Abstract
It is widely accepted that the growth of protective α-Al2O3 scales on Ni-based alloys is governed by the inward diffusion of oxygen through the oxide grain boundaries (GB). However, there is also some outward diffusion of metal ions to the surface, but it is difficult to quantify. In this work we apply atomic force microscopy, scanning electron microscopy and transmission electron microscopy to investigate the outward flux of Al, which manifests as the growth of small ridges along the alumina GBs after the removal of the outermost oxide layer by mechanical polishing or focused ion beam techniques followed by additional oxidation. As a model alumina-former, NiAl with Hf and Zr additions was investigated. In comparison to Zr, Hf was found to reduce the outward Al diffusion. This outward diffusion was six orders of magnitude smaller than the O inward diffusion.
Highlights
Alumina is important as a structural material, as well as a functional material in electronics and as a thermally grown external oxide or scale on Al-containing hightemperature alloys
In this work we apply atomic force microscopy, scanning electron microscopy and transmission electron microscopy to investigate the outward flux of Al, which manifests as the growth of small ridges along the alumina grain boundaries (GB) after the removal of the outermost oxide layer by mechanical polishing or focused ion beam techniques followed by additional oxidation
This study describes the initial work aimed at improving the understanding of transport processes in alumina by conducting well-controlled oxidation exposures and applying high-resolution microscopy techniques for characterization of grain boundary chemistry
Summary
Alumina is important as a structural material, as well as a functional material in electronics and as a thermally grown external oxide or scale on Al-containing hightemperature alloys. Oxid Met (2017) 88:469–479 diffusion of O along grain boundaries when a reactive element (RE) is present [1,2,3,4,5,6,7,8,9,10,11]. Some prior growth mechanism studies were confounded by the formation of cubic alumina phases such as h-Al2O3 [7], which grow by outward transport [12]. Recent studies have noted that some concurrent outward Al diffusion along grain boundaries does occur in RE-doped a-Al2O3 [13,14,15]. In order to study outward diffusion, the isotope must be in the alloy or in the oxide beneath a previously formed oxide
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