## Abstract

A new iron-nitrogen phase, $\alpha ^{\prime \prime}$, occurs as an intermediate in the decomposition nitrogen-martensite ($\alpha ^{\prime}$) $\rightarrow $ Fe$_{4}$N($\gamma ^{\prime}$). Although it is a transition phase, $\alpha ^{\prime \prime}$ is relatively stable and has a structural unit which contains eight (2 $\times $ 2 $\times $ 2) distorted and expanded body-centred tetragonal units of the original martensite structure. The contents of the unit cell, which has dimensions a = 5$\cdot $72, c = 6$\cdot $29 angstrom, c/a = 1$\cdot $10, are ideally Fe$_{16}$N$_{2}$ 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 Fe$_{4}$N, and the mechanisms of its formation from nitrogen-martensite and of its subsequent transformation to Fe$_{4}$N are discussed. For a case where solute atoms are separated by such relatively large distances, the ordering observed in $\alpha ^{\prime \prime}$-iron nitride appears to be unique. The unknown nitride reported by previous workers to be precipitated from nitrogen-ferrite is identified as the $\alpha ^{\prime \prime}$-phase. Its relationship with the transition phase of tempered carbon-martensite and the technical significance of its existence are mentioned.