Unidirectional solidification of Al-Cu alloy films between solid plates

To clarify the solidification mechanism of thin alloy films between solid plates, the effects of the film thickness and the growth conditions on the unidirectional solidification structures of Al-Cu alloys were studied. The primary arm spacings of the thin film specimens, whose thicknesses were less than or equal to the secondary arm spacings, were twice or three times larger than those of bulk Al-15%Cu specimens. This phenomena were interpreted through the quantitative estimation of the influence of solid plates on the solute diffusion process in the liquid near dendrite tips. Two diffusion models were proposed, and the calculated ratios (D_h/D₁) of the primary arm spacing D_h of the film and D₁ of the bulk specimen were: (1) Columnar diffusion layer model: (D_h/D₁) = {(4π/3)/ (2h/D₁)}^1/2, (2) Hemispherical diffusion layer model: (D_h/D₁) = {(1/(2h/D₁)}^1/3. The intermediate value of the above models showed good agreement with the experimental ratios (d_h/d₁) of the film d_h and the bulk d₁.