Twinning in Ceramics

Deformation by slip or twinning. In ceramics there are both crystallographic and electrostatic considerations. The slip direction is usually along a close packed direction. The slip plane will usually be a closely packed plane or one that does not put like charges in... 

Fig. 2. RepHca electron micrograph of the fracture surface of enstatite—P-spodumene—zirconia glass-ceramic, showing twinning in the enstatite grains...

Arlt, G. (1990) Twinning in ferroelectric and ferroelastic ceramics stress relief. J. Mater. Sci. 25, 2655-66. 

Slip is generally easier than twinning in metals, but the reverse is true for many ceramic materials. In polycrystalline solids, both twinning and slip may operate. 

Most twin-screw extruders used in ceramic production were chosen either to produce the high requisite extrusion pressures or to introduce the high requisite or desirable shear forces for kneading and homogenizing ceramic compounds. For decades now, Werner Pfleiderer - now COPERI-ON - have been supplying such extruders to well-known producers. Werner Wiedmann s and Maria Holzel s contribution describes the operation and technical makeup of Twin-screw extruders in ceramic extrusion . 

Ceramics are usually used in a polycrystalline form. GBs in ionic and covalent materials must be better understood to improve the science of processing of many modem ceramic materials the properties of polycrystalline ceramics depend directly on the geometry and composition of GBs. The types of GBs commonly found in ceramic materials range from situations in which the distance between the grains is >0.1 pm and such grains are separated by a second phase (glass), to the basal twin boundary in AI2O3, which is atomically abrupt and potentially very clean. 

In this chapter we are concerned with the deformation of ceramics leading to a permanent shape change. This is known as plastic deformation and is both nonrecoverable and irreversible. There are several mechanisms that are responsible for plastic deformation in crystalline materials dislocation motion, vacancy motion, twinning, and phase transformation. In metals at room temperature dislocation motion is the most important of these mechanisms. In Chapter 12 we already noted that dislocations do not move easily in ceramics and this is the reason for their inherent brittleness. Nevertheless, dislocation motion is observed in ceramics under specific loading conditions. In general, plastic deformation of ceramics requires high temperatures and this is important because... 

The classic example of a reconstructive phase transformation in ceramics is the transformation between the low and high forms of SiOi the distorted form of quartz structure is stable at the lower temperature. Twins are again often formed during reconstructive phase transformations when these lead to a decrease in symmetry since the change can often occur in symmetry-related ways the twins are then related by the lost symmetry element. 

Following the above observation on twinning in BT, it may be of interest to visualize the nature and characteristics of twinning in a single crystalline of this ceramic. 

The serration curves in ceramics are analogous with those observed in HCP and BCC metals during low-temperature deformation. Serration is formed by the movement of partial dislocations this motion converts part of a crystal to a twin orientation. It is believed that dislocations are involved in twinning, but the mechanism is not yet clear. 

Figure 89. SiC varistor ceramic etched with modified Murakami s solution, BF. The a-SiC has a bright appearance. Twins in the a-SiC have been rendered visible. The Si02 phase is gray.

Figure 2-12 Microstructure of enstatite- 3-spodumene glass-ceramics showing lamellar twinning in enstatite.

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