Growth stability of twinned dendrites in directionally solidified Al-4.5 wt% Cu alloy

Based on the twin planes formed at 3000 μm/s, the growth stability of twinned dendrites in Al-4.5 wt% Cu alloy was studied in detail at a low growth rate during Bridgman solidification. Experimental results show that the twinned dendrite growth could still keep stable even after decreasing the rate to 20 μm/s. This is far below the usual rate threshold for the formation of twinned dendrites (∼1000 μm/s). The dominated twinned structures at 20 μm/s have their (1 1 1) twin plane and 〈1 1 0〉 dendritic trunk closest to thermal gradient direction. Importantly, distinct stacking fault (SF) structures directly connecting two adjacent twin boundaries provide a direct experimental evidence for the close correlation between the SFs and the twinned dendrite growth in aluminum alloys.