Direct observations on channel segregate formation in aluminium alloys

Direct in situ observations of channel initiation and development in Al-25Ge and Al-21 Cu alloys revealed that channels always started from entrapped liquid at the growth front. The entrapment arose from either the preferential growth of some dendrites in a columnar array or from the agglomeration of equiaxed dendrites at the advancing interface. The reduced permeability in the vicinity of the entrapment enhanced fluid flow, and channels developed both downstream and upstream of the initiation point. Irregularities in growth always occurred at the slow growth rates used in this work. Interdendritic fluid flow extensively modified the structure and a fine dendrite mesh resulted when solute-rich liquid was able to flow freely from the mushy zone after the initial stage of growth.