Papers by Keyword: Solute Segregation

Abstract: To describe the thermal stability of the nanocrystalline solid solution with weak segregation such as Cu-Zn system, we developed a hybrid model combining the first principles calculation and thermodynamic evaluation. The dependence of the solute segregation behavior on the solute concentration, grain size and temperature were demonstrated. We found that the segregation energy does not change with the solute concentration monotonically. At a constant solute concentration and a given temperature, a nanograin structure can remain stable if the initial grain size is kept in a critical range. The model predictions were confirmed by the experimental measurements that a state of steady nanograin growth can be achieved by designing a certain solute concentration and a proper initial grain size.

Abstract: It is generally well recognized that in the course of a grain boundary (GB) diffusion experiment the diffusion of solute atoms in grain boundaries must exhibit a strong time-dependent segregation. But there has been no clear understanding of exactly how this time dependence develops. In this chapter, we review and analyse transient solute GB diffusion by means of the computer simulation technique of Lattice Monte Carlo (LMC). This technique has been successfully used on numerous occasions for the purposes of systematically studying the GB transition regimes that occur between the principal well-defined Harrison GB kinetics regimes (A, B and C-Types). Recently, the analysis using LMC has been extended to the case of solute GB diffusion when the segregation factor is independent of time. In the present paper, we analyse two cases of solute segregation in GB diffusion: first, where the solute atoms are homogeneously distributed along the tracer source plane but their mobility is not high at this plane; and the second, where the mobility of the solute atoms along the tracer source plane is comparable to their mobility along the GB. It is shown that the time dependence of the segregation can contribute significantly into the resulting values of the triple-product that is usually obtained experimentally in the Harrison Type-B kinetics regime.

Abstract: Phase field method (PFM) offers the prospect of carrying out realistic numerical calculation on dendrite growth in metallic systems. The dendritic growth process of multiple dendrites and direcitonal solidification during isothermal solidifications in a Fe-0.5mole%C binary alloy were simulated using phase field model. Competitive growth of multiple equiaxed dendrites were simulated, and the effect of anisotropy on the solute segregation and microstructural dedritic growth pattern in directional solidification process was studied in the paper. The simulation results showed the impingement of arbitrarily oriented grains, and the grains began to impinge and coalesce the adjacent grains with time going on, which made the dendrite growth inhibited obviously. In the directional solidification, the maximum concentration gradient showed in the dendrite tip, and highest solute concentration existed at the bottom of the dendrites. With the increasing of the anisotropy, dendrite tip radius became smaller, and the crystal structure is more uniform and dense.

Abstract: The effect on heterogeneous nucleation of solute segregation was studied with segregating energy and segregating coefficient. The results showed that the segregating energyΔG^γ_segr of solute played a more vital role in the nucleating process, specially in the case that (1-k) is small. A critical criterion with the parameter was suggusted for estimating the nucleating abilitiy of the external particles in binary alloy, which had the vital theoretical or practical significance for choicing refiner.

Abstract: Al₂O₃·SiO_2f/A356 composites contained La were fabricated by squeeze casting, and effect of La on the solute segregation and microstructure were researched. The results indicate that the microstructure in the composite can be refined by La, which enriches near the interface to be favorable to improve the wettability between the fiber and Al alloy. The phases RE-richening do not formed at the interface. There is no special influence of La on the Mg segregation in the matrix alloy. The distribution of Mg and La in the composite are near the interface.

Abstract: A detailed comparison between kinetic and thermodynamic effects on grain growth in nanocrystalline Gd doped ceria ceramics was given. From the thermodynamic standpoint, the evolution of grain size with annealing time can be described using an analytical model for grain boundary segregation upon isothermal grain growth. From the kinetic standpoint, the single isothermal grain growth can be understood in terms of a single, thermally-activated rate process with constant grain boundary energy, σ_b. The solute excess in grain boundaries increases with grain growth and tends to its saturated value, therefore, the inhibition of grain growth can be attributed to the reduction of grain boundary energy σ_b through solute segregation.

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: The knowledge of the structure/property relationship in polycrystalline materials is the basis for successful application of Grain Boundary Engineering. We demonstrate this relationship in the reverse way: from the selective corrosion attack observed in unique sample – loops from excavated necklace dated to the 10th century and manufactured from a Ag–1%Cu alloy – we can deduce the method of manufacturing the objects. Individual grain boundaries in this object were identified by electron back-scattering diffraction. Crystallographic maps of the grain boundaries are confronted with the level of the long-termed selective corrosion attack in the soil electrolyte under conditions of decomposing human body. It is shown that general grain boundaries, which are highly segregated by copper, are preferably attacked by corrosion. The segregated layers represent a less-noble material comparing to the surrounding bulk in this environment. In contrast, the twin and other special grain boundaries are significantly more resistant against this attack.

Abstract: In this study, the role of solute segregation on the strength and the evolution behavior of dislocation junctions is studied by utilizing kinetic Monte Carlo and 3D dislocation dynamics simulations. The different solute concentrations and the character of the junctions are all included in the simulations in an effort to make a parametric investigation. The results indicate that solute segregation can lead to both strengthening and weakening behavior depending upon the evolution of the dislocation junctions.