Observation of the initiation and propagation of solidification cracks by means of in situ synchrotron X-ray radiography

We report on the in situ, time resolved, observation of solidification cracking in a thin sample of an Al-15wt%Sn alloy at ESRF BM05. During the experiment, solidification cracking was seen to occur during natural cooling of the sample at a solid fraction of ∼95%, between directionally agglomerated dendritic networks. Through detailed analysis, three stages of crack growth were observed: crack initiation, which typically occurs asymmetrically with the liquid film separating from one side of the dendritic network prior to full detachment; crack propagation, where the liquid film generally detaches from both sides simultaneously; and crack coalescence. We correlate our observations using scanning electron microscopy, which shows that voids between grains, and also spikes, can appear asymmetrically on either side of the crack surface.