Binary and ternary interstitial alloys III. The iron-carbon system: the characterization of a new iron carbide

Cementite and iron percarbide are conveniently prepared by the methods described in the previous paper (part II). Iron percarbide has a narrow range of composition near 31 atomic % carbon and is identical with a carbide, assumed to be Fe 2 C, which was previously obtained by Hägg (1934). The iron percarbide unit cell is probably either orthorhombic, with dimensions a, 9.04(3); b = √3.a, 15.66(3); c, 7.92(1) kX, or hexagonal, with dimensions a' = 2a, 18.08(6); c' = c, 7.92(1) kX. There two possible units of structure are simply related, the volume of the latter being twice that of the former. The orthorhombic unit cell contains approximately 80 iron atoms, and the empirical formula for the carbide is Fe 20 C 9 . Although the existence of a carbide Fe 2 C is not disproved, it is unlikely that it has been prepared in this or any previous work. The cementite is identical with specimens formed below 700° C in steel and obtained by the action of carbon monoxide on ferric oxide. Both iron carbides are metastable. Iron percarbide decomposes giving cementite and carbon, and cementite breaks down less rapidly to give α iron and carbon.