Strengthening mechanism

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. 

At critical stress, two adjacent bowed segments joined, leaving a dislocation shear loop around the particles. The increase in flow stress to bypass particles of interparticle spacing H is expressed as (Dieter, 1961) 

particle shape, size, and distribution affect the strengthening process of the MMCs (Dieter, 1961 Kumar, 2009 Cheng et al., 2001 Yu, 2010 Suresh et al., 2007 Rao et al., 2012 Goh et al., 2008). For example, in the case of pulsed electric current sintered CU-AI2O3 (5 vol%) composite, there is an increase in yield stress by 370 MPa due to grained boundary strengthening compared with the one without any particles, which is the case for pure Cu (ZeinEddine et al., 2013). 

In the case of direct strengthening, the load from the matrix gets directly transferred to the reinforcement. Because the reinforcement is stronger than the matrix, it increases 

The crystalline nature of the lattice itself inhibits the dislocation motion. This is called lattice friction or Peierls stress. The Peierls stress for FCC metal is lO GPa, for HCP metal it is 5 x 10 GPa, and for BCC metal it is approximately 0.1 — 1 GPa. This also indicates why some types of nanoparticles are getting more attention compared with others in the MMCs (Cao et al., 2007). 

The strength of a material is related to its defect structure as well as to its bond energy. As stated previously, dislocations can begin to move through the grains when the resolved shear stress reaches or exceeds the critical resolved shear stress. The primary strengthening mechanism then involves inhibiting the motion of dislocations. This can be accomplished in a variety of ways as will be shown. 

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. 

A dramatic example of work hardening can be demonstrated by taking a piece of soft copper wire, such as a short length of 8 groimding wire. Bend it sharply in half and try to straighten it. Continued flexing will cause the wire to become harder and brittle imtil it eventually breaks from fatigue. 

Plastic deformation of metals is mainly determined by the mobility of dislocations. To design engineering materials with high strength, dislocation movement has to be impeded. In this section, we want to discuss possible mechanisms to do this by different obstacles and to see what amount of strengthening (or hardening, as it is also called) can be achieved. 

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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. 

Table 2. Strengthening Mechanisms in High Temperature Alloys...

Because pure aluminum is n picaUy too soft to be drawn into a fine wine, it is often alloyed with 1° o sihcon or 1° o magnesium to provide a sofid solution-strengthening mechanism. The resistance of Al-1° o Mg wine to fatigue failure and to degradation of ultimate strength after exposure to elevated temperatures is superior to that of Al—1° o Si wine. 

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. 

To strengthen mechanical properties and vary the transition temperature, hydrophobic monomers such as BMA were introduced into poly(IPAAm). The unique swelling properties with varying BMA composition are shown in Figure 11. 

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. 

TABLE 2. STRENGTHENING MECHANISMS IN HIGH TEMPERATURE ALLOYS... 

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. 

Fig. 8.2. Representative data associated with four of the key strengthening mechanisms within a material, (a) Increase in flow stress as a function of concentration of substitutional impurities, (b) Dependence of flow stress on mean particle size for material in which there are second-phase particles, (c) Dependence of yield stress on mean grain size of material, (d) Relation between yield stress and mean dislocation density. (Adapted from (a) Neuhauser and Schwink (1993), (b) Reppich (1993), (c) Hansen (1985), (d) Basinski and Basinski (1979).)...

From a mechanistic perspective, what transpires in the context of all of these strengthening mechanisms when viewed from the microstructural level is the creation of obstacles to dislocation motion. These obstacles provide an additional resisting force above and beyond the intrinsic lattice friction (i.e. Peierls stress) and are revealed macroscopically through a larger flow stress than would be observed in the absence of such mechanisms. Our aim in this section is to examine how such disorder offers obstacles to the motion of dislocations, to review the phenomenology of particular mechanisms, and then to uncover the ways in which they can be understood on the basis of dislocation theory. 

As noted above, our working hypothesis concerning the various hardening mechanisms is that chemical impurities, second-phase particles and even other dislocations serve as obstacles to the motion of a given dislocation. As a result of the presence of these obstacles, the intrinsic lattice resistance tp is supplemented by additional terms related to the various strengthening mechanisms. We further assume that the flow stress can be written as... 

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. 

Embury J. D., Lloyd D. J. and Ramachandran T. R., Strengthening Mechanisms in Aluminum... 

The presence of infiltrated epoxy has imparted significant increases in strength, hardness, elastic modulus, and toughness. The improvement in these properties can be attributed to the pore-filling effect and the concomitant reduction in the overall residual porosity. Porosity has been established by various workers to have a profound influence on hardness, strength, and elastic modulus [10]. This identical strengthening mechanism has also been observed for epoxy-modified HTSC materials [7],... 

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