Metal strengthening mechanisms

A hardness indentation causes both elastic and plastic deformations which activate certain strengthening mechanisms in metals. Dislocations created by the deformation result in strain hardening of metals. Thus the indentation hardness test, which is a measure of resistance to deformation, is affected by the rate of strain hardening. 

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. 

E. V. Clougherty, D. Kalish, in Strengthening Mechanisms. Metals and Ceramics, Syracuse Univ. Press, Syracuse, 1966, p. 431. 

Webb, W. W., H. D. Bartha, and T. B. Shaffer (1966). Strength eharacteristics of whisker crystals, macrocrystals and microcrystals, pp. 329-354. In Strengthening Mechanisms, Metals and Ceramics. Burke, J. J., N. L. Reed, and V. Weiss, Eds. Syracuse University Press, New York. 

The Plastic Deformation of Metals by R. W. K. Honeycombe, Edward Arnold, London England, 1984. Honeycombe s book is especially appealing because of its emphasis on the metallurgical applications of dislocation theory to the various strengthening mechanisms. 

Figure 5. Influence of length scale on strengthening in nanoscale metals. The three length scales, D, d and < influence different strengthening mechanisms and can provide additive...

It is shown that superposition of external magnetic field on the melting zone makes it possible to control the depth and shape of the metal pool. Mechanical properties of a new class of titanium alloys with an intermetallic strengthening, produced by MEM method, are given. 

Metal products are made by distinct manufacturing routes, leading to specific product forms to suit end-user needs. Fundamental differences between wrought, cast, powder metal and weld filler metals have led to different specification systems for such products. Heat treatment is the most important method of strengthening metals although mechanical work is also used. See Figure 5-1. 

Knowledge of the ways in which solids fail can be exploited to improve the mechanical properties, particularly the strength, of solids. Pure metals tend to be soft and relatively weak. Ductility can be reduced and the metal strengthened by restricting dislocation movement. However, if this is continued too far the metal will become brittle. A compromise is often required. 

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). 

The term superalloy is used for a group of nickel-, iron-nickel-, and cobalt-based high-temperature materials for applications at temperatures > 540 °C. It is useful to compare the main subgroups in terms of the strengthening mechanisms applied and stress-rupture characteristics achieved, as shown in Fig. 3.1-127. In this section iron-nickel- and nickel-based superalloys are covered whereas cobalt-based superalloys are dealt with in Sect. 3.1.6.3. Nickel-based superalloys are among the most complex metallic materials with numerous alloying elements serving particular functions, as briefly outlined here. 

In general, the addition of nanoparticles into the metal matrix increases its strength. The main strengthening mechanisms of nanoparticle-reinforced MMCs are presented in the following sections. 

Cao, G., Chen, X., Kysar, J.W., Lee, D., Gan, Y.X., 2007. The mean free path of dislocations in nanoparticle and nanorod reinforced metal composites and implication for strengthening mechanisms. Mechanics Research Communications 34, 275—282. 

It is difficult for a dislocation to move past another dislocation, so one strengthening mechanism involves simply creating more dislocations. As the dislocations pile up on one another, it becomes very difficult for additional dislocations to move through the material and metal becomes much harder. One way to create more dislocations is to work harden the material by flexing, hammering, or cold rolling it. 

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. 

Methods for mechanical testing of materials are briefly introduced along with various strengthening mechanisms. The number and siu-face area of the slip systems in metals and in ceramics are shown to be responsible for the ductility (or the lack of it) and for ductile-to-brittle transitions. Griffith s theory of brittle fracture is used to introduce fracture mechanics and to develop the concept of fracture toughness. The viscoelastic behavior of polymers is briefly discussed. 

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