Influence of Water Vapor and Temperature on the Oxide Scale Growth and Alpha-Case Formation in Ti-6Al-4V Alloy

The Ti-6Al-4V alloy is extensively used in aerospace, automotive and biomaterial applications. In the aerospace industry, the service temperature of Ti-6Al-4V is currently limited to 350 °C due to its insufficient oxidation resistance. Oxidation at higher temperatures causes the formation of a fast-growing oxide scale and an oxygen-enriched subsurface layer, which is known as the “alpha-case.” Additionally, the effect of water vapor on the oxidation behavior is critical. In the present study, the oxidation behavior of Ti-6Al-4V in dry air and air containing 10 vol.% H2O at 500, 600 and 700 °C for up to 500 h has been investigated. The main focus of this study is the examination of the different oxide scale morphologies along with the oxygen enrichment in the subsurface zone. It has been observed that spallation of the oxide scale is more severe in a water vapor-containing environment. In dry air, the oxide morphology shows the typical layered TiO2/Al2O3 structure after exposure at 700 °C for 300 h, while Al2O3 precipitates are present in the outermost part of the TiO2 scale when oxidized in wet air. This indicates that the solubility and diffusivity of Al3+ ions in TiO2 are influenced by water vapor. In addition, the extent of oxygen enrichment in the subsurface zone (alpha-case) as a function of temperature and time is determined by nanoindentation profiles. It was shown that in contrast to the scale formation, the alpha-case thickness is not affected by the presence of water vapor in the atmosphere.

Effect of Fe (oxyhydr)oxide morphology on phytic acid transport under saturated flow condition

<p>Natural iron (oxyhydr)oxides are ubiquitous in subsurface environments. Phytic acid (myo-inositol hexaphosphate, IHP), a dominant form of organic phosphate (OP) in organic carbon-rich surface soils, strongly binds with Fe (oxyhydr)oxide. The cotransport of IHP and Fe (oxyhydr)oxide with different morphology under acid and alkaline conditions in the subsurface is mostly overlooked. These cotransport processes are critical for P (bio)geochemical processes in the subsurface that is rich in Fe (oxyhydr)oxides. Three Fe (oxyhydr)oxides (ferrihydrite, hematite, and goethite) were chosen in this study, and the cotransport of IHP and Fe (oxyhydr)oxide was investigated in saturated columns by injecting Fe (oxyhydr)oxide under different IHP concentrations (0, 10, 25, 50, and 100 µM) at pH of 5 and 10. The presence of IHP significantly enhanced the mobility of Fe (oxyhydr)oxide at both pH 5 and 10 due to the stronger electrostatic repulsion between Fe (oxyhydr)oxide and quartz sand. At low IHP concentrations (< 50 µM IHP), goethite with a rod-like morphology showed strong mobility due to its orientation transport along with the water flow streamline. The mobility of amorphous Fe (oxyhydr)oxide, ferrihydrite, was much slower than the goethite. However, ferrihydrite could facilitate more IHP transport due to its sorption capacity for IHP that is higher than goethite and hematite. At high IHP concentrations (> 50 μM), surface precipitation might have occurred on ferrihydrite because of its poorly ordered crystallinity, which contributed to its less negatively charged surface and weak ferrihydrite facilitated IHP transport. The new insight provided in this study is important for evaluating the transport behavior and impact of IHP in a saturated solum rich in Fe (oxyhydr)oxides.</p>

Tailoring the Anodic Hafnium Oxide Morphology Using Different Organic Solvent Electrolytes

Highly ordered anodic hafnium oxide (AHO) nanoporous or nanotubes were synthesized by electrochemical anodization of Hf foils. The growth of self-ordered AHO was investigated by optimizing a key electrochemical anodization parameter, the solvent-based electrolyte using: Ethylene glycol, dimethyl sulfoxide, formamide and N-methylformamide organic solvents. The electrolyte solvent is here shown to highly affect the morphological properties of the AHO, namely the self-ordering, growth rate and length. As a result, AHO nanoporous and nanotubes arrays were obtained, as well as other different shapes and morphologies, such as nanoneedles, nanoflakes and nanowires-agglomerations. The intrinsic chemical-physical properties of the electrolyte solvents (solvent type, dielectric constant and viscosity) are at the base of the properties that mainly affect the AHO morphology shape, growth rate, final thickness and porosity, for the same anodization voltage and time. We found that the interplay between the dielectric and viscosity constants of the solvent electrolyte is able to tailor the anodic oxide growth from continuous-to-nanoporous-to-nanotubes.