On the Growth Dynamics of Nearly-Locked Grain in the Three-Phase In-Bi-Sn Eutectic System

Solidification microstructures are significantly affected by the anisotropy of crystal/crystal interphase energy. A recent experimental work on a three-phase eutectic system by the authors suggested that two distinguishable eutectic grains, i.e., quasi-isotropic and locked, form when crystal/crystal interphase energies contain negligible and strong anisotropy, respectively (Mohagheghi and Şerefoğlu in Acta Mater 2018, vol. 151, pp. 432–42, 2018). In two-phase eutectic systems, in addition to these two grain types, another class of eutectic grain called nearly-locked (NL) was reported. In order to investigate the existence of the NL grain in three-phase eutectic systems, real-time directional solidification (DS) and rotating directional solidification (RDS) experiments are performed on thin samples of In-Bi-Sn eutectic alloy. It is found that NL grains also form in three-phase eutectics and they contain some characteristic features of both quasi-isotropic and locked grains. The anisotropy is strong enough to tilt the lamellar pattern with respect to the thermal gradient axis, as in the case of locked grains; however, the NL grains also retain some of the characteristic features of the quasi-isotropic grains, such as λ-diffusion, systematic eutectic spacing adjustment, and recovery mechanisms. As a result, these grains tend to form a relatively uniform ABAC-type growth pattern, similar to quasi-isotropic grains. Using the equilibrium shapes extracted from the interphase traces of RDS patterns, the γ plot of the anisotropic interphase, which contains 2 twofold smooth and distinct minima, is determined.