Improved mechanical performances of dynamically solidified Mg_97.7Y_1.4Al_0.9 alloy by three dimensional ultrasounds

Three dimensional (3D) ultrasounds with 20 kHz frequency and 14–22 μm amplitudes were applied to the solidification process of Mg_97.7Y_1.4Al_0.9 alloy. With the increase of ultrasonic dimension and amplitude, both the primary Al₂Y particles and the principal α-Mg phase were significantly refined in combination with an increased proportion of Al₂Y/α-Mg parallel growth interfaces, while the eutectic Al₂Y phase changed from long strip to fine granular shape. The dynamic solidification mechanism was further explained on the basis of in-situ acoustic spectrum measurements. It was found that once tiny primary Al₂Y and α-Mg nuclei formed, they immediately strengthened acoustic field by facilitating bubble formation at their solid/liquid interfaces, and the subsequent bubble oscillation and collapsing in turn promoted their further nucleation by improving wettability and increasing local undercooling. The ultrasonically solidified alloy showed superior mechanical performances as compared with static solidification structures. The maximum tensile strength and plasticity achieved 271 MPa and 12.8% which were 1.41 and 1.36 times higher than static values, and the high temperature creep properties were enhanced as well.