| A new model for the continuous structural evolution during rapid solidification which includes the interactions between the external cooling, release of latent heat, solute diffusion and the capillary effect of solid-liquid interface was suggested. This model where the heat transfer and the solute diffusion procedure were calculated by a numerical and an analytic method each other, was based on the facts that the boundary layer thickness of the thermal diffusion field is much larger than the cell or dendrite spacing, while that of the solute diffusion field is smaller. The microstructural evolutions in the gas -atomized Al-Fe(2∼8wt%) droplets were obtained by this model. The main results are as follows. The discontinuous microstructural change from `Zone A` to `Zone B` with increasing solid fraction was predicted and their microstructural scales were calculated quantitively. In `Zone A`, the interface velocity is governed mainly by the solutal diffusion and the non-steady state solidification occurs. In `Zone B`, the interface velocity is determined by the heat removal rate of external coolant rate irrespective of the solute concentration and initial undercooling, and the steady state solidification occurs. Irrespective of the initial undercooling, droplet size(＞10㎛) and Fe Concentration(2∼8wt%), the `Zone A` and `Zone B` structures appear simultaneously during gas-atomization of AI-Fe alloys. |
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