A simple model for nitrogen-induced lattice expansion ofγ'NandγNphases 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 gamma(N)and gamma'(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) gamma(N)is more pronounced than in the long-range ordered (LRO) gamma'(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 gamma'(N)-Me4N (Me = Fe,Cr,Ni).