The structure of nitrogen-supersaturated ferrite produced by ball milling

Highly supersaturated solid solutions of nitrogen in ferrite (bcc) were produced by ball milling of various powder mixtures of α-iron and ε-Fe₃N_1.08. The microstructure and the crystal structure of the product phases were examined as a function of nitrogen content using X-ray powder diffraction, high-resolution electron microscopy and Mössbauer spectroscopy. It was found that the grain size decreases with increasing nitrogen content. Unexpected shifts of the reflections in the X-ray powder diffraction patterns of the supersaturated N-ferrites, depending on the hkl values of the reflections and nitrogen content, were observed. These shifts cannot be explained by tetragonal distortion of the bcc unit cell, but they are in accordance with the occurrence of a certain type of stacking faults on bcc {211} planes. This result, together with the observation of some isolated fcc crystals (by high-resolution electron microscopy) and a drop in microstrain for high nitrogen contents, demonstrates that unconventional deformation mechanisms are operative in these materials below a certain grain size, leading to a breakdown of the classical Hall–Petch relation for mechanical strengthening.