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Grain Growth and Microstructure Control
Published in M. N. Rahaman, Ceramic Processing and Sintering, 2017
To achieve a basic understanding of how grains grow and how such growth can be controlled, it is often convenient to first consider a fully dense, single-phase solid where the complicating effects of pores are absent. In ceramics, grain growth is divided into two main types: normal and abnormal. In normal grain growth the grain sizes and shapes occur within a fairly narrow range, and except for a magnification factor, the grain size distribution at a later time is similar to that at an earlier time. Abnormal grain growth is characterized by the rapid growth of a few larger grains at the expense of the smaller ones. Simple models have been developed to predict the kinetics of normal grain growth, but many of them analyze an isolated grain boundary or a single grain and neglect the topological requirements of space filling. In recent years, computer simulation has begun to play an important role in exploring the complexities of grain growth.
Sintering and Microstructure Development
Published in Mohamed N. Rahaman, Ceramic Processing, 2017
Microstructures of polycrystalline ceramics that have been heated for some time at a sufficiently high temperature often show very large (abnormal) grains in a matrix of finer grains. A well-known microstructure in the ceramic literature involves the growth of a relatively large single-crystal Al2O3 in a fine-grained Al2O3 matrix, which appears to show the single-crystal grain growing much faster than the matrix grains (Figure 9.25). Earlier explanations considered that the grain size distribution of the starting material was the major factor leading to abnormal grain growth. They were based on the Hillert theory of grain growth [28] which predicted that any grain with a size greater than twice the average grain size would be predisposed to growing abnormally. This explanation is not supported by recent computer simulations [29] and theoretical analysis [30], which show that although the large grains grow, they do not outstrip the normal grains. The normal grains grow at a faster relative rate so that the large (abnormal) grains eventually return to the normal size distribution (Figure 9.26). The size effect is therefore not a sufficient criterion for abnormal grain growth. Inhomogeneities in chemical composition, liquid phases, and particle packing have long been suggested as possible causes of abnormal grain growth.
Principles of Sintering and Microstructural Development
Published in Mohamed N. Rahaman, Ceramic Processing, 2017
Microstructures of polycrystalline ceramics that have been heated for some time at a sufficiently high temperature often show large (abnormal) grains in a matrix of finer grains. A well-known microstructure in the ceramic literature involves the growth of a relatively large single-crystal Al2O3 in a fine-grained Al2O3 matrix that appears to show the single-crystal grain growing much faster than the matrix grains (Figure 13.20). Earlier explanations considered that the grain size distribution of the starting material was a major factor that dictated whether abnormal grain growth would occur or not. Any grain with a size greater than twice the average grain size would be predisposed to growing abnormally [14]. This explanation is not supported by recent computer simulations [15] and theoretical analysis [16], which show that although the large grains grow, they do not outstrip the normal grains. The normal grains grow at a faster relative rate. Consequently, the large (abnormal) grains eventually return to the normal size distribution (Figure 13.21). The size effect is, therefore, not a sufficient criterion for abnormal grain growth. Inhomogeneities in chemical composition, liquid phases, and particle packing have long been suggested as possible causes of abnormal grain growth.
Microstructure and texture evolution during grain growth of AM30 magnesium alloy
Published in Philosophical Magazine, 2022
D. Panda, R. Kushwaha, R. K. Sabat, S. Suwas, S. K. Sahoo
Figure 1 shows the microstructure and texture of the rolled sample before annealing. It could be observed from the figure that the microstructure comprises of both small and large grains along with substantial twins in the samples (Figure 1a). These deformation twins are generally observed during rolling of Mg and its alloys [31–34]. Figure 1(b) shows the normalised grain size distribution of the initial sample before annealing, using the line intercept method [28,35,36]. The average grain size of the initial sample was found to be 7.12 µm (Figure 1b). The breadth of the grain size distribution, which is an indicative of normal or abnormal grain growth, was found to be 1.320 (Figure 1b). A constant value of this breadth suggests normal grain growth behaviour (NGG), whereas a continuous increase in its value suggests abnormal grain growth behaviour (AGG) during the course of annealing [28–30]. The texture of the initial sample is presented in Figure 1(c) as (0002) pole figure. A strong basal texture, i.e. (0002) poles parallel to the normal direction (ND) of the sample was observed (Figure 1c). Similar basal textures are observed during deformation of Mg alloys containing high Al, such as AM60, AM70, AM80, AZ61, and AZ80 Mg alloys [23,24,37,38]. Figure 1(d) shows the texture of the initial sample in the form of ODFs, at constant φ2= 0° and 30° sections. The sample had a dominant basal fibre, as can be seen from Figure 1(d).