Deformation in Polycrystalline Ceramics

On several occasions throughout this book, no distinction was made whether single or poly crystalline ceramics were used to exemplify certain topics. Single and polycrystalline ceramics were interchangeably used to emphasize the subjects under consideration. Now, this section will focus on several cases of deformation in polycrystalline ceramics not yet discussed in depth. First among the topics to be considered below is that of preferred orientation, a well-known aspect influencing not only the physical properties of ceramics, but also their mechanical behavior. 

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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. 

Although superplasticity is defined as the ability of a polycrystalline material to exhibit large elongations, in many ceramics-related materials and ceramic composites superplasticity is also said to occur even though the polycrystal is deformed in compression, or in three- or four-point bending conditions, as long as GBS is the primary deformation process.4-7... 

Sintered ceramics made of lead-zirconium titanate (PZT Pb(Tii jZr,)03 x S 0.5) are usually used for phoioacoustic experiments [105, 106]. The unit cell of the lead-zirconium titanate has a perovskite structure. Below the Curie temperature (328 °C for the PZT-4 (Vemitron) used by us [24]), the cells are tetragonally deformed, i.e., positive and negative charges are shifted and electric dipole moments are produced. In analogy to ferromagnetism, domains with randomly distributed polarization direction are formed. By the application of an electric field, these can be orientated in a preferred direction, and the sintered polycrystalline ceramic is then remanently polarized. The properties of these anisotropic piezoelectric materials are described by various parameters which depend on the polarization and deformation direction. In the common terminology, the < ordinate system shown in Fig. 3 is obtained for the cylindrical piezoelectric crystals [24]. 

Many polycrystalline ceramics consist of a random array of single crystals and, in these cases, the materials are elastically isotropic. As shown in Table 2.3, only two elastic constants are needed to describe a linear elastic deformation, c, and Cl2- For convenience, the engineering elastic constants can be used, and one obtains Eqs. (2.65)-(2-70). There is, however, an additional relationship between c,... 

Time-dependent hysteresis effects can also occur in crystalline materials and these lead to mechanical damping. Models, such as the SLS and the generalized Voigt model, have been used extensively to describe anelastic behavior of ceramics. It is, thus, useful to describe the sources of internal friction in these materials that lead to anelasticity. The models discussed in the last section are also capable of describing permanent deformation processes produced by creep or densification in crystalline materials. For polycrystalline ceramics, creep is usually considered from a different perspective and this will be discussed further in Chapter 7. 

The critical temperatures, Tab and Tbc, vary greatly for different ceramics. For polycrystalline MgO the brittle-to-ductile transition occurs at 1700°C (0.6 T ). There is no plastic deformation in P-SiC below 2000°C. Talc, M0S2, and graphite all deform at room temperature. M0S2 and graphite are widely used as solid lubricants. 

The mechanisms responsible for fracture in structural ceramics at elevated temperatures have been reviewed [154]. Sensitivity to flaws or microstructural inhomogeneities which nucleate microcracks are among the failure mechanisms. The flaws which control failure under creep conditions are different from those responsible for fast fracture at room temperature. A common feature is the development of cracks through gradual damage accumulation, depend on the microstructure. The role of cracks in the deformation and fracture behavior of polycrystalline structural ceramics have been reviewed [155]. 

Deformation at elevated temperatures is the commonly observed case in ceramics. Once again using the example of polycrystalline ceramic MgO, the following stress-strain curves are illustrated (Fig. 4.8). One of the possible differences in these stress-strain curves reflects the difference in grain-size the different grain size before deformation is shown in Fig. 4.9. The composition and porosity were also different in the otherwise nominally pure and dense specimens, as seen in Table 4.1. 

In a polycrystalline ceramics, such as BT, when structural phase transition occurs, a crystallite (grain) experiences slight deformation. This deformation is... 

At this point, it is constmctive to observe the dislocation structure in MgO polycrystalline ceramics, as revealed by TEM. In Fig. 6.36, a typical dislocation structure is shown for creep deformation, not unlike that found in metals, with the presence of subgrains, in which a 3D dislocation network may be seen. Note that the long dislocation segments seldom run in a straight line from one node to another, but are bowed out, often in only one plane, though, sometimes, as seen in Fig. 6.36a,... 

A study of Vickers hardness of polycrystalline ceramics revealed that cracking may cause critical transition points in the Vickers ISE trends. The transition point was associated with extensive cracking in and around the indentation and a shift in the energy balance during indentation. Different ratios of the indentation work are expended on volumetric deformation and surface fracture processes above and below the transition point. The transition point was very distinct for brittle materials such as silicon carbide. The Vickers hardness transition point was related to a new index of ceramic brittleness defined as ... 

C. K. L Davies and S. L. Sinha Ray, High temperature creep deformation of polycrystalline alumina in tension, in Special Ceramics Vol. 5, edited by P. Popper (Br. Ceram. Res. Assoc., Stoke on Trent, 1972) 193 207. 

Unlike structural steels and alloys wherein the energy dissipation occurs at the tip of a running crack when a metal is being plastically deformed, the mechanism of the plastic deformation in its classic understanding is absent in brittle materials (e.g. ceramics, polycrystalline SH1 /I and rocks), but the energy dissipation takes place here as a result of micro-cracking in the vicinity to the crack tip (Fig.I)o This region is named dissipative (1,2) o... 

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