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Gaseous nitriding of steel and iron can significantly improve their properties, for example corrosion resistance, fatigue endurance and tribological properties. In order to obtain a better understanding of the early stages of formation of the initial cubic primitive γ′-Fe4N, the mechanism and crystallography of the α–γ′ phase transformation was investigated under simplified conditions. Single-crystal α-Fe whiskers were nitrided at 823 K and a nitriding potential of 0.7 atm−1/2for 20 min. The resulting microstructure and phases, as well as the crystallographic orientation of crystallites belonging to a particular phase, were characterized by scanning electron microscopy coupled with electron backscatter diffraction. The habit planes were investigated by single- and two-surface trace analysis. The α-Fe whiskers partly transform into γ′-Fe4N, where γ′ grows mainly in a plate-like morphology. An orientation relationship close to the rational Pitsch orientation relationship and {0.0780.4320.898}αand {0.3910.3670.844}γ′as habit planes were predicted by the phenomenological theory of martensite crystallography (PTMC), adopting a {101}α〈101〉αshear system for lattice invariant strain, which corresponds to a {111}γ′〈112〉γ′shear system in γ′. The encountered orientation relationship and the habit planes exhibit excellent agreement with predictions from the PTMC, although the transformation definitely requires diffusion. The γ′ plates mainly exhibit one single internally untwinned variant. The formation of additional variants due to strain accommodation, as well as the formation of a complex microstructure, was suppressed to a considerable extent by the fewer mechanical constraints imposed on the transforming regions within the iron whiskers as compared to the situation at the surface of bulk samples.

The crystallography and mechanism of the early stages of the formation of cubic primitive γ′-Fe4N in single-crystal α-Fe whiskers during gaseous nitriding were investigated by electron microscopy and electron backscatter diffraction. The α–γ′ phase transformation could be observed under simplified conditions owing to the lack of geometric constraints for the transformation., Gaseous nitriding of steel and iron can significantly improve their properties, for example corrosion resistance, fatigue endurance and tribological properties. In order to obtain a better understanding of the early stages of formation of the initial cubic primitive γ′-Fe4N, the mechanism and crystallography of the α–γ′ phase transformation was investigated under simplified conditions. Single-crystal α-Fe whiskers were nitrided at 823 K and a nitriding potential of 0.7 atm−1/2 for 20 min. The resulting microstructure and phases, as well as the crystallographic orientation of crystallites belonging to a particular phase, were characterized by scanning electron microscopy coupled with electron backscatter diffraction. The habit planes were investigated by single- and two-surface trace analysis. The α-Fe whiskers partly transform into γ′-Fe4N, where γ′ grows mainly in a plate-like morphology. An orientation relationship close to the rational Pitsch orientation relationship and {0.078 0.432 0.898}α and {0.391 0.367 0.844}γ′ as habit planes were predicted by the phenomenological theory of martensite crystallography (PTMC), adopting a {101}α〈101〉α shear system for lattice invariant strain, which corresponds to a {1 1 1}γ′〈1 12〉γ′ shear system in γ′. The encountered orientation relationship and the habit planes exhibit excellent agreement with predictions from the PTMC, although the transformation definitely requires diffusion. The γ′ plates mainly exhibit one single internally untwinned variant. The formation of additional variants due to strain accommodation, as well as the formation of a complex microstructure, was suppressed to a considerable extent by the fewer mechanical constraints imposed on the transforming regions within the iron whiskers as compared to the situation at the surface of bulk samples.

The crystallographic features of a range of diffusional-displacive phase transformations was described by the phenomenological theory of martensite crystallography (PTMC). While the PTMC accounts for the crystallography of γ'-Fe4N formation in bulk-aged N-supersaturated α-Fe or nitrided α-Fe whiskers, there is still a lack of understanding of the morphological and crystallographic features of γ'-Fe4N directly formed on bulk polycrystalline α-Fe substrates, in spite of its importance to the microstructure development during nitriding. In this study, the morphology and crystallography of γ'-Fe4N, formed directly by gaseous nitriding of pure polycrystalline α-Fe substrates at 823 K, were investigated by means of EBSD and serial sectioning. The α-Fe grains partly transform into γ'-Fe4N, whereby γ' grows in several morphologies consisting of different variants. Characteristic features of the thus formed γ' phase are pronounced orientation gradients accompanied by different α-γ' orientation relationships (OR) and consequently deviations from the near Pitsch OR and habit plane predicted by PTMC with a lattice invariant shear (LIS) according to { 101 } α 〈 10 1 ¯ 〉 α in the case of bulk-aged samples. Another feature of the γ' grains are twin boundaries and it is made likely that the incompatibility of the twin boundaries with the near Pitsch OR predicted by PTMC is one of the main factors leading to the formation of an OR close to Nishiyama-Wassermann or Kurdjumov-Sachs. It is shown that the orientation gradients and the change of the habit plane are mostly compatible with modified PTMC predictions involving a change of the LIS by variation of the LIS direction while maintaining the { 1 ¯ 0 1 ¯ } α LIS plane.