Multigrain junctions

Fig. 4.14 Cavity formation in silicon nitride, (a) Lenticular cavities form at grain boundaries for grades of silicon nitride bonded by highly refractory glasses, (b) Crack-like cavities form at boundaries for grades of silicon nitride that are less refractory, (c) In addition to these interfacial cavities, more irregular, /urn-size cavities are also observed at multigrain junctions in silicon nitride. From Hockey and Wiederhom,94 Luecke et a/.101 and Wiederhom et al.100...

Figure 13.11 Multigrain junction cavities of different shapes in SN 88 after creep for 1693 h at 1300°C under stress of 155 MPa. [15, 37].

As a consequence, the network of grains must move apart, giving rise to dilatational stresses which are responsible for cavitation within the material. Cavitation occurs when the dilatational stress at a multigrain junction exceeds a critical stress for cavity nucleation. Subsequent cavity growth occurs via redistribution of the secondary phases from the cavity towards the uncavitated material. As the secondary phase flows away from the cavitated junction, the stress required for cavity nucleation relaxes, thus releasing the stressed contacts between silicon nitride grains. The rearrangement of dilatation stresses then results in a new round of cavity formation. 

Fig. 6.110 The presence of lenticular cavities along two-grain junctions in the 00 was confirmed by a SEM of secondary fracture surface and b TEM of longitudinal section, c TEM also revealed extensive multigrain junction cavitation in the NT164. d The absence of two-grain junction cavitation in the NT164 was attributed to the nearly complete elimination of the intergranular phase [46]. With kind permission of John Wiley and Sons...

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