CHAPTER 1Part 1

MATERIAL STRUCTURE ANDBINARY ALLOY

SYSTEM

Atomic Structure

nucleus

Electron : charge magnitude : -1.60x10-19 C

Proton : + 1.60x10-19 C

Neutron : neutral, no charge

• Is the basic unit of matter.

• 3 basic particles of atoms are : ELECTRON, PROTON AND NEUTRON

Remember an atom will contain equal number of P and E !!

Element structure• Is the substance that is made up from one type of atom. • Cannot be broken down into any simpler substances by

chemical method.

• Has its own atomic mass.

• Has equal number of protons and number of electrons.

Example : Hydrogen is made up of one type of atom containing 1 proton and 1 electron.

MOLECULE AND COMPOUND

MOLECULE : 2 OR more atoms that are chemically joined together.Example : H2, O2 , H20

COMPOUND : contains more than 1 type element. ( at least 2 different elements)Example : H20 , CO2, NH3

Remember : All compounds are molecules but not all molecules are compounds!!

H2 O2 H20Molecules but not compound because each is composed of a single element

Molecule and compound

The Periodic Table• Is a tabular arrangement of chemical elements.

• Is organized on the basis of their atomic number,

electron configuration and recurring chemical

property.

• Elements are presented in order of increasing

atomic number ( number or proton)>> row

• Elements arrayed in similar electron structure

as well as chemical and physical properties >>

column

HOW TO FIND NUMBER OF PROTON ,

ELECTRON AND NEUTRON OF AN

ELEMENT ?

6

CCARBON12.011

 

 

Try this

CRYSTALLINE STRUCTURES• Atoms in crystalline solids : positioned in an order and repeated

pattern.

• The repetitive entities in such structure is called the unit cells.

• Most common metals exists in at least one of three relatively

simple crystal structures; which is

a) BCC – body centered cubic b) FCC – face-centered cubic

b)HCP – hexagonal close-packed

Example : Atomic Radii and Crystal

Structure for few metals

BCC Structure• Has cubic unit cell with atoms located at all 8 corners and a single atom at

the cube center

• Has two atoms per unit cell, one in the center and 1/8 of each of the 8 corner atoms.

• Lattice parameter @ length of unit cell can be calculated by using the following formula.

cellt2atoms/uni1)(18)8

1(atomsofNumber

a

a

R 3

4Ra

Where,

R = atomic radius

a = unit cell length

FCC Structure• Has atoms located at each of the corners and centers of all the cube

faces.

• Has 4 atoms per unit cell, one-half at each of six faces and 1/8 of

each of the 8 corner atoms.

• Lattice parameter @ length of unit cell can be calculated by using

the following formula , where, a=unit cell length and

R= atomic radius.

cellt4atoms/uni)2

1(68)

8

1(atomsofNumber

a

a

4R

R

22Ra

HCP Structure• Has a unit cell that is hexagonal.

• The top and bottom faces consists 6 atoms that form a regular

hexagons and surround a single atom in the center.

• Another plane consists 3 additional atoms which is situated in

the middle.

• Has 6 atoms per unit cell, 1/6 of each of the 12 top and bottom

face corner atoms, ½ of each of the center face atoms and 3

midplane interior atoms.

 

Where a and c represents the short and long unit cell dimensions respectively

Terms in Recrystallization Process

• Is the formation of a new set of strain-free grains

within a previously cold-worked material usually by an

annealing heat treatment.

• Crystal structure – the manner in which atoms/ions are

arrayed in space.

• Space lattice – the regular geometrical arrangement of

atoms

Lattice Not Lattice

Space Lattice and Crystal

Terms in Recrystallization Process

2 different orientations of grains

Grain Boundary

•Grain – an individual crystal in a polycrystalline metal or ceramic.

•Grain Boundary – the interface separating two adjoining grains

having different crystallographic orientations.

Atomic Bonding in Solids• Atomic bonding is the interatomic forces that bind the

atoms together.• There are primary and secondary bonds found in solids:

Primary / Chemical Bonding > stronger bonds 3 types of primary bond:-

Ionic Covalent Metallic

Secondary / Physical Bonding > weaker bonds 2 types of secondary bond:-

Van der Waals Hydrogen

Ionic Bonding• Electrically charged ions are formed by the transfer of

electron from one atom to another.

• Found in compounds composed of both metallic and

nonmetallic elements.

• Example : NaCl , MgO

• Characteristics :

stable, hard, brittle,

electrically and

thermally insulative.

Tendency to lose outermost electrons( donor > +ve ion)

Tendency to gain electrons

(acceptor > -ve ion)

Covalent Bonding• Involving sharing of electrons between adjacent atoms.

• 2 atoms that are covalently bonded will each contribute at least 1e

to the bond.

• Nonmetallic elemental molecules ( H2, Cl2, F2) and molecules

containing dissimilar atoms ( CH4, H2O, HNO3, HF) are covalently

bonded.

• Characteristics : Varies

Diamond ( strong, hard, high melting temperature)

Bismuth ( very weak, low melting temperature)

(Methane)

• Valence electrons form a “sea of electrons” that is uniformly dispersed

around the metal ion cores and act as a form of glue for them.

• Found in metals and their alloys. Ex: Fe (iron), W (tungsten)

• Characteristics:

Good electrical and thermal conductors due to their free valence electrons

Opaque neither transparent (allowing all light to pass through) nor

translucent (allowing some light to pass through)

Ductile (material's ability to deform under tensile stress(stretched into a

wire)

Metallic Bonding

Metal ions / Ion cores

Sea of electrons / Electron cloud

• Van der Waals resulted from attractive forces between electric dipoles. Weakest bond Exists between all atoms or molecules, inert gas (which has

stable e structure, and between covalently bonded molecules.

Ex : Ar, Cl2

• Hydrogen bonding Most common in covalent bonded molecules. Found to exist between some molecules that have hydrogen

as one of the constituents. Ex : NH3, H2O

Secondary Bonding

+ - + -

Atomic or molecular dipoles

Van der Waals bonding

between two dipoles

THE END