Literature Survey on Diffusivities of Oxygen, Aluminum, and Vanadium in Alpha Titanium, Beta Titanium, and in Rutile

Investigation of the Effect of Stress on Oxygen Diffusion in Pure Titanium Using a Phase-Field Model

Diffusion plays a vital role during the fabrication of many materials. It is a well-known fact that stress can influence diffusion behavior. In order to optimize material processing techniques, a quantitative evaluation of the effect of stress on diffusion is essentially required. By analyzing the free energy change in a Ti-O system during diffusion, a phase-field model was developed to address this issue. Using this model, the diffusion of oxygen atoms in pure titanium under different stress states was investigated. It was observed that the true equilibrium concentration of oxygen was proportional to its hydrostatic pressure. Tensile stress can increase the oxygen concentration. This raise in concentration decreased with temperature. However, the promotion of diffusion can be attained in deeper regions at a higher temperature. On the contrary, compressive stress inhibited the diffusion of oxygen in pure titanium. Under a certain compressive stress, the decrease in the oxygen concentration at the surface layer was more significant at a lower temperature, while a decrease could be observed at a deeper distance from the surface at a higher temperature. A thermodynamic explanation of the effect of stress on diffusion was given based on the proposed phase-field model.

Investigating the thermal oxidation behavior of Ti–6Al–7Nb alloy in dry air

The thermal oxidation behavior of a forged Ti–6Al–7Nb (Ti67) alloy was investigated in dry air conditions. The alloy specimens were oxidized for 50 h at different temperatures (650°C, 750°C, and 850°C) and the oxidation kinetics were analyzed. The microstructure of Ti67 after oxidation and the different phases and compounds were identified. A parabolic equation based on weight gain was used to define the oxidation rate. The parabolic rate constant increased from 1.10 × 10−8 at 650°C to 2.10 × 10−8 and 1.53 × 10−7 g2/(cm4·h) at 750°C and 850°C, respectively. At 650°C, the oxidation behavior resumed the parabolic behavior until 40 h, a breakaway phenomenon. At 750°C, a linear behavior was observed in the periods of 20–50 h. This linear trend appeared also after 35–50 h at 850°C. The activation energy was found to be 238.4 kJ/mole. At the lowest oxidation temperature (650°C), a disconnected oxide layer was observed. These variations in the response of Ti67 alloy to the different thermal oxidation conditions are worthy of consideration when modifying the surfaces for biomedical applications.

Study on the Surface Modification of Nanostructured Ti Alloys and Coarse-Grained Ti Alloys

Commercial purity titanium (CP-Ti) and a Ti-6Al-4V alloy (Ti64) were processed by high-pressure torsion (HPT) for 10 and 20 turns. The HPT processing produced a nanostructured microstructure and a significant strength enhancement in the CP-Ti and Ti64 samples. After 20 turns, the samples of HPT-processed CP-Ti and Ti64 were subjected to laser surface treatments in an air atmosphere using different scanning speeds and laser powers. The surface roughness of the laser-modified samples increased with increasing laser power and this produced hydrophilicity due to a lower contact angle. After a holding time of 27 days, these samples underwent a hydrophilic-to-hydrophobic transformation as the contact angle increased from 13° to as much as 120° for the CP-Ti sample, and for the Ti64 sample the contact angle increased from 10° to 126°. In addition, the laser surface modification process was carried out with different atmospheres (air, vacuum and O2) on heat-treated but unstrained CP-Ti and Ti64 samples and the contact angle changed due to the surface element content. Thus, as the carbon content increased from 28% to 47% in CP-Ti in a vacuum environment, the surface contact angle increased from 22° to 140°. When a laser surface modification process is conducted under oxygen-less conditions, it is concluded that the contact angle increases rapidly in order to control the hydrophobic properties of Ti and the Ti alloy.

Characterization of Spatter and Sublimation in Alloy 718 during Electron Beam Melting

Due to elevated temperatures and high vacuum levels in electron beam melting (EBM), spatter formation and accumulation in the feedstock powder, and sublimation of alloying elements from the base feedstock powder can affect the feedstock powder's reusability and change the alloy composition of fabricated parts. This study focused on the experimental and thermodynamic analysis of spatter particles generated in EBM, and analyzed sublimating alloying elements from Alloy 718 during EBM. Heat shields obtained after processing Alloy 718 in an Arcam A2X plus machine were analyzed to evaluate the spatters and metal condensate. Comprehensive morphological, microstructural, and chemical analyses were performed using scanning electron microscopy (SEM), focused ion beam (FIB), and energy dispersive spectroscopy (EDS). The morphological analysis showed that the area coverage of heat shields by spatter increased from top (<1%) to bottom (>25%), indicating that the spatter particles had projectile trajectories. Similarly, the metal condensate had a higher thickness of ~50 μm toward the bottom of the heat shield, indicating more significant condensation of metal vapors at the bottom. Microstructural analysis of spatters highlighted that the surfaces of spatter particles sampled from the heat shields were also covered with condensate, and the thickness of the deposited condensate depended on the time of landing of spatter particles on the heat shield during the build. The chemical analysis showed that the spatter particles had 17-fold higher oxygen content than virgin powder used in the build. Analysis of the metalized layer indicated that it was formed by oxidized metal condensate and was significantly enriched with Cr due to its higher vapor pressure under EBM conditions.

Non-Isothermal Oxidation Behaviors and Mechanisms of Ti-Al Intermetallic Compounds

Non-isothermal oxidation is one of the important issues for the safe application of Ti-Al alloys, so this study aimed to illustrate the non-isothermal oxidation behaviors and the corresponding mechanisms of a TiAl-based alloy in comparison with a Ti₃Al-based alloy. The non-isothermal oxidation behaviors of Ti-46Al-2Cr-5Nb and Ti-24Al-15Nb-1.5Mo alloys in pure oxygen were comparatively investigated with a thermogravimetry-differential scanning calorimetry (TGA/DSC) simultaneous thermal analyzer heating from room temperature to 1450 °C with a heating rate of 40 °C/min. When the temperature rose above 1280 °C, the oxidation rate of the Ti-46Al-2Cr-5Nb alloy sharply increased and exceeded that of the Ti-24Al-15Nb-1.5Mo alloy owing to the occurrence of internal oxidation. When the temperature was higher than 1350 °C, the oxidation rate of the Ti-46Al-2Cr-5Nb alloy decreased obviously due to the generation of an oxygen-barrier β-Al₂TiO₅-rich layer by a chemical reaction between Al₂O₃ and TiO₂ in the oxide scale. Based on Wagner’s theory of internal oxidation, the reason for the occurrence of internal oxidation in the Ti-46Al-2Cr-5Nb alloy is the formation of the α phase in the subsurface, while no internal oxidation occurred in the Ti-24Al-15Nb-1.5Mo alloy due to the existence of the β phase in the subsurface with the enrichment of Nb and Mo.

Non-Isothermal Oxidation Behavior and Mechanism of a High Temperature Near-α Titanium Alloy

Non-isothermal oxidation is one of the important issues related to the safe application of high-temperature titanium alloys, so this study focuses on the non-isothermal oxidation behavior and mechanism of near-α titanium alloys. The thermogravimetry-differential scanning calorimetry (TGA/DSC) method was used to study the non-isothermal oxidation behavior of TA29 titanium alloy heated from room temperature to 1450 °C at a heating rate of 40 °C/min under pure oxygen atmosphere. The results show that non-isothermal oxidation behavior can be divided into five stages, including no oxidation, slow oxidation, accelerated oxidation, severe oxidation and deceleration oxidation; for the three-layer TiO₂ scale, Zr, Nb, Ta are enriched in the intermediate layer, while Al is rich in the inner layer and Sn is segregated at the oxide-substrate interface, which is related to their diffusion rates in the subsurface α case. The oxidation mechanism for each stage is: oxygen barrier effect of a thin compact oxide film; oxygen dissolution; lattice transformation accelerating the dissolution and diffusion of oxygen; oxide formation; oxygen barrier effect of recrystallization and sintering microstructure in outer oxide scale. The alloying elements with high valence state and high diffusion rate in α-Ti are favorable to slow down the oxidation rate at the stage governed by oxide formation.

Structural characterization of semi-heusler/light metal composites prepared by spark plasma sintering

A composite of powders of semi-Heusler ferromagnetic shape memory and pure titanium was successfully prepared by spark plasma sintering at the temperature of 950 °C. Sintering resulted in the formation of small precipitates and intermetallic phases at the heterogeneous interfaces. Various complementary experimental methods were used to fully characterize the microstructure. Imaging methods including transmission and scanning electron microscopy with energy dispersive X-ray spectroscopy revealed a position and chemical composition of individual intermetallic phases and precipitates. The crystalline structure of the phases was examined by a joint refinement of X-ray and neutron diffraction patterns. It was found that Co38Ni33Al29 decomposes into the B2-(Co,Ni)Al matrix and A1-(Co,Ni,Al) particles during sintering, while Al, Co and Ni diffuse into Ti forming an eutectic two phase structure with C9-Ti2(Co,Ni) precipitates. Complicated interface intermetallic structure containing C9-Ti2(Co,Ni), B2-(Co,Ni)Ti and L21-(Co,Ni)(Al,Ti) was completely revealed. In addition, C9-Ti2(Co,Ni) and A1-(Co,Ni,Al) precipitates were investigated by an advanced method of small angle neutron scattering. This study proves that powder metallurgy followed by spark plasma sintering is an appropriate technique to prepare bulk composites from very dissimilar materials.

A Novel Investigation of the Formation of Titanium Oxide Nanotubes on Thermally Formed Oxide of Ti-6Al-4V

Traditionally, titanium oxide (TiO2) nanotubes (TNTs) are anodized on Ti-6Al-4V alloy (Ti-V) surfaces with native TiO2 (amorphous TiO2); subsequent heat treatment of anodized surfaces has been observed to enhance cellular response. As-is bulk Ti-V, however, is often subjected to heat treatment, such as thermal oxidation (TO), to improve its mechanical properties. Thermal oxidation treatment of Ti-V at temperatures greater than 200°C and 400°C initiates the formation of anatase and rutile TiO2, respectively, which can affect TNT formation. This study aims at understanding the TNT formation mechanism on Ti-V surfaces with TO-formed TiO2 compared with that on as-is Ti-V surfaces with native oxide. Thermal oxidation–formed TiO2 can affect TNT formation and surface wettability because TO-formed TiO2 is expected to be part of the TNT structure. Surface characterization was carried out with field emission scanning electron microscopy, energy dispersive x-ray spectroscopy, water contact angle measurements, and white light interferometry. The TNTs were formed on control and 300°C and 600°C TO-treated Ti-V samples, and significant differences in TNT lengths and surface morphology were observed. No difference in elemental composition was found. Thermal oxidation and TO/anodization treatments produced hydrophilic surfaces, while hydrophobic behavior was observed over time (aging) for all samples. Reduced hydrophobic behavior was observed for TO/anodized samples when compared with control, control/anodized, and TO-treated samples. A method for improved surface wettability and TNT morphology is therefore discussed for possible applications in effective osseointegration of dental and orthopedic implants.

Direct diffusion through interpenetrating networks: oxygen in titanium

How impurity atoms move through a crystal is a fundamental and recurrent question in materials. The previous models of oxygen diffusion in titanium relied on interstitial lattice sites that were recently found to be unstable--leaving no consistent picture of the diffusion pathways. Using first-principles quantum-mechanical methods, we find three oxygen interstitial sites in titanium, and quantify the multiple interpenetrating networks for oxygen diffusion. Surprisingly, all transitions contribute to diffusion.