Co-existence of γ'N phase and γN phase on nitrided austenitic Fe-Cr-Ni alloys - III. An investigation of the evolution of long-range ordered domains

Experimentally and theoretically, the basic structures of disordered expanded austenite, γ_N, and ordered expanded austenite, γ^'_N, have been confirmed to consist of Cr-N short-range ordering (SRO) and Fe₄N-like long-range ordering (LRO) in addition to Cr-N SRO, respectively. So far, the transition from γ_N to γ^'_N during low temperature nitriding has not been elucidated. In present work, four f.c.c. Fe-Cr-Ni alloys with 0, 6, 12 and about 18 wt.% Cr in low-temperature nitrided condition were investigated with transmission electron microscopy (TEM) to explore the evolution from γ_N to γ^'_N. The Fe₄N-like LRO is present as small ordered domains separated by antiphase boundaries. Ordering of the interstitial nitrogen involves nucleation, growth and, after impingement, coarsening of the ordered domains. The domain size decreases with Cr content and increases with N content in the investigated alloys. A 3-dimensional Cellular Automaton (CA) was developed to simulate nucleation and growth until impingement into full LRO was reached; also the subsequent coarsening stage was simulated. The simulated distributions of ordered domains for the range of Cr contents are consistent with the TEM results. Cr-N SRO plays both a role of promoting nucleation and limiting growth of the ordered domains. Diffraction simulation by Fourier transforming the CA-simulated nitrogen distribution is consistent with the diffraction data and clarifies the different roles of high-density antiphase boundaries and Cr-N SRO on the superlattice reflections. The simulation is further validated by comparison of the predicted distributions of occupied interstices to literature data for Mössbauer spectroscopy and Extended X-ray Absorption Fine Structure.