Influence of growth velocity variations on the pattern formation during the directional solidification of ternary eutectic Al-Ag-Cu

In order to control the evolving microstructure in complex eutectics and other multi-phase systems, it is important to understand the adjustment mechanisms and the parameters that determine pattern evolution. Here, a combined experimental and simulation approach is taken to investigate the response of a three-phase eutectic system to changes in solidification velocity. Using Al-Ag-Cu as a model system, large scale three-dimensional phase-field simulations are compared to directionally solidified samples containing both, velocity increases and decreases. The experimental results are obtained by synchrotron tomography for detailed consideration of the microstructure directly before and after a targeted velocity change and by traditional SEM analysis of sample cross sections to capture effects over longer length scales. In addition to qualitative analysis of the images, the microstructures are statistically assessed using phase fractions, shape factor and nearest neighbor statistics. Both, simulation and experiment show an immediate change in phase fraction as a result of a change in growth velocity. Adjustment of the microstructural pattern occurs more slowly over a relatively long length scale, due to splitting, merging and overgrowing events. Novel quantification techniques emphasize that ternary eutectic phase arrangements are complex and continuously evolving structures which, even under ideal conditions, do not reach steady state growth as quickly as previously believed.