Milan Bizjak

Internal Oxidation of an Ag–1.3 at.% Te Alloy

The internal oxidation of Ag–1.3 at.% Te was studied at 750, 800, and 830°C in pure oxygen (1 atm). The internal oxidation under such high oxygen pressure resulted in formation of two different types of oxide particles and two different fronts of internal oxidation in the internal oxidation zone. The coarser Ag2TeO3 particles were formed through the in situ internal oxidation of Ag2Te particles and the tiny oxide precipitates (most probably also Ag2TeO3) were formed through internal oxidation of tellurium from solid solution. Considering the mechanism of internal oxidation, both diffusionless and diffusive modes were found to be present simultaneously in the oxidation of Ag–1.3 at.% Te alloy. These results were examined with regard to the solubility of tellurium in silver, which was found to be 0.1 at.% Te at 750°C and 0.26 at.% Te at 830°C, as well as the presence and dissolution of Ag2Te particles.

The characterisation of microstructural changes in rapidly solidified Al-Fe alloys through measurement of their electrical resistance

Abstract Microstructural changes of rapidly solidified Al alloy ribbons of various thicknesses and containing different concentrations of iron were analysed. The kinetics and sequence of microstructural changes occurring at a constant heating rate were analysed through the measurement of electrical resistance. This method allowed detection of the transition between metastable and stable phases of rapidly solidified Al – Fe alloy ribbons with various concentrations of iron and various thicknesses. After determining the temperature regions throughout the heating cycle, the quenched microstructure was analysed at transition points using both optical and transmission electron microscopy. Precipitation is the most common reaction for these types of alloys, and the most important alloy formed is Al13Fe4.

The effect of sol–gel boehmite coatings on the corrosion and decarburization of C45 steel

AbstractSol–gel boehmite coatings were produced on the polished surfaces of hypo-eutectoid C45carbon steel by dip coating. After the deposition the coatings were heat treated by drying at RT, 150 and 200 °C. It was found that for crack-free coatings, the drying temperature needs to be ≥150 °C. Properly heat-treated boehmite coatings then improve the corrosion resistance of steel, while proposed alumina coatings derived from the boehmite coatings during annealing of the steel affect the decarburization. The coated steel samples were annealed at temperatures that are typical for the heat-treatment and thermomechanical process of C45 steel, i.e., AC3 (austenitization, thermomechanical process). The sol–gel alumina coating decreased the decarburization rate of the quenched steel during the tempering (≤600 °C), which meant that the hardness reduction on the steel surface was smaller. Alumina coatings have a smaller effect on the decarburization at higher temperatures; however, the metallographic analyses indicate that the coatings can retard the oxidation rate in comparison to the decarburization rate and thus, somewhat increase the visual level of the decarburization. Under annealing conditions (T = 950 °C, t = 0.5–2 h) at which the oxidation rate is higher than the decarburization rate the decarburization process is not important since the decarburized layer is peeled off the steel surface together with the scale. No visual decarburization is then observed.

The effect of pH, fluoride and tribocorrosion on the surface properties of dental archwires

Nickel-titanium and stainless steel are the most commonly used alloys for orthodontic treatments. Even though both are known to be resistant to corrosion, there are circumstances that can lead to undesired situations, like localized types of corrosion attack, wear during sliding of an archwire though brackets and breakdowns due to iatrogenic causes. The aim of this research was to analyse the influence of environmental effects on the corrosion and tribocorrosion properties of NiTi and stainless steel dental alloys. The effects of pH and fluorides on the electrochemical properties were studied using the cyclic potentiodynamic technique. The migration of ions from the alloy into saliva during exposure to saliva with and without the presence of wear was analysed using ICP-MS analyses. Auger spectroscopy was used to study the formation of a passive oxide layer on different dental alloys. It was found that lowering the pH preferentially affects the corrosion susceptibility of NiTi alloys, whereas stainless steel dental archwires are prone to local types of corrosion. The NiTi alloy is not affected by smaller increases of fluoride ions up to 0.024M, while at 0.076M (simulating the use of toothpaste) the properties are affected. A leaching test during wear-assisted corrosion showed that the concentrations of Ni ions released into the saliva exceeded the limit value of 0.5μg/cm2/week. The oxide films on the NiTi and stainless steel alloys after the tribocorrosion experiment were thicker than those exposed to saliva only.