Grain Boundary Chemistry and Transport Through Alumina Scales on NiAl Alloys

Abstract

It is widely accepted that the growth of protective a-Al 2 O 3 scales on Nibased 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.

Y/Hf-doped AlCoCrFeNi high-entropy alloy with ultra oxidation and spallation resistance

“…It is well known that the equiaxed zone is formed by outward aluminum diffusion, and meanwhile the columnar zone is formed by inward oxygen diffusion [5,18]. It has been also demonstrated by Boll and Pint et al [49] that outward flux of Al is much smaller than the flux for the inward diffusion of O in the oxide scale for the Hf or Zr-doped NiAl and thus the contribution of the outward Al flux has negligible effect on the increase of oxide thickness on doped NiAl. Therefore, the ratio of the thickness of the characteristic equiaxed zone to the total oxide thickness can show respective contribution of outward diffusion of aluminum and inward diffusion of oxygen during oxidation [18].…”

Transition metal attenuated mechanism for protective alumina formation from first principles

“…In this context, a benecial transient rapidly growing "messy" alumina was recently described, 8 where the cause for the initial accelerated oxidation was the cooperative effect of water and yttria. Along this line, oxygen diffusivities in RE doped scales are several orders of magnitude greater than the Al 3+ diffusivities 9 despite the fact that activation barriers for the two have been reported to be of similar size ($6 eV). 10 Indeed, the discrepancy between measured and calculated values for oxygen vacancy activation barriers has been coined the "corundum conundrum" that is partly owing to shortcomings in employing sintered alumina to model thermally grown alumina.…”

Oxidation behavior and outward diffusion of Al along oxide grain boundaries of FeCrAl alloys overdoped with Zr and Hf by means of Atom Probe Tomography (APT)

“…So, the analysis of the new oxide formed along the GB ridge volume as a function of time and alumina scale thickness can give quantitative data on the magnitude of the GB ux and aluminum diffusivity. Nychka's paper [29] and Torben's paper [30] give a quantitive calculation of the Al GB ux based on GB ridge volume, where the presented data in both papers agree with the 1st Fick's law.…”