Dendrite growth mechanism comparison of Al-4.5%Cu alloy under positive and negative temperature gradients

<p indent="0mm">The dendritic growth kinetics and microstructural evolution mechanisms of Al-4.5%Cu alloy under positive and negative temperature gradients were explored by constrained directional solidification and free rapid solidification techniques. In the case of the positive temperature gradient, the cooling rate at the solid/liquid interface was only 1.1×10^–3~2.3 K/s, while the constitutional supercooling played a dominant role but was below the theoretical freezing temperature interval of <sc>78 K.</sc> In the situation of a negative temperature gradient, the cooling rate of alloy droplets increased rapidly with the reduction of their diameters and was elevated by six orders of magnitude if compared with a positive temperature gradient condition. Meanwhile, droplet supercooling also rose remarkably to attain a maximum of <sc>170 K.</sc> If a positive temperature gradient was kept constantly the continuous increase of growth velocity resulted in the successive “planar—cellular—dendritic” interface transitions. The growth velocity thresholds for these two structural transitions increased with temperature gradient rise, which was accompanied by the obvious reduction of both primary and secondary dendrite arm spacings of α-Al phase. But the corresponding dendrite tip radius showed a slowly increasing tendency, and the interfacial solid concentration was below the initial alloy composition. Under the influences of the negative temperature gradient, the α-Al phase exhibited a kind of directionally growing dendrite structure originating from the single nucleation site plus the ordinary equiaxed dendrites formed from multiple nucleation sites. Similar to the directional solidification process, the sufficient increase of growth velocity and liquid supercooling within tiny alloy droplets induced the successive “dendrites—cellular dendrites—cells” structure transitions after single-site nucleation. On the other hand, an abrupt grain refinement effect from equiaxed dendrites into spheroid crystals was initiated in multiply nucleated alloy droplets. Once droplet supercooling exceeded <sc>100 K,</sc> microsegregationless dendrite growth was realized for α-Al phase during rapid solidification.