Strengthening mechanisms hardening

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. 

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. 

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... 

Strengthening Mechanism PH precipitation hardening SH solution hardening CW cold working... 

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. 

Small particles of a second phase, evenly distributed in the grains of the first phase, form a strong barrier to dislocation motion. This was previously discussed in section 6.3, and we saw there that there are two possible ways to overcome such obstacles, the Orowan mechanism and cutting of the particles. The mechanism actually occurring depends on the strength of the obstacles and on their distance. This strengthening mechanism is frequently called precipitation hardening, because the particles are usually created by a precipitation process, described below. 

Surveying the strengthening mechanisms discussed in chapter 6, we see that grain boundary strengthening is not suitable in creep applications because we need large grains as explained above. Work hardening can also not... 

Strengthening mechanisms Solution hardening (Mo) + precipitate hardening M23C6 Precipitate hardening carbonitrides TiC, TiN, M23C6... 

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. 

In summary, we have discussed the three mechanisms that may be used to strengthen and harden single-phase metal alloys strengthening by grain size reduction, solid-solution strengthening, and strain hardening. Of course, they may be used in conjunction with one another for example, a solid-solution strengthened alloy may also be strain hardened. 

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