JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II MATERIAL SCIENCE AND METALLURGY Previous Question Paper (March - 2022 - R18)

The Atomic Packing Factor (APF) is defined as the ratio of the volume of atoms in a unit cell to the total volume of the unit cell.

For a BCC (Body-Centered Cubic) structure, there are 2 atoms per unit cell. The atoms touch along the body diagonal, where the relationship between the lattice parameter a and atomic radius R is 4R = a√3. The APF is calculated as:

For an FCC (Face-Centered Cubic) structure, there are 4 atoms per unit cell. The atoms touch along the face diagonal, where 4R = a√2. The APF is calculated as:

Imperfections in solids, or crystal defects, are deviations from the perfect periodic arrangement of atoms in a crystal lattice. These are classified based on their dimensionality into point, line, surface, and volume defects.

1. Point Defects: These are zero-dimensional defects involving one or a few atoms. Examples include vacancies (a missing atom from a lattice site) and interstitials (an extra atom occupying a space between normal lattice sites). These defects are often thermally activated and influence diffusion and electrical conductivity.

2. Line Defects (Dislocations): These are one-dimensional defects that represent a misalignment of atoms along a line. The two primary types are edge dislocations, where an extra half-plane of atoms is inserted into the lattice, and screw dislocations, which result from a shear stress causing a spiral ramp of atoms around the dislocation line. These defects are critical in determining the mechanical properties of metals, specifically their ability to undergo plastic deformation.