A STUDY OF THE GROWTH OF ALPHA-AL2O3 SCALES USING HIGH-RESOLUTION IMAGING SECONDARY-ION MASS-SPECTROMETRY

The oxide morphology and transport processes occurring during the growth of alpha-aluminum oxide (alpha-Al2O3) at 1,200-degrees-C on an iron-aluminum (Fe-25 wt% Al) alloy and a nickel-aluminum (Ni-32 wt% Al) alloy (beta-NiAl) were investigated. Imaging secondary ion mass spectrometry (SIMS) was used to locate the different isotopic species following two-stage oxidations in O-16(2) and O-18(2). These images (with 0.1 mum lateral resolution) showed the growth of ridges of alpha-Al2O3 occurred by outward transport of Al for NiAl. As the ridges grew taller and wider, inward diffusion of oxygen played a role in the growth process, Mature alumina scales thickened by outward transport of Al and inward diffusion of oxygen. Alpha-Al2O3 formed on Fe-25 wt% Al also grew through a combination of Al and oxygen diffusion, the latter appearing to take place along grain boundaries in the coarse-grained oxide.