DECHEMA

About: DECHEMA is a nonprofit organization based out in Frankfurt am Main, Germany. It is known for research contribution in the topics: Corrosion & Oxide. The organization has 756 authors who have published 1307 publications receiving 25693 citations.

The initial stages in the oxidation of TiAl

Abstract: The oxidation behaviour of titanium aluminides containing 36 wt.-% Al (Ti36Al) and 35 wt.-% Al plus 5 wt.-% Nb (Ti35Al5Nb) has been investigated by electron microscopic methods with emphasis on transmission electron microscopy (TEM). The oxidation experiments were carried out at 800 to 1000°C in laboratory air for 0.5 h to 4 h. In addition thermogravimetric measurements were made. It has been shown that the shortterm oxidation of TiAl can be divided into two stages. In stage I the preferred formation of aluminium oxide leads to an aluminium depletion of the metal subsurface zone and the subsequent formation of titanium nitrides which enhances the oxidation rate. After consumption of the depletion layer a repeated cycle of aluminium oxide formation, subsequent local depletion of the metal subsurface zone in Al and consumption of the Ti-rich metal phase by nitride formation is observed leading to linear oxidation behaviour (stage II). In the niobium containing alloy the dissolution of alumina in titania is decreased and thus the formation of aluminium oxide at the metal/oxide interface is favoured. By electron diffraction it has been found that the aluminium oxide formed at the metal/oxide interface most probably is an aluminium oxynitride Al27O39N. The aluminium depleted metal phase has been analyzed to consist of α2-Ti3Al and a new cubic phase with a composition between of α2-Ti3Al and γ-TiAl. Die Anfangsstadien bei der Oxidation von TiAl Das Oxidationsverhalten von Titanaluminiden mit einem Aluminiumgehalt von 36 Gew.% (Ti36Al) und 35 Gew.%Al, sowie 5 Gew.% Nb (Ti35Al5Nb) wurde mit Hilfe von elektronenmikroskopischen Methoden untersucht, wobei ein Schwerpunkt auf der Transmissionselektronenmikroskopie (TEM) lag. Isotherme Auslagerungsversuche wurden bei 800 bis 1000°C fur 0,5 bis 4 h an Laborluft durchgefuhrt. Zusatzlich wurden thermogravimetrische Messungen vorgenommen. Es wurde gezeigt, das die Anfangsoxidation von TiAl in zwei Stadien eingeteilt werden kann. Im Stadium I fuhrt die bevorzugte Oxidation von Aluminium zu einer Aluminiumverarmung in der Metallrandzone und der nachfolgenden Bildung von Titannitriden, mit einer erhohten Oxidationsgeschwindigkeit als Folge. Nach dem Aufbrauchen der Verarmungszone wird ein wiederholter Zyklus aus Aluminiumoxidbildung, nachfolgender lokaler Aluminiumverarmung der Metallrandzone und dem Aufbrauchen des Ti-reichen Metalls durch Nitridbildung beobachtet, der lineares Oxidationsverhalten zur Folge hat (Stadium II). In der Legierung, die Niob enthalt, wird die Auflosung des Aluminiumoxids im Titanoxid vermindert und dadurch die Bildung von Aluminiumoxid an der Metall/Oxid-Grenzflache begunstigt. Mit Hilfe der Elektronenbeugung wurde gefunden, das das Aluminiumoxid, das sich an der Metall/Oxid-Grenzflache bildet, sehr wahrscheinlich ein Aluminiumoxynitrid Al27O39N ist. Bei der an Aluminium verarmten Metallphase wurde festgestellt, das sie aus α2-TiAl und einer neuen kubischen Phase mit der Zusammensetzung zwischen α2-Ti3Al und γ-TiAl besteht.

Development of coatings for protection in specific high temperature environments

Abstract: Metallic and intermetallic coatings are widely used in jet engines and land-based gas turbines for oxidation and corrosion protection in the hotter parts of the engines. However there is a significant number of industrial processes where the use of protective coatings at high temperatures could contribute to a significant extension of life-time or an increase in operation temperature and thus efficiency. Examples of such industries are incineration and gasification of waste, biomass and coal, chemical process industries and petrochemical plants where highly aggressive environments are encountered containing species of e.g. carbon, chlorine, sulphur, vanadium or alcalines. Since most of these process environments contain only very low oxygen partial pressures or exhibit high concentrations of extremely aggressive compounds, the conventional, uncoated materials come to their limits. In recent years in laboratory work a number of new types of coatings have been developed for high-temperature applications which include diffusion coatings, overlay coatings and nanotechnological approaches for sealing porosity. In the paper the background of this development and the thermodynamic fundamentals are discussed together with some more recent solutions based on synergistic effects of multi-element coatings. Some results of performance tests of these coatings in sulfidizing, carburizing, chloridizing and vanadate environments will be presented. At the end conclusions can be drawn on the suitability of the different types of coatings for their specific applications.