Morphology of Stripes Created in the (Zn) – Single Crystal
Polish Academy of Sciences, Institute of Metallurgy and Materials Sciences (IMIM PAN), Reymonta 25, Kraków 30-059, Poland
Some experiments dealing with the (Zn)–hexagonal single crystal growth have been performed with the imposed positive thermal gradient and a set of the constant growth rates. Experiments were developed under stationary state created by means of the Bridgman system. The apparatus was equipped with a moving temperature field. A graphite crucible had a sophisticated geometry that the germs could be positioned within. The (Zn) – single crystal was doped by the small amount of titanium and copper. Titanium forms the Zn16Ti inter-metallic compound with the zinc. Copper does not form any inter-metallic compound but is solved in the zinc / titanium solid solution, (Zn).
Some eutectic stripes were generated periodically in the single crystal. The stripes consisted of strengthening inter-metallic compound, Zn16Ti and (Zn) – solid solution.
Three ranges of growth rates were distinguished in the (Zn) - single crystal formation: a/ at some low growth rates, the L – shape irregular eutectic structure appears, (with branches), b/ at some middle growth rates the regular lamellar structure is observed, c/ at some elevated growth rates the regular rod-like structure is the morphology which exists exclusively in the stripes.
Since the discussed single crystal growth occurs under stationary state the proper criterion which defines this state is to be applied. The thermodynamics of irreversible processes supposed that the only condition which describes the stationary state is the criterion of minimum entropy production.
Based on the mentioned criterion a new procedure of pattern selection has recently been formulated. According to the procedure this pattern is selected which exhibits the lower minimum entropy production in the envisaged system. In the current model, the procedure will be applied to select the stripes morphology. Additionally, a growth rates range within which L-shape irregular rods were transforming continuously into the regular lamellae was observed. The progressive vanishing of branches occurs within the range. The formation of branches is referred to the creation of marginal stability in the system and continuous alteration of this stability into the stationary state. The existence of marginal stability is accompanied by the formation of a perturbation wave at the solid / liquid interface of the non-faceted (Zn) - eutectic phase which always appears in the stripes.
Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 1, by Waldemar Wołczyński
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-01-24 07:27 Revised: 2013-01-24 07:27
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