Phase separation and structure evolution of ternary Al–Cu–Sn immiscible alloy under ultrasonic levitation condition

► Liquid Al–15%Cu–55%Sn alloy is containerlessly solidified under ultrasonic levitation. ► The microstructure transforms into the lateral-type macrosegregation pattern. ► Stokes motion is suppressed under ultrasonic levitation condition. ► Centrifugal migration is the major dynamic mechanism of macrosegregation evolution. The containerless rapid solidification of liquid Al–15%Cu–55%Sn immiscible alloy is accomplished by single-axis ultrasonic levitation. A maximum undercooling of 209K (0.23TL) is obtained for the alloy melt at a cooling rate of about 100K/s. The phase separation pattern transforms from the vertical-type macrosegregation under normal condition into the lateral-type macrosegregation under ultrasonic levitation. The enhancement of both undercooling and cooling rate induces the refinement of microstructure and the remarkable extension of solute solubility in (Sn) phase. Theoretical analyses show that the morphology transition mainly depends on the density and surface tension differences between Al-rich and Sn-rich liquid phases. The centrifugal migration of Sn-rich droplets caused by sample rotation is the major dynamic mechanism responsible for the macrosegregation evolution under ultrasonic levitation condition.