Transmission electron microscopy investigation of acicular ferrite precipitation in γ′-Fe4N nitride

Acicular-shaped crystals precipitate from γʹ-Fe4N nitride in an iron–nitrogen alloy and were identified by electron microdiffraction as α-ferrite. Acicular ferrite develops both the Nishiyama–Wassermann and the Kurdjumov–Sachs orientation relationships with γʹ-Fe4N nitride. These orientation relationships were discussed in terms of the symmetry theory. The driving force for acicular ferrite formation was related to the increasing nitrogen content of γʹ-Fe4N, in equilibrium with α-ferrite, with decreasing temperature. The passage from lamellar to acicular structure in Fe–N system was proposed.