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 tran sformations. In the final stages of phase development, the metastable Theta-alumina transforms to the equilibrium alpha-alumina phase. Image s formed using the photoluminescence from trace Cr3+ impurities in the scale, in conjunction with scanning electron microscopy, indicate tha t the ct-alumina forms by a nucleation and growth process within the T heta-alumina matrix. Lateral impingement of the growing alpha-alumina islands coincides with the reported drop in oxidation kinetics to thos e characteristic of long-term oxidation of NiAl. Unlike most interface -controlled transformations, the kinetics of the Theta-to-alpha transf ormation in the lateral growth are found to be logarithmic, Possible c auses of these unusual kinetics are proposed. (C) 1997 American Insitu te of Physics.