Strengthening Mechanisms

Anything done to the crystal to impede dislocation movement should cause strengthening .There are generally four methods to increase the strength of metal as following:

• Work (Strain) Hardening: When in the crystal more than one slip is activated, and then dislocation intersections will occur. This generates a complex stress state at their junction and their motion is impeded. In a polycrystalline sample, some crystals statistically will be oriented for multiple slip therefor more and more dislocation interaction occurs. Therefore an increasing stress is required to continue dislocation movement.
• Grain Size Effect: When a dislocation moving on the slip plane encounters a grain boundary, movement is retarded because of the orientation change. To move another dislocation on the same plane toward the boundary requires an increase in stress because two dislocations of like sign repel each other.
• Particle Effect: If hard particles are present in a softer matrix they hinder dislocation motion. If the stress becomes sufficiently high the dislocation may cut through the particle, or the dislocation may move out of the slip plane onto another and climb around the particle.
• Solid Solution Effect: Most pure metals can be strengthened by the addition of solute atoms, either substitutionally or interstitially. There are several reasons for this, but most important and obvious one is that the strain energy of the crystal will be reduced if the solute atom is located along the core of the dislocation. Thus, it requires more stress to move the dislocation away from the dislocation than the pure matrix.