# INTRODUCTION TO STRUCTURE OF METALS AND ALLOYS

## Crystalline Materials:

• A crystalline material is one in which the atoms are situated in a repeating (or) periodic array over large atomic distances. ## Non Crystalline Materials:

• Materials that do not crystallize are called non-crystalline (or) Amorphous materials

## Space Lattice | Structure of Metals and Alloys

• Lattice is the regular geometrical arrangement of points in crystal space. • The atoms arrange themselves in distinct pattern in space is called a Space Lattice.
• Atoms in crystalline materials are arranged in a regular 3 – Dimensional repeating pattern known as Lattice Structure.
• They are divided by network of lines in to equal volumes, the points of intersection are known as Lattice Points.

## Unit Cell | Structure of Metals and Alloys • It is the smallest portion of the lattice which repeated in all directions.
• 3D visualization of 14 Space Lattices are known as Bravai’s Space Lattice.
• If a unit cell contains lattice points only at it’s corners, then it is called Primitive Unit Cell (or) Simple Unit Cell.
• Three edge length x,y, & z and three interaxial angles α, β, & γ are termed as Lattice Parameters.

## Crystal System For The Structure of Metals and Alloys

• It is a scheme by which crystal structures are classified according to unit cell geometry.

• Cubic
• Tetragonal
• Hexagonal
• Orthorhombic
• Rhombohedral
• Monoclinic
• Triclinic

### Crystal Systems # Simple Crystal Structure:

### Body Centered Cubic Structure (BCC)

• Unit cell contains 2 atoms
• Lattice Constant a= 4r / √3, where r is atomic radius
• Atomic packing factor APF = 0.68
• Metals are Vanadium, Molybdenum, Titanium, Tungsten ### Face Centered Cubic (FCC)

• Unit cell contains 4 atoms
• Lattice Constant a= 4r / √2, where r is atomic radius
• Atomic packing factor APF = 0.72
• FCC structures can be plastic deformed at severe rates
• Metals are Copper, Aluminum, Phosphorous, Nickel, Cobalt etc ### Hexagonal Closed Packed Structure (HCP)

• Unit cell contains 3 atoms
• Axial ratio c/a, where ‘c’ is Distance between base planes, ‘a’ is Width of Hexagon
• Axial Ratio varies from 1.58 for Beryllium to 1.88 for Cadmium (Therefore  a=2.9787, c=5.617)
• Atomic packing factor APF = 0.74
• Metals are Zinc, Cadmium, Beryllium, Magnesium etc   ## Crystallographic Planes and Directions

The Layers of atoms in the planes along which atoms are arranged is known as “Atomic” (or) “Crystallographic planes”.

## Miller Indices:

Miller Indices is a system of notation that denotes the orientation of the faces of a crystal and the planes and directions of atoms within that crystal.

## Miller Indices for Planes:

### 1. The (110) surface Intercepts :   a , a , ∞

Fractional intercepts :   1 , 1 , ∞

Miller Indices :   (110)

### 2. The (111) surface Intercepts :   a , a , a

Fractional intercepts :   1 , 1 , 1

Miller Indices :   (111)

The (100), (110) and (111) surfaces considered above are the so-called low index surfaces of a cubic crystal system.

### 3. The (210) surface Intercepts :   ½ a , a , ∞

Fractional intercepts :   ½ , 1 , ∞

Miller Indices :   (210)