5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

Eutectic alloys are self-organising composite materials with a wide variety of microstructural features. Size, shape, distribution, and orientation of these features can be modified by process parameters (temperature gradient, velocity of interface) as well as material parameters (volume fractions, anisotropy of interfacial energies, diffusivities, impurity nature and its percentage etc.). The objective of this work is to study the role of solid-solid and solid-liquid anisotropies in eutectic structure formation particularly the internal structure of colonies that arise due to a two-phase growth instability. We have chosen the Sn-Te eutectic system as the base binary system which has SnTe and Te phases, with Ag/Sb as impurity additions for triggering the colony formation. The binary and ternary (Ag/Sb addition) alloys are directionally solidified at different interfacial velocities to study the morphological evolution. In case of binary Sn-Te eutectic, broken labyrinth/ rod morphologies are observed (see left fig.). Upon addition of a third component, a diffusive instability (similar to a Mullins-Sekerka instability) leads to the formation of two-phase colonies that arise beyond a particular velocity. Critical velocities beyond which instabilities form are determined for each of the alloying additions. In our experiments we find colony structures having an internal sub-structure upon additions of both Ag/Sb. The internal structure due to Ag addition however, is different from Sb Addition (See figues). We explain the role of anisotropies in the free-energy of the interphase boundaries that lead to the formation of the different eutectic colony structures using results obtained from experimental characterization techniques (using SEM/TEM/EBSD).