Microstructural study of ε-iron-carbonitrides formed by a glow discharge technique

Abstract

The microstructure and phase transformations occurring inε-iron-carbonitrides have been studied by means of X-ray and electron diffraction, electron microscopy and Mössbauer spectrometry. Ordering of the interstitial atoms, N or C, results in a hexagonal unit cell for Fe₃(C, N) with parametersa′=a√3 andc′=c wherea andc are the lattice parameters of the hexagonal close-packed (h c p) iron unit cell. Stacking faults on (0001)_∈ planes and partial dislocations with Burgers vector b=1/3〈1 0 ¯1 0〉 are observed in quenchedε-Fe₄ (C, N). After quench-aging, the carbonitrides show a structural hardening due to the precipitation of a metastable phase. Slow cooling ofε-carbonitrides with less than 25 at% interstitials leads to the precipitation ofγ′-carbonitride and ferrite inε-phase grains which allows the orientation relationships between theε-,γ′- andα-phases to be defined and a model of theε-phase-γ′-phase transformation to be proposed.