In situ observations of stress-induced defect formation at the solid-liquid interface

Directional solidification experiments have been carried out to examine in situ the stress generated at the interface which lead to the formation of defects. A transparent system, which is also birefringence, is selected for the study to observe simultaneously both the morphology of the interface and the stress fringes. For a cusp orientation, the instability of the interface is shown to give rise to stress generation, and when sufficient stress accumulates, a grain boundary is formed to relieve the stress. A stress-induced parallel grain boundary formation is shown to be the mechanism for planar-to-cellular interface transition. The same mechanism is shown to cause cellular tip splitting to adjust the cellular spacing. A simple model of stress generation due to concentration gradients near the perturbed interface is developed to show the possible mechanism of stress generation.