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CHAPTER 1

INTRODUCTION
1.1 Motivation
Materials with small grain sizes have been the focus of attention for scientists and engineers since it has been proved by the work of Hall [Hall51] and Petch [Pet53] that strength increases as we decrease the grain size of the material. This finer grain size also helps in achieving superplastic deformation at higher strain rates and improves the fracture toughness of the material. Some of the traditional methods involving mechanical processes lead to a grain size above 10 µm. In present scenario, some of the techniques also lead to a grain refinement to submicron or nano range like ball milling, vapour deposition etc. But the techniques are not appropriate for large scale applications as they …show more content…
1.4.1.2 Edge dislocation

An edge dislocation moves in response to a shear stress applied in a direction perpendicular to its line;
Figure - 1.4: Schematics of Edge dislocation
The tendency by which the plastic deformation is induced via dislocation motion is known as slip. The dislocation line propagates along the crystallographic plane which is termed as slip plane (Figure -1.4). Movement of dislocation leads to the permanent plastic deformation, in response to applied shear stresses.
1.4.1.3 Screw dislocation

The motion of a screw dislocation in response to the applied shear stress is shown in Figure 1.5; the direction of movement is perpendicular to the stress direction. For an edge, motion is parallel to the shear stress.
Figure - 1.5: Schematics of Screw dislocation
Basically, all strengthening techniques based on this simple principle: restricting or hindering dislocation motion renders a material harder and stronger.

The mechanism for crystalline material includes:

1. Work hardening or strain hardening

2. Grain boundary straightening

3. Solid solution straightening

4. Particle/precipitate …show more content…
When the large numbers of dislocations get accumulated near the grain boundary, dislocation 'pile up' occurs. These pile up causes dislocation diffusion across the grain boundary, allowing further deformation in the material. Now the larger grain brakes into two or more parts to reduce the additional pile up across the grain boundary. More grains in the structure means more number of grain boundaries, therefore, these additional grain boundaries itself act as a barrier for the movement of dislocations and ultimately the yield strength of material increases. Thus, there is then an inverse relationship between grain size and yield strength, as demonstrated by the Hall–Petch equation. Eq.