Reprecipitation, liquid phase sintering

This mechanism is similar to the one occurring during liquid-phase sintering, where the dissolution of crystalline material into the glassy phase occurs at the interfaces loaded in compression and their reprecipitation on interfaces loaded in tension. The rate-limiting step in this case can be either the dissolution kinetics or transport through the boundary phase, whichever is slower. This topic was discussed in some detail in Chap. 10, and will not be repeated here. 

Fig. 5.43 The three mechanisms of grain-shape accommodation and neck growth during solution reprecipitation controlled densification of liquid-phase sintering a contact flattening, b dissolution of small grains, and c solid-state bonding. Reproduced with permission from [73]. Copyright 2009, Springer...

Kur] Kuroki, H., Han, G., Shinozaki, K., Solution-Reprecipitation Mechanism in Fe-Cu-C During Liquid Phase Sintering , Int. J. Powder Met., 35(2), 57-62 (1999) (Phase Relations, Thermodyn., Calculation, 14)... 

The mechanisms of liquid-phase sintering processes have been described in detail by Kingery (213, 214], Petzow (215), and German [216]. The sintering process can be treated as a sequence of three stages particle rearrangement solution-reprecipitation and skeleton sintering. 

For solid phase sintering, there are fonr ways of diffusion i) surface diffusion, ii) volnme diffusion (often called lattiee diffusion), iii) vapor phase transport (evaporation-eondensation), and iv) grain boundary diffusion the boundaries are very disturbed areas, which allow diffusion short-circuits . For liquid phase sintering, we must add dissolution-reprecipitation effects or a vitreous flow. Finally, for pressure sintering the pressure exerted allows the plastic deformation of the crystallized phases and the viscous flow of the amorphous phases. 

Both modifications a and /i are present in different amounts in the starting powder (Table 10) mostly a-rich powders are used. At sintering temperature the metastable a-phase and the subcritical /i-particles (dcrit <0.5 pm) dissolve in the liquid and reprecipitate during cooling as /i solid solutions on the initial stable /1-particles which act as nuclei, while the homogeneous and heterogeneous nucleation can be neglected in most cases (Sect. 6.1). 

Nevertheless, the microstructure cannot be directly correlated to the initial /1-content in the starting powder. The experimentally determined particle density in sintered samples indicates that only a part of the initial /I particles are able to grow [283]. The number of growing /1-particles depends on a critical particle diameter dcrit (Fig. 18). Particles below dcrit will dissolve in the oxide nitride liquid during phase transformation and reprecipitate on the overcritical /1-particles according to an anisotropic Ostwald ripening process [284, 292, 293]. 

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