Abstract
Experimentally and theoretically, the basic structures of disordered expanded austenite, γN, 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 γN 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 γN 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.
Original language | English |
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Article number | 118971 |
Journal | Acta Materialia |
Volume | 253 |
Number of pages | 12 |
ISSN | 1359-6454 |
DOIs | |
Publication status | Published - 2023 |
Keywords
- Fe-Cr-Ni alloys
- Nitriding
- Order-disorder phenomena
- Antiphase boundary
- Cellular automaton