Metric and Topological Characterization of the Advanced Stages of Loose Stack Sintering
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I.
INTRODUCTION
THE geometric
approach to the study of sintering focuses on the evolution of microstructure, which is quantified by measuring the metric and topological properties of the pore and solid phases. Metric properties are global averages of volumes, areas, lengths, and curvatures per unit volume of structure and are thus size-dependent; those measured in this study are listed in Table I. Standard techniques of quantitative stereology are employed in the measurement of the metric properties. Topological properties of present interest are the number of isolated pores per unit volume and the connectivity (the number of redundant connections) of the pore network per unit volume, as illustrated in Figure 1. The microstructural properties described above form the basis for a definitive visualization of the evolution of the geometry of sintering. There are three more of less distinct geometrical stages t" traversed by a sintered body. The initial stage is signified by the formation of interparticle contacts or necks that grow until they begin to impinge on each other (Figure 2). During this stage, the decrease in pore volume and area is accompanied by an increase in the connectivity, the latter brought about by formation of additional interparticle contacts as the particles are brought closer by densification.[2] Pore channels that are formed as a result of impingement of growing necks form an interconnected network. During the intermediate stage, this network shrinks and suffers a decrease in the connectivity via closure of pore channels or network coarsening.131 Eventually, the number of redundant connections becomes sufficiently small so that some of the channel closure events lead to isolation of separate pores. The third stage is characterized by further decrease in the connectivity and a continuation of isolation and removal of pores as the volume fraction of porosity approaches zero. A.S. WATWE, formerly with the Department of Materials Science and Engineering, University of Florida, is with Inco Alloys International, Inc., Huntington, WV 27505. R.T. DeHOFF, Professor, is with the Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611. Manuscript submitted July 24, 1987. METALLURGICAL TRANSACTIONS A
Microstructural analysis of the advanced stage of sintering involves the study of low porosity sintered bodies. Mechanical and physical properties of a powder-processed component in any range of porosity are strongly influenced by the microstructure. A variety of commercial products such as high-strength refractories, magnetic materials, and nuclear fuel pellets that are made by powder technology have low porosity requirements. Hence, microstructural study of high-density sintered components is of considerable practical interest. In spite of this, previous investigations of this type have been sparse because long sintering times are required and the topological analysis is effort intensive. A detailed geometrical study of the advanced stage of sintering is thus of bot
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