A model is proposed for the evaluation of the stacking fault energy (SFE) in Fe–Mn–C austenitic alloys, at different temperatures. It accounts for the variation of the Gibbs energy of each element during the austenite to e martensite transformation, plus their interactions. The Gibbs energy due to the antiferromagnetic to paramagnetic transition is also taken into account. The required data have been obtained from the literature. The result shows a decrease of the SFE with temperature, with a saturation below the austenite Neel temperature. The result agrees with the mechanical and thermal martensitic transformation limits proposed by Schumann. The plasticity mechanisms depend on the SFE. The mechanical martensitic transformation occurs below 18 mJ/m 2 , and twinning between 12 and 35 mJ/m 2 , in agreement with the tensile tests and the deformation microstructures observed in an Fe–22 wt.% Mn–0.6 wt.% C alloy at 77, 293 and 693 K.

Correlations between the calculated stacking fault energy and the plasticity mechanisms in Fe–Mn–C alloys

Twinning-induced plasticity (TWIP) steels

Dependence of tensile deformation behavior of TWIP steels on stacking fault energy, temperature and strain rate

A subregular solution thermodynamic model was used to calculate the stacking fault energies (SFEs) of high-manganese (10 to 35 wt pct) steels with carbon contents of 0 to 1.2 wt pct. Based on these calculations, composition-dependent diagrams were developed showing the regions of different SFE values for the mentioned composition range. These diagrams were called SFE maps. In addition, variations in the SFE maps were observed through increasing the temperature, aluminum content, and austenite grain size. These changes were seen either as an increasing trend of SFE caused by raising the temperature and aluminum content, or as a decreasing behavior caused by increasing the grain size. The SFE value of 20 mJ/m2 within these diagrams was introduced as the upper limit for the strain-induced martensite formation. The variations in this limit caused by increasing the temperature and aluminum content were mathematically evaluated to find out the minimum amount of manganese that was required to avoid the martensitic transformation. By introducing the isocarbon and isomanganese diagrams of the SFE, it was seen that both temperature and aluminum had a greater effect on the SFE when added to the steels with the lower manganese contents. Moreover, by adding more aluminum to the composition of the high-manganese steels, its influence on the SFE decreased continuously.

Derivation and Variation in Composition-Dependent Stacking Fault Energy Maps Based on Subregular Solution Model in High-Manganese Steels

Thermodynamic modeling of the stacking fault energy of austenitic steels

Four Fe-30Mn-0.9C-XAl alloys are employed to investigate the influence of aluminium content to the stacking fault energy in Fe-Mn-Al-C alloy system. The range of aluminium content is zero to 8.47 wt%. Based on the thermodynamic model, the stacking fault energy can be obtained through calculation. Increasing the aluminium content will make the stacking fault energy of Fe-30Mn-0.9C based alloys increase at 300 K.

The influence of aluminium content to the stacking fault energy in Fe-Mn-Al-C alloy system

The formation of AIN was studied using a series of Fe-Mn-Al-C alloys with manganese contents from 20 to 40 wt% All investigations were carried out in air and nitrogen at 1000° C The permeability (D
N
N
N) of nitrogen in Fe-Mn-Al-C alloys was found to be a major factor influencing the formation of AIN precipitates which are always observed underneath the oxide scales of the specimen The formation of AIN is related to the lower formation rate of Al2O3 and high permeability of nitrogen; therefore AIN can only be formed in those alloys with a higher manganese concentration because the specimens are treated in air and in pure nitrogen at 1000 C

W. S. Yang

Papers

The influence of aluminium content to the stacking fault energy in Fe-Mn-Al-C alloy system

High temperature studies of Fe-Mn-Al-C alloys with different manganese concentrations in air and nitrogen