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

Abstract

During the early stages of oxidation of (111) NiAl at 1100 °C, an alumina scale forms and undergoes a series of allotropic phase transformations. In the final stages of phase development, the metastable Θ-alumina transforms to the equilibrium α-alumina phase. Images formed using the photoluminescence from trace Cr3+ impurities in the scale, in conjunction with scanning electron microscopy, indicate that the α-alumina forms by a nucleation and growth process within the Θ-alumina matrix. Lateral impingement of the growing α-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 θ–to–α transformation in the lateral growth are found to be logarithmic. Possible causes of these unusual kinetics are proposed.