Reassessment of low-temperature Gibbs energies for T0-temperature evaluation

Abstract

Bainitic steels are of great technological interest due to their particular microstructure combining high strength and good toughness. Since the beginning of the 80s’, some controversy exists as to the formation mechanism of this phase, which is partially attributable to difficulties in experimental characterization of this metastable microstructure. In the past, several attempts for predictive modeling of bainite start temperatures, retained austenite phase fraction, morphology and strain-induced martensitic transformation are reported. Nevertheless, these different models often fail when they are transferred from one specific alloy group to another. As the bainite transformation is mainly driven by the thermodynamics of the system, our aim in the present work is to assess the multicomponent mc_fe [1] database and revise the thermodynamic modeling according to experimental evidence of phase stability as well as bainite and martensite start temperatures. Our current Fe-base database includes 24 elements, but for the current project we focus on the typical alloying elements for high strength bainitic steel involving C, Ni, Mn, Cr, Al, Si and Fe. Our assessment shows that some binaries, such as Fe-Ni, Fe-Mn, Fe-C need to be revised. In a first step of re-evaluation of the thermodynamic modeling parameters, the revision of the magnetic contribution especially in Fe-Ni and Fe-Mn is performed. Although the current work could be seen as “old-fashion” Calphad modeling, it is well known that the modeling at low temperature of high temperature alloys is always a challenge as there are not much reliable thermodynamic data available. Thus, for the present work, we use a combination of available data, such as experimental thermodynamic data, DFT, stacking fault energy as well as available bainite and martensite temperatures available in the literature. In the present contribution, we will show an improvement of the mc_fe database for the low temperature phase transformation of austenite in the multicomponent system Fe-C-Cr-Mn-Ni-Al-Si. This work is a first corner stone for Matcalc kinetic modeling of the bainite transformation.

[1] E. Povoden-Karadeniz, mc_fev2.060, (2020) https://www.matcalc.at/index.php/databases/open-database.