The Cellular Breakdown of the Planar Interface in Unidirectional Solidification of Al–Cu Alloy

Abstract

The interfacial transition from the plane to the cellular structure and the segregation pattern of Al–Cu alloys were investigated by means of a quenching method during unidirectional solidification.The following results were obtained: (1) The preferred grain boundary segregation occurred prior to the breakdown of the planar interface. (2) Small holes appeared at random on the solid-liquid interface at the biginning of the transition. (3) The holes were aligned or distributed regularly to form an elongated cell. (4) The solidification proceeded entrapping the liquid phase at the bottom of the holes and resulted in the “rosary-like segregation”. (5) The “rosary-like segregation” was also observed in hexagonal cell nodes.These phenomena can hardly be explained by the constitutional supercooling or by the M–S theory. In the current paper these were discussed in terms of the stability of the small pits on the solid-liquid interface.