Microstructure growth kinetics and mechanical property improvement of directionally solidified Al-4.5 %Cu-4.5 %Si ternary alloy

A microstructure selection map was established for Al-4.5 %Cu-4.5 %Si ternary alloy within a broad directional growth velocity range of 0.1–500 μm·s⁻¹ under a constant temperature gradient of 200 K·cm⁻¹ temperature gradient. This covered the whole solid/liquid interface evolution spectrum from planar front through cellular morphology to dendritic structures. The critical growth velocity to maintain planar solid/liquid interface was determined to be 0.28 μm·s⁻¹, whereas the growth velocity threshold to initiate cell-to-dendrite transition was found to be 8.1 μm·s⁻¹. It was found that the successive increase of directional growth velocity transferred alloy phase constitution from single α-Al through double (αAl +Al₂Cu) into triple (αAl +Al₂Cu+Si) phases. The experimental results validated the linear superposition of classical CS interface stability criterion and the KGT dendritic growth model to elucidate the directional solidification kinetics of dilute ternary alloys. Compared with isothermal solidification, the tensile strength increased by 27.9 %, and ductile elongation improved by fivefold.