Question

List, draw and explain the strengthening mechanisms for metals. Why we are using this techniques?

Answer 1

1.Work hardening

The primary reason for work hardening are dislocations. Dislocations interact with each other by generating stress fields. These interaction could be repulsive or attractive in nature. Additionally, if two dislocations cross, dislocation line entanglement occurs which opposes dislocation motion. These entanglements act as pinning points, which oppose dislocation motion. On increasing the dislocation density the yeild strength increases.

2 Alloying:

For this strengthening mechanism, solute atoms of one element are added to another, resulting in either substitutional or interstitial point defects in the crystal (figure1 in attached file). This solute atoms cause lattice distortions that impede dislocation motion, which increases the yield stress of the material.But there is a limit to the amount of solute that can be added, and one should look at the phase diagram for the material and the alloy to make sure that another phase is not created.

3.Grain boundary strengthening

In a polycrystalline metal, grain size has a tremendous influence on the mechanical properties. Because grains usually have varying crystallographic orientations, grain boundaries arise. While undergoing deformation, slip motion will take place. Grain boundaries act as an restriction to dislocation motion due to change in the direction of motion because of different orientation of grains( please refer fig 2 for image)

4.Precipitation hardening

In most binary systems, when we add above a a concentration given by the phase diagram it will cause the formation of a second phase. However a second phase can also be created by mechanical or thermal treatments. The particles that compose the second phase precipitates act as pinning points in a similar manner to solutes, though the particles are not necessarily single atoms(please refer to fig 3).