Atom probe tomography evidence of nitrogen excess in the matrix of nitrided Fe–Cr

Excess nitrogen uptake is a key point in the understanding of nitriding process. The increased nitrogen solubility in the matrix of nitrided Fe–Cr alloys is recognised as one possible source of nitrogen excess, and investigated by means of atom probe tomography. The nitrogen concentration in the matrix of as-nitrided specimens, containing a high density of nanometre scale chromium nitride platelets, is measured significantly higher than the one in re annealed samples, and than in the calculated equilibrium values. These results bring direct experimental evidence that the additional amount of nitrogen dissolved in the matrix of nitrided alloys is a significant part of the so-called excess nitrogen.