tailieunhanh - Kinetics of Materials - R. Balluff S. Allen W. Carter (Wiley 2005) WW Part 15

Tham khảo tài liệu 'kinetics of materials - r. balluff s. allen w. carter (wiley 2005) ww part 15', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 548 CHAPTER 22 SOLIDIFICATION instability is increased by increasing the rate of solidification fully formed dendrites develop as in Figs. and . As this transition progresses crystallographic factors play a role and the cell-growth direction deviates toward the preferred growth direction 100 for cubic metals as in Fig. . Also ridges develop along the dendrite sides as in Fig. . Finally secondary dendrite branches appear on the primary dendrites as in Fig. . Figure Transition from cellular to dendritic growth as the growth velocity is increased a Cellular growth at low velocities b Cellular growth deviated to the fast growing 100 direction c Appearance of ridges along the primary dendrites d Start of secondary dendrite branch formation. After Flemings 2 . Solute Segregation during Dendritic Solidification During dendritic growth extensive solute segregation occurs in the interdendritic spaces this phenomenon is a serious problem in the casting of alloys. The segregation occurs because of the tendency of the solidifying solid to reject excess solute into the remaining liquid and can be understood using the model developed to analyze plane-front solidification. However the geometry of the dendritic liquid solid interface and the adjacent diffusion field is complex. As a reasonable approximation the dendritic structure may be represented by the diagram in Fig. . The solidification in the interdendritic space can be described by constructing the cell shown dashed and assuming that solidification proceeds in a manner similar to the plane-front solidification of a bar as discussed in Section . Under typical casting conditions k k. Therefore the segregation Figure Simplified cell model for analyzing interdendritic segregation. STRUCTURE OF CASTINGS AND INGOTS 549 profile along X follows Eq. approximately with k k and L equal to half the interdendritic spacing. A typical profile is illustrated in Fig. for a .

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