Estudo da transformação martensítica nas ligas Fe18Mn3Ti e Fe18Mn processadas por laminação a frio

Abstract

In the present work, the microstructural behavior of a high manganese maraging steel called Fe18Mn3Ti was investigated, and compared it with another manganese steel, Fe18Mn. FeMn alloys with high contents of this element represent a recent development in austenitic steels, serving as alternatives to traditional maraging steels mainly due to their production costs. Their mechanical properties are obtained by the relationship between their deformation mechanisms and phase transformations. Combining the benefits of TRIP and TWIP steels in a single material can result in significant improvements in mechanical properties, merging high hardness with toughness, thus expanding and enhancing the range of applications of these materials. The Fe18Mn3Ti alloy is susceptible to homogeneous phase decomposition accompanied by segregation, presenting instability in composition fluctuations close to the established aging temperature (450°C). Therefore, the study of the microstructural evolution of the Fe18Mn3Ti and Fe18Mn alloys cold rolling at different reduction levels was carried out, with characterization by various techniques such as OM, SEM/EDS, XRD, EBSD, TEM, and ferritoscopy, as well as hardness tests by micro and nanoindentation. Both materials exhibited the phases austenite (FCC), -martensite (HCP), and ’- martensite (BCC) in their microstructures. As straining increased, both austenite and -martensite were consumed, leading to the formation of more α’-martensite. The Fe18Mn alloy had a higher amount of -martensite in its initial microstructure compared to the Fe18Mn3Ti alloy, as titanium acted as an inhibitor to -martensite formation, thereby favoring the formation of more ’-martensite from austenite (TRIP effect). The calculated SFE values of 8.8 mJ/m2 for the Fe18Mn3Ti alloy and 11.5 mJ/m2 for the Fe18Mn alloy, based on the XRD data, indicate the predominance of the TRIP effect. Nanoindentation analysis identified the presence of interruptions (pop-ins) in the load-displacement curves in both alloys, indicating possible sites of phase transformation. For the Fe18Mn3Ti alloy, ’-martensite nucleation occurred mainly at linear defects and shear bands inside austenitic grains, and also in - martensite laths. In the Fe18Mn alloy, ’-martensite nucleation occurred mainly in the -martensite laths and at intersections of these laths.