Features of bicrystal growth during the directional crystallization of metal melts
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Features of Bicrystal Growth during the Directional Crystallization of Metal Melts V. V. Gubernatorov, T. S. Sycheva*, V. M. Gundyrev, and Yu. N. Akshentsev M.N. Mikheev Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620137 Russia *e-mail: [email protected] Received December 7, 2015
Abstract—The factors responsible for the formation of different configurations of boundaries between adjacent crystallites during their growth from melt by Bridgman and Czochralski methods have been considered by an of example Fe–20 wt % Ga alloy and Ni bicrystals. It is found that the configuration of intercrystallite boundary is related to the features of crystallite growth, caused by the strained state of intercrystallite and interphase (crystal–melt) boundaries, the difference in the linear thermal expansion coefficients of the crystallite boundaries and bulk, and the shape (geometry) of the bicrystal cross section. It is suggested that the strained state of boundaries and the formation of substructure in crystallites during directional crystallization from metal melt are significantly affected by their deformation under the melt weight. DOI: 10.1134/S1063774517020134
INTRODUCTION A conventional (without any special operations) directional crystallization of metal melts results generally in a competitive and/or noncompetitive growth of several crystallites [1]. According to the observations of the sample macrostructure, the intercrystalline boundaries (ICBs) acquire various configurations. The ICB configuration in cast metal materials is of great practical importance, because it significantly affects the mechanical properties of polycrystalline metals and alloys. In particular, the occurrence of winding and wavy ICBs in steels (Fe–C alloys) subjected to thermomechanical treatment hinders crack propagation during intercrystallite fracture; as a result, the temper brittleness of steels is suppressed, and their strength and plasticity increases [2–4]. The purpose of this study was to establish the mechanism of the formation of some ICB configurations observed in bicrystals grown by directional crystallization. The variety of ICB configurations in cast bicrystals is related to the specific features of their growth from melt. The data of [5–7] suggest that they are due to a great extent to the behavior of interphase boundaries (IPBs) and ICBs, which is determined by the crystallization and recrystallization conditions, respectively. Note that a boundary can be considered as a phase composed of (i) a layer with a distorted crystalline structure and a thickness of two to three interatomic distances and (ii) two adjacent layers (which are also distorted, differ in the chemical (at least, impurity) composition from the environment, and expand into
the crystallite bulk (CB) at several hundreds of nanometers) [8]. Since this phase is characterized by large extension and certain thickness, it has all properties inherent in solids, which differ from the characteristics of the surrounding metal. I
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