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Thermodynamic justification for the intermetallic layers formation within the hexagonal single crystal

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Many eutectic systems exhibit either a lamellar or rod-like structure depending on solidification conditions, Elliott [1]. Especially, growth rate plays a crucial role in the lamella/rod transformation. Some impurities also involve the transition, Steen and Hellawell [2]. The impurities change the specific surface free energies and finally modify a mechanical equilibrium at the triple point of the solid/liquid interface. According to the current model assumptions, the mechanical equilibrium varies with solidification conditions (growth rate) and no effect of impurities is observed. A given orientation of crystal manifests a proper growth rate. Some changes of the orientation from an initial state into a final one give also an effect on the lamella/rod transtition, [3]. The theory developed by Jackson and Hunt [4] has tried to predict the threshold rate at which transformation should occur. It yields: a m a m a m a m E P L L R R α α β β α α β β ξ ξ ξ / / / / / / / . + + > + ( ) ( ) (4 )(1 (1 ) ) 1 5 (1) The discerning analysis shows that the above inequality can predict whether an eutectic alloy manifests lamellar or rod-like structure, only. E, P and aj L, aj R - parameters applied in theory [4], j = α, β, ζ = Sβ/Sα, mj - slope of the liquidus lines, respectively, j = α, β. Thus, the inequality characterizes a given phase diagram. Thus, Eq. (1) cannot be applied to describe the structural transformation. It is evident that the Eq. (1) is completely misleading in the case of some predictions connected with structural transformation. Therefore, a new condition for transformation based on the calculation of the minimum entropy production will be applied in the current analysis. All the threshold growth rates will be determined for the (Zn)-Zn16Ti eutectic system by means of the current theorem. According to the current theorem th[...]

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