5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

Feathery grains have attracted considerable research interest since they were first observed in semi-continuous casting of industrial aluminum alloys about 70 years ago. Different from the regular columnar or equiaxed morphologies observed in alloy castings, this special crystal structure exhibits the alternant twinned and untwinned lamellae morphology. Previous studies focus on the analysis and characterization of the twinned dendrite growth appearing in direct-chill (DC) or similar semi-continuous casting processes. And the twinned dendrite growth during these solidification processes is believed to occur when the following solidification conditions are met: (i) a relatively high thermal gradient (typically 100 K/cm); (ii) a large growth speed (at least 1 mm/s); (iii) the presence of convection in the melt. In this work, we first successfully produced twinned dendrites in Al-4.5 wt.% Cu alloys during Bridgman solidification. Based on the twin planes already formed at 3000μm/s, by introducing an abrupt decrease in growth speed, the twinned growth could be produced till the end of solidification after abruptly decreasing the growth speed to 100, 50, 20, 10, 5, 3, 1 and 0.3 μm/s, respectively. The twinned patterns in aluminum alloys are richer than previously expected during a wide range of solidification processes. The corresponding pattern selection map was further revealed. Besides, an interesting orientation rotation phenomenon was verified by EBSD on either side of the coherent twin boundary during twinned cell growth. In high-resolution transmission electron microscopy (HRTEM) views, successive stacking faults (SFs) occurred nearby the twin boundaries (TBs) at 1μm/s, while the distortion around the TBs at 3000μm/s was quite slight. Importantly, distinct SF structures directly connecting two adjacent TBs, with a spacing of about 221nm, provides a direct experimental evidence for the close correlation between the SFs and the twinned dendrite growth in aluminum alloys.