Atom-probe- and x-ray diffraction analysis of the composition and structure of precipitates formed on tempering of ternary iron-carbon-nitrogen martensites

Abstract

X-ray diffraction analysis showed that both α"-type precipitates (as in Fe-N) and ε/η-type precipitates (as in Fe-C) develop in ternary Fe-C-N martensite upon tempering below 450 K. Atom-probe analysis was used to study the distribution of the interstitial atoms on a local scale. Most of the enriched regions formed after tempering contain both nitrogen and carbon atoms, but in an enriched region one type of interstitial is dominant. The carbon-rich regions contain a significantly larger amount of total interstitial atoms than the nitrogen-rich regions. The total interstitial amounts are close to that in ε/η for the carbon-rich regions and close of that in α" for the nitrogen-rich regions. This suggests that upon tempering Fe-C-N martensite nitrogen atoms reside preferentially in an α"-type structure and carbon atoms reside preferentially in an ε/η-type structure, but that coprecipitation of nitrogen and carbon atoms occurs to a limited extent as well.