The Fe-Mn system has been used to demonstrate the key role of anti-ferromag
netic contributions to the martensitic transformation behaviour and the sta
cking fault energy of fee Fe-Mn alloys. An analysis of previous work leads
to the conclusion that the optimum magnetic characterisation in the low tem
perature region may differ substantially from the behaviour deduced from a
high temperature analysis. An alternative model has been developed, which l
eads to the satisfactory quantitative prediction of the relevant M-s, A(s)
and T-0 temperatures as well as stacking fault energies and the model is ca
pable of being extended to ternary systems. The results also have a bearing
on current expressions for the phase stability of the gamma-epsilon transf
ormation in pure iron.