The structure of nitrogen-supersaturated ferrite produced by ball milling

Jochen Aufrecht (1) , Andreas Leineweber (2) , Eric Jan Mittemeijer (1) , Jacques Foct (3)

1 Max Planck Institute for Metals Research
2 Department Mittemeijer
3 LMPGM - Laboratoire de Métallurgie Physique et Génie des Matériaux

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 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. It could be shown that 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.

Mots clés: Physical Sciences

Jochen Aufrecht, Andreas Leineweber, Eric Jan Mittemeijer, Jacques Foct. The structure of nitrogen-supersaturated ferrite produced by ball milling. Philosophical Magazine, 2008, 88 (12), pp.1835-1855. ⟨10.1080/14786430802322198⟩. ⟨hal-00513934⟩