ESSC & DUPLEX 2019

The formation of σ phase in various technological steel grades is crucial for materials properties due to embrittlement caused by the precipitation of σ phase at the grain boundaries. Thus, understanding and prediction of its temperature and composition dependent stability is required, which can be obtained by thermodynamic modeling. Its phase description should represent the correct crystal chemistry, thermodynamic stability and phase composition and extend properly to multi-component systems. Originally, the σ phase of Fe-based alloys [1] was modelled as (Fe)8(Cr)4(Cr,Fe)18 with extension to multicomponent systems where for example Ni can substitute Fe on 1st and 2nd sublattice Mo substitute Cr on 2nd and 3rd sublattice. However, the site occupancies of this model are not respectful to the known crystal structure of σ phase [2]. This discrepancy between modeling and physical base leads to questionable predictions of the phase stability in complex systems. We developed a new sublattice model for Fe-Cr system first [3] which is written (Cr,Fe)10(Cr,Fe)4(Cr,Fe)16 and correctly describes the crystal chemistry of the σ phase [3]. This model is being extended to multicomponent systems including Mo, Si and Ni within other elements which are soluble and stabilize the σ phase in steel. Validation of the predicted phase stability and composition is obtained by experimental close-to-equilibrium observations. Based on the revised model, we have carried out thermokinetic simulation of precipitation of σ phase in duplex steels, which allows us to understand the nucleation and growth behavior of σ phase formation as function of steel composition, temperature and time, targeting at optimised alloy and process design for improved materials properties. [1] J.-O. Andersson, B. Sundman, Thermodynamic properties of the Cr-Fe system, Calphad. 11 (1987) 83–92. [2] J.-M. Joubert, Crystal chemistry and Calphad modeling of the σ phase, Prog. Mater. Sci. 53 (2008) 528–583. [3] A. Jacob, E. Povoden-Karadeniz, E. Kozeschnik, Revised thermodynamic description of the Fe-Cr system based on an improved sublattice model of the σ phase, Calphad 60 (2018) 16–28.