Papers by Keyword: Abnormal Grain Growth

Abstract: The texture evolution due to grain growth that takes place during annealing was investigated in nanocrystalline Fe-Ni alloys fabricated by using an electroforming method. In the current materials, the as-deposited textures were of fibre-type characterized by strong <100>//ND and weak <111>//ND components, and the occurrence of grain growth during annealing resulted in the strong development of the <111>//ND components with a significant decrease of the <100>//ND components. It was clarified that abnormal grain growth plays an important role on the evolution of the microstructures and textures. The abnormally grown grains were observed using orientation imaging microscopy in the early stages of grain growth, and their morphological features have been discussed.

Abstract: The nucleation and growth behavior of magentoplombite has been observed during the abnormal grain growth in the presence of liquid phase. Abnormal grain growth was explained in terms of grain orientation due to liquid phase. It has been observed that the nuclei formation for abnormal grain growth was driven by the capillary force due to the liquid phase. Because magnetoplumbite crystal structure has an anisotropic crystal nature, abnormal grain growth with faceted grain shape was observed depending on its chemical composition. The formation of nuclei and particle orientation was important for the occurrence of abnormal grain growth.

Abstract: Use of silver (Ag) nanoparticle suspension for various applications such as ink-jet printing of electronic circuits has been of prime interest. We observed the microstructure evolution of the inkjet-printed Ag thin films on Si substrates under various annealing conditions using the field-emission scanning electron microscopy (FE-SEM). Abnormal grain growth characteristics were identified when annealed at about 240 oC under ambient air. Growth characteristics of pores were found to be in accordance with that of grains. Competition between grain and pore growth is attributed to small grain sizes, low packing density and high porosity, which are characteristic of inkjet-printed Ag films as dried.

Abstract: Phase-field model (PFM) in multiple orientation fields was used to simulate the grain growth in three-dimensions (3-D) for isotropic and anisotropic grain boundary energy. In the simulation, the polycrystalline microstructure was described by a set of non-conserved order parameters and each order parameter describes each orientation of grains. For isotropic grain boundary energy, the simulation showed the microstructure evolution of normal grain growth. For anisotropic grain boundary energy, however, the simulation showed that certain grains which share a high fraction of low energy grain boundaries with other grains have a high probability to grow by wetting along triple junctions and can grow abnormally with a growth advantage of solid-state wetting. The PFM simulation shows the realistic microstructural evolution of island and peninsular grains during abnormal grain growth by solid-state wetting.

Abstract: Abnormal grain growth (AGG) proceeds in case that normal grain growth is inhibited. It has long been known that the inhibition involves finely dispersed particles and/or the development of specific textures. There is another strong obstacle against the grain boundary (GB) motion; the solute atoms can reduce their energy by moving from the bulk into a GB. Resultant interaction between the solute atoms and a GB makes the GB motion more difficult. However the role of the GB segregation effect on AGG has not been clarified. In this study we simulate the 2D and 3D grain growth accompanying boundary segregation of solute atoms by using a phase-field model. It is shown that the segregation plays an important role on the occurrence of AGG. The boundary-segregation-induced AGG can take place when the average driving force of grain growth approaches a critical condition for pinning-depinning transition in solute-drag atmosphere.

Abstract: When a polycrystal is in chemical equilibrium, the microstructure evolves as a result of grain growth under the capillary driving force arising from the interface curvature. As the growth rate of an individual grain is the product of the interface mobility and the driving force, the growth of the grain can be controlled by changing these two parameters. According to crystal growth theories, the growth of a crystal with a rough interface is governed by diffusion and its interface mobility is constant. In-contrast, the growth of a crystal with faceted interfaces is governed by the interface reaction and diffusion for driving forces below and above a critical value, respectively. As the growth rate is nonlinear for the regime of interface reaction control, the grain growth is nonstationary with annealing time. Calculations reveal that the types of nonstationary growth behavior including pseudo-normal, abnormal, and stationary are governed by the relative value of the maximum driving force, gmax, to the critical driving force for appreciable growth, gc. Recent experimental observations showing the effects of critical processing parameters on microstructural development also support the theoretical prediction. The principles of microstructural design are deduced in terms of the coupling effects of gmax and gc.

Abstract: In order to save natural resources and to reduce production costs, many industries have conducted studies on new developed steels and manufacturing processes. For instance, the use of high-strength bolts can decrease the number, size and weight of bolts used in a car, thereby decreasing fuel consumption. For this reason, steel makers are developing boron-added steel for high-strength bolts, which eliminates production processes. However, it has been pointed out that this boron-added steel is sensitive to austenite grain coarsening. The austenite grain coarsening does not occur uniformly, hence it can affect mechanical properties such as tensile, fatigue and so on. Therefore, the aim of this study is to investigate the abnormal behavior of grain growth and to determine the optimum amount of alloying elements in boron-added steel for use in making high-strength bolts.

Abstract: Grain growth may occur in two forms, normal grain growth, characterized by a constant grain size distribution during growth, and abnormal grain growth, where one or more abnormally large grains may form in the microstructure. The presence of abnormally large grains in an otherwise uniform microstructure may be detrimental to the mechanical properties of a polycrystalline structure. Little is understood of the exact cause of abnormal grain growth. The annealing conditions leading to the onset of abnormal grain growth have been investigated via a series of grain growth experiments carried out on an Al-4wt%Cu alloy. The structure of which consisted of equiaxed grains (<8μ) pinned by a fine dispersion of sub-micron second phase particles, which may dissolve upon annealing. Minority texture components may experience accelerated growth due to a higher energy and mobility compared to the surrounding grain structure. The combination of these two events may result in the abnormal growth of some grains. SEM imaging and EBSD data has then made it possible to characterize the influence of particle dissolution and grain boundary misorientation on the onset of abnormal grain growth. The stability of ‘island grains’ found to exist internally in abnormally large grains has also been investigated in relation to the misorientation relationship and localized second phase volume fraction found there. There was only weak evidence of special misorientation relationships between the island grains and the abnormally large grains in which they exist, and although there was evidence of an enhanced fraction of pinning particles at island grain boundaries, this was also true of boundaries in general. The larger size of island grains is their dominant characteristic, and grains which become island grains may have been incipient abnormal grains.

Abstract: Theories of abnormal grain growth (AGG) in three dimensions usually approximate an abnormal grain by a sphere. The abnormal grain is then represented by its spherical equivalent grain radius. This study, by contrast, treats AGG in terms of concepts that include both the boundary curvature and the number of faces of the abnormal grain. We treat AGG for the case of pinned matrices, including the phenomena of initiation and growth kinetics. The influence of interfacial energy and mobility of the abnormal grain boundary are also discussed.

Abstract: The multiplicity and variety of grain shapes in three-dimensional polycrystalline metals makes their energetic and kinetic analyses difficult. To help simplify the analysis of isotropic polycrystals, average N-hedra (ANHs) (N=3,4,5,…∞) were created as a set of regular polyhedra, consisting of N identical faces, which act as topological “proxies” for analyzing the corresponding class of irregular grains containing mixed faces of the same number. This paper outlines a further generalization of the ANH concept that extends three-dimensional analysis to include the growth or shrinkage of a small population of grains embedded in a textured matrix.