Interstitial atom configurations in stable and metastable Fe-N and Fe-C solid solutions

Abstract

Mössbauer Fe⁵⁷ spectroscopy allows comparison of Fe−N and Fe−C interstitial solid solutions. The spectra of Fe−N retained austenite indicate that nitrogen atoms are randomly distributed on octahedral sites in the austenite and in the virgin martensite. On heating, austenite decomposes directly to the equilibrium phases α iron and Fe₄N at temperatures above 160°C. Virgin martensite ages at room temperature by local ordering of nitrogen atoms. In that process, three new iron atom environments develop, characteristic of the Fe₁₆N₂ (α″) structure. However, the excessive width of the peaks indicate the perfect order of the Fe₁₆N₂ precipitate is not achieved, except after very long times. Further aging at 100°C leads to the complete decomposition of the virgin martensite to the discrete phases α iron and Fe₁₆N₂. This two phase structure is stable up to 160°C, above which the precipitation of Fe₄N occurs. These results are in contrast to Fe−C data. Carbon atoms in retained austenite tend to be far apart in their octahedral sites, and this nonrandom distribution is inherited by the virgin martensite. Fe−C austenite decomposes by the formation of ∈ carbide below 160°C and precipitation of Fe₃C above 180°C. The carbon atoms in virgin martensite agglomerate at room temperature and regions of ordered Fe₄C are believed to result. Subsequently ∈ carbon is formed at 80°C and Fe₃C precipitates above 160°C.¹