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Binary and ternary interstitial alloys I. The iron-nitrogen system: the structures of Fe4N and Fe2N By K. H. Jack, Tht British Iron and Steel Research (Communicated by Sir Charles Goodeve, F.R.S.—Received 17 December 1947) [Plate 1J The £-iron nitride phase (Fe2N), the existence of which is confirmed, is prepared by passing ammonia over iron at temperatures not exceeding 450° C and under such conditions that the partial pressure of hydrogen is negligible. The positions of the nitrogen atoms in y'(Fe4N) and in £ are determined.
Binary and ternary interstitial alloys. I 35 to the c axis with an interplanar distance c/2, and with a two-dimensional close packing in each layer (see figure 2). For Fe3N, it was considered that one-third of the number of interstices in each layer is filled with nitrogen atoms in such a way that <-------------a -------------► structure I structure II N at N atfii Figure 1. Possible structures for Fe4N, according to Brill (1928) and Hagg (19286, 1929). • Iron atom, x unoccupied octahedral interstice, ® nitrogen atom, O unoccupied tetrahedral interstice.
36 K. H. Jack Hagg (1928 a, 1929) found that the £-phase in which Fe2N occurs has a base- centred orthorhombic structure, formed by a small anisotropic distortion of the e lattice when the nitrogen concentration exceeds 11-1 %.
Binary and ternary interstitial alloys. I 37 Nitrogen analysis Three methods were used: (i) a modified macro-Kjeldahl method (Jones & Morgan 1932); (ii) semi-micro-Kjeldahl (Beet& Belcher 1938); (iii) loss in weight on heating the nitride in vacuo at 1000° C (Hagg 1929).
Binary and ternary interstitial alloys. I 39 It contains eight iron atoms at 0 0 0, 0 0,. f \0, f \0, 0f f f f,f f f, f f f, and four nitrogen atoms at f 1 f, ff f,§f f, Off. Relative intensities calculated for this structure are shown in table 2 to be in good agreement with observed values for superlattice reflexions as well as ‘iron’ lines.