High-energy ball-milling-induced non-equilibrium phase transformations

α-Fe powder has been ball milled in an anhydrous atmosphere at two different temperatures (room temperature and 200 °C). The composition and structure of milled samples were investigated using X-ray diffraction, Mössbauer spectroscopy, combustion analysis, and thermal analysis. A series of non-equilibrium phase transformations was observed during milling. The nitriding reaction sequence at room temperature established as a function of milling time (an increase of nitrogen in the powder) corresponds to a high-temperature reaction sequence in the equilibrium phase diagram. In contrast, hot milling at 200 °C leads to a lower average nitrogen content in milled powders. It seems that formation of metastable nitride phases with high N content induced by ball impacts is in competition with thermally activated decomposition of these phases to equilibrium states with low N content during milling at 200 °C.