FCC/HCP MARTENSITIC TRANSFORMATION TEMPERATURES AND THERMAL CYCLING EVOLUTION IN FE-MN-CR ALLOYS

Fe-Mn based alloys are a low cost alternative to NiTi as shape memory alloys. Technologically interesting alloy compositions contain between 15 and 30 wt.% Mn in order to obtain the fcc to hcp martensitic transformation by cold deformation, while inhibiting the competing fcc to bcc transition. These alloys usually contain up to 6 wt.% Si to control the stacking fault energy and magnetic ordering temperatures of the austenitic structure, and 5-9 wt.% Cr and 0-5 wt.% Ni to achieve good corrosion resistance. In order to attempt alloy design for these systems it is necessary, at least, to have information about the binary and ternary systems involved. Although the binary Fe-Mn, Fe-Si and Fe-Cr and ternary Fe-Mn-Si systems have been extensively studied in the past, the information about the other Fe-Mn based ternary systems involved (namely Fe-Mn-Cr and Fe-Mn-Ni) is scarce. In this work we present information about the fcc/hcp martensitic transformation temperatures of selected Fe-Mn-Cr alloys measured by means of dilatometry and electrical resistivity techniques. We have found that these transformation temperatures suffer an evolution when samples are thermally cycled, up to an asymptotic state. In the case of the studied samples, this asymptotic state is reached after a small number of cycles (n < 7). Transmission electron microscopy observations were performed to elucidate the microstructural origin of this phenomenon. Finally, we present a comparative analysis of the thermal cycling behavior of these samples with the results found for Fe-Mn and Fe-Mn-Si alloys.