Rapid eutectic growth kinetics of binary Ni–Ge alloy under acoustic levitation and dynamic microgravity states

Abstract

The rapid eutectic growth mechanism within liquid Ni33Ge67 alloys was investigated by both acoustic levitation and drop tube techniques, where the maximum liquid undercoolings achieved 136 K (0.13 TL) and 252 K (0.24 TL), respectively. A crystal growth kinetics transition from NiGe + (Ge) lamellar eutectics through NiGe dendrites, finally to anomalous eutectics, was accomplished by modulating the liquid undercooling level. Under the acoustic levitation state, NiGe dendrites nucleated primarily at 80 K undercooling. When alloy undercooling further increased to 109 K, the anomalous eutectic grew preferentially and achieved the maximum growth velocity of 36.3 mm s−1 at 136 K undercooling. Furthermore, complex flow patterns were induced within the levitated liquid alloy, which maximized at the alloy surface and promoted the independent nucleation of two constituent phases. Under microgravity conditions, liquid undercooling and cooling rate increased with the reduction of alloy droplet diameter. As droplet diameter decreased to 101 μm, the cooling rate attained 4.1 × 104 K s−1, substantially greater than the 163.3 K s−1 value for acoustic levitation processing, resulting in significant structural refinement.