Thermodynamic calculation of stacking fault energy and its effect on fcc→hcp phase transformation in nitrogen alloyed stainless steels

The observed fcc-->hcp phase transformation in some iron-based alloys, such as Fe-Mn-based shape memory alloys, is due to the formation of stacking faults, expansion of the faults by the movement of Shockley partial dislocations followed by ordered overlapping of the faults. Therefore, alloy systems with low stacking fault energy (SFE) are naturally more prone to the fcc-->hcp martensitic phase transformation. Nitrogen (N) is known as an austenite stabilizing element; however, some existing experimental data in the literature have shown that it decreases the stacking fault energy of austenitic steels. Therefore, one would expect that "N" alloying should promote the fcc(gamma)-->hcp(epsilon) martensitic transformation and thus have a destabilizing effect on austenite. To resolve the above discrepancies we have considered the effect of "N" on the stacking fault energy of fee iron-based alloys. Our calculations have shown that by increasing "N" in solid solution, the stacking fault energy of the system initially increases to a maximum and then decreases. Using a phenomenological description for fcc(gamma)-->hcp(epsilon) heterogeneous martensitic nucleation kinetics, we have demonstrated that increasing nitrogen concentration decreases the driving force for the fcc(gamma)-->hcp(epsilon) martensitic transformation and increases the friction work due to both nitrogen solid solution strengthening and segregation of nitrogen to stacking faults for fee iron-based alloys.