ATOM-PROBE AND X-RAY-DIFFRACTION ANALYSIS OF THE COMPOSITION AND STRUCTURE OF PRECIPITATES FORMED ON TEMPERING OF TERNARY IRON-CARBON-NITROGEN MARTENSITES

X-ray diffraction analysis showed that both alpha ''-type precipitates (as in Fe-N) and epsilon/eta-type precipitates (as in Fe-C) develop i n ternary Fe-C-N martensite upon tempering below 450 K. Atom-probe ana lysis was used to study the distribution of the interstitial atoms on a local scale. Most of the enriched regions formed after tempering con tain both nitrogen and carbon atoms, but in an enriched region one typ e of interstitial is dominant. The carbon-rich regions contain a signi ficantly larger amount of total interstitial atoms than the nitrogen-r ich regions. The total interstitial amounts are close to that in epsil on/eta for the carbon-rich regions and close of that in alpha '' for t he nitrogen-rich regions. This suggests that upon temper ing Fe-C-N ma rtensite nitrogen atoms reside preferentially in an alpha ''-type stru cture and carbon atoms reside preferentially in an epsilon/eta-type st ructure, but that coprecipitation of nitrogen and carbon atoms occurs to a limited extent as well.