Microstructural evolution of ternary Ag33Cu42Ge25 eutectic alloy inside ultrasonic field

Ultrasonic field with a frequency of 20 kHz is introduced into the solidification process of ternary Ag₃₃Cu₄₂Ge₂₅ eutectic alloy from the sample bottom to its top. The ultrasound stimulates the nucleation of alloy melt and prevents its bulk undercooling. At low ultrasound power of 250 W, the primary ε₂ phase in the whole alloy sample grows into non-faceted equiaxed grains, which differs to its faceted morphology of long strip under static condition. The pseudobinary (Ag+ε₂) eutectic transits from dendrite shape grain composed of rod type eutectic to equiaxed chrysanthemus shape formed by lamellar structure. By contrast, the ultrasound produces no obvious variation in the morphology of ternary (Ag+Ge+ε₂) eutectic except a coarsening effect. When ultrasound power rises to 500 W, divorced ternary (Ag+Ge+ε₂) eutectic forms at the sample bottom. However, in the upper part, the ultrasonic energy weakens, and it only brings about prominent refining effect to primary ε₂ phase. The microstructural evolution mechanism is investigated on the cavitation, acoustic streaming and acoustic attenuation.