Strengthening of Ni-Co-Cr-Al alloy by ultrasonic processing

Three-dimensional ultrasounds were applied during the solidification and solid phase transition processes of Ni₅₀Co₂₃Cr₁₄Al₁₃ alloy. The ultrasounds not only transformed primary γ(Ni) phase from coarse dendrites to tiny equiaxed grains with higher Al solute content, but also led to the formation of (γ(Ni)+ α(Cr)) eutectic structure within inter-grain region during solidification. Meanwhile, enormous strain energy in solidified structure was generated by ultrasonic vibration, which decreased the nucleation energy for γ′(Ni₃Al) precipitation and facilitated its growth into a larger size with up to 45 % volume fraction. The yield and ultimate strength after ultrasonic processing elevated to 780 and 1080 MPa, respectively, even higher than those reported data after complex heat treatment procedures. The most significant strengthening factor was ultrasound-induced precipitation strengthening, which obstructed dislocation movement and improved strain hardening. Meanwhile, the formation of α(Cr) eutectic and the refinement of primary γ(Ni) grains also made a secondary level contribution.