Introduction

A low interfacial energy between ferrite and inclusions is preferred to stimulate the formation of ferrite and obtain fine ferrite microstructure. Following the calculation of interfacial energy of B1 type inclusions (denoted as MI) with austenite, the interfacial energy of MI with ferrite with Baker-Nutting relationship is calculated, including both chemical and structural components.

Chemical interfacial energy

The chemical component of interfacial energy comes from the composition difference across the interface. The Baker-Nutting orientation relationship is assumed between ferrite() and inclusion. By compressing the [001] of MI, the /MI interface can be treated as a coherent cube-on-cube b.c.c./b.c.c. interface. Thus, the nearest neighbor broken bond (NNBB) methods developed previously is applicable to calculate chemical interfacial energy for /MI interface, although in this case second nearest interaction has to be incorporated. The calculated /TiC chemical energy are shown in Fig.1 as (100) polar plot and corresponding Wulff construction. It is apparent that a deep cusp appears in [001] orientation.





Fig.1 (100) Wulff construction for ferrite/TiC chemical interfacial energy


Fig.2 (001) interfacial energy between ferrite and inclusions

Structural interfacial energy

The structural component of interfacial energy arises from the presence of misfit dislocation at semi-coherent interfaces. The Bollmann's O-lattice theory is employed to determine the dislocation structure and then the structural interfacial energy of /MI interface is calculated from Hirth and Lothe's equations for a few low index interfacial planes, by neglecting the interstitial atoms in MI as an approximation. In Baker-Nutting relationship, (001) /MI interface contains much fewer misfit dislocations than other orientations, resulting in lowest (001) structural interfacial energy. For different inclusions, vanadium compounds have small lattice misfit with ferrite, and may have a low structural energy.

Total interfacial energy

The total interfacial energy as the summation of the chemical component and structural component is shown in Fig. 2 for (001) interfaces with some carbides, nitrides and oxides. The result indicates both chemical and structural component contribute significantly to the total interfacial energy. Generally, vanadium and titanium compounds have lower interfacial energy than niobium and zirconium compounds due to small misfit, and oxides have lower interfacial energy than carbides and nitrides due to small elastic constants.

Zhi-Gang Yang (Dept. of Materials Science, Ibaraki University, Hitachi, Ibaraki,316- 8511, Japan)

# This paper published in

Current Advances in Materials and Process (Report of the Iron and Steel Institute of Japan (ISIJ) Meeting, Tokyo, Mar, 1999, Vol.12(3):527