On the crystal structure of Cr2N precipitates in high-nitrogen austenitic stainless steel. II. Order–disorder transition of Cr2N during electron irradiation

Abstract

The crystal structure and order–disorder transition of Cr2N were investigated utilizing transmission electron microscopy (TEM). Based on the analyses of selected-area diffraction (SAD) patterns, the crystal structure of the ordered Cr2N superstructure was confirmed to be trigonal (P\overline 3 1m), characterized by three sets of superlattice reflections (001), (1\over 31\over 30) and (1\over 31\over 31). During electron irradiation, the superlattice reflections gradually disappeared in the regular sequence (001), (1\over 31\over 30) and (1\over 31\over 31), indicating that the order–disorder phase transition of Cr2N occurred. The convergent-beam electron diffraction (CBED) observation revealed that the space group of disordered Cr2N is P63/mmc, which corresponds to an h.c.p. (hexagonal close packed) sublattice of metal atoms with a random distribution of N atoms in six octahedral interstices. The redistribution model of N atoms through the order–disorder transition is discussed based on the characteristics and disappearing sequence of superlattice reflections.