JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II MATERIAL SCIENCE AND METALLURGY Previous Question Paper (September - 2021 - R18)

Point defects are imperfections that extend only over a few atomic positions in a crystal lattice. The primary types include:

• Vacancy: An atom is missing from a regular lattice site. This occurs due to thermal vibrations or during solidification.
• Self-interstitial: An extra atom of the same type is crowded into an interstitial void space of the crystal lattice.
• Substitutional impurity: A foreign atom replaces a host atom at a lattice site.
• Interstitial impurity: A smaller foreign atom occupies the void space between host atoms.
• Frenkel defect: A pair consisting of a vacancy and an interstitial atom, common in ionic solids where a cation leaves its site for an interstitial position.
• Schottky defect: A pair of oppositely charged ion vacancies, maintaining charge neutrality in ionic crystals.

Dislocation strengthening, or strain hardening, is the process of increasing the yield strength of a metal by increasing its dislocation density through plastic deformation. As a metal is plastically deformed, the number of dislocations increases significantly due to dislocation multiplication.

The mechanism relies on dislocation-dislocation interactions:

1. As dislocation density increases, the average distance between dislocations decreases.
2. Dislocations moving through the lattice encounter stress fields from other stationary or moving dislocations.
3. These stress fields act as obstacles, impeding the motion of dislocations.
4. Since plastic deformation is the result of dislocation motion, hindering this motion requires a higher applied stress to continue deformation, thereby increasing the yield strength of the material.

The relationship between yield strength and dislocation density is expressed as:

Where σy is the yield stress, σ0 is the friction stress, α is a constant, G is the shear modulus, b is the Burgers vector, and ρ is the dislocation density.