ABSTRACT
Rapid solidification allows for very high thermal undercooling with potential homogeneous nucleation and spontaneous nondendritic solidification when the energy required for the nucleation becomes less than the energy required for branching. Not all alloys can be easily undercooled to such a degree. Nondendritic structure due to undercooling as a result of high cooling rate was obtained in Ni alloys upon melt atomization with cooling rates above 106 K/s [3]. Combination of rapid solidification with ultrasonic melt processing has been investigated as a potentially efficient approach since the 1980s [4]. Atomization of liquids by ultrasonic oscillations has been reported in the first works of Wood and Loomis. The physics of this process is well covered elsewhere [5, 6]. Ultrasonic-aided atomization when the Ni-based melt was dispersed by an oscillating Ar or He flow at 16-20 kHz allowed the formation of nondendritic granules 50-100 µm in size at cooling rates about 106 K/s [7].
