Phase-field modeling of microstructural pattern formation during directional solidification of peritectic alloys without morphological instability - art. no. 031504

During the directional solidification of peritectic alloys, two stable soli d phases (parent and peritectic) grow competitively into a metastable liqui d phase of larger impurity content than either solid phase. When the parent or both solid phases are morphologically unstable, i.e., for a small tempe rature gradient/growth rate ratio (G/upsilon (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 solidificati on of a dilute binary alloy. In contrast, when Glu, is large enough for bot h 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 reg ime (in small samples where convection effects are suppressed) include alte rnate layers (bands) of the parent and peritectic phases perpendicular to t he growth direction, which are formed by alternate nucleation and lateral s preading of one phase onto the other as proposed in a recent model [R. Triv edi, Metall. Mater. Trans. A 26, 1 (1995)], as well as partially filled ban ds (islands), where the peritectic phase does not fully cover the parent ph ase which grows continuously. We develop a phase-field model of peritectic solidification that incorporates nucleation processes in order to explore t he formation of these structures. Simulations of this model shed light on t he morphology transition from islands to bands, the dynamics of spreading o f the peritectic phase on the parent phase following nucleation, which turn s out to be characterized by a remarkably constant acceleration, and the ty pes of growth morphology that one might expect to observe in large samples under purely diffusive growth conditions.