Question : With the help of neat sketch explain different types of  crystal structure.

Answer: 

A. Crystal Structure : 

 Crystal Structure

1. The crystal structure is formed by associating every lattice point with an assembly of atoms or molecules or ions, which are identical in composition, arrangement and orientation.
2. Crystals have the three types of unit cells which are mostly being used: 

a .Body Centred Cubic Structure (BCC):

Body Centred Cubic Structure (BCC)

1. BCC structure has atoms at its each corner and one atom in its center. So, the BCC consists of net total of two atoms.   Total atoms in BCC = (8/1)*8+1 =2 atoms
2. The coordination number of BCC arrangement is 8 and packing factor is 0.68.
3. The BCC structure can be generally seen in Lithium, Potassium, Sodium etc.

b. Face Centered Cubic Structure (FCC):

1. It consists of atoms at its each corner and one atom at center of each face. 

Classify the crystal imperfections and give their brief introduction.

Que . Classify the crystal imperfections and give their brief introduction.

Answer : 

All defects and imperfections in crystals can be classified in following types:

a.       Point Imperfections:

Point Imperfections:

1. 1 . Point defects are where an atom is missing or is in an irregular place in the lattice structure.
2. 2 . These defects are completely local in effect.
3. 3. Point imperfections are always present in crystals and their presence results in a decrease in the free energy.
4. 4. The number of defects at equilibrium concentration at a certain temperature can be computed as,

Where     N=Ne-EdikT

                                         n = Number of imperfections,

                                                     N = Number of atomic sites per mole,

                                   k = Boltzmann's constant,

                                                                             Ed = The free energy required to form the defect, and

                                   T  = Absolute temperature.

b.      Line Imperfection or Dislocations:

Line Imperfection or Dislocations

1. 1.   A linear disturbance of the atomic arrangement, which can very easily occur on the slip plane       through the crystal, is known as dislocation.
2. 2.   Dislocation is a two-dimensional line defect and is a very important crystal imperfection. It is        responsible for the deformation of metals by
3.       slip phenomenon.
4. 3.   It may also be concluded that it is region of localized lattice disturbances separating the                  slipped and unslipped regions of a crystal.
5. 4.   These are formed in the process of solidification of metals and mainly in their plastic                    deformation of strain hardening, yield point, creep and fatigue and brittle fractures.
6. 5. Causes of dislocation are:

•     a. Thermal stresses or external stresses causing plastic flow.
•     b. Crystal growth.
•     c . Phase transformation.
• d. Segregation of solute atoms causing mismatches.

1. 6. There are two types of dislocation:
2.      a. Edge dislocations, and
3.      b. Screw dislocations.
4. 
   
5. 7.    These dislocations are responsible for the useful property of ductility in metals, ceramic and        polymers.

C. Surface and Grain Boundary Imperfections:

Surface and Grain Boundary Imperfections:

1. 1.   These defects are two-dimensional and are due to a change in the stacking of atomic planes            on  or across a boundary, twin boundary, stacking faults,
2.       etc.
3. 2.   The external surface of the material is an imperfection itself because the atomic bonds do not        extend beyond it.
4. 3.  Since these surface atoms are not entirely surrounded by other , they possess higher energy           than internal atoms.
5. 4.  Surface atoms have neighbours on only one side while atoms inside the crystal have neighbour on both sides.

Explain the properties and use of types of stainless steels commonly used in industries. Explain how the "stainless" properties are obtained in these materials.

 Que 1.35. Explain the properties and use of types of stainless steels commonly used in industries. Explain how the "stainless" properties are obtained in these materials.