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Abstract

Some eutectic stripes have been generated in a hexagonal (Zn) - single crystal. The stripes are situated periodically with the constant interstripes spacing. The eutectic structure in the stripes consists of strengthening inter-metallic compound, Zn16Ti, and (Zn) – solid solution. The rod-like irregular eutectic structure (with branches) appears at low growth rates. The regular lamellar eutectic structure is observed at middle growth rates. The regular rod-like eutectic structure exists exclusively in the stripes at some elevated growth rates. A new thermodynamic criterion is recommended. It suggests that this eutectic regular structure is the winner in a morphological competition for which the minimum entropy production is lower. A competition between the regular rod-like and the regular lamellar eutectic growth is described by means of the proposed criterion. The formation of branches within irregular eutectic structure is referred to the state of marginal stability. A continuous transitions from the marginal stability to the stationary state are confirmed by the continuous transformations of the irregular eutectic structure into the regular one.
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Abstract

Specimens of Si single crystals with different crystal orientation [100] and [110] were studied by Electro-Ultrasonic Spectroscopy (EUS) and Resonant Ultrasonic Spectroscopy (RUS). A silicon single crystal is an anisotropic crystal, so its properties are different in different directions in the material relative to the crystal orientation. EUS is based on interaction of two signals: an electric AC signal and an ultrasonic signal, which are working on different frequencies. The ultrasonic wave affects the charge carriers' transport in the structures and the intermodulation electrical signal which is created due to the interaction between the ultrasonic wave and charge carriers, is proportional to the density of structural defects. RUS enables to measure natural frequencies of free elastic vibrations of a simply shaped specimen by scanning a selected frequency range including the appropriate resonances of the measured specimens.
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Abstract

The analysis of influence of mould withdrawal rate on the solidification process of CMSX-4 single crystal castings produced by Bridgman method was presented in this paper. The predicted values of temperature gradient, solidification and cooling rate, were determined at the longitudinal section of casting blade withdrawn at rate from 1 to 6mm/min using ProCAST software. It was found that the increase of withdrawal rate of ceramic mould results in the decrease of temperature gradient and the growth of cooling rate, along blade height. Based on results of solidification parameter G/R (temperature gradient/solidification rate), maximum withdrawal rate of ceramic mould (3.5 mm/min), which ensures lower susceptibility to formation process of new grain defects in single crystal, was established. It was proved that these defects can be formed in the bottom part of casting at withdrawal rate of 4 mm/min. The increase of withdrawal rate to 5 and 6 mm/min results in additional growth of susceptibility of defects formation along the whole height of airfoil.
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