Papers by Keyword: Back-Diffusion

Abstract: . An improvement of the Brody & Flemings theory has been proposed to study not only the solute segregation but the solute redistribution during the single crystal growth as well. The redistribution is treated as a phenomenon superposed upon the segregation phenomenon. The crystal growth has been performed by the closed Bridgman system with a constant growth rate and an imposed temperature gradient. A hypo-eutectic Zn-Ti alloy has been subjected to oriented growth and precipitation of the Zn-16Ti intermetallic compound was observed. The precipitates have been placed as some layers distributed within the single crystal with a constant spacing. Some data from the Zn-Ti binary phase diagram was introduced into the theory to show possibility of the control the phenomenon of the reinforcement by precipitation during a single crystal growth.

Abstract: Structures obtained during Zn-Cu-Ti single crystal growth by the Bridgman method were investigated. The alloys of composition Zn-Cu0.1wt.%-Ti0.1wt.% were used for single crystals growth, which were produced at rate in the range of 1.8mm/h to 16mm/h. The Zn-Cu-Ti alloys were characterized by multiphase structure; in the solute-hardened matrix by Cu atoms exists a needle-shaped intermetallic phase Zn16Ti. The strong anisotropy of the hexagonal structure gives a significant influence on a second phase distribution in the matrix volume. In the range of the low growth rates (from 1.8mm/h to 10mm/h) oscillatory structures were observed. That effect vanished at the growth rates higher than 10mm/h. The observed oscillatory structures were characterized by existing periodic layers rich in the intermetallic phase of Zn16Ti interlaid with pure matrix (alpha phase Zn with Cu solute).

Abstract: An Al foil of 25 µm thickness was used to produce a Ni/Al/Ni interconnection under isothermal conditions at 680°C. After dissolution of the Al filler metal in the Ni substrate, and Ni from the substrate in the Al liquid foil, a solidification process occurred. First of all, the solidification was stopped just at the disappearance of the Al liquid metal, but also at different steps of the progress of solidification. Sub-layers of the two phases Al₃Ni₂ and Al₃Ni were revealed within the frozen joints. A solidification - segregation model was developed to describe the formation of the sub-layers of both mentioned phases. The model is able to predict solute segregation profiles across the sub-layers of the Ni/Al/Ni interconnection, taking into account a superposition of some phenomena like partitioning, back-diffusion into the solid and peritectic reactions. The value of the diffusion coefficient into the solid for the Al₃Ni₂ phase formation in the presence of the liquid is also estimated.

Abstract: Direct simulations of solidification processes that account for all space and time scales are often beyond the reach of current computational power. To overcome this limitation micromacro approaches that incorporate the effects of small-scale phenomena into large-scale process models have been developed. An important small-scale solidification phenomenon is microsegregation —the redistribution of rejected solute components at the scale of the solid crystal morphology. This paper outlines a general microsegregation model that not only accounts for many of the critical small-scale phenomena in alloy solidification but also well suited as the micro component of a micro-macro model of metal casting. In the development of this microsegregation model, particular emphasis and testing is placed on alternative treatments of modeling the microscale solute diffusion in the solid phase—the so-called “back-diffusion.”