Modification of alumina scale formation on FeCrAlY alloys by minor additions of group IVa elements D. Naumenko Æ V. Kochubey Æ L. Niewolak Æ A. Dymiati Æ J. Mayer Æ L. Singheiser Æ W. J. Quadakkers Received: 30 November 2007 / Accepted: 8 April 2008 / Published online: 23 April 2008 Ó Springer Science+Business Media, LLC 2008 Abstract The effect of Ti, Zr and Hf minor additions on the alumina scale formation on a high-purity, FeCrAlY model alloy has been studied. Thermogravimetry at 1,200– 1,300 °C in Ar–20%O 2 and two-stage oxidation using 18 O-tracer were combined with characterisation by electron microscopy and sputtered neutral mass spectroscopy. After oxidation, the incorporation of Hf and Zr into the scale was far more substantial than that of Ti. This is explained by the higher thermodynamic stability of the Zr- and Hf-based oxides because the incorporation occurred to a large extent via an internal oxidation process. The scale growth kinetics is accelerated by incorporation of zirconia precipitates that provide short-circuit paths for oxygen diffusion, reduce the scale grain size and cause formation of porosity. In con- trast, the incorporation of Hf-containing oxides has no such accelerative effect on the scale growth kinetics. Introduction FeCrAl alloys are materials used in many high-temperature applications where a high oxidation resistance is required. The alloys are commonly used as construction materials for heating elements, hot zone furnace furniture and more recently in car catalysts and industrial gas heaters. The oxidation resistance is provided by formation of an alumina surface scale, which protects the alloy from rapid envi- ronmental degradation. The protective properties of the alumina scale, in particular scale adherence during cyclic oxidation, are improved by the minor additions (less than 1 mass%) of reactive elements (RE), such as Y, La, Ce, Zr, Hf either in metallic form or as oxide dispersions (ODS) [1, 2]. This positive RE effect is attributed to gettering alloy sulphur impurity [3, 4], thus preventing deleterious sulphur segregation to the alumina/alloy interface and to the sup- pression of outward cationic transport through the alumina scale [5, 6], thus reducing vacancy injection into the oxide/ metal interface. Recent microstructural studies [7] indicate that the extent of the cationic transport suppression differs between different REs and depends on their exact contents and combinations. The minor alloy chemistry has been recognised as a crucial factor, which significantly affects the oxidation limited lifetime of the FeCrAl materials, i.e. the time to critical Al depletion for the scale formation followed by the catastrophic breakaway oxidation. Large lifetime differences were reported for alloys of virtually the same major composition, however, with variations of RE and/or impurity contents [8, 9]. Commercial FeCrAl alloys frequently contain several reactive elements [10, 11]. Most of the advanced FeCrAl materials are alloyed with group IIIa elements, Y (less commonly La or Ce) along with group IVa elements (Ti, Zr, Hf). While the group III elements are added to improve the oxidation resistance as discussed above, the primary reason for adding group IVa elements is to facilitate the manufacturing of wrought alloy products, such as ultra thin foils for car catalyst substrates. Ti, Zr and Hf have been found to decrease the DBTT temperature and to prevent the formation of Cr carbides at the alloy grain boundaries, which lead to embrittlement problems during hot rolling D. Naumenko (&)  V. Kochubey  L. Niewolak  L. Singheiser  W. J. Quadakkers Forschungszentrum Ju ¨lich GmbH, IEF-2, 52425 Ju ¨lich, Germany e-mail: d.naumenko@fz-juelich.de A. Dymiati  J. Mayer Gemeinschaftslabor fu ¨ r Elektronenmikroskopie, RWTH-Aachen, 52074 Aachen, Germany 123 J Mater Sci (2008) 43:4550–4560 DOI 10.1007/s10853-008-2639-5