Nitride Formation and Excess Nitrogen Uptake After Nitriding Ferritic Fe-Ti-Cr Alloys

The microstructure of the nitrided zone of Fe-Ti-Cr alloys, containing a total of 0.30 at. pct (Ti + Cr) alloying elements, with varying Ti/Cr atomic ratio (0.45, 0.87, and 1.90), was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The stable TiN and CrN nitrides did not precipitate after nitriding. Instead, ultrafine, metastable, mixed Ti_1–x Cr _x N nitride precipitates developed in the nitrided zone: The precipitates were of platelet morphology (length ≤30 nm and thickness ≤3 nm) and of cubic, rock-salt, crystal-structure type. The misfit strain around the nitride platelets in the ferrite matrix increases with increasing Ti/Cr atomic ratio. As a consequence, most pronouncedly for the highest Ti/Cr atomic ratio, a tetragonally distorted ferrite matrix surrounds the precipitates, as evidenced both by XRD and TEM. The amount of nitrogen taken up was determined quantitatively by measuring the so-called nitrogen-absorption isotherms. It follows that the absorbed amount of so-called excess nitrogen dissolved in the matrix and adsorbed at the nitride-platelet faces increases distinctly with increasing Ti/Cr atomic ratio. The results were discussed in terms of the dependence of misfit strain on the Ti/Cr atomic ratio and the higher chemical affinity of Ti for N than of Cr for N.