SKM 2021 – wissenschaftliches Programm
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MM: Fachverband Metall- und Materialphysik
MM 4: Topical Session Interface-Dominated Phenomena - Diffusion
MM 4.3: Vortrag
Montag, 27. September 2021, 16:00–16:15, H2
Theoretical aspects of the reactive element effect — Andy Chen1, Arthur Heuer1, Matthew Foulkes2, and •Michael Finnis2 — 1CWRU, Cleveland OH, U.S.A — 2Imperial College London, UK
We are interested in the mechanism of growth of alumina scales on aluminium-containing alloys, and how the presence of certain elements such as Hf, Zr and Y operates to retard the growth of such scales, thereby improving their adherence and protective quality. These questions have been open for several decades, and are still not completely resolved [1]. We report here some recent results of experiments and theory, and in-sights that we have obtained. From tracer diffusion experiments, followed by Time of Flight - Secondary Ion Mass Spectroscopy (TOF-SIMS), diffusion coefficients of O and Al through alumina scales have been estimated and compared with measurements of weight-gain. Noting that grain boundaries are now believed to be the route for ion and electron diffusion, both of which are necessary for oxidation, we have applied Density Functional Theory to calculate the segregation energy of these reactive elements to a range of sites on grain boundaries in α-Al2O3,using our previous models of grain boundary structure [2]. Segregation of Hf and Zr and oxygen vacancies to the grain boundaries is favoured, with a spread of energies that we assess, and our calculations suggest how this may indirectly reduce the mobility of ions or electrons. Shortcomings of the classical theory of oxidation [3], originally due to Wagner, are discussed. The evidence is now overwhelming that the conventional hopping of cation and anion vacancies is not a relevant process.
[1] W. T. Chen, B. Gleeson, and A. Heuer. Oxidation of Metals, 92(3-4):137-150, 2019. [2] Hannes Guhl, Hak-Sung Lee, Paul Tangney, W. M. C. Foulkes, Arthur H. Heuer, Tsubasa Nakagawa, Yuichi Ikuhara, and Michael W. Finnis. Acta Materialia, 99:16-28, 2015. [3] A. Atkinson. Reviews of Modern Physics, 57:437-470, 1985.