Effect of Mn content on the phase transitions and microstructure of Fe-Mn alloys

In a recent study, a change in morphology with a very narrow range of Mn composition (2-12 wt %) was observed which also resulted in a very different mechanical behavior but kept the BCC structure. The underlying mechanism that is responsible for this occurrence is unknown and will be the focus of this study. Aside from the changes in morphology with respect to the Mn content, the effect of the cooling rate on the Fe-2Mn, Fe-3Mn, Fe-4.5Mn and Fe-12Mn alloys were also studied. Three samples were heat treated for each alloy and one of each were then furnace cooled, air cooled and water quenched. The different Fe-Mn alloys were examined using several characterization techniques. SEM, EBSD and TEM analysis were performed to look into the microstructure. Dilatometry with a fast cooling rate of 116 °C/s was conducted to determine the phase transformation temperatures and to be compared with the previous findings, which had a cooling rate of 1 °C/s. Furthermore, hardness and tensile tests were done to obtain the mechanical properties. A metastable BCC phase was formed for the Fe-2Mn, Fe-3Mn, Fe-4.5Mn and Fe-12Mn alloys regardless of the cooling rate. A change in morphology from the equiaxed ferrite structure of the Fe-2Mn to the lamellar structures of the other three alloys was also observed for all cooling rates employed. Furthermore, a more pronounced change in morphology was seen with increasing Mn content as compared to the effect of the cooling rate. The difference in morphology manifested in the mechanical properties of the Fe-Mn alloys wherein the Fe-2Mn showed the lowest yield strength σy and tensile strength σu but the highest ductility. It can be seen that with just 1 % change in Mn composition, the yield strength is higher by 48% and the ductility lower by 43% for the Fe-3Mn, which is a significant difference. The results of the dilatometry tests, on the other hand, showed a decrease in phase transformation temperatures with increasing Mn content and increasing cooling rate. However, this does not necessarily translate to a change in morphology. Looking deeper into the microstructure of Fe-3Mn water quenched through the TEM, the specimen revealed the presence of laths and a high density of dislocations. If compared to the Fe-2Mn alloy which predominantly have equiaxed grains for all cooling rates, low density of dislocations is expected since this type of grain structure have the reconstructive phase transformation which are allowed to grow by diffusion. With this observation, a possible cause of the change in morphology can be a change in phase transformation mechanism.