Effect of stacking fault energy and austenite strengthening on martensitic transformation in Fe-Mn-Si alloys

In the Fe-Mn-Si alloys with very low stacking fault energy ($\gamma_0$), the $\gamma$(fcc)$\to$(hcp) martensitic transformation substantially relies on an overlapping-of-stacking-faults mechanism. $\gamma_0$ strongly influences its $M_s$ temperature. $M_s$ gets lowered when $\gamma_0$ increases. It has been shown that both Mn and Si strengthen the austenitic matrix. However, Mn in a certain concentration range raises $\gamma_0$ of the alloy and thus lowers its $M_s$ while Si acts in the other way around. Carefully summarizing and treating the available theoretical and experimental data, an analytical expression is established to describe the $M_s$ as a function of $\gamma_0$ and the strengthening effect that is related with the strain energy. The results show that stacking fault energy plays an overwhelming role on the transformation in such alloys, even though the austenite strengthening also lowers $M_s$ to some extent