  First-principles alloy theory is used to establish the $\gamma$-surface of
Fe-Cr-Ni alloys as function of chemical composition and temperature. The
theoretical stacking fault energy (SFE) versus chemistry and temperature trends
agree well with experiments. Combining our results with the recent plasticity
theory based on the $\gamma$-surface, the stacking fault formation is predicted
to be the leading deformation mechanism for alloys with effective stacking
fault energy below about 18 mJ m$^{-2}$. Alloys with SFE above this critical
value show both twinning and full slip at room temperature and twinning remains
a possible deformation mode even at elevated temperatures, in line with
observations.
