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, gamma N, and ordered expanded austenite, gamma'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 gamma N to gamma'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 gamma N to gamma'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 impinge-ment, 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 nucle-ation 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 Mo center dot ssbauer spectroscopy and Extended X-ray Absorption Fine Structure.

Automated summary

Both experimental and theoretical studies have confirmed that disordered expanded austenite, gamma N, and ordered expanded austenite, gamma'N, consist of Cr-N short-range ordering (SRO) and Fe4N-like long-range ordering (LRO) in addition to Cr-N SRO, respectively. However, the transition from gamma N to gamma'N during low temperature nitriding has not been fully understood. In this study, four Fe-Cr-Ni alloys were investigated to explore the evolution from gamma N to gamma'N using transmission electron microscopy (TEM). Results showed that Fe4N-like LRO exists as small ordered domains separated by antiphase boundaries, and the size of the ordered domains is influenced by the Cr and N content in the alloys. A 3-dimensional Cellular Automaton (CA) simulation was developed to simulate the nucleation, growth, and coarsening of the ordered domains, and the simulated distributions were consistent with the TEM results.