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Diffusion-plasticity coupling during selective oxidation of metal alloys

Subject Area Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
Term from 2017 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 392017294
 
Final Report Year 2023

Final Report Abstract

The aim of this common ANR-DFG proposal was to study the effect of plastic deformation on bulk and grain boundary diffusion of Cr in pure Ni and in Ni-Cr alloys in an extended temperature range including low temperatures and using two complementary techniques: radiotracer method (UM, Germany) and SIMS (Mines, France). Solid-state diffusion is one of the key phenomena in many time- and temperature-dependent metallurgical processes involved either in the manufacturing, joining, or thermo-mechanical treatment. At low temperatures (< 500 °C), accelerated diffusion-related phenomena are observed in industrial alloys but their kinetics cannot often be explained by an extrapolation of the high temperature data. The DIPLOX project proposed to extend the previous studies devoted to the effect of plastic deformation on Cr diffusion in Ni, focusing more specifically on grain boundary (GB) diffusion and using the complementarity of the radiotracer and SIMS techniques with the final aim to integrate the measured diffusion coefficients in an oxidation kinetics code in order to take into account the effect of plastic deformation in the Cr selective oxidation. The project aimed at (i) providing information on the relationship between GB structure and GB diffusion rates, (ii) providing direct experimental estimates of Cr segregation factor at Ni high-angle grain boundaries, (iii) studying the effect of a low and severe plastic deformation on bulk and GB diffusion and its consequence on alloy oxidation. The Münster team was primarily focused on the radiotracer measurements of Cr diffusion in selected low-sigma Ni grain boundary (using a bicrystal) and in deformed Ni polycrystals. All these goals were successfully approached. First at all, a close cooperation of German and France teams is now established which resulted in four common publications (and two further common papers are in preparation now). A versatile (tracer diffusion, electron microscopy, DFT calculations, MD simulation) investigation of Cr diffusion in Ni ∑11(110) grain boundary verified a structure heterogeneity of the interface. The combination of the two methods of diffusion analysis allowed unprecedented insights into hierarchy of diffusion paths in severely deformed materials and a direct proof of co-existence of high-angle grain boundaries in both relaxed and non-equilibrium states after severe plastic deformation. Furthermore, structure-property relations for grain boundaries in multi-component alloys were elaborated with unprecedented details including an intricate interplay of segregation, diffusion, elastic stresses, phase decomposition and precipitation. In sum, the DIPLOX project provided new fundamental results on the coupling between the microstructure, strain, grain boundary diffusion, and oxidation. The outputs of the project contributes to better understanding of accelerated degradation mechanisms observed on industrial components at low temperatures, which are related to the accelerated diffusion processes. In particular, the large variability in grain boundary diffusion coefficients depending on their “metallurgical state” explains the uneven oxidation front observed when working on materials with surface states more representative of industrial conditions.

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