Maria Marcu

Effect of niobium alloying level on the oxidation behavior of titanium aluminides at 850°C

This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 + γ(Ti3Al + TiAl) and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.

Microstructure, surface characterization and long-term stability of new quaternary Ti-Zr-Ta-Ag alloy for implant use

The novel Ti-20Zr-5Ta-2Ag alloy was characterised concerning its microstructure, morphology, mechanical properties, its passive film composition and thickness, its long-term electrochemical stability, corrosion resistance, ion release rate in Ringer solution of acid, neutral and alkaline pH values and antibacterial activity. The new alloy has a crystalline α microstructure (by XRD). Long-term XPS and SEM analyses show the thickening of the passive film and the deposition of hydroxyapatite in neutral and alkaline Ringer solution. The values of the electrochemical parameters confirm the over time stability of the new alloy passive film. All corrosion parameters have very favourable values in time which attest a high resistance to corrosion. Impedance spectra evinced a bi-layered passive film formed by the barrier, insulating layer and the porous layer. The monitoring of the open circuit potentials indicated the stability of the protective layers and their thickening in time. The new alloy releases (by ICP-MS measurements) very low quantities of Ti, Zr, Ag ions and no Ta ions. The new alloy exhibits a low antibacterial activity.

Functional Effects of the Deposition Substrate on the Electrochemical Behavior of Platinum Particles

In the present work, polytyramine (PTy) and cobalt oxide (Co3O4) were used as substrates for platinum electrodeposition, in order to obtain electrode systems with electrocatalytic properties. PTy and Co3O4 were previously electrochemically deposited on graphite and boron-doped diamond (BDD) supports, respectively. Anodic oxidation of methanol was used as a test-reaction for assessing possible functional effects of the substrate on the electrochemical behavior of the Pt particles. It was found that, when deposited on PTy or cobalt oxide, the electrocatalyst exhibits higher activity and is less susceptible to fouling, via strong adsorption of reaction intermediates. X-ray photoelectron spectroscopy (XPS) measurements suggested that these features can be ascribed to the presence of –OH functional groups at the surface of the Pt particles. It was also found that electrodeposition of platinum on BDD resulted only in metallic Pt and Pt(OH)2, whereas when graphite was used as substrate the presence of platinum oxides was also evident. During prolonged anodic oxidation of methanol, a higher stability of the –OH groups from the platinum surface was observed for graphite-supported Pt particles, compared to the case when bare BDD was used as support. # 2012 The Japan Society of Applied Physics