Austenite Grain Refinement and Subsequent Martensite Transformation of Ternary Cu–Al–Ni Alloy Solidified Within Ultrasonic Field

A power ultrasound with different amplitudes was applied during the solidification of ternary Cu-14wt pct Al-4.5 wt pct Ni alloy, which was then quenched to obtain martensite phases. It effectively promoted the nucleation of primary austenite β (AlCu₃) phase, thereby reducing its grain size by almost one order of magnitude. This refined austenite structure induced by ultrasound also had influence on the martensite transformation during subsequent quenching. In the coarse β grain formed without ultrasound, only very little amount of β₁^′ martensite phase with 18R structure existed and part of it consisted in the coarse γ₁^′ martensite phase with 2H structure. As ultrasound amplitude rose up, the martensite phases were both refined and the relative content of β₁^′ phase increased. This microstructure modification led to the enhancement of mechanical properties of Cu-14 pct Al-4.5 pct Ni alloy as well. The yield strength of the quenched alloy solidified at the maximal ultrasonic amplitude was 1.5 times higher than that without ultrasound. The microhardness and the storage modulus were also enhanced with the increase of ultrasound amplitude.