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Materials Frontier No.184

Title:Expanded austenite: from fundamental parameters to predicting composition- and stress-profiles
Speaker:Prof. Marcel Somers, Full Professor of Physical Metallurgy at Technical University of Denmark (Kgs. Lyngby, DK) 
Venue:Room302,Materials building H
Inviter:Prof. GU Jianfeng
Academic degrees
1985:  Ir. (= civ. ing.) Physical Metallurgy, Delft University of Technology (DUT), The Netherlands
1989:  Dr.ir. in Physical Metallurgy from DUT, The Netherlands
1989-1990: Philips Centre for Materials Technology and Innovation (Eindhoven, NL)
1990-1997: assistant professor in Physical Chemistry of the Solid State at DUT (Delft, NL)
1997- now: Full Professor of Physical Metallurgy at Technical University of Denmark (Kgs. Lyngby, DK)
Scientific focus areas
In general thermodynamics, kinetics and mechanics of solid state phase transformations in metallic materials, mostly steels, both diffusive and martensitic. Extensive experience in materials characterization with reflected light and electron microscopy, X-ray and electron diffraction as well as optical and electron spectroscopy. More specifically, gas-metal interactions as a basis for understanding and optimizing thermochemical surface engineering and high temperature corrosion.Thermodynamic and kinetic materials modelling in relation to gas-metal interaction (surface engineering and oxidation) as well as modelling of materials characterization with X-ray diffraction and X-ray photo electron spectroscopy.
The case developing during low temperature surface hardening of austenitic stainless steel  by nitriding, carburizing or nitrocarburizing consists of a supersaturated interstitial solution of nitrogen and/or carbon in austenite. The favorable properties of this so-called expanded austenite depend on the profiles of interstitial concentration and associated composition-induced residual stress over the case. The prediction of composition and stress profiles for a certain steel grade from process parameters as gas composition, temperature and time would enable targeted surface engineering of stainless steels. Furthermore, such a numerical model would enable the design of stainless steel compositions that are tailored for optimal performance during low temperature surface hardening.
In the present contribution a numerical model is presented to predict the interdependent composition and stress profiles over the expanded austenite case as developing during low temperature nitriding of austenitic stainless steel. The model departs from fundamental parameters as determined experimentally on homogeneous powders and foils and a diffusion model that incorporates the interaction of composition and stress as well as the elastic-plastic accommodation of composition-induced lattice expansion in the case.
The presentation includes the following topics:
-        Interstitial solubility, interstitial diffusion and phase stability of homogeneous expanded austenite;
-        Crystallography, thermal expansion and magnetism of homogeneous expanded austenite;
-        Residual-stress in expanded austenite cases;
-        Numerical modelling aspects of composition- and stress profiles.

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