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Unformatted text preview: pore channels at the grain edges. d. Final stage: Pores are tetrahedral inclusions at the corners where four tetrakaidecahedra meet. 10 Tetrakaidecahedron A tetrakaidecahedron is constructed from an octahedron by trisecKng each edge and joining the points to remove the six edges. The resulKng structure has 36 edges, 24 corners, and 14 faces (8 hexagonal and 6 square). The volume of the tetrakaidecahedron Each pore is shared by 3 tetrakaidecahedra, and if r is the radius of the pore, then the total volume of the porosity per unit cell is The porosity of the unit cell, Vp/Vt 11 Desifying vs. Nondensifying Only densifying mechanisms are important in the intermediate and ﬁnal stages 12 The Intermediate Stage – Lattice Diffusion
la[ce diﬀusion coeﬃcient relaKve density grain size 13 The Intermediate Stage – Grain Boundary Diffusion
Grain Boundary diﬀusion Grain Boundary coeﬃcient thickness 14 Idealized Models for The 3 Stages of Sintering a. Ini*al stage: Model structure represented by spheres in tangen*al contact. b. Near the end of the ini*al stage: Spheres have begun to coalesce. The neck growth illustrated is for center to center shrinkage of 4%. c. Intermediate stage: Dark grains have adopted the shape of a tetrakaidecahedron, enclosing white pore channels at the grain edges. d. Final stage: Pores are tetrahedral inclusions at the corners where four tetrakaidecahedra meet. 15 The Final Stage
LaJce Diﬀusion relaKve density Grain Boundary diﬀusion la[ce diﬀusion coeﬃcient grain size Grain Boundary diﬀusion Grain Boundary coeﬃcient thickness 16 Sintering with External Pressure A common diﬃculty that oien arises in the sintering of materials...
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This document was uploaded on 12/23/2013.
 Fall '13

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