Brittle and Semibrittle Solids

Dislocations are present but essentially immobile except at very high temperatures, typically in excess of 1000°C. 

This is an important point. A sublattice phase with the FCC structure should not, generally speaking, be considered CCP with regards to slip. The atoms or ions on one sublattice may very well be in a CCP-hke arrangement, but they can be kept apart by large atoms or ions residing on the other sublattice (the interstitial sites). Slip is easiest along tmly close-packed layers of identically sized spheres that are in contact and, preferably, without obstacles such as interstitials. Thus, another reason for low ductility in intermetallics and ceramics is the lack of a sufficient number of active slip systems to allow plastic deformation. 

Lately, however, some surprising exceptions have been found to the general rule of low plasticity in ceramics. One is the perovskite oxide strontium titanate, SrTiOs. Recent studies on single crystals have revealed a transition from nonductile to ductile behavior in this material not only at temperatures above 1000°C, but again, below 600°C. Even more unexpectedly, it reached strains of 7 percent at room temperature with flow stresses comparable to those of copper and aluminum alloys. At both the high and low temperatures, the plasticity appears to be owing to a dislocation-based mechanism (Gumbsch et al., 2001). 

Normally, dislocation-based plastic deformation is irreversible, that is, it is not possible to return the material to its original microstructural state. Remarkably, fully reversible dislocation-based compressive deformation was recently observed at room temperature in the layered ternary carbide Ti3SiC2 (Barsoum and El-Raghy, 1996). This compound has a hexagonal stmcture with a large cja ratio and it is believed that the dominant deformation mechanism involves dislocation movement in the basal plane. 

In general, however, the low plasticity of intermetallics, glasses, and ceramics hinders their use in many engineering applications and impedes their fabrication by 

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