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Chapter 5 - Imperfections: Interfacial and Volumetric Defects



In Chapter 4, we dealt with point and line defects. There is another class of defects called interfacial, or planar, defects. These imperfections, as the name signifies, occupy an area or surface and so are two-dimensional, as well as being of great importance. Examples of such defects are free surfaces of a material, grain boundaries, twin boundaries, domain boundaries, and antiphase boundaries. Of all these, grain boundaries are the most important from the point of view of the mechanical properties of the material. In what follows, we consider in detail the structure of grain and twin boundaries and their importance in various deformation processes, and, very briefly, the structure of other interfacial defects. Details regarding the strengthening of a material by grain boundaries are given in Section 5.3. Volumetric defects, such as voids, also play a major role in the mechanical properties of materials, affecting the strength and elastic properties of the material significantly. Volumetric defects are briefly described in Section 5.7. In Section 5.8, we present the defects occurring in polymers.

Grain Boundaries

Crystalline solids generally consist of a large number of grains separated by boundaries. Most industrial metals and ceramics are polycrystalline aggregates, and the mechanical properties of these polycrystals can be radically different from those of the monocrystals that form the individual grains. Figure 5.1 illustrates a polycrystalline aggregate, in which each grain has a distinct crystallographic orientation.

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Suggested reading
Interfacial defects
Gleiter, H.. “On the Structure of Grain Boundaries in Metals.Mater. Sci. Eng., 52 (1982) 91.
H. Gleiter and B. Chalmers. “High-Angle Grain Boundaries,” Progress in Materials Science, vol. 16, Chalmers, B., Christian, J. W., and Massalski, T. B., eds. Elmsford, NY: Pergamon Press, 1972.
Murr, L. E.. Interfacial Phenomena in Metals and Alloys. Reading, MA: Addison-Wesley, 1975.
Sutton, A. P. and Baluffi, R. W.. Interfaces in Crystalline Materials. New York, NY: Oxford University Press, 1994.
Reed-Hill, R. E., Hirth, J. P., and Rogers, H. C., eds., Deformation Twinning. TMS-AIME Conf. Proc. New York, NY: Gordon and Breach, 1965.
Christian, J. W. and Mahajan, S.. Deformation Twinning. New York, NY: Oxford University Press, 1995.
Grain-size effects
R. W. Armstrong. “The Influence of Polycrystal Grain Size on Mechanical Properties” in Advances in Materials Research, vol. 4, Herman, H., ed. New York, NY: Wiley Interscience, 1971, p. 101.
R. W. Armstrong, in Yield, Flow, and Fracture of Polycrystals, Baker, T. N., ed. London: Appl. Sci. Publ., 1983, p. 1.
Gleiter, H., “Nanocrystalline Materials,” Progress in Materials Science, 33 (1989) 223.