Eutectic structure modulation of directionally solidified Mg-32.4%Al alloy within ultrasonic field

Ultrasounds with different amplitudes were introduced into the directional solidification process of Mg-32.4%Al eutectic alloy. Under 0.8 μm ultrasonic amplitude, herringbone-shaped (Mg)+ γ-Mg₁₇Al₁₂ eutectic structure coarsened with increased eutectic cell width and enlarged lamellar spacing. Raising the amplitude to 3.8 μm brought in a periodic banded structure formation along temperature gradient, consisting of alternating spherical (Mg)+ γ eutectic cells and fine spherical γ grains. Theoretical calculation showed that, in the former case, weak convection led to the coarsening of eutectic structure by enhancing solute mixing and reducing the diffusion flux at the solid/liquid (S/L) interface, thereby maintaining its steady-state growth. In the latter case, transient cavitation enhanced the nucleation rate, while acoustic streaming effect induced symmetric flow/temperature fields and promoted the formation of spherical eutectic cells. Then the acoustic streaming continuously reduced the thickness of solute boundary layer and enriched the Al solute at S/L interface, resulting in preferential γ-phase growth, which formed fine spherical grains under cavitation. During their further growth, the consumption of Al solute reduced the interface growth rate, thereby enhancing the undercooling level ahead of growing interface. When the liquid composition and undercooling simultaneously satisfied the criteria for eutectic nucleation, the spherical eutectic cells formed again and initiated a new banded-growth cycle. The alloy compression strength was increased from 185 to 225 MPa under 3.8 μm ultrasonic amplitude, due to the periodic distribution of γ-phase aggregates enhancing resistance to dislocation slip and motion.