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

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 (P31m), characterized by three sets of superlattice reflections (001), ((11/33)0) and ((11/33)1). During electron irradiation, the superlattice reflections gradually disappeared in the regular sequence (001), ((11/33)0) and ((11/33)1), 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 P6(3)/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.