“Colossal” interstitial supersaturation in delta ferrite in stainless steels: (II) low-temperature nitridation of the 17-7 PH alloy

Low-temperature gas-phase nitridation has been studied in interstitially-hardened 17-7 precipitation-hardening stainless steel. After nitridation, conventional transmission electron microscopy reveals that delta ferrite grains in such alloys show a uniform weak diffraction contrast. Chemical analysis reveals that the weak-contrast ferrite grains contain an enormous interstitial nitrogen supersaturation (>20 at.%). Surprisingly, there is no significant tetragonality in these weak-contrast ferrite grains. Such weak diffraction contrast is attributed to a nanometer-scale nitridation-induced spinodal decomposition of delta ferrite grains. In addition, nitride nanoparticles are observed in the steel, and were identified as rocksalt-structured MN nitride (M: Fe, Cr, Ni, and Al). Further, platelets of hexagonal M2N1-x are observed at low nitriding temperatures (623 and 653 K), but were absent during nitridation at a higher temperature (713 K).