MEL group focuses on experimental projects related to the formation dynamics of ternary eutectic microstructures with an emphasis on three-phased eutectic patterns and anisotropy. This includes a multidisciplinary experimental research program in the field of solidification science with various impacts in nonlinear physics, metallurgy, materials science, and engineering. Eutectics are naturally grown composites, which are generally solidification-processed. The eutectic microstructures basically consist of nearly periodic arrangements of different crystal phases on the micrometer scale. Because of their low melting points, and the remarkable mechanical, optical, and electrical properties that they owe to their fine microstructures, eutectics are extensively used in metallurgical industries including casting and soldering. In brief, they are solidification-processed high-performance composites. However, a substantial disadvantage of eutectic materials is apparently the uncontrollable variability of their microstructural features on a scale larger than a few tens of a micrometer. Would these microstructures be perfectly periodic on a macroscopic scale, the solid would present extraordinary properties. However, for as yet unknown reasons, eutectic microstructures always exhibit a large density of defects, which destroy the long-range periodicity and reduce the quality of the material.

Solidification is usually studied under directional solidification condition, where a sample is pulled at fixed velocity V, toward the cold side of an imposed unidirectional thermal gradient G. A fundamental property of directional out-of-equilibrium patterns is their multi-stability. They are not subjected to any selection principle, meaning that a continuum of steady patterns, including branches of periodic, symmetric, or symmetry-broken solutions, can be reached under the same solidification conditions, depending on the history through which these conditions were reached. Due to this history dependency, special attention has been paid to the initial stages of eutectic grain formation and to perturbations during growth. Furthermore, the stable steady-state microstructures in thin (2D), thick (3D), and intermediate-thick (quasi-2D) specimens differ substantially.

Here are more specific information regarding the ongoing projects:

Dynamics of three-phased eutectic growth of isotropic systems

The study of growth dynamics includes examination of periodic steady-state microstructures in 2D, quasi-2D, and 3D specimens in model alloys such as In-Bi-Sn system. Limits of morphological stability, material constants such as diffusion, Jackson and Hunt constants are also within the interest of this group. Determination of a stability map of microstructures as functions of the characteristic eutectic spacing, crystal growth velocity, specimen thickness, and alloy composition is the ultimate goal which will enable us to predict and hence to control the microstructural features. This will create new solidification paths for novel structures and hence lead to remarkable physical properties of materials.

Effects of anisotropy on ternary eutectic systems

The contact surfaces of crystals generally exhibit a certain degree of anisotropy. In other words, the interphase surface energy is a function of the crystallographic orientation of the phases. This implies that the eutectic-solidification dynamics should depend on the orientation of the eutectic phases with respect to each other and to the temperature gradient. Indeed, many alloys exhibit orientation relationships, such as Kurdjumov-Sachs relation, corresponding to low-energy interphase boundaries at fixed crystal orientations. The competition between isotropic and anisotropic grain dynamics is conjectured to be a key factor for the formation of solidification textures in epitaxial systems. Hence, the aim of this project is to investigate the effects of anisotropy in In-Bi-Sn and Al-Cu-Ag ternary eutectic systems using 2D and 3D specimens and various solidification techniques.

Growth of faceted/nonfaceted eutectic structures

There exist many remaining questions regarding faceted/nonfaceted eutectic alloys although they have been extensively used in the industry. The goal of this project is to investigate and characterize various microstructures obtained in Cu-B and AMPD-SCN systems employing different compositions and growth conditions. Crystallographic orientation of the boron phases is also examined to understand the relation between the microstructure and the crystallographic orientation. After establishing the stability map and the physical properties of the system, new application areas are expected to be generated for boron products.