Morphological characterization of the directionally solidified AlAgCu ternary eutectic

Abstract

Ternary eutectic microstructures have a level of complexity far exceeding that of binary eutectics. While developing from the liquid, three phases are attempting to minimize the interfacial areas and the diffusion distances between them, and thus they cannot develop simple structures like lamellae or fibres as are often observed in nf-nf eutectics such as AlCu or AgCu. In sections taken perpendicular to the growth direction, microstructures have been observed with various morphologies, including a highly organized structure which resembles stacked bricks. There is no detailed description of ternary eutectic structures in the literature and only a limited model made by Himemiya available that extends the classical Jackson and Hunt theory to simple geometrical arrangements of ternary eutectics. We therefore performed directional solidification experiments with the ternary eutectic AlCuAg alloy. The temperature gradient ahead of the eutectic interface was kept constant at 3 K/mm and the solidification velocity varied from 0.2 to 5 µm/s. The microstructures observed contain an Al-rich phase and the Al2Cu and Ag2Al phases with nontrivial shapes. We analyze in cross section the distance distributions between the phases, the distribution of interphase separations, the volume fraction and surface area of all phases and determine correlation functions. The variation of these parameters with solidification velocity, along with further experimental observations and analyses, will be discussed with respect to eutectic growth theory.