Plastic deformation of metals and ceramics

Metals are normally ductile, and plastic deformation can be considerable. This is a valuable property of metals and is used, for example, to fabricate dish shapes of one sort or another by impressing a die into a sheet of metal. The metal deforms plastically and retains the dished shape when the load on the die is removed. Ceramics are regarded as brittle, and normally the same fabrication process applied to a ceramic would shatter it. However, at high temperatures, many ceramic materials can deform plastically, and at low temperatures many metals become brittle and lose ductility. Thus, it is convenient to treat these two apparently dissimilar crystalline materials together. Polymers, which contain crystalline and amorphous regions, are considered in Section 10.1.11. 

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. 

The movement of dislocations is constrained by crystallography. Some planes allow movement to take place more easily than do others, and these preferred planes are called slip planes. Similarly, dislocation movement is easier in some directions than it is in others. These preferred directions are called slip directions. The combination of a slip plane and a slip direction is called a slip system. 

Slip in hexagonal metal crystals occurs mainly parallel to the basal plane of the unit cell, normal to the c axis. The slip systems can be described as 000 1 (1 1 20), of which there are three. Body-centred cubic metals have slip described by 1 1 0 (I 1 1), giving 12 combinations in aU. Other slip systems also occur in metals, but those described operate at lowest energies. 

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