Crystallography of γ′-Fe₄N formation in single-crystalline α-Fe whiskers

Abstract

<jats:p>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′-Fe<jats:sub>4</jats:sub>N, 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<jats:sup>−1/2</jats:sup> 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′-Fe<jats:sub>4</jats:sub>N, where γ′ grows mainly in a plate-like morphology. An orientation relationship close to the rational Pitsch orientation relationship and {0.078 <jats:overline>0.432</jats:overline> <jats:overline>0.898</jats:overline>}<jats:sub>α</jats:sub> and {0.391 <jats:overline>0.367</jats:overline> <jats:overline>0.844</jats:overline>}<jats:sub>γ′</jats:sub> as habit planes were predicted by the phenomenological theory of martensite crystallography (PTMC), adopting a {<jats:overline>1</jats:overline>0<jats:overline>1</jats:overline>}<jats:sub>α</jats:sub>〈<jats:overline>1</jats:overline>01〉<jats:sub>α</jats:sub> shear system for lattice invariant strain, which corresponds to a {<jats:overline>1</jats:overline> <jats:overline>1</jats:overline> <jats:overline>1</jats:overline>}<jats:sub>γ′</jats:sub>〈<jats:overline>1</jats:overline> <jats:overline>1</jats:overline>2〉<jats:sub>γ′</jats:sub> 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.</jats:p>

Fe Nitride Formation in Fe–Si Alloys: Crystallographic and Thermodynamic Aspects

“…[19] Table II. Crystallographic Orientation Relationships (ORs) Between a-Fe and c¢ As Well As a-Fe and e Reported in the Literature and Relevant to the Current Study Nishiyama-Wassermann OR [44,45] Pitsch OR [39] Near-Pitsch OR [42] Burgers OR [46][47][48] 101…”

“…The boundary traces of the plates agree with {0.08 4.3 9} a as habit plane (Figure 5(d)), i.e., the habit plane associated with the near-Pitsch OR. [42] Hence, the nitride plates observed after 1 hour most likely consist of c¢ and exhibit the near-Pitsch OR with a-Fe. The microstructure observed after 1 hour is, therefore, considered to represent an early stage of the plate-type growth of c¢ frequently reported upon nitriding Fe-Si alloys.…”

“…Frequently, a midrib exists in the center of the nitride plates (Figure 4(g)), analogous to the midrib of martensite plates as reported for c¢ plates formed in pure a-Fe. [42] After nitriding for 16 and 48 hours, the compound layer growth seems to be preceded rather by conventional planar-type growth than by the protrusion of nitride plates into a-Fe (Figures 4(c) and (d)). Neighboring nitride plates have largely impinged due to lateral thickening.…”

“…However, the literature has shown by means of the phenomenological theory of martensite crystallography that the near-Pitsch OR correctly describes the OR between c¢ plates and a-Fe in nitrided and heat-treated pure Fe. [40][41][42][43] Therefore, the Pitsch OR will not be further addressed in the following.…”

“…[39][40][41] Such near-Pitsch OR (Table II) governs also the early stage of c¢ growth during nitriding pure a-Fe, resulting in the formation of c¢ plates characterized by {0.08 4.3 9} a ||{3.9 3.7 8.4}c¢ habit planes. [42] For detailed information regarding the a/c¢ ORs, the reader is referred to the References 42 and 43.…”