Metastable alumina modifications

Furthermore there is a need of detailed information about the formation of metastable alumina modifications during the isothermal oxidation of p-NiAl and about the influence of reactive elements on the. alumina modifications. At temperatures below 900°C and in the initial state the formation of -y-Al203 was reported by Doychak [14] on Zr-doped 0-NiAl and on undoped 0-NiAl by Brumm and Grabke [4]. The initial formation of -Al2Oa on 0-NiAl and its transformation to a-Al203 at temperatures higher than 950°C was reported by Brumm and Grabke [4], Rybicki and Smialek [8] and Doychak [14]. 

The formation and conversion of the metastable alumina modification is also indicated by the time dependent changes of the parabolic rate constant kp. From initially high values corresponding to the fast growth of the metastable alumina modification, the kp values decrease to a lower level corresponding to the slow growth of the a-... 

Values of the oxidation rate constants kp were determined from the slope of plots of the square of mass gain per area versus time. It is possible to separate the regions for the growth of metastable alumina modifications (I) in the first hours of the experiments and for protective a-alumina growth (II) after longer oxidation times. The... 

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