Dmitry Naumenko

Current Thoughts on Reactive Element Effects in Alumina-Forming Systems: In Memory of John Stringer

In memory of John Stringer (1934–2014), one of the leaders in studying the reactive element (RE) effects, this paper reviews the current status of understanding of the effect of RE dopants on high-temperature oxidation behavior, with an emphasis on recent research related to deploying alumina-forming alloys and coatings with optimal performance in commercial systems. In addition to the well-known interaction between indigenous sulfur and RE additions, effects have been observed with C, N, and O found in commercial alloys and coatings. While there are many similarities between alumina-forming alloys and coatings, the latter bring additional complicating factors such as the effects of O incorporation during thermal spraying MCrAlY coatings, coating roughness, and heat treatments that must be considered in optimizing the beneficial dopant addition. Analogies can be seen between RE effects in alloys and in the substrates beneath diffusion M–Al coatings. Recently, there has been more interest in the influence of mixed oxidant environments, since these may modify the manifestation of the RE effect. Finally, some thoughts are provided on optimizing the RE benefit and modeling oxidation of RE-doped alloys.

Batch to batch variations in the oxidation behaviour of alumina forming Fe-based alloys

Two batches of a commercial alumina forming Fe based ODS alloy were studied in respect to their oxidation behaviour in the temperature range 1100–1300°C. In the early stages of exposure the oxidation kinetics of the two materials appeared to be virtually identical, whereas strong differences in oxide growth rate occurred after longer exposure times. Extensive microstructural studies using optical metallography, SEM, TEM and MCs+-SIMS showed that these differences are related to a larger volume fraction of Ti(C,N) precipitates in one of the studied batches. The Ti(C,N) precipitates present in this batch were found to become incorporated into the growing oxide scale. Transformation of these particles into Ti-rich oxides leads to a volume change resulting in microcracking of the scale and subsequent enhanced oxygen access. The results show that the question whether Ti alloying additions are beneficial or detrimental in respect to alumina scale properties strongly depends on the form in which Ti prevails in the alloy. Unterschiede im Oxidationsverhalten zwischen verschiedenen Chargen von aluminiumoxidbildenden Fe-Basislegierungen Das Oxidationsverhalten von zwei Chargen einer kommerziellen FeCrAl-Legierung wurde untersucht im Temperaturbereich zwischen 1100 und 1300°C. Wahrend bei kurzen Auslagerungszeiten die Oxidationskinetik beider Chargen nahezu gleich war, wurden nach langeren Zeiten starke Unterschiede in den Oxidschichtwachstumsraten beobachtet. Mikrostrukturelle Untersuchungen mittels optischer Metallographie, REM, TEM und MCs+-SIMS ergaben, das diese Unterschiede bei langen Auslagerungszeiten mit der Anwesenheit von feinen Ti(C,N)-Ausscheidungen in einer der untersuchten Chargen korreliert ist. Die Carbonitridteilchen werden in die wachsende Oxidschicht aufgenommen und anschliesend in Ti-reiche Oxide umgewandelt. Die Umwandlung ist mit einer Volumenanderung verknupft und fuhrt zur Bildung von Poren und Mikrorissen in der Deckschicht, und anschliesend zu einer beschleunigten Oxidation. Die Ergebnisse zeigen, das die Frage, ob Ti-Zusatze einen positiven oder negativen Einflus auf die Deckschichteigenschaften haben, abhangt von der Form in der das Ti in der Legierung vorliegt.