Structure of the Fe-Mn-Si alloys submitted to γ ⇆ ε thermocycling

The accumulation of defects and internal stresses and related changes of the direct and reverse martensitic transition (MT) on thermocycling over the phase transition in different Fe-Mn-Si alloys are investigated by in situ neutron diffraction, TEM, SEM-EBSD, DSC and mechanical spectroscopy. The temperature of direct MT in Fe22Mn-3Si, Fe-26Mn-4Si and Fe-26Mn-7Si alloys decreases during thermal cycling, while that of the inverse MT increases, which causes a widening of the transformation hysteresis. Transmission electron microscopy shows that a large number of dislocations and stacking faults are formed in both martensitic and austenitic phases in the Fe-22Mn-3Si annealed alloy submitted to gamma <-> epsilon thermocycling. From in situ neutron diffraction tests on Fe22Mn-3Si annealed alloy and classical Williamson-Hall analysis, we found that the microstrains in the epsilon-martensite after 1 and 12 thermocycles are roughly twice higher as compared with the austenite phase.