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

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.