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

Acicular-shaped crystals precipitate from γ'-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 γ'-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 γ'-Fe4N, in equilibrium with α-ferrite, with decreasing temperature. The passage from lamellar to acicular structure in Fe–N system was proposed.

Research Highlights
► Acicular crystals precipitate from pearliticγ'-Fe4N nitride in an iron-nitrogen alloy and were identified by electron microdiffraction as acicular ferrite.
► The crystal structure, orientation relationships with the matrix and morphologies of acicular ferrite, were studied by transmission electron microscopy.
► The driving force for the formation of acicular ferrite is related to the temperature dependence of nitrogen content of γ'-Fe4N, in equilibrium with ferrite.
► The passage from the pearlitic structure to the acicular structure in the present iron-nitrogen alloy was proposed.