Solids strengthening mechanisms

Dieter, G.E., Hardening Effect Produced with Shock Waves, in Strengthening Mechanisms in Solids, American Society of Metals, Metals Park, Ohio, 1962, pp. 279-340. 

Dislocations multiply in a facile manner during a plastic deformation process, and several mechanisms for this have been observed by electron miscroscopy. Dislocations are destroyed by the processes of recovery and recrystallization during annealing after plastic deformation. Since dislocations cause low-yield stresses in metals and other solids, solid strengthening is accomplished either by eliminating dislocations or by immobilizing them. 

Other strengthening mechanisms include solid solution formation and strain hardening. Solid solution strengthening involves replacing a small number of atoms in the lattice with substitutional impurities of a slightly different size. This creates strain in the crystal. 

The active strengthening mechanism at room temperature of the studied material was identified as solid solution strengthening, grain boundary strengthening, and Al3Zr precipitate strengthening. 

Nanoindentation tests of the samples show that hardness of (Ti,Zr)N films (Fig. 3) increases in comparison with TiN and ZrN films approaching to the hardness of boron carbide B4C (40GPa). The effect of increased hardness is probably due to a solid-solution strengthening mechanism. 

The values of obtained from Eq. (6.4) are usually found to be less than the experimental yield strength values in many materials and, thus, one concludes these materials must contain strengthening mechanisms. The frictional stress is clearly very sensitive to the dislocation width and, thus, it is important to identify the material properties that govern this parameter. Dislocation width is governed primarily by the nature of the atomic bonding and crystal structure. In covalent solids, the bonding is strong and directional and, hence, dislocations are very narrow (w b). In ionic solids and bcc metals, the dislocations are moderately narrow whereas in fee metals dislocations are wide (w>106). 

Volume defects consist of inclusions or precipitates of a second phase material or voids. Voids can be formed by vacancy clusters or from the nucleation of bubbles from dissolved gases or from components with high vapor pressures. Such defects can range in size from microscopic to gross. Bear in mind that not all such defects are unwanted. Many are purposely introduced into the final solid to tailor certain electrical, optical, and magnetic properties, or to serve as strengthening mechanisms. These topics are discussed in later chapters. 

Alloying the metals with other components offers other strengthening mechanisms. Adding a second metal as substitutional atoms in the lattice is known as solid solution hardening. The size difference between the solvent (host) atoms and the solute (impurity) atoms strains the lattice and makes it difficult for dislocations to move. Adding smaller atoms that can go into the interstices produces a similar hardening in the lattice. This is the role that carbon plays in the strengthening of steel. 

Properties Solid solutions, dislocations V Mechanical properties Y Dislocations, slip sterns, strengthening mechanisms Y Phase equilibria, the iron-iron carbide phase diagram Mechanical properties of Fe-C alloys Y ... 

Describe in your own words the three strengthening mechanisms discussed in this chapter (i.e., grain size reduction, solid-solution strengthening, and strain hardening). Explain how dislocations are involved in each of the strengthening techniques. 

Frenkel S computed the force required to shear two planes of atoms past each other in a perfect crystal and showed that the critical yield stress (or elastic limit) is of the order G/ln, where G is the shear modulus. Experimental values of the elastic limit are 1(X)-1(K)0 times smaller than the above estimate. By considering the form of the interatomic forces and other configurations of mechanical stability, the theoretical shear strength could be reduced to G/30, still well above the observed values in ordinary materials. It is now firmly established that crystalline imperfections, such as dislocations, microscopic cracks, and surface irregularities, are primarily the reasons for the observed mechanical weakness of crystalline solids. This aspect of the mechanical behavior of solids, including a discussion of strengthening mechanisms, is discussed in Volume 2, Chapter 7. In this section the chemical and structural aspects of mechanical behavior, i.e., bonding and crystal structure, are emphasized. 

R. Hill, Theory of Mechanical Properties of Fibre-Strengthened Materials - III. Self-Consistent Model, Journal of the Mechanics and Physics of Solids, August 1965, pp. 189-198. 

Despite the vast number of reported syntheses, crystal structures, properties and applications of monomeric Pcs, it is still difficult to clarify clearly the synthetic mechanisms of Pcs in various conditions, to predict fully the supramolecular structures of Pcs in the solid state (except for a very few types of simple Pcs), and to elucidate completely the correlation between molecular structure and properties. The large-scale preparation and separation of some novel Pcs with interestingly properties is still very hard. On the basis of the great potential applications of Pcs in high-tech fields, exploitation of multi-functional Pc materials needs to be strengthened in the future. There is still plenty of room for further investigation of Pc chemistry. 

Precipitates have important effects on the mechanical, electronic, and optical properties of solids. Precipitation hardening is an important process used to strengthen metal alloys. In this technique, precipitates are induced to form in the alloy matrix by carefully controlled heat treatment. These precipitates interfere with dislocation movement and have the effect of hardening the alloy significantly. 

Mickel-llase Superalloys. The nickel-basc superalloys are Ihe most complex in composition and microslrueuirex and. in must respects, the most successful high temperature alloys. The earliest superalloys were wrought, ie fabricated to final size by a mechanical working operation. Later alloys have incorporated higher aluminum plus tilanium contents, as well as molybdenum lor solid-solution strengthening (Nimonics 115 and 1201. 

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