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Real-time study of pattern formation dynamics in nonvariant eutectic solidification microstructures

Final Report Summary - NEUSOL (Real-time study of pattern formation dynamics in nonvariant eutectic solidification microstructures)

This project concerns a multidisciplinary experimental research program in the field of solidification science with various impacts in nonlinear physics, metallurgy, materials science, and engineering. The problem studied is the formation dynamics of multiphase microstructures in eutectic alloys. Eutectics are naturally grown composites, which are generally solidification-processed. These microstructures basically consist of nearly periodic arrangements of different crystal phases on the micrometer scale. Because of their low and constant melting temperatures, 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. However, a substantial challenge for eutectic materials is to control the variability of the microstructure on a scale larger than a few tens of a micrometer. If the microstructures were perfectly periodic on a macroscopic scale, the solid, including lead-free solders, 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. The irregularities in the microstructure are much more likely to be intrinsic properties of the nonlinear dynamics of lamellar eutectic patterns rather than responses to extraneous defects as was previously believed. Further ill-understood important features of eutectic solidification are the formation of sharp crystallographic textures and the mechanisms leading to the observed features of eutectic patterns after long solidification times. Whereas these problems have been addressed to some extent in two-phase eutectics, they represent the state-of-the-art in three-phase systems, which were the objectives of this project.

PI became a totally independent researcher by establishing a lab named as Microstructural Evolution Laboratory (MEL) which basically includes sample preparation setups, state-of-the-art directional solidification setups, and a double-sided microscope. All these equipment were designed, specified, and sourced by MEL group members. Using these solidification setups and the microscope, real-time images were obtained to examine the dynamics of three-phase eutectic growth and the large majority of the project goals are achieved. Detailed examination of the periodic steady-state microstructures in 2D samples has been performed and it is concluded that the ABAC structure is the stable configuration for the three-phase isotropic eutectic grains in thin samples (Fig. 1). At the lower and upper bound of these stability limits, elimination and branching are observed, respectively. Oscillation of the ABAC pattern is observed as a transient state. The characterization of the 2D three-phase alloys in terms of periodic steady-state microstructures, eutectic spacing, phase diffusion coefficient and material’s constants (Kr, Kc and λ2V constants of Jackson and Hunt theory) are determined and this work is published in Acta Materialia [doi:10.1016/j.actamat.2016.01.065]. After obtaining the 2D steady-state microstructures, the morphological stability limits of the ABAC pattern, as well as the spacing adjustment mechanisms, are determined. These results are also included in the same journal article.

Fig. 1 The stable configuration for the three-phased isotropic eutectic grains in thin samples is observed to be ABAC pattern where In2Bi (black) phase is denoted as A, β-In (white) phase is denoted as B, γ-Sn (gray) phase is denoted as C. Basic state and spacing adjustment mechanisms observed in three-phase microstructures are shown. Growth direction is upwards.

Obtaining the ABAC structure back after the spacing adjustment mechanisms was not straightforward in 2D samples. After elimination mechanism, most of the times the ABAC structure is recovered, but after branching, it was rarely the case. Study on the quasi-2D samples revealed some very interesting recovery mechanisms of the ABAC structure. These results are reported in the article entitled as “Dynamics of spacing adjustment and recovery mechanisms of ABAC-type growth pattern in ternary eutectic systems” published in the Journal of Crystal Growth [doi:10.1016/j.jcrysgro.2017.04.008]. ABAC pattern recovery by phase-exchange mechanisms is shown in Fig. 2.

Fig. 2 Top and bottom views of the phase-exchange mechanism used to recover the ABAC pattern after elimination, which happened after a velocity decrease from the initial velocity of 0.26 µm/s to the final velocity of 0.22 µm/s. The time difference between the top and bottom images is 345 seconds. Sample thickness is 23.0 µm. Growth direction is upwards.

Due to the anisotropy of crystal/crystal interphases, very interesting three-phase microstructures are obtained, as shown in Fig. 3. Results showed that AB interphase boundary anisotropy and dynamics play very critical role in microstructural evolution. It may change both the microstructure of three-phase structure as well as the morphology of each phase. Additionally, anisotropic (locked) ABAC structures can become separated where AB grows on one side of the sample and C on the other side. Effects of anisotropy on three-phase eutectics are shown for the first time in the literature.

Fig. 3 Example of microstructures obtained due to the anisotropy of crystal/crystal interphases. Rotation (ω) or growth (V) velocity and sample thickness (δ) information are given under the images. Top-row images show locked structure. Middle-row images show the microstructure obtained by rotation the top-row images. “Kinks” are formed due to forbidden orientations. Bottom-row images show separated structure. Growth direction is upwards.

Reaching the vast majority of the project objectives deeply enhanced our fundamental understanding of the solidification pattern formation in three-phase eutectic systems, which impacts both materials science and nonlinear physics communities internationally. On the technological perspective, this understanding will open the way to control and hence to optimization of the properties of solidification-processed composite materials, including lead-free solders. For instance, enhancing the mechanical properties of lead-free solders by producing more homogeneous micro-composite structure will make them more resistant to crack formation during thermal cycling, which will enable the use of lead-free solders in the avionics industry. Currently, the lack of fundamental understanding limits, for instance, the usage of lead-free solders in the electronics industry. Finally, socio-economically, increased use of lead-free solders will reduce the lead contamination of landfills, where it can leach into groundwater and can be a very high potential risk both to human health and to the environment.

In summary, this study is the first experimental study on the investigation of dynamics and stability of three-phase eutectics as well as the effect of anisotropy on three phase eutectics in real-time and it made major contributions to the fundamental understanding of the solidification characteristics of three-phase eutectics. These improvements will have a strong impact on the international community by opening the way to the control and hence to the optimization of the properties of three-phase eutectics, including lead-free solders. Further optimization will spread the usage of three-phase eutectics to other applications such as avionics where consistency is vital.

Regarding dissemination and training activities, the PI or her graduate students made 19 presentations at international conferences or meetings, attended various workshops and training sessions. One M.S. student supervised by the PI was graduated in 2015; another M.S. and Ph.D. students are expected to graduate at the end of the semester. 2 journal articles are already published in respected journals and 3 more are expected to be published within a year. The PI made many connections both with academia and industry in Europe and currently leading part of a European Space Agency project. To conclude, both the project and the integration period is successfully completed.

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