A novel approach for non-destructive residual life-time estimation of coatings for high temperature applications is presented. Coatings of ferromagnetic Cr-doped AlN are applied on a paramagnetic metallic substrate (Ni-base alloy) using a diffusion treatment. The Cr-content in AlN achieved by this procedure is about 2at.%. Applied on a metallic plant component (turbine blade, chemical reactor, boiler tubes etc.) these coatings serve simultaneously two functions. On the one hand they deliver protection against high temperature corrosion because they act as a reservoir phase for alumina and chromia scale formation, while on the other hand due to their magnetic properties they can be used as a depletion sensor for the reservoir of the protective elements. Knowledge about the oxidation kinetics combined with monitoring of the coating's magnetic moment before and after oxidation can allow prediction of its residual life-time. Magnetic investigations of the coating indicate soft ferromagnetic behaviour. The ferromagnetic domains are visualised with the help of magnetic force microscopy (MFM). During oxidation of the coating a change in the microstructure of the alloy subsurface zone takes place. This is characterised by a gradual decrease of the amount of the ferromagnetic Al(Cr)N-phase and by the formation of new ferromagnetic precipitates of Ni and Fe throughout the diffusion zone. Their influence on the net magnetic moment of the coating–alloy system is discussed.

Kurzfassung Auf Grund ihrer guten mechanischen Eigenschaften bei hohen Temperaturen und ihrer geringen Dichte besitzen Legierungen auf der Basis der intermetallischen TiAl-Phase ein hohes Potential in einer Reihe industrieller Anwendungen. Mit einer Dichte von rund 4 g/cm 3 sind Bauteile aus TiAl-Legierungen etwa um die Hälfte leichter als diejenigen aus aktuell verwendeten hochtemperaturbeständigen Werkstoffen (Nickelbasislegierungen oder Stähle). Abhängig von Zusammensetzung und Herstellungsprozess können unterschiedliche Mikrostrukturen entwickelt werden. Die Gefüge von verschiedenen Ti- und TiAl-Legierungen (α-Ti, α2-Ti3Al, orthorhombisches Ti2AlNb und γ-TiAl) wurden metallographisch untersucht, wobei verschiedene Ätzmittel zur Anwendung kamen. Zusätzlich wurden die Legierungen mit anderen analytischen Verfahren (Rasterelektronenmikroskopie, Elektronenstrahlmikroanalyse, Röntgendiffraktometrie) charakterisiert.

Because of their good mechanical properties at high temperatures and their low density, the intermetallic TiAl phase based alloys have great potential for a use in a range of industrial applications. Having a density of about 4 g/cm(3), TiAl-alloy components are about half of that of the weight of the high temperature resistant materials currently in use (nickel based alloys or steels). Depending upon the composition and manufacturing process used, a number of different microstructures can be adjusted for these alloys. In this article, the microstructures of different Ti and TiAl alloys (alpha-Ti, alpha(2)-Ti(3) Al, orthorhombic Ti(2)AlNb and gamma-TiAl) were investigated using different etchants. In addition the alloys were also characterised using other analytical methods (scanning electron microscopy, electron probe microanalyser and X-ray diffraction).