Atomprobe analysis of initial decomposition of Fe-N martensite

Abstract

The initial decomposition of Fe-N martensite (containing 4.4 at.-%N, ¹⁵N isotope enriched) was studied by atom probe analysis. Different stages of decomposition were obtained by giving three Fe-N specimens dissimilar heat treatments (from room temperature aging to tempering at 373 K). The composition-depth profiles and the associated autocorrelation functions of the specimen analysed show the nature and the extent of the atomic scale composition changes during the martensite decomposition. Tempering leads to the development of nitrogen rich regions in a nitrogen depleted matrix. The nitrogen rich regions could be interpreted as α″ transition nitride precipitates. The composition data suggest that nitrogen deficient α″-Fe₁₆N_2-x at an intermediate stage of tempering transforms into perfect α″-Fe₁₆N₂ by continued tempering, beyond completed precipitation.