JNTU B.Tech II MATERIAL SCIENCE AND METALLURGY Previous Question Paper (March - 2021 - R18)

A unit cell is the smallest repeating structural unit of a crystal lattice that, when stacked in three-dimensional space, reproduces the entire crystal structure. It is defined by its lattice parameters, including the lengths of its edges and the angles between them.

The three basic cubic crystal structures are:

• Simple Cubic (S.C.): Atoms are located only at the eight corners of the cube. Each atom is shared by eight adjacent unit cells, resulting in an effective number of 1 atom per unit cell. The atomic packing factor is approximately 0.52.
• Body-Centered Cubic (B.C.C.): Atoms are located at the eight corners of the cube plus one atom at the center of the body. The effective number of atoms is 2. This structure is common in metals like iron and tungsten.
• Face-Centered Cubic (F.C.C.): Atoms are located at the eight corners of the cube and at the center of each of the six faces. The effective number of atoms is 4. This structure is found in ductile metals like copper, aluminum, and gold.

Dislocations are linear crystal defects that significantly influence the mechanical properties of metals, particularly their plastic deformation. The two primary types are:

• Edge Dislocation: This is formed by the insertion of an extra half-plane of atoms into the crystal lattice. The dislocation line runs along the edge of this extra plane. Above the slip plane, the atoms are in compression, while below it, they are in tension. The Burgers vector is perpendicular to the dislocation line.
• Screw Dislocation: This is formed by a shear stress applied to a crystal, causing the crystal lattice to distort in a spiral or helical ramp-like arrangement around the dislocation line. In this case, the Burgers vector is parallel to the dislocation line.

These dislocations allow for plastic deformation at stresses much lower than the theoretical strength of a perfect crystal by moving through the lattice via slip.