João Luiz Lopes Rezende

Phase-Field Modeling of the Coarsening in Multi-component Systems

A thermodynamically consistent method for the investigation of the coarsening behavior and in particular for the prediction of the secondary dendrite arm spacing (SDAS) in multi-component alloys is proposed which is based on the numerical simulation by means of a phase-field model. Existing variants of the phase-field model equations for multi-component systems were considered and their advantages and disadvantages were discussed. For the investigation of the coarsening behavior the variant described by the mixture composition and the entropy change was chosen. The method is applied to a high-alloy tool steel where it was found that elements such as C, Si, Mn decrease the SDAS whereas Cr increases. The resulting dependencies of the SDAS on alloy composition were compared to the analytical prediction of the coarsening model. For this aim the analytical model of the coarsening behavior in multi-component alloys [Rappaz and Boettinger, Acta Mater. 47 (1990)] was extended by taking into account the cross dependencies between the components in multi-component diffusion and the case of slow diffusion in the solid phase. The equilibrium parameters used in the phase-field model and in the analytical model were obtained from Thermo-Calc through global equilibrium calculations using the database TCFE7. The difference between both methods was found to be smaller than 2 pct in the investigated composition region.

Prediction of Microstructure and Microsegregation in a Fe-Mn-C Austenitic Steel based on Phase-field Microstructure Simulations

This paper presents an experimental and theoretical investigation of the microstructure formation and segregation behaviour of a new austenitic steel based on the Fe-Mn-C alloy system. In order to accomplish the simulations, a modification of the model by Cha et al. is introduced. It includes a correction which compensates for artificial solute-trapping. A good qualitative agreement is found between the simulated 2D microstructures and the corresponding micrographs. Both simulations and experiments show that this alloy is very prone to manganese segregation. The further investigation on this field is scheduled.

Hot ductility behaviour of high manganese steels with varying aluminium contents

The influence of aluminium on the hot ductility of high manganese steels between 700°C and Tliq has been examined, using the hot tensile test machine. Five materials with 17 wt-% [Mn], 0.3 wt-% [C] and 0–8 wt-% [Al] were cast. To detect the cause of cracking, optical microscopy and scanning electron microscopy analysis of the fracture surfaces have been conducted and compared to thermodynamic modelling. By adding more than 3 wt-% [Al] the fully austenitic structure changed to a two-phase structure (δ-Fe, γ-Fe). In case of the one- and also two-phase structured materials two regions of lower ductility (<30% RA) are present. Complex (AlN) and Mn(S,Se) precipitates were formed with different cores in dependence of the [Al] content.