Microstructural phase evaluation of high-nitrogen Fe-Cr-Mn alloy powders synthesized by the mechanical alloying process

In this study, the formation of Fe18Cr8MnxN alloys by mechanical alloying (MA) of the elemental powder mixtures was investigated by running the milling process under nitrogen and argon gas atmospheres. The effect of the milling atmosphere on the microstructure and phase contents of the as-milled powders was evaluated by X-ray diffraction and transmission electron microscopy. The thermal behavior of the alloyed powders was also studied by differential scanning calorimetry. The results revealed that in the samples milled under nitrogen, three different phases, namely ferrite (alpha), austenite (gamma), and a considerable amount of amorphous phase are present in the microstructure. In contrast, in the samples milled under argon, the structure contains the dominant crystalline ferrite phase. By progression of MA under the nitrogen atmosphere, the ferrite-to-austenite phase transformation occurs; meanwhile, the quantity and stability of the amorphous phase increase, becoming the dominant phase after 72 h and approaching 83.7 wt% within 144 h. The quantitative results also showed that by increasing the milling time, grain refinement occurs more significantly under the nitrogen atmosphere. It was realized that the infused nitrogen atoms enhance the grain refinement phenomenon and act as the main cause of the amorphization and alpha-to-gamma phase transformation during MA. It was also found out that the dissolved nitrogen atoms suppress the crystallization of the amorphous phase during the heating cycle, thereby improving the thermal stability of the amorphous phase.