Papers by Keyword: Grain Boundary Energy

Abstract: On the basis of Hillerts model of grain growth, a new model of Goss secondary recrystallization in silicon steel has been developed in which inhibitor and grain boundary energy are taken into account. An analysis shows that these two parameters synergistically affect secondary recrystallization and Goss grain evolves to a coarse grain as inhibitor intensity increases and statistical grain boundary energy decreases. This model successfully explains Goss secondary recrystallization.

Abstract: Low energy grain boundaries were considered to be important in abnormal grain growth by theoretical deduction. The disorientation angles and coincidence site lattice grain boundaries distribution of more than 20 Goss grains and their neighboring matrix grains in primary recrystallized Fe-3%Si alloy were investigated using an electron backscatter diffraction method. It was found that the frequency of low energy grain boundaries of Goss grains which are more likely to abnormally grow are higher than their neighboring matrix grains, which indicated that low energy grain boundaries play a dominant role in the abnormal grain growth of Fe-3%Si alloy. The result meets well with the abnormal grain growth theory.

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: A Monte Carlo Potts′ model with different ratios of grain boundary energy is proposed for the sintering process of two-phase nanocomposite ceramic materials. The grain growth process is successfully investigated, and the effect of grain boundary energy on the microstructure is discussed. The simulation results show that the competition between grain boundary and interfacial energies can affect the pinning effect of nano-particles, lead to various microstructure. The simulation results are in accordance with the experiments.

Abstract: The influence of external mechanical stresses on agglomeration and bending of solidifying crystals has been investigated by microstructural characterisation of hypoeutectic Al cast specimens. The samples were produced by near-static cooling, gravity die casting and high pressure die casting (HPDC), where the solidifying crystals experience different levels of mechanical stresses. Electron backscatter diffraction (EBSD) technique was used to acquire grain misorientation data which can be linked to crystal agglomeration and bending behaviour during solidification. The length fraction of low-energy grain boundaries in HPDC samples was substantially higher than in gravity diecast and ‘statically cooled’ samples. This is related to the high amount of shear applied on the solidifying alloy, which promotes crystal collisions and agglomeration. In-grain misorientations were significant only in branched dendritic crystals which were subjected to significant shear stresses. This is attributed to the increased bending moment acting on long, protruding dendrite arms compared to more compact crystal morphologies.

Abstract: Recently, the necessity to grade grain size to ultrafine and nano scale for understanding the mechanical behavior of these materials has been recognized. However, the nature of such classification has remained unclear. As an example, ultrafine (100 nm -1 μm) and nano (<100 nm) grained FCC metals, compared to their coarse grained counterparts, exhibit a grain size strengthening that may deviate from the Hall-Petch relationship. To explain the mechanism of such deviation, previous dislocation theories seem insufficient. To solve this problem, a critical grain size criterion governing the shift of deformation mechanism is proposed in this work. This model employs an energetic approach; it relates the grain boundary energy density to certain critical energy values; and it permits, for the first time, a quantitative grading of grain sizes. Predictions based on this model were evaluated. The prediction on copper polycrystals of various grain sizes showed a very good agreement with experimental results. It is thus wished that the grain size theory on plastic deformation mechanism could be unified with the dislocation theory. In this study, such unification is attempted by using a parameter defined as the defect energy density. The possibility of such generalization is further reasoned upon the fact that the defect energy approach should be a unique but common form applicable for both dislocations and grain boundaries.

Abstract: The grain boundary energy anisotropy in BCC Fe-based polycrystals is considered. The correlation between the energy in BCC random grain boundaries and the distribution of grain boundary planes in the bulk was examined with a special attention on the presence of low index (low surface energy) planes in the internal surfaces. For a BCC structure, {100} and {110} planes are known to be the lowest energy planes dominating the equilibrium crystal shapes. Experimental evidences demonstrated that these planes were predominant in the texture of surfaces controlled by surface energy [2]. Moreover, the relation between the grain boundary character distribution and the crystallographic dependence on the grain boundary energy in the bulk after annealing treatment was studied. The grain character boundary distribution (GCBD) was calculated using the crystallographic information obtained from OIM-EBSD maps from samples showing columnar grains. Preliminary results showed no particular distribution trend within the standard stereographic triangle (001-101-111).

Abstract: The grain boundary structure and its energy are necessary for the fundamental understanding of the physical properties of materials. In aluminum, three distinct atomic structures of a Σ9(221)[110] tilt grain boundary have been reported in previous studies using atomistic simulations and a high-resolution transmission electron microscopy (HRTEM). In this work, we studied the atomic structure and energy of the Σ9 tilt grain boundary in aluminum using first-principles calculations. A comparison of the grain boundary energies among the three distinct Σ9 tilt grain boundaries determined through first-principles calculations allowed us to identify the most stable atomic structure of Σ9 tilt grain boundary in aluminum.

Abstract: The (relative) grain boundary energy of random high angle boundaries has been measured in several Fe-based polycrystals. Crystallographic data obtained by orientation contrast microscopy (OIM) are combined with the geometrical configuration of grain boundaries at triple junctions. A two-parameter representation of the relative grain boundary energy in terms of misorientation angle and misorientation axis is presented. In the applied procedure a variation of the energy values assigned to one boundary was observed depending on the triangulation path chosen by the operator to connect the arbitrary initial boundary with the boundary under consideration. Results show no evidence of correspondence between the observed energy cusps and the presence of CSL boundaries.

Abstract: The kinetics and morphology of the grain boundary grooving of Zn bicrystals with 16° <1010> tilt GB by Sn(Zn) melt has been studied at 325°C in equilibrium conditions in vacuum. It is shown that grooving process is interface controlled at least on the first stage. Groove walls mobility is evaluated. Changing of GB grove shape from “faceted walls” corner for annealing time < 78 h to concave “Mullins type” groove for annealing time > 78 h at the same experimental temperature was observed for the first time.