Modulating Phase Constitution and Copper Microsegregation for FeCoNiCuAl High-Entropy Alloy by Optimized Ultrasonic Solidification

The introduction of one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) ultrasounds into solidifying FeCoNiCuAl high-entropy alloy was efficiently optimized, which realized the maximum input of acoustic energy and the effective adjustment of the energy proportion between stable and transient cavitation effects. In addition to the ordinary advantage of grain refinement, the superiority of power ultrasound in modulating such Cu-containing high-entropy alloys with dendritic structures mainly lay in the significant regulation of phase volume fraction and the elimination of severe Cu element microsegregation. As the main energy transmission form under 1D ultrasound, stable cavitation slightly increased the nucleation rate of α and γ₁ phases, which jointly contributed to suppressing the Cu solute enrichment from 41.6 to 36 at pct through the acoustic streaming during the subsequent growth of γ₁ phase. When 2D and 3D ultrasounds were applied, the intensive transient cavitation dominated the solidification process. The induced local high undercooling resulted in the competitive nucleation and growth between α and γ₁ phases, leading to the more than one order of magnitude reduction in their grain sizes and the significant rise of γ₁ phase volume fraction from 13 up to 50 pct. Meanwhile, it strikingly reduced the final Cu content difference between these two phases from over 30 to around 3.8 at pct by decreasing the Cu composition in competitively formed γ₁ nuclei. The above microstructure modification brought in excellent compressive property for 3D ultrasonically solidified alloy, whose strength and ductility were simultaneously enhanced by 27 and 24 pct.