Calculation of the interfacial energies between α and γ iron and equilibrium particle shape

The interfacial energies between α and γ iron were calculated for Kurdjumov-Sachs (K-S), Nishiyama-Wassermann (N-W), and one orientation relationship (OR), which may represent irrational OR, with varying interface orientation. The relaxation of atomic structure in the vicinity of the interface was performed by the Monte Carlo method using an embedded atom method (EAM) potential. Whereas the polar plot of calculated interfacial energies exhibited small and large cusps for the K-S and N-W ORs, an almost equiaxed energy surface with shallow cusps was obtained for the irrational ORs. The equilibrium shape of an α particle, N-W oriented with the γ matrix, was a thick rectangular plate with broad facets containing monatomic ledges. In contrast, the equilibrium shape of a particle, K-S oriented with the γ matrix, was elongated nearly in the closed-packed directions with {112}_y type and another broad facet that achieves relatively good matching. The volume of these shapes in the Wulff space tends to be smaller for K-S than that of the N-W OR. The equilibrium shape of the grain boundary α particle at the γ grain boundary was calculated under the assumption that the α particle has K-S OR with one of the two γ grains using the modified Wulff construction proposed earlier. The variation of the shape of the α particle with γ grain boundaries at an early stage of precipitation may primarily be ascribed to the change in the variant of the smallest nucleation activation energy and less prominently to the change in the γ grain boundary energy. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee.