Morphology of the solid-liquid interface and redistribution of solutes in unidirectionally solidified aluminum alloys

Abstract

The commercial high-purity 99.997% aluminum and its alloys ( k < 1 type Al-Mn, Al-Zn, Al-Fe and k > 1 type Al-Cr, Al-Ti alloys) were unidirectionally solidified and decanted to examine the solid-liquid interface morphology. The entrapping of solutes into the solid at interface forms segregating regions and releases the onset of constitutional supercooling. According to the degree of constitutional supercooling, these regions increase their areas in forming depressions, cell nodes or cell walls. The degree of segregation at these regions almost does not change during the morphological transition. The morphological transitions in various alloy systems occur under constant values of C 0 R/ G which are governed strongly by the distribution coefficient. Under conditions near the transition, grains having different morphologies are sometimes observed to coexist in one interface; and those having orientation near 〈111〉 are the earliest to transit to other morphology. A characteristic inner-substructure is observed in Al-Ti alloys. It is not found in other alloy systems. Its formation is suggested to be due to the functioning of undissolved TiAl 3 nuclei in the constitutionally supercooled region ahead of the freezing interface.