A simple model for nitrogen-induced lattice expansion of γ'_N and γ_N phases in Fe–Cr–Ni alloys with different chromium contents

A recent publication on low-temperature nitriding of Fe–Cr–Ni alloys with various combinations of Cr and Ni contents showed experimentally the co-existence of γN and γ'N phases. An important result of the investigations was that with increasing Cr-content in the alloy, the lattice expansion induced by nitrogen dissolution in short-range ordered (SRO) γN is more pronounced than in the long-range ordered (LRO) γ'N phase. Eventually, for Cr contents beyond about 12 wt.%, the lattice expansions of the two co-existing phases become inseparable with X-ray diffraction investigation. The present contribution gives a simplified visualisation and prediction of the lattice expansion to explain this phenomenon. It is demonstrated that 25% of the N atoms trapped by Cr resides in LRO positions of γ'N-Me4N (Me = Fe,Cr,Ni).