Effect of V content on microstructure and mechanical property of a TiVCuNiAl composite fabricated by spark plasma sintering

Ultrafine-grained (Ti₄₀Cu_22.9Ni_19.4Al_17.7)_x(Ti₈₀V₂₀)_1−x (x = 0.35 and 0.55) composites were fabricated by spark plasma sintering and crystallization of amorphous phase. Outstanding difference in microstructure and mechanical property is found for the two composites. Microstructure observation shows that the two composites all consist of body-centered cubic β-Ti and face-centered cubic (Cu, Ni)–Ti₂ regions but have opposite matrix and reinforcing phases. Meanwhile, mechanical property examination indicates that the composites for x = 0.35 exhibit far higher fracture strength and far larger fracture strain compared with the composites for x = 0.55 fabricated under the same parameter conditions. The different mechanical properties for the two composites can be explained by the different fracture mechanisms resulted from their different microstructures.     

In vitro corrosion behaviour of plasma nitrided Ti–6Al–7Nb orthopaedic alloy in Hanks solution

Titanium alloy (Ti–6Al–7Nb) used for orthopaedic applications was nitrided using a conventional dc plasmatechnique. Load-dependent microhardness measurements exhibit a hardness of 2087 H_v at 25 g load for the alloy nitrided at 900 8C for 8 h. Cyclic polarization measurement in Hanks solution show maximum corrosion rate and minimum areaof repassivation for the alloy nitrided for 8 h at 900 8C. Electrochemical impedance measurements show an increase in charge transfer resistance and decrease in double layer capacitance when compared to untreated alloy.     

Thermodynamic assessment of Fe–Ti–S ternary phase diagram

A thermodynamic analysis of the Fe-Ti-S ternary system was performed by incorporating first-principles calculations into the calculation of phase diagrams (CALPHAD) method. To evaluate the Gibbs energy, the Debye-Grüneisen model was applied for some sulfides of the Ti-S binary system. In addition, the cluster expansion and cluster variation methods were used for the solid solution phases in the Ti-S binary and (Fe,Ti)S phases. The calculated Ti-S binary phase diagram showed good agreement with the experimental results. The very low solubility of the Ti solid solution in the Ti-S system, as reported by Murray, agreed well with our calculated results. A binodal phase decomposition of the liquid phase was expected in the S-rich region. The Gibbs energy curve of (Fe,Ti)S between FeS and TiS was found to be convex downward. This is characteristic of an isomorphic solid solution, attributed to the attractive interaction between Fe and Ti in (Fe,Ti)S. The vertical phase diagram between FeS and TiS, obtained using the thermodynamic database, was in good agreement with the experimental results of Mitsui et al. The solubility products of (Fe,Ti)S have been experimentally estimated previously. The calculated solubility product agreed with the experimental value of TiS.     

Effect of the cooling rate in the corrosion behavior of a hot worked Ti-6Al-4V extra-low interstitial alloy

This work discusses the effect of the cooling rate during a forging process on the microstructure and corrosion behavior of a Ti–6Al–4V extra-low interstitial (ELI) alloy, which is commonly used as biomaterial. The samples were hot forged at two different temperatures, both of them within the dual phase field (α + β) and a constant strain rate of 4 × 10⁻³ s⁻¹ was employed during the tests. The samples were cooled in three different cooling media (water, air and clay) and the microstructure was analyzed using scanning electron microscopy (SEM). The corrosion resistance was determined by cyclic polarization tests in Ringer’s solution at 37 °C. Comparison between the results obtained for forged and commercial samples allowed to establish some correlations between cooling rate, microstructure and corrosion resistance. It was found that the clay as a cooling medium is a good candidate to obtain a proper microstructure and properties for biomedical applications, eliminating the requirement of subsequent heat treatment and reducing costs.     

Gradient microstructure of TC21 alloy induced by hydrogen during hydrogenation

Through melt hydrogenation, a gradient microstructure (α″ + α′)/(α + β_H) has been observed in TC21 alloy. The addition of hydrogen induces martensite transformation and increases the volume fraction of β. It is found that the absorption process of hydrogen atoms can be divided into melting and cooling stages. During cooling, the continuous absorption of hydrogen and the corresponding decrease of freezing point of melt extend solidification time of melt and lead to hydrogen enrichment in the upper of the specimen, which induces the formation of the gradient structure. The hydrogenated TC21 alloy shows higher thermoplasticity compared with the unhydrogenated TC21 alloy. The flow stress of the upper part of the hydrogenated alloy is lower than that of the center part. A gradual variation has been observed in the microhardness along the gradient direction due to variation in the microstructure. The microhardness of the upper surface drops about 45% with 14.6 at.%H.     

Development of a processing route for carbon allotrope-based TiC porous nanocomposites

Ti-foils are currently used as a spallation target material to produce radioisotopes for physics research at the ISOLDE facility at CERN. However, radioisotope production rates often decrease over time due to material degradation from high operation temperatures. Due to enhanced release rates, porous nanomaterials are being studied as spallation target materials for isotope production. TiC is a material with a very high melting point making it an interesting material to replace the Ti-foils. However, in its nanometric form it sinters readily at high temperatures. To overcome this, a new processing route was developed where TiC was co-milled with graphite, carbon black or multi-wall carbon nanotubes in order to hinder the sintering of TiC. The obtained nanocomposite particle sizes, density, specific surface area and porosity were characterized and compared using ANOVA. All carbon allotropes mixed with the TiC, were able to successfully stabilize the nanometric TiC, hindering its sintering up to 1500 °C for 10 h.     

Optical investigation of the behavior of the electric arc and the metal transfer during vacuum remelting of a Ti alloy

High-speed camera imaging and optical emission spectroscopy have been used for investigating the structure of the electric arc and the transfer mechanisms of the liquid metal during vacuum arc remelting (VAR) of Ti alloys. The arc exhibited a similar operating regime to that described in the previous literature for the case of Inconel 718 and zirconium alloy electrodes. The arc behaved in a diffuse mode with many separate and rapidly moving cathode spots. Several parameters of the cathode spots, including their current, size and apparent velocity were evaluated. The application of an external axial magnetic field tended to encourage the cathode spots to locate themselves on the base of the electrode. A large density ratio of Ti⁺ ions and Ti atoms in the interelectrode plasma was evaluated, suggesting that the plasma was strongly ionized. The calculated excitation temperature of Ti⁺ ions (1–1.2 eV) was about 1.5–2 times greater than that obtained for Ti atoms. The transfer mechanisms of the drops of liquid metal might be classified into three main modes depending on the gap length: drop falling, drip short and drop erosion induced by the cathode spots. The importance of the influence of the arc on the metal transfer mechanisms was inversely related to the gap length.     

Dynamic globularization prediction during cogging process of large size TC11 titanium alloy billet with lamellar structure

The flow behavior and dynamic globularization of TCll titanium alloy during subtransus deformation are investigated through hot compression tests. A constitutive model is established based on physical-based hardening model and phenomenological softening model. And based on the recrystallization mechanisms of globularization, the Avrami type kinetics model is established for prediction of globularization fraction and globularized grain size under large strain subtransus deformation of TC11 alloy. As the preliminary application of the previous results, the cogging process of large size TCI 1 alloy billet is simulated. Based on subroutine development of the DEFORM software, the coupled simulation of one fire cogging process is developed. It shows that the predicted results are in good agreement with the experimental results in forging load and microstructure characteristic, which validates the reliability of the developed FEM subroutine models.     

Microstructural characterisation of fatigue crack growth fracture surfaces of lamellar Ti45Al2Mn2Nb1B

Investment cast Ti45Al2Mn2Nb1B with fine lamellar microstructures was subject to fatigue crack propagation testing at 650 °C and a stress ratio of R = 0.1. The fracture surfaces were examined under SEM and the observed features are correlated with both stress intensity range (ΔK) and lamellar orientation. Translamellar fracture is primary fracture mode for most of the lamellar orientations. Interlamellar fracture is influenced by a combination of the ΔK and lamellar orientation. At low ΔK only the lamellar colonies with their lamellar interfaces almost perpendicular to the stress axis fractured via interlamellar fracture mode. At high ΔK interlamellar fracture can occur in lamellar colonies with any orientations.