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John Ågren Dept. of Matls. Sci. & Engg. Royal Institute of Technology Stockholm, 100 44 Sweden Acknowledgement: Reza Naraghi, Samuel Hallström, Lars Höglund and Malin Selleby Oxidation and diffusion in oxides – a progress report

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability MGI ICME 3D-MS New Research directions in Materials science and Engineering:

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Content 1.Aim of work 2.Issues in modelling of oxidation 3.Bulk diffusion in oxides a)Summary and present stage b)Some issues in the diffusion modelling c)Defect structures for oxygen diffusion and results 4.Grain boundary diffusion 5.Kirkendall porosity

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Predict oxidation: -Sharp-interface methods – DICTRA -Diffuse-interface methods – phase-field For example: - Oxidation of steels - Degradation of superalloy coatings We need: - Mathematical expressions for oxidation rate in terms of diffusional flux as function of gradients in composition or chemical potentials. -Parameters that characterize a given material 1. Aim of work

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability 2. Issues in modelling of oxidation Predict: Rate of oxidation -External (growth of external layers) -Internal (internal oxide particles) -Grain boundaries in metal and in oxides What oxides form? Porosity -Kirkendall effect Jonsson et al. 2006 Giggins and Pettit 1971 Internal oxidation of Mn, Si, Cr etc during carburization (Holm et al. 2010)

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability External oxidation - general Inward and outward growth Atmosphere with O 2 Metal Oxide Me O xMe+yO->Me x O y - Oxygen diffusion in the oxide layer gives inward growth - Metal diffusion in the oxide layer gives outward growth

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Growth of external layers Metal Oxide Atmosphere with O 2 Oxygen content Distance x Diffusion and flux balances in sharp-interface modelling!

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Growth of internal oxide particles Oxygen must diffuse through external oxide layers and dissolve in the metallic matrix. Rate may be controlled by oxygen diffusion, alloy element diffusion or both.

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability 3. Bulk diffusion in oxides 3.a) Summary and present stage Kinetic parameters from model. Darken’s thermodynamic factor, e.g. from Calphad analysis. Base models on a vacancy mechanism!

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Present stage The models have been implemented in DICTRA. Data base now contains diffusional mobilities of -Wüstite (Halite) Fe and O -Magnetite (Spinel) Fe Cr O -Hematite (Corundum) Fe, Cr, O

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability 3.b) Some issues in the diffusion modelling Phase equilibria and driving forces – Calphad thermodynamics Defect structures -Vacancies and interstitials -Oxygen substitutional -Fe, Cr etc interstitial Diffusing species -Ions -Neutral atoms -Positive holes Rate equations Fe-O Calculated from Sundman 1991.

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Diffusing species: in ionic systems – two extremes Electronic conduction compared to diffusion: -Much faster (charge does not need to be included) -Much slower (ions diffuse as species) If electronic conduction and diffusion are about the same rate (electronic conduction needs to be accounted for)

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Fe diffusion in spinel (lattice-fixed frame of reference, Hallström et al. 2011) (Fe +2,Fe +3 ) 1 (Fe +2,Fe +3,Va) 2 (Va,Fe +2 ) 2 (O -2 ) 4 Thermodynamic model (Sundman 1991): Extra octahedral Va Extra octahedral Fe +2 octahedral fcc tetrahedral Interstitial sites 1/8*8 sites 4 sites

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Normal octahedral extra octahedral sites sites

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Experimental data on Fe tracer diffusion in spinel - Optimization of Fe mobilities (Hallström et al. 2011)

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Töpfer et.al. 1995 Alloy elements in spinel (lattice fixed frame of reference) Töpfer et.al. 1995 Cr content of inward growing spinel will inherit Cr content of alloy.

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability 3.c) Defect structures for oxygen diffusion and results Wüstite (Halite) Yamagochi et al. 1982 found that oxygen diffusion rate increases with oxygen potential. Thus oxygen vacancies cannot be the dominating mechanism. We thus postulate that oxygen diffuses on interstitial (cation ) sites:

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Oxygen tracer diffusion in Wüstite. Experiments from Yamaguchi et al 1982

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability 3.c) Defect structures for oxygen diffusion and results Magnetite (Spinel) O tracer diffusion in spinel (lattice-fixed frame of reference) T= 1150°C Millot and Niu 1996

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability At low oxygen potentials it seems reasonable that oxygen diffusion is assisted by anion vacancies. The lower the oxygen potential, the higher fraction of vacancies and rate of diffusion. (Fe +2,Fe +3 ) 1 (Fe +2,Fe +3,Va) 2 (Va,Fe +2 ) 2 (O -2, Va) 4 Modification of Sundman’s thermodynamic model: octahedral fcc tetrahedral Interstitial sites 1/8*8 sites 4 sites

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability (Fe +2,Fe +3 ) 1 (Fe +2,Fe +3,Va) 2 (Va,Fe +2,O -2 ) 2 (O -2,Va) 4 Modification of Sundman’s thermodynamic model: octahedral tetrahedral Interstitial sites 1/8*8 sites 4 sites Alternativ 1 At high oxygen potentials this model predicts very low vacancy fractions. The lower the oxygen potential, the higher fraction of vacancies and rate of diffusion. This is not in agreement with experiments! fcc

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability This may work but gives too a strong increase in vacancy content at high oxygen potentials. It also requires a complete re-assessment of Fe- O! Schematic diagram (Millot and Niu 1996)

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Modification of Sundman’s thermodynamic model: Alternativ 2 (Fe +2,Fe +3 ) 1 (Fe +2,Fe +3,Va) 2 (Va,Fe +2 ) 2 (O -2, Va) 4 octahedral fcc tetrahedral Interstitial sites 1/8*8 sites 4 sites Could the vacancy formation energy be chosen such that:

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability 1073 K1423 K At present we have (Millot and Niu 1996) (Giletti and Hess 1988)

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability 3.c) Defect structures for oxygen diffusion and results Hematite (Corundum) ”End members” and plane of electro neutrality

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability We postulate

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Experiments: Amami et al. 1999

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability 4. Grain boundary diffusion NiO

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability 5. Kirkendall porosity

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Maruyama et al. 2004 Voids form as a consequence of a divergence in the oxygen flux.

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Schematics of Kirkendall effect in magnetite μ Fe Distance

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Oxygen mobility Distance Oxygen flux Distance

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Distance Pores

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability T. Jonsson et al. 2008 FeO Fe 3 O 4 Fe 2 O 3

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NIST Diffusion workshop 2012 Excellence – Relevance - Availability Summary DICTRA can now handle diffusion in oxides allowing prediction of oxidation. Data base now contains diffusional mobilities of Wüstite (Halite): Fe and O Magnetite (spinel): Fe Cr O Hematite (Corundum): Fe, Cr, O Grain boundary diffusion is taken into account in a simplified manner. Oxygen diffusion may cause Kirkendall effect and porosity.

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