Atom probe tomography characterization of CrN precipitation in low temperature 15N-enriched plasma nitrided 316L austenitic stainless steel.


Atom probe tomography characterization of CrN precipitation in low temperature 15N-enriched plasma nitrided 316L austenitic stainless steel.

Martinavičius, A.; Danoix, R.; Drouet, M.; Abrasonis, G.; Hannoyer, B.; Danoix, F.

Nitriding of austenitic stainless steel (ASS) at moderate temperatures (~400°C) leads to the formation of a modified surface layer which shows increased hardness and induced magnetism, without compromising the corrosion resistance. The exact nature of this layer is still a matter of debate.

In this study, ASS 316L has been plasma nitrided with a mixture of 14N and 15N for 30 min at 400°C. Only a single phase, usually called the S phase or expanded austenite, is detected by X-ray diffraction. Transmission electron microscopy shows a high density of stacking faults and lattice distortions in this S phase, but does not provide any direct information regarding its composition, in particular regarding nitrogen and chromium distributions at the nanometer scale. Atom probe tomography and field ion microscopy reveal the presence of nanometric chromium nitride precipitates, with irregular oblate-spheroid-like shape. The preferential precipitation of chromium nitride at grain boundaries and/or stacking faults is investigated. These observations suggest that incorporation of large amounts of N provides strong driving force for chromium nitride formation, even at such a short nitriding time and rather low temperature.

Because of the presence of silicon and obviously iron in this industrial stainless steel, direct composition measurement of these chromium nitrides, in particular their nitrogen content, is not possible. In order to solve this classical analytical issue, specimens were nitrided using different 14N/15N ratios. Possible N+/Si2+ and N2+/Fe2+ overlaps at 14 and 28 Da respectively are investigated in order to estimate the actual chromium nitride composition. Results will be compared with the one obtained with the newly developed high mass resolution Flextap, potentially allowing N+/Si2+ and N2+/Fe2+ peak discrimination.

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Publ.-Id: 19128