Interaction of nitrogen atoms in expanded austenite formed in pure iron by intense nitrogen plasma pulses

The paper presents the results of experiments on modification of pure iron by high-intensity nitrogen pulsed-plasma treatment. The duration of nitrogen plasma pulses is approximately 1 mi s, and the energy density amounts to about 5 J/cm2. Such pulses are capable to melt the surface layer of the substrate (1- 2 mi m) and to introduce a significant concentration of nitrogen into the molten layer. Nuclear reaction analysis (NRA), X-ray diffraction (XRD) and conversion electron Mössbauer spectroscopy (CEMS) were used for characterisation of the treated samples. The main results of the data analysis are as follows: it has been stated that such treatment leads to gradual transformation of initial alfa-phase into austenitic gamma structure in which expanded austenite gammaN is present. Treatment with 20 pulses results in almost complete transformation and introduces a retained dose of nitrogen estimated as 5.5 × 1017 N/cm2. The susceptibility for expansion of the lattice transformed to austenite in this way is smaller than in the case when the steel subjected to conventional nitriding is originally of austenite type. The analysis of the ratio of alfa to gammaN as a function of the nitrogen content provides a firm evidence that strong repulsion forces act between the first and the second nearest-neighbour nitrogen atoms in the fcc austenitic structure formed as a result of nitriding of pure iron by intense nitrogen plasma pulses.