Nonlinear ultrasonic characterization of γ΄ phase evolution in DZ411 alloy after long-term aging

To explore the feasibility of the nonlinear ultrasonic method in the aging microstructure evolution application of directionally solidified nickel-based superalloy， DZ411 directionally solidified nickel-based superalloy is studied.The microstructure observation， γ΄ phase quantitative characterization， nonlinear ultrasonic detection， and lattice mismatch analysis of DZ411 alloy under different aging conditions are carried out by using nonlinear ultrasonic detection technology， combined with SEM and XRD. The correlation between nonlinear ultrasonic coefficient and quantitative parameters of microstructure evolution is discussed. The results show that with the extension of aging time， the γ΄ phase size increases， the γ΄ phase changes from cubic to spherical， the cubic degree decreases， the particle density decreases， and the normalized ultrasonic nonlinear coefficient increases exponentially. The nonlinear ultrasonic coefficient is positively correlated with the lattice mismatch， and the γ΄ phase equivalent diameter， and negatively correlated with the γ΄ phase area fraction. The analysis shows that the above phenomenon is attributed to the full diffusion of alloying elements and the redistribution of solute atoms at the two-phase interface during the aging process， increasing the γ΄ phase size and the lattice strain at the two phase intorface， enhancing the interference of the local stress-strain field on the propagation characteristics of the ultrasonic wave and aggravating the distortion of the ultrasonic wave at the two-phase interface， thus promoting the generation of harmonic components and the enhancement of nonlinear effects.