Shape accommodation

Fig. 5.39 Conceptual outline of the changes associated with solution-reprecipitation densification where both grain growth (Ostwald ripening) and grain-shape accommodation act to release liquid to fill residual pores. Reproduced with permission from [73]. Copyright 2009, Springer...

The content of the liquid phase plays a key role in determining whether there is grain-shape accommodation or not in a liquid-phase sintering. In a given system, there is a critical content of the liquid phase, below which grain-shape accommodation occurs, while above the grain-shape accommodation is absent. When the... 

At low concentrations of the liquid phase, the elimination of isolated pores is continued at a reduced rate, due to the solution precipitation and grain-shape accommodation. If the volume fraction of the liquid is sufficiently high, the filling of the isolated pores could be discontinuous [95, 100]. In this case, grain growth, rather than grain-shape accommodation, is responsible for the filling of the pores. 

Lee, D. D., Kang, S.-J. L. and Yoon, D. N., A direct observation of the grain shape accommodation during liquid phase sintering, Scripta Metall., 24, 927-30, 1990. 

Kaysser, W. A., Zivkovic, M. and Petzow, G., Shape accommodation during grain growth in the presence of a liquid phase, J. Mater. Sci., 20, 578-84, 1985. 

Densification and grain shape accommodation by solution-precipitation... 

Figure 10.27 Schematic diagram illustrating densification accompanied by Ostwald ripening. Grain shape accommodation can also occur when the liquid volume fraction is low.

Grain shape accommodation is clearly observed in the micrographs of Fig. 10.28 for the W(Ni) system containing 4 wt% Ni. However, many microstructures of materials produced by liquid-phase sintering also show rounded (spheroidal) grains. As an example. Fig. 10.29 shows the microstructure for the same W(Ni) system described in Fig. 10.28 but with a higher amount of liquid (14 wt% Ni). 

It is clear from Figs. 10.28 and 10.29 that the volume fraction of liquid is a key factor in determining whether grain shape accommodation will occur. 

The occurrence of grain shape accommodation is normally favored when the amount of liquid is relatively small. In this case, the liquid is insufficient to completely fill the voids between the grains as long as the grains take up a rounded or sphCToidal shape. Grain shape accommodation produces a polyhedral grain... 

When the volume of the liquid phase is fairly low, solution-precipitation and grain shape accommodation lead to a slow, continuous elimination of the isolated... 

The polyether urethane disc is essentially an annular ring with a steel shear-resisting pin in the center. A 1/16 in. gap separates the pin from the inside vertical edge of the annular disc. The outer edge of the disc is V-shaped as depicted in Figure 1.12. The V-shape accommodates bulging under load. Each leg of the V forms a 30° angle with the vertical. 

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