Crystallography of γ′-Fe4N formation on bulk polycrystalline α-Fe substrates

Abstract

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〈101¯〉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¯01¯}α LIS plane.