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Brief description of five methods to increase the strength of materials

1.Work hardening - It is the strengthening of a metal or polymer by plastic deformation. This occurs because of dislocation movements and dislocation generation within the crystal structure of the material. Many non-brittle metals with high melting points and various polymers can be strengthened by this method. Alloys not amenable to thermal hardening, such as low-carbon steel, are work-hardened. In fact, certain metals like such as pure copper and aluminum can only be strengthened via work hardening

2.Alloying or solid solution strengthening -The addition of solute atoms of one element to another results in either substitutional or interstitial point defects in the crystal. The solute atoms cause lattice distortions that impede dislocation motion so as to enhance the yield stress of the material. Solute atoms have stress fields around them which can interact with those of dislocations. The presence of solute atoms impart compressive or tensile stresses to the lattice, depending on solute size which interfere with nearby dislocations, causing the solute atoms to act as potential barriers. In general, the solid solution strengthening depends on the concentration of the solute atoms, shear modulus of the solute atoms, size of solute atoms, valency of solute atoms and the symmetry of the solute stress field.

3.Precipitation Strengthening -For many binary systems, alloying above a concentration given by the phase diagram will cause the formation of a second phase. A second phase can also be created by mechanical or thermal treatments. The particles that compose the second phase precipitates act as pinning points. If the precipitate atoms are small, the dislocations would cut through them. As a result, new surfaces of the particle would get exposed to the matrix and the particle-matrix interfacial energy would increase. For larger precipitate particles, looping or bowing of the dislocations would occur and result in dislocations getting longer.

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4.Grain Boundary Strengthening-In a polycrystalline metal, grain size influences on the mechanical properties. Because grains usually have varying crystallographic orientations, grain boundaries arise and slip motion will ensue during deformation,. Grain boundaries act as an impediment to dislocation motion. The stress required to move a dislocation from one grain to another in order to plastically deform a material depends on the grain size in such a way that average number of dislocations per grain decreases with average grain size. A lower number of dislocations per grain results in lower dislocation pressure building up at grain boundaries. This makes it more difficult for dislocations to move into adjacent grains and renders a higher yield strength to the material.

5.Dispersion strengthening- It is a type of particulate strengthening in which incoherent precipitates attract and pin dislocations. These particles are typically larger of larger order of size. The effect of dispersion strengthening is effective at high temperatures whereas precipitation strengthening described in step 3 above is typically limited to temperatures much lower than the melting temperature of the material.

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