Segregation, precipitation, and α-α’ phase separation in Fe-Cr alloys: a multi-scale modelling approach

Kuronen, A. and Granroth, S. and Heinonen, M. H. and Perälä, R. E. and Kilpi, T. and Vitos, Levente (2015) Segregation, precipitation, and α-α’ phase separation in Fe-Cr alloys: a multi-scale modelling approach. PHYSICAL REVIEW B, 92 (21). ISSN 2469-9950

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Abstract

Iron-chromium alloys, the base components of various stainless steel grades, have numerous tech- nologically and scientifically interesting properties. However, these features are not yet sufficiently understood to allow their full exploitation in technological applications. In this work, we investigate segregation, precipitation, and phase separation in Fe-Cr systems analysing the physical mechanisms behind the observed phenomena. To get a comprehensive picture of Fe-Cr alloys as a function of composition, temperature and time the present investigation combines Monte Carlo simulations us- ing semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy. In order to obtain a general picture of the relation of the atomic interactions and properties of Fe-Cr alloys in bulk, surface, and interface regions several complementary methods has to be used. Using Exact Muffin-Tin Orbitals method the effective chemical potential as a function of Cr content (0–15 at.% Cr) is calculated for a surface, second atomic layer and bulk. At ∼10 at.% Cr in the alloy the reversal of the driving force of a Cr atom to occupy either bulk or surface sites is obtained. The Cr containing surfaces are expected when the Cr content exceeds ∼10 at.%. The second atomic layer forms about 0.3 eV barrier for the migration of Cr atoms between bulk and surface atomic layer. To get information on Fe-Cr in larger scales we use semiempirical methods. Using combined Monte Carlo molecular dynamics simulations, based on semiempirical potential, the precipitation of Cr into isolated pockets in bulk Fe-Cr and the upper limit of the solubility of Cr into Fe layers in Fe/Cr layer system is studied. The theoretical predictions are tested using spec- troscopic measurements. Hard X-ray photoelectron spectroscopy and Auger electron spectroscopy investigations were carried out to explore Cr segregation and precipitation in Fe/Cr double layer and Fe0.95Cr0.05 and Fe0.85Cr0.15 alloys. Initial oxidation of Fe-Cr was investigated experimentally at 10−8 Torr pressure of the spectrometers showing intense Cr2O3 signal. Cr segregation and the formation of Cr rich precipitates were traced by analysing the experimental spectral intensities with respect to annealing time, Cr content, and kinetic energy of the exited electron.

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