The occurrence and the crystal structure of α"-iron nitride; a new type of interstitial alloy formed during the tempering of nitrogen-martensite

A new iron-nitrogen phase, α'', occurs as an intermediate in the decomposition nitrogen-martensite (α') → Fe4N(γ'). Although it is a transition phase, α'' is relatively stable and has a structural unit which contains eight (2 x 2 x 2) distorted and expanded body-centred tetragonal units of the original martensite structure. The contents of the unit cell, which has dimensions a ═ 5⋅72, c ═ 6⋅29 Å, c/a ═ 1⋅10, are ideally Fe16N2 with the atoms occupying special positions of space group I4/mmm. The iron atoms are displaced from sites occupied in the ferrite lattice by the presence of nitrogen atoms which occupy one twenty-fourth of the number of octahedral interstices in a completely ordered manner. Up to one-half of the number of nitrogen-atom sites may be vacant. The structure is also simply related to that of Fe4N, and the mechanisms of its formation from nitrogen-martensite and of its subsequent transformation to Fe4N are discussed. For a case where solute atoms are separated by such relatively large distances, the ordering observed in α'' -iron nitride appears to be unique. The unknown nitride reported by previous workers to be precipitated from nitrogen-ferrite is identified as the α'' -phase. Its relationship with the transition phase of tempered carbon-martensite and the technical significance of its existence are mentioned.