigue behavior of cp-Ti and Ti–6Al–4V laser-marked biomaterials

The aim of this work was to determine the influence of laser surface modification treatments on mechanical and electrochemical behavior in Ti and Ti–6Al–4V implants. For each metal, different samples were laser modified simulating the markings according to the international requirements. (It is necessary in each metallic biomaterial to mark the serial, batch and company numbers.) Microstructural changes produced by this treatment were observed: (a) the melting zone with small grain sizes and martensitic structures in above-mentioned metals and (b) the heat-affected zone (HAZ) with alpha phase in cp-Titanium with bigger grain sizes and Widmanstatten structure in Ti–6Al–4V. Positive tensile residual stress was determined by means X-ray analysis in the zones marked by laser. Furthermore, corrosion behavior was studied in a simulated body fluid at 37°C. Pitting was observed in different zones near the HAZ and the results showed a decrease of the corrosion resistance in the laser treated samples. Residual stresses and the martensitic microstructures favoured the decrease of the corrosion-fatigue life around 20% of both metals under physiological conditions.     

Comparisons of immersion and electrochemical properties of highly biocompatible Ti–15Zr–4Nb–4Ta alloy and other implantable metals for orthopedic implants

Abstract

Metal release from implantable metals and the properties of oxide films formed on alloy surfaces were analyzed, focusing on the highly biocompatible Ti–15Zr–4Nb–4Ta alloy. The thickness and electrical resistance (R_p) of the oxide film on such an alloy were compared with those of other implantable metals. The quantity of metal released during a 1-week immersion test was considerably smaller for the Ti–15Zr–4Nb–4Ta than the Ti–6Al–4V alloy. The potential (E₁₀) indicating a current density of 10 μA cm^?2 estimated from the anodic polarization curve was significantly higher for the Ti–15Zr–4Nb–4Ta than the Ti–6Al–4V alloy and other metals. Moreover, the oxide film (4–7 nm thickness) formed on the Ti–15Zr–4Nb–4Ta surface is electrochemically robust. The oxide film mainly consisted of TiO₂ with small amounts of ZrO₂, Nb₂O₅ and Ta₂O₅ that made the film electrochemically stable. The R_p of Ti–15Zr–4Nb–4Ta was higher than that of Ti–6Al–4V, i.e. 0.9 Ω cm² in 0.9% NaCl and 1.3 Ω cm² in Eagle's medium. This R_p was approximately five-fold higher than that of stainless steel, which has a history of more than 40 years of clinical use in the human body. Ti–15Zr–4Nb–4Ta is a potential implant material for long-term clinical use. Moreover, E₁₀ and R_p were found to be useful parameters for assessing biological safety.     

Mechanical properties and defective effects of bcc V–4Cr–4Ti and V–5Cr–5Ti alloys by first-principles simulations

V–(4–5) wt.% Cr–(4–5) wt.% Ti alloys are important candidate structural materials for the first-wall and blanket in future fusion reactor. Thus it is necessary to study the fundamental mechanical properties and the irradiation effects of the V-based alloys. Within a random solid solution model, the elastic constants and ideal strength of the V–4Cr–4Ti and the V–5Cr–5Ti alloys were calculated and compared with those of pure V solid. According to the theoretical Cauchy pressure and the ratio of bulk modulus and shear modulus, both alloys exhibit good ductility. Within the 250-atom supercell, inclusion of one vacancy defect or one interstitial H (He) atom will further enhance the ductility of these alloys.     

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.     

Effects of thermal treatment on precipitate shape and mechanical properties of Mg–8Gd–4Y–Nd–Zr alloy

Precipitation reactions in a Mg–8Gd–4Y–Nd–Zr alloy have been investigated using TEM, HREM, hardness measurements and tensile testing. Globular β′ precipitates, which were different from the typical plate-shaped β′ precipitates usually observed in Mg–Gd-based alloys, were detected in the 160 °C/192 h-aged sample. Instead of dissolution and then precipitating as plate-shaped β′ precipitates, the formed globular β′ precipitates grew up when further aged at 215 °C, which resulted in the decrease in strength comparing with that of the 215 °C single-stage aged samples. Two-stage ageing treatments on the alloy demonstrated that ageing 192 h at 150 °C plus 16 h at 215 °C made the ultimate strength and the yield strength improved 17 MPa and 13 MPa, respectively.     

Study of Structural and Magnetic Properties of Superparamagnetic Fe3O4–ZnO Core–Shell Nanoparticles

In this work, Fe₃O₄–ZnO core–shell nanoparticles have been successfully synthesized using a simple two-step co-precipitation method. In this regard, Fe₃O₄ (magnetite) and ZnO (zincite) nanoparticles (NPs) were synthesized separately. Then, the surface of the Fe₃O₄ NPs was modified with trisodium citrate in order to improve the attachment of ZnO NPs to the surface of Fe₃O₄ NPs. Afterwards, the modified magnetite NPs were coated with ZnO NPs. Moreover, the influence of the core to shell molar ratio on the structural and magnetic properties of the core–shell NPs has been investigated. The prepared nanoparticles have been characterized utilizing transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and vibrating sample magnetometer (VSM). The results of XRD indicate that Fe₃O₄ NPs with inverse spinel phase were formed. The results of VSM imply that the Fe₃O₄–ZnO core–shell NPs are superparamagnetic. The saturation magnetization of prepared Fe₃O₄ NPs is 54.24 emu/g and it decreases intensively down to 29.88, 10.51 and 5.75 emu/g, after ZnO coating with various ratios of core to shell as 1:1, 1:10 and 1:20, respectively. This reduction is attributed to core–shell interface effects and shielding. TEM images and XRD results imply that ZnO-coated magnetite NPs are formed. According to the TEM images, the estimated average size for most of core–shell NPs is about 12 nm.     

Microstructure and mechanical properties of Si3N4–TiC nanocomposites

Abstract

Si₃N₄–TiC nanocomposites are fabricated by hot press sintering from silicon nitride nanopowders and ultrafine TiC powders. The microstructure and mechanical properties are analysed and discussed. Scanning electron microscopy images show that the microstructure consists of equiaxed grains and grain boundary phase. The TiC added as a dispersed phase reacts with the nitrogen from Si₃N₄ during the liquid phase sintering, with the formation of TiC_0.7 N_0.3 , trace of SiC and N₂. The adding of a proper amount of TiC powders increases the flexural strength and has little influence on fracture toughness. The hardness increases with increasing TiC content.     

Liquid-Phase Sintering and Properties of Si3N4–TiN Composites

Densification during liquid-phase sintering of Si₃N₄–TiN was studied in the presence of Y₂O₃. The content of TiN was varied from 0–50 mass%. During the densification Y-silicate was formed. The amount of silicate increased with both decreasing fraction of TiN and increasing isothermal heating time. Density, fracture toughness, and electrical resistivity were measured as a function of TiN content. It was found that the density and fracture toughness increased with increasing TiN content. The electrical resistivity drops drastically, from 10¹⁰ m for sintered Si₃N₄ to 10^–3 m for sintered Si₃N₄–TiN composite containing 30 vol% TiN.     

Microstructure and mechanical properties of wire arc additive repairing Ti–6.5Al–2Sn–2Zr–4Mo–4Cr titanium alloy

Wire arc additive manufacture (WAAM) technology was employed to repair Ti–6.5Al–2Sn–2Zr–4Mo–4Cr (TC17) titanium alloy, which is widely used as compressor blades and blisk. The microstructure evolution and mechanical properties of the repaired specimen were investigated. The results show that the repaired specimen has a good verticality, and it forms good metallurgical bonding with the base metal. The ultimate strength of the interface specimen can reach 88.2% of the base metal. The elongation is slightly lower than the base metal due to the fact that half of the tensile interface specimens with little deformation are base material. Therefore, WAAM technology might be a potential and economical method to repair damaged blade by optimising the WAAM process.     

Microstructure,mechanical properties and yield asymmetry of Mg–4Al–2Sn–xY alloys

The effect of 0, 0.3, 0.6, 0.9?wt-% Y addition on the microstructure and mechanical properties of extruded Mg–4Al–2Sn alloys were investigated. The results show that α-Mg, Mg₁₇Al₁₂, Mg₂Sn and Al₂Y phases form in the extruded Y-containing alloys. Mg₁₇Al₁₂ phase, containing trace amounts of Y, tends to distribute on the grain boundaries in the form of needles. When the Y content is 0.6?wt-%, the alloy has the best combination mechanical properties. Its tensile yield strength, ultimate tensile strength and tensile elongation are 172?MPa, 270?MPa and 11.2%, respectively. As the Y content increases, the tensile and compressive asymmetries in the Mg–4Al–2Sn–xY alloy decrease, due to grain refinement and the weakening of texture.     

Comparison about as-cast microstructures and mechanical properties of Mg–4Y–1.2Mn–0.9Sc and Mg–4Y–1.2Mn–1Zn (wt%) magnesium alloys

In this article, the as-cast microstructures and mechanical properties of the Mg–4Y–1.2Mn–0.9Sc and Mg–4Y–1.2Mn–1Zn (wt%) magnesium alloys are investigated and compared. The results indicate that the Sc-containing alloy is mainly composed of α-Mg and fine particle-like Mg₂₄Y₅, Mn₁₂Y, and Mn₂Sc phases, while the Zn-containing alloy mainly consists of α-Mg and coarse Mg₁₂YZn phases with a continuous network. Furthermore, the grains of the Zn-containing alloy are relatively finer than those of the Sc-containing alloy. In addition, the Sc-containing alloy exhibits relatively higher tensile properties at room temperature and 300 °C than the Zn-containing alloy. However, the creep properties at 300 °C and 30 MPa for 100 h for the Sc-containing alloy are relatively lower than those for the Zn-containing alloy.     

Investigation of microstructure and chemical composition of Ni contacts to n-type 4H–SiC

Structure and chemical compositions of the interface layer obtained after nickel deposition on silicon carbide surface and subsequent annealing have been analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray diffraction (XRD) and Raman spectroscopy. Nickel silicide (Ni₂Si) were characterized as the main product of reaction between nickel and silicon carbide after annealing at temperatures range 700–1000 °C. Raman spectroscopy and TOF-SIMS profiling results confirmed carbon precipitation within contact layer. Obtained results indicate graphitic form of carbon and its non-uniform distribution in the contact layer. Moreover, TOF-SIMS analysis showed modification of nitrogen distribution in the contact area upon Ni/SiC contact annealing.     

Comparison of residual stresses in Ti–6Al–4V and Ti–6Al–2Sn–4Zr–2Mo linear friction welds

Abstract

In this paper, the levels of residual stress in the vicinity of linear friction welds in Ti–6Al–4V (Ti-64), a conventional α–β titanium alloy, and Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near α titanium alloy with higher temperature capability, are mapped and contrasted. The alloys have significantly different high temperature properties and the aim of this work was to investigate how this might affect their propensity to accumulate weld residual stresses and their response to post-weld heat treatment. Measurements are reported using high energy synchrotron X-ray diffraction and the results are compared to those made destructively using the contour method. The strain free lattice plane d ₀ variation across the weld has been evaluated using the biaxial sin²Ψ technique with laboratory X-rays. It was found that failure to account for the d ₀ variation across the weld line would have led to large errors in the peak tensile stresses. Contour method measurements show fairly good correlation with the diffraction results, although the stresses are underestimated. Possible reasons for the discrepancy are discussed. The peak tensile residual stresses introduced by the welding process were found to be greater for Ti-6242 (～750 MPa) than for Ti-64 (～650 MPa). Consistent with the higher temperature capability of the alloy, higher temperature post-weld heat treatments have been found to be necessary to relieve the stresses in the near α titanium alloy compared to the α+β titanium alloy.     

Workability of Ti–6Al–4V alloy at high temperatures and strain rates

Hot workability of Ti–6Al–4V has been investigated by means of hot compression tests carried out in the 880–950 °C temperature range and 1–50 s⁻¹ strain rate range. The effect of microstructural characteristics of the deformed specimens have been studied and correlated with the test temperature, total strain and strain rate. A constitutive equation for the flow stress has been defined and the test conditions for a homogeneous deformation evaluated. The machine employed for testing allowed to reach very high strain rates by means of a uniform compression for long strains (until 0.9), whereas data extracted from the scientific literature are significantly limited in comparison. In this way, a higher accuracy could be obtained in material behaviour modelling for forging process simulation.     

Interface microstructure and strength properties of Ti–6Al–4V and microduplex stainless steel diffusion bonded joints

The diffusion bonding of Ti–6Al–4V alloy and micro-duplex stainless steel was carried out in the temperature range of 850–1000 °C for 45 min in vacuum. The influence of bonding temperature on the microstructural development, micro-hardness and strength properties across the joint region was determined. The layer wise σ phase, λ + FeTi and λ + FeTi + β-Ti phase mixtures were observed at the bond interface when the joint was processed at 900 °C and above temperature. The maximum tensile strength of ∼520.1 MPa and shear strength of ∼405.5 MPa along with 6.8% elongation were obtained for the diffusion couple processed at 900 °C. Fracture surface observation in scanning electron microscopy (SEM) using energy dispersive X-ray spectroscope (EDS) demonstrates that, failure takes place through λ + FeTi phase when bonding was processed at 900 °C, however, failure takes place through σ phase for the diffusion joints processed at and above 950 °C.     

Production and characterization of AA6061–B4C stir cast composite

This work focuses on the fabrication of aluminum (6061-T6) matrix composites (AMCs) reinforced with various weight percentage of B₄C particulates by modified stir casting route. The wettability of B₄C particles in the matrix has been improved by adding K₂TiF₆ flux into the melt. The microstructure and mechanical properties of the fabricated AMCs are analyzed. The optical microstructure and scanning electron microscope (SEM) images reveal the homogeneous dispersion of B₄C particles in the matrix. The reinforcement dispersion has also been identified with X-ray diffraction (XRD). The mechanical properties like hardness and tensile strength have improved with the increase in weight percentage of B₄C particulates in the aluminum matrix.     

Microstructural characteristics and mechanical properties of friction stir welded joints of Ti–6Al–4V titanium alloy

The α + β titanium alloy, Ti–6Al–4V, was friction stir welded at a constant tool rotation speed of 400 rpm. Defect-free welds were successfully obtained with welding speeds ranging from 25 to 100 mm/min. The base material was mill annealed with an initial microstructure composed of elongated primary α and transformed β. A bimodal microstructure was developed in the stir zone during friction stir welding, while microstructure in the heat affected zone was almost not changed compared with that in the base material. An increase in welding speed increased the size of primary α in the stir zone. The weld exhibited lower hardness than the base material and the lowest hardness was found in the stir zone. Results of transverse tensile test indicated that all the joints had lower strength and elongation than the base material, and all the joints were fractured in the stir zone.     

Effect of Zn concentration on the microstructures and mechanical properties of extruded Mg–7Y–4Gd–0.4Zr alloys

Microstructures and mechanical properties of the Mg–7Y–4Gd–xZn–0.4Zr (x = 0.5, 1.5, 3, and 5 wt.%) alloys in the as-cast, as-extruded, and peak-aged conditions have been investigated by using optical microscopy, scanning electron microscope, X-ray diffraction, and transmission electron microscopy. It is found that the peak-aged Mg–7Y–4Gd–1.5Zn–0.4Zr alloys have the highest strength after aging at 220 °C. The highest ultimate tensile strength and yield tensile strength are 418 and 320 MPa, respectively. The addition of 1.5 wt.% Zn to the based alloys results in a greater aging effect and better mechanical properties at both room and elevated temperatures. The improved mechanical properties are mainly ascribed to both a fine β′ phase and a long periodic stacking-ordered structure, which coexist together in the peak-aged alloys.     

Evolution of microstructures and phases of Al–Mg alloy under 4 GPa high pressure

Optical microscope (OM), energy dispersive X-ray (EDX) analysis, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscope (TEM) were applied to investigate the solidification microstructures and phases of Al–9.6 wt.% Mg alloy which solidified under 4 GPa high pressure with the melting temperature 1,153 K. Fine dendritic microstructures were obtained, and the second dendritic arm spacing reduced. Area fraction of the primary α-Al phase increased and that of the second phase decreased. In addition, the solid solubility of Mg in α-Al phase increased. The lattice constant of α-Al phase increased. Specially, the new double phase regions (α-Al′ + Al _x Mg _y ) formed besides a small amount of Al₃Mg₂ phases under high pressure. The Al _x Mg _y phase presented a mean size of about 20 nm, and had the hexagonal structure with the lattice constant of a = 0.288 nm, c = 0.8165 nm probably. Wherein the lattice constant of α-Al′ phase differed from that of α-Al phase greatly. Moreover, evolution mechanism of microstructures and phases under 4 GPa high pressure was discussed.