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>The influence of interfacial toughness, as influenced by sulfur and reactive elements, and stress on the cracking and spallation of alumina scales from metallic substrates.
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The influence of interfacial toughness, as influenced by sulfur and reactive elements, and stress on the cracking and spallation of alumina scales from metallic substrates.
Ni-based superalloys, such as single crystal PWA 1480 and 1484, are being used in aircraft engines as turbine blades and vanes. Fe-base alloys (FeCrAl) are used in automobiles (in catalytic converters) and in high temperature furnaces as heating elements. The protection of these alloys against oxidation is provided by the formation of a slow-growing alumina scale. The adherence of the alumina scale to the alloy is necessary to maintain the oxidation resistance under isothermal and cyclic conditions.; It has been established that small additions of reactive elements (Y, Hf and Ce) and removal of impurities such as sulfur from the alloy substantially improve the adherence of the alumina layer to the alloy. Despite a significant amount of research over the past 50 years, there is still much controversy regarding the detailed mechanisms responsible for the improvement of the adherence.; Following previous research reported in the literature, a program was carried out to investigate the factors which affect the adherence or cracking and spallation of protective alumina scale and how the reactive elements and sulfur content affect these factors for Ni-based superalloys (PWA 1480, 1484) and Fe-based alloys (FeCrAl, +Ti, +Y). New advanced techniques such as high resolution SEM, EDS, TEM, STEM, and XRD techniques have been employed to elucidate the "reactive element" and "sulfur" effects. Also, state of the art XRD equipment has been used to measure the strain or stress in the alumina scale at room temperature and at high temperature. The adhesion of the alumina scale to alloys was measured using indentation testing such as Rockwell C.; It was found that the major benefit of adding a reactive element is to tie up sulfur in the alloy and lower residual sulfur which is free to segregate to the alumina/alloy interface. Residual sulfur in the alloy diffuses to the intact alloy/alumina interfaces and voids, resulting in weakening of an otherwise strong interfacial bond.; Classical stress measurement techniques such as Tilting and Rocking techniques were modified with respect to the alumina. In addition, new techniques, the Fixed Incidence Multiplane (FIM) and the Fixed Incidence Tilting (FIT) techniques were developed and shown to be useful for thin oxide scales (t {dollar}<{dollar} 0.5 {dollar}mu{dollar}m thickness). Stresses (e.g. growth stresses) in alumina scales on FeCrAl alloys at 850-1100{dollar}spcirc{dollar}C were measured using the newly developed FIM technique. Until recently, growth stresses were thought to be negligible compared to the total residual stresses. Growth stresses measured in alumina scales on FeCrAl alloys at 1000{dollar}spcirc{dollar}C and 1100{dollar}spcirc{dollar}C were in the range of {dollar}-{dollar}1 GPa, which was higher than previously measured growth stresses. In situ stress measurement results indicated that growth stresses in alumina scales were still high in the presence of reactive elements (Y) and the addition of reactive element did not reduce growth stresses. Rather, it improves the adhesion of the alumina scale and prevents spalling of the alumina scale.
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