The microstructural scales of dendrites, such as primary, secondary and tertiary ami spacings, control the segregation profiles and the formation of secondary phases within interdendritic regions, which determine the properties of cast structures. Investigations on tertiary dendritic arms are scarce in the literature, and only a few studies have been conducted in order to analyze the effects of melt convection within the interdendritic region during transient solidification. In this article, the influence of thermosolutal convection on the tertiary dendrite arm spacing is experimentally examined in the downward vertical unsteady-state directional solidification of Al-Cu and Al-Si hypocutectic alloys. (c) 2006 Elsevier B.V. All fights reserved.601518711874