2016 Volume 102 Issue 3 Pages 170-178
Time-resolved X-ray imaging has been adopted to directly observe the microstructural evolution under the influence of ultrasonic vibration in Sn-21 mass%Bi alloys. Simultaneously with the circulating convection ahead of dendrite tips (10 mm/s), the longitudinal oscillation of dendrites with low frequency (~20 Hz) occurred immediately after an imposition of ultrasonic vibration. The convection ahead of dendrites caused morphological change from dendritic to cellular at the dendrite tip, where the fragmentation rarely occurred. The tip radius of primary dendrite is approximately 3.5 times as large as that prior to the ultrasonic vibration. The growth velocity remarkably decreased due to the promotion of heat flow. In the mushy region, where the dendrite morphology remained mostly unchanged, the fragmentation of primary and secondary arms occurred. Some detached dendrite arms moved to the downstream side along the convection oscillating in the mushy region. The dendrite fragmentation frequently occurred in the downstream side. It is likely that the agitation of liquid induced by the dendrite oscillation as well as high accumulation of solute contributed to the frequent dendrite fragmentation. The solute concentration distribution indicated that the difference in the solute concentration of liquid flowing into the mushy region caused microstructural changes, such as dendrite fragmentation and change in the solid fraction.
