The electron and field ion metallography of zones in nitrided Fe-Mo alloys

Abstract

Constant activity nitriding of Fe-1.7 and 3 at.%Mo alloys in the temperature range 500–600°C produces homogeneous precipitation of fine coherent disc-shaped zones on {001} ferrite planes giving rise to continuous streaking in 〈001〉 α directions in the electron diffraction patterns and high hardness increments. Transmission electron microscopy shows that the mean zone size and number density increase gradually throughout the nitriding process to nitrogen saturation. An analysis of the coherency strain field images indicates that the zones are of interstitial character and produce elastic displacements of 0.3 to 0.9 Å normal to the habit plane. The zone thicknesses and structures have been studied in greater detail by field ion microscopy (F.I.M.). The field evaporation technique demonstrates that the zones have an apparent interplanar spacing of 2.7 ± 0.5 A ̊ normal to the {001} α habit plane and thicknesses of ≈ 6, 9 and 12 Å. The F.I.M. and E.M. observations are interpreted on the basis that the zones have a body centred tetragonal structure in which some Fe atoms are replaced by Mo atoms in a random manner and in which N atoms are partially or completely ordered on alternate (002) α planes as in α″ Fe 16N 2.