Ultrasounds Induced Microstructure Transition and Improved Mechanical Property of Directionally Solidified Ternary Cu–Al–Ni Alloy

Two ultrasonic modes, i.e., continuous and pulsed ultrasounds, were introduced into the directional solidification process of Cu_68.3Al_27.6Ni_4.1 alloy. A columnar-to-equiaxed structure transition occurred to primary β(Cu₃Al) phase within continuous ultrasonic field, which was accompanied with a grain size reduction by 7.5 times. Under pulsed ultrasound, β phase maintained the fine columnar structures with a similar grain size. In the former case, numerous β phase nucleation sites formed ahead of solid/liquid (S/L) interface because of the large local undercoolings induced by transient cavitation. Meanwhile, intensive acoustic streaming suppressed the liquid temperature gradient from 120 to 85 K/cm, which interrupted the solute transportation along heat flow direction and resulted in equiaxed microstructures. Under the intermittent ultrasonic action in the latter case, fewer nucleation sites were generated near S/L interface but small columnar β grains were split from the original ones under stable cavitation. Since no steady convection was driven, the liquid temperature gradient of 110 K/cm remained almost constant, making those grains grow into refined columnar structures. Under the action of pulsed ultrasound, the yield strength was enhanced by a factor of 1.5 because of grain refinement strengthening, together with 94 pct shape recovery rate due to columnar grain structures.