Mechanical properties dislocations

A. H. Cottrell, The Mechanical Properties of Matter, Wiley, 1964, Chap. 9. D. Hull, Introduction to Dislocations, 2nd edition, Pergamon Press, 1975. W. T. Read, Jr., Dislocations in Crystals, McGraw Hill, 1953. 

Tinplate and Solder. Metallurgical studies were performed to determine the effect of irradiation at low temperature on the corrosion resistance of tinplate and on the mechanical properties and microstructure of tinplate and side-seam solder of the tinplate container. The area of major interest was the effect of low-temperature irradiation on the possible conversion of the tin from the beta form to the alpha form. In the case of pure tin, the transition occurs at 18 °C. It was feared that low-temperature irradiation would create dislocations in the crystal lattice of tin and enhance the conversion of tin from the silvery form to a powdery form rendering the tin coating ineffective in protecting the base steel. Tin used for industrial consumption contains trace amounts of soluble impurities of lead and antimony to retard this conversion for several years. 

Changes in electrical resistivity (V3) and mechanical properties (V3, V4) of graphite fibers upon nitration have been studied. Increases in elastic modulus, and decreases in tensile strengths, have been related to removal of boundary dislocations by the intercalation process proposed elsewhere iN4). 

The vast subject of dislocations, particularly with respect to mechanical properties, will not be considered in this book, and only a few aspects of dislocations, especially interactions with point defects, will be explored. 

The behavior of polycrystalline materials is often dominated by the boundaries between the crystallites, called grain boundaries. In metals, grain boundaries prevent dislocation motion and reduce the ductility, leading to hard and brittle mechanical properties. Grain boundaries are invariably weaker than the crystal matrix, and... 

In a similar fashion, the line and planar defects described above are all, strictly speaking, volume defects. For the sake of convenience it is often easiest to ignore this point of view, but it is of importance in real structures, and dislocation tangles, for instance, which certainly affect the mechanical properties of crystals, should be viewed in terms of volume defects. 

An elastic continuum model, which takes into account the energy of bending, the dislocation energy, and the surface energy, was used as a first approximation to describe the mechanical properties of multilayer cage structures (94). A first-order phase transition from an evenly curved (quasi-spherical) structure into a... 

Despite the similarities in brittle and ductile behavior to ceramics and metals, respectively, the elastic and permanent deformation mechanisms in polymers are quite different, owing to the difference in structure and size scale of the entities undergoing movement. Whereas plastic deformation (or lack thereof) could be described in terms of dislocations and slip planes in metals and ceramics, the polymer chains that must be deformed are of a much larger size scale. Before discussing polymer mechanical properties in this context, however, we must first describe a phenomenon that is somewhat unique to polymers—one that imparts some astounding properties to these materials. That property is viscoelasticity, and it can be described in terms of fundamental processes that we have already introduced. 

Dislocations are linear defects and were first invoked to account for the mechanical properties of solids, particularly the shear strengths. Dislocations play an important role in a variety of solid state phenomena from phase transitions to chemical reactions and the subject has been investigated and reviewed widely (Fine, 1973 Nembach, 1974). The effect of dislocations on the transformations and properties of organic solids has been recognized in recent years (Thomas Williams, 1971 Jones Thomas, 1979). 

Hard and soft solids are generally classified as having high- and low-energy surfaces, respectively. These criteria must be used cautiously since the mechanical properties of a solid depend on the concentration of defects and dislocations in the bulk. 

The essential difference between treatments of chemical processes in the solid state and those in the fluid state is (aside from periodicity and anisotropy) the influence of the unique mechanical properties of a solid (such as elasticity, plasticity, creep, and fracture) on the process kinetics. The key to the understanding of most of these properties is the concept of the dislocation which is defined and extensively discussed in Chapter 3. In addition, other important structural defects such as grain boundaries, which are of still higher dimension, exist and are unknown in the fluid state. 

Fracture often determines the reliability of a material in its practical applications. Brittle fracture of a material is the reason for a sudden catastrophe. The mechanical property ductile or brittle determines, in essence, whether or not a tool can be made from a given material. Let us identify the imperfections of a crystal and the chemical processes which cause ductility and brittleness. We distinguish two limiting cases of failure 1) A crystal, under external stress, deforms by forming a narrowing neck until eventually ductile rupture occurs. Dislocations are the only imperfections involved in this process of failure. 2) Crystals fracture suddenly. A sharp crack propagates and causes the failure. 

By metallurgists in terms of the mechanical properties, such as modulus, fracture toughness, ultimate tensile strength. And they came up with a theory that deals with dislocation, fracture mechanic and continuum mechanics. 

Benlahsen, M., Lepinoux, L. and Grilhe, J. (1993), Image forces on dislocations the elastic modulus effect , Materials Science and Engineering, A164, 428 132. Bolshakov, A., Oliver, W.C. and Pharr, G.M. (1996), Influences of stress on the measurement of mechanical properties using nanoindentation Part II. Finite element simulations , Journal of Materials Research, 11, 760-768. 

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