Ultrasounds induced eutectic structure transition and associated mechanical property enhancement of FeCoCrNi_2.1Al high entropy alloy

One dimensional (1D) and three dimensional (3D) ultrasound sources were applied to modulate the solidification process of FeCoCrNi_2.1Al high entropy alloy whose liquidus temperature is up to 1614 K with the acoustic signal in liquid alloy in-situ measured. It was found that 1D ultrasound significantly refined the lamellar eutectic structure by decreasing both the eutectic cell width and the aligned α(B2)/γ(FCC) phase spacing. If ultrasound amplitude and dimension were increased, lamellar eutectic gradually transformed into anomalous eutectic without synergetic orientation. Meanwhile, a metastable σ phase enriched with Cr precipitated from γ phase in anomalous eutectic under 3D ultrasounds. Based on the combined analysis of eutectic structural evolution and sound field characteristics, the ultrasonic solidification mechanism was further clarified. The promoted dependent nucleation by stable cavitation and the fast growth rate caused by acoustic streaming were the main reasons for the significantly refined eutectic structure under 1D ultrasound. The induced independent eutectic nucleation and growth by transient cavitation, together with the enhanced solute and thermal diffusion by the intensive acoustic streaming under 3D ultrasounds were responsible for the formation of anomalous eutectic and the precipitation of metastable σ phase. The maximum ultimate strength and total elongation under ultrasound were simultaneously increased by 28% and 93%, showing comprehensive mechanical performance superior to those prepared by conventional approaches. The prominent strain hardening in γ phase and increase in α/γ interfaces induced by refining effect were the strengthening mechanisms, and the improvement of ductility was attributed to the intensified strain hardening in γ phase.