On the crystal structure of Cr₂N precipitates in high-nitrogen austenitic stainless steel. III. Neutron diffraction study on the ordered Cr₂N superstructure

The ordered structure of Cr₂N precipitates in high-nitrogen austenitic steel was investigated utilizing high-resolution neutron powder diffractometry (HRPD). On the basis of the Rietveld refinement of neutron diffraction patterns, the ordered Cr₂N superstructure was confirmed to be trigonal (space group ), with lattice parameters a = 4.800 (4) and c = 4.472 (5) Å, as suggested in previous transmission electron microscopy studies [Lee, Oh, Han, Lee, Kim & Takaki (2005). Acta Cryst. B61, 137-144; Lee, Kim & Takaki (2006). Acta Cryst. B62, 190-196]. The occupancies of the N atoms in four crystallographic sites [1(a), 1(b), 2(d) and 2(c) Wyckoff sites] were determined to be 1.00 (5), 0.0, 0.74 (9) and 0.12 (3), respectively, reflecting a partial disordering of N atoms along the c axis. The position of the metal atom was specified to be x = 0.346 (8) and z = 0.244 (6), corresponding to a deviation from the ideal position (x = 0.333 and z = 0.250). This deviation caused the -type superlattice reflection to appear. A comparison between the ideal and measured crystal structures of Cr₂N was performed using a computer simulation of selected-area diffraction patterns.