Lamella-rod pattern transition and confinement effects during eutectic growth

We present an in situ experimental study of rod-like and lamellar growth patterns during directional solidification of a model eutectic transparent alloy, namely the succinonitrile-(d)camphor (SCN-DC) system. Coupled-growth patterns, and their morphological transformations upon changes of the pulling velocity V (0.0035 − 0.07 µms −1), were observed in real time in a series of flat-wall samples with different thicknesses δ from 15 (thin samples) to 350 µm (thick samples). While rod-like patterns formed systematically at relatively large velocity, both lamellae and rods were observed at sufficiently low velocity, regardless of the sample thickness. In confined-geometry samples (δ = 30 − 140 µm), lamellar patterns were stabilized over a large range of the interphase spacing λ at low V. In this thickness range, the lamella ends in contact with the sample walls were stabilized, and lamellar breakup into rods was mediated by secondary instabilities such as a zigzag mode and a spatial-oscillation mode. The (reverse) transition from rods to lamellae involved both rod elimination and rod elongation instabilities. In the bistable range, a hysteresis upon increasing or decreasing V was observed, as well as complex phenomena involving oscillatory, mixed and hybrid patterns. A discussion on the elementary mechanisms that determine the lamellar-rod transition, and their dependence on λ and V is initiated.