Lateral growth kinetics of alpha-alumina accompanying the formation of a protective scale on (111)NiAl during oxidation at 1100 degrees C

During the early stages of oxidation of (111) NiAl at 1100 degrees C, an alumina scale forms and undergoes a series of allotropic phase transformations. In the final stages of phase development, the metastable Theta-alumina transforms to the equilibrium alpha-alumina phase. Images formed using the photoluminescence from trace Cr3+ impurities in the scale, in conjunction with scanning electron microscopy, indicate that the ct-alumina forms by a nucleation and growth process within the Theta-alumina matrix. Lateral impingement of the growing alpha-alumina islands coincides with the reported drop in oxidation kinetics to those characteristic of long-term oxidation of NiAl. Unlike most interface-controlled transformations, the kinetics of the Theta-to-alpha transformation in the lateral growth are found to be logarithmic, Possible causes of these unusual kinetics are proposed. (C) 1997 American Insitute of Physics.