THE INFLUENCE OF COMPOSITION ON EQUIAXED CRYSTAL-GROWTH MECHANISMS AND GRAIN-SIZE IN AL-ALLOYS

The role of solute concentration on equiaxed crystal growth mechanisms and grain size has been investigated. The study was undertaken on hyp oeutectic concentrations in the systems Al-Si-Ti and Al-Si-Fe-Ti. The equiaxed structure was ensured by adding heterogeneously nucleating Ti B2 particles in excess of that needed for a maximum grain-refining eff ect. It was found that two principally different growth models have to be applied in different parts of an alloy system. Cellular type of cr ystals in the low alloy region develop in a clover-leaf form and the d iffusion profile builds up front-wise ahead of the cellular interface. The alloying elements cooperate by growth restriction in promoting a fine-grained structure. On the high alloy side, however, dendrites wit h orthogonal branches develop lancet-like tips causing a major part of the diffusion to take place side-wise instead of in front of the grow ing tip. Increasing the solute concentration in this regime will incre ase the growth rate and consequently the grain size.