On the nucleation of cementite on bainitic ferrite–austenite interphase boundaries

Abstract

Among all possible variants of the Isaichev orientation relationship between cementite and ferrite, a single major cementite variant has been observed to appear in bainite. Interphase boundary nucleation of cementite on ferrite–austenite semi-coherent interfaces is considered a plausible reason for this observation. With the aid of known crystallographic relations and habit planes of the ferrite–cementite, ferrite–austenite and austenite–cementite phases, a model for cementite nucleation has been proposed. The interphase-boundary nucleus is assumed to form on a semi-coherent ferrite–austenite interface and to possess ferrite–cementite and austenite–cementite habits as two main facets of the nucleus. It is shown that interphase cementite nucleation will be viable if the energies of all facets of the nucleus are in the semi-coherent range.

Keywords:

• bainite
• cementite nucleation
• crystallography
• interfaces

Acknowledgement

The authors gratefully acknowledge the financial support of the Natural Science and Engineering Research Council of Canada (NSERC).

Notes

1. Ordinarily, bainite is known as a mixture of ferrite and cementite. Addition of certain elements such as Si inhibits cementite precipitation and the carbon-enriched, stable austenite then may remain in the microstructure at room temperature. Therefore, typically two types of bainitic ferrite microstructures are known: carbide-free bainite and bainite.

2. Weatherly and Zhang have shown that for a KS OR, O-line condition will be satisfied when all ∆gs are parallel in the zone axis of 〈1 1 0〉γ//〈1 –1 1〉α.

3. Jong Lee et al. in [24] showed that, unless the ratio of strain energy to the driving force, ∆G_v, exceeds 0.75, the minimization of interfacial energy is enough to predict the equilibrium shape and in most precipitation cases this ratio is not reached.

4. Thermodynamic data were extracted from Thermo-Calc, TCFE6 database. The driving force unit was converted from J/mol to J/m³ using a molar volume of 2.33 × 10⁻⁵ for cementite based on the lattice parameters used in this work.