Dm. Lipkin et al., LATERAL GROWTH-KINETICS OF ALPHA-ALUMINA ACCOMPANYING THE FORMATION OF A PROTECTIVE SCALE ON (111)NIAL DURING OXIDATION AT 1100-DEGREES-C, Applied physics letters, 70(19), 1997, pp. 2550-2552
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.