Tak Shing Lo

Two-phase microstructure selection in peritectic solidification: from island banding to coupled growth

Abstract We report the first experimental observation in directionally solidified peritectic Fe–Ni alloys of two-phase island banding microstructures that consist of rows of islands of one solid phase (either peritectic or primary) inside the continuous matrix of the other phase. These microstructures form under predominantly diffusion-limited growth conditions and when both phases are morphologically stable, as recently predicted by numerical simulations of a phase-field model. They are observed either as transients that seed the formation of coupled growth structures, or as the final microstructure. Phase-field simulations are reported that shed light on the relationship of island banding and coupled growth as well as on the growth conditions and nucleation parameters that control the dynamical selection of these two basic microstructures in peritectic alloys.

Phase-field modeling of microstructural pattern formation during directional solidification of peritectic alloys without morphological instability.

During the directional solidification of peritectic alloys, two stable solid phases (parent and peritectic) grow competitively into a metastable liquid phase of larger impurity content than either solid phase. When the parent or both solid phases are morphologically unstable, i.e., for a small temperature gradient/growth rate ratio (G/v(p)), one solid phase usually outgrows and covers the other phase, leading to a cellular-dendritic array structure closely analogous to the one formed during monophase solidification of a dilute binary alloy. In contrast, when G/v(p) is large enough for both phases to be morphologically stable, the formation of the microstructure becomes controlled by a subtle interplay between the nucleation and growth of the two solid phases. The structures that have been observed in this regime (in small samples where convection effects are suppressed) include alternate layers (bands) of the parent and peritectic phases perpendicular to the growth direction, which are formed by alternate nucleation and lateral spreading of one phase onto the other as proposed in a recent model [R. Trivedi, Metall. Mater. Trans. A 26, 1 (1995)], as well as partially filled bands (islands), where the peritectic phase does not fully cover the parent phase which grows continuously. We develop a phase-field model of peritectic solidification that incorporates nucleation processes in order to explore the formation of these structures. Simulations of this model shed light on the morphology transition from islands to bands, the dynamics of spreading of the peritectic phase on the parent phase following nucleation, which turns out to be characterized by a remarkably constant acceleration, and the types of growth morphology that one might expect to observe in large samples under purely diffusive growth conditions.