Significance of Minor Alloying Additions and Impurities on Alumina Scale Growth and Adherence in FeCrAl Alloys

Ultra-high purity Fe-Cr-Al-Y model alloys with controlled additions of impurities such as phosphorus and carbon, and potentially more beneficial elements such as titanium and zirconium have been prepared by induction melting in water-cooled, silver crucibles. 1 mm thick samples were then prepared by hot and cold rolling and annealing prior to cyclic oxidation in air at temperatures in the range 1100-1300degreesC. Other impurities were kept to a minimum of <10 ppm. scanning electron microscopy, Auger surface analysis and Scanning Transmission Electron Microscopy were used to characterise the samples both before and after oxidation.Weight gain studies during oxidation showed that the high phosphorus containing alloy went into breakaway very quickly, after only 200 hours at 1300degreesC, while the Ti and Zr rich samples lasted for 1900 hours and 3300 hours respectively. In some cases, chromium or titanium rich precipitates were found along the alloy grain boundaries, often associated with carbon, while in other cases precipitates were found along the oxide metal interface. Although phosphorus was found at this interface in some of the samples, it was not always present, and may not be the only contributing factor to the premature breakaway failure of the oxides. A complete review of the microstructural evolution of these samples during oxidation will form the main topic of this paper

Effect of Carbon Content on the Oxidation Behaviour of FeCrAlY Alloys in the Temperature Range 1200-1300°C

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm respectively) were investigated in respect to their oxidation behaviour at 1200 and 1300 degrees C in air. Oxidation tests, with exposure times ranging from a few hundred to several thousands of hours, revealed that the growth rate of the protective alumina scale was hardly affected by the alloy C-content. However, the time to occurrence of breakaway oxidation for the specimens (1 mm thickness) was substantially shorter for the high-than for the low-C alloy. This was primarily caused by poorer oxide scale adherence but additionally by a higher critical Al-content for occurrence of breakaway of the high-C alloy compared to the low-C alloy.Extensive microstructural studies revealed formation of Cr-carbides at the grain boundaries in both alloys. The high-C alloy additionally showed carbide formation at the scale/metal interface, thus deteriorating scale adhesion. Furthermore, inter- and intra-granular carbide precipitation is considered to induce strengthening of the metal, thus hindering relaxation of the thermally-induced oxide stresses by substrate creep. In a series of experiments with variations in the cooling rates, it was verified that carbide formation very likely occurs during specimen cooling