Papers by Keyword: FeCrAl Alloys

Abstract: First-principles calculations have been carried out to investigate the incipient oxidation mechanism of FeCrAl alloy. It is found that Al,Y,Cr atoms energetically prefer surface sites, and the driving force of Y segregation to surface is strongest. The surface segregation of Y, Al and Cr will suppress the outward diffusion of Fe, form the tight coherent films of Al₂O₃,Cr₂O₃ and RE oxides which can restrain oxygen inward diffusion, as a result, leading to the decrease of the growth of oxide films. The O adsorption process at Fe surface are found to be spontaneous, and our calculations predict Al, Cr, Y segregation at Fe surface is beneficial for decreasing the oxidation rate of FeCrAl alloy. The interaction between O and Fe, Cr, Al, Y atoms exists both ionic and covalent binding characteristics. Also Al, Y alloying increases ionic and covalent binding between Al, Y and O, which speed selective oxidation of Al and Y, and hence improves the oxidation performance of FeCrAl containing Y alloy.

Abstract: The early stages of the scale growth process were studied of two FeCrAl alloys: one synthetic (Fe23Cr5Al) and one commercial (Kanthal APM alloy). In addition, Yttrium was implanted to the Fe23Cr5Al alloy. Oxidation exposures were carried out at 1000°C using two-stage-oxidation exposures in atmospheres containing significantly different amounts of ¹⁸O₂-tracer. The scales were analyzed in terms of SIMS, PLS and SEM. The distribution of oxygen isotopes which corresponded to the location of new oxide formation, the scale phase composition, scale morphology and microstructure were determined which enabled description of the scale evolution on all studied alloys. Similar evolution stages were observed, but minor differences were related to the rate of disappearing of the transient aluminium oxides.

Abstract: In order to contribute to a better understanding of the processes, which occur in the structure of FeCrAl alloys during oxidation, in situ – studies by two-dimensional high temperature X-ray diffraction (2D-XRD) using a global area detector and grazing incidence with a monocapillary have been performed. The 2D-XRD yields simultaneously with the identification of the oxides and their formation kinetics information about the grain size, grain shape, stresses, texture as well as grain movements during the oxidation process of both oxide and metal. Two commercial FeCrAl alloys with different reactive element additions were investigated in the temperature range of 850°C to 1100°C. In the range of 1100°C already in the first 5 min the alloy grains become coarse and appear as single spots along the lateral profile in the 2D-XRD pattern. Dynamic displacement of these spots along the 2θ – axis during the exposure indicates the formation of stresses, which differ from grain to grain. Initially, re-crystallisation and grain growth dominate and grains disappear and new grains appear. On further exposure the grains twist continuously with 1° to 3° per hour, depending on the alloy. The “dancing grain” effect of the alloy is probably related with growth stresses in the oxide scale and influenced by the bulging of the foil. Simultaneously, α-Al2O3 is detected from the first pattern after 5 min and shows an enhanced formation rate in the first 15 min of the oxidation. The α-Al2O3 grains are with 0.3 to 0.4 4m extremely fine and, a dense well adherent scale is observed even after 1 h.