Ductility in Single Crystal Ceramics

Bear in mind that tests providing mechanical property data, such as ultimate strength, etc., may be considered extremely sensitive to the experimental conditions and the data are usually more scattered than those in similar tests of metallic materials. 

Load versus deflection diagrams for Y-FSZC-10 and Y-PSZC-3 are compared in Fig. 2.74. 

Additional experiments on stress and strain, providing information on ductility, may be seen in Fig. 2.75. This figure illustrates the brittle and ductile failure modes and the strain-rate dependence of yield stress. These different failure 

Material (orientation of the specimen axis) Density (g/cm ) Ultrasonic velocity (m/ s) Elastic modulus (GPa) Bending strength (MOR) (MPa) Ultimate strain (x 10 m/m) 

Material Veh (mm/ min) Elastic limit (MPa) Upper yield point (MPa) Lower yield point (MPa) Strain at elastic limit (xl0 m/m) Strain at upper yield point (MPa) Elastic modulus (GPa) 

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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). 

The stress-strain behavior of ceramic polycrystals is substantially different from single crystals. The same dislocation processes proceed within the individual grains but these must be constrained by the deformation of the adjacent grains. This constraint increases the difficulty of plastic deformation in polycrystals compared to the respective single crystals. As seen in Chapter 2, a general strain must involve six components, but only five will be independent at constant volume (e,=constant). This implies that a material must have at least five independent slip systems before it can undergo an arbitrary strain. A slip system is independent if the same strain cannot be obtained from a combination of slip on other systems. The lack of a sufficient number of independent slip systems is the reason why ceramics that are ductile when stressed in certain orientations as single crystals are often brittle as polycrystals. This scarcity of slip systems also leads to the formation of stress concentrations and subsequent crack formation. Various mechanisms have been postulated for crack nucleation by the pile-up of dislocations, as shown in Fig. 6.24. In these examples, the dislocation pile-up at a boundary or slip-band intersection leads to a stress concentration that is sufficient to nucleate a crack. 

Low-temperature ductility is rarely observed in ceramics, which are inherently brittle, but some bulk ceramics show plasticity at ambient temperatures. One example of low-temperature plasticity in MgO is considered here. First, consider a single crystal, where i orientation-dependent properties are of interest. Orientation is one of the factors that influence mechanical properties. It was observed (by etch-pit technique) that the flow in MgO occurs on the 110 (110) slip system. However, it was also found [28] that the 110 (110) slip system contributes to deformation above 600 °C. Details on Plastic deformation in MgO single crystals were presented in Sect. 2.2, Figs. 2.33 and 2.38. Consequently, some information on deformation in polycrystalline ceramics may be of interest. 

Considerable work was done on MgO, both single crystals and polycrystalline materials alike, toward an understanding of deformation behavior in ceramics. Furthermore, when polycrystalline MgO is ductile, its strain hardening is comparable to that of (111) oriented single crystals [16]. 

Single crystals of MgO have received attention due to their use in ductile ceramic studies. Extreme purity is required in this area. Periclase windows are also of potential interest in infrared applications due to their transmission characteristics. 

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