chapter one 1 jf302

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Diploma in Mechanical Engineering (Material)

JF 302 MATERIAL TECHNOLOGY 1

www.pis.edu.my

peneraju ilmu sejagat

Week 3


CLO 1 : Explain various types of materials used in

manufacturing industry.


Understand the materials structure

Understand the Element Periodic Table

Explain the crystal structure

Explain the primary inter-atomic bonds

Describe metal and alloy solidification

Define solid solution formation

Explain the equilibrium phase diagram


materials structure Element Periodic Table crystal structure primary bonds

metal and alloy solidification solid solution formation equilibrium phase diagram

Three different types of primary inter-atomic bond are found in solids which are:-

i. Ionic bond

ii. Covalent bond

iii. Metallic bond

The primary bond is stronger and stable.

However, van der Waals or secondary bond is weak in comparison to the primary bond.




i. Ionic Bond

It is always found in compounds that are composed of both metallic and non-

metallic elements.

Atoms of a metallic element easily give up their valence electrons to the non-metallic atoms.




Na Cl

Example: when atoms of sodium and chloride are brought together, the valence electron of the sodium is transferred to the chloride, producing a

positive sodium ion and negative chloride ion, which are attached to each

other electro-statically and form a stable compound.

2:8:1 2:8:7

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Formula unit: NaCl

Cl - Na+

2:8 2:8:8




Think critically 1.1

Lets do an example by combining

calcium and phosphorus:

Ca P 2:8:8:2 2:8:5

Solution




Properties of ionic compound: a. hard and brittle

b.held together by strong electrostatic forces

c. good insulators in solid form because there are no free electrons/ions

d. conduct electricity when it is dissolved or melted

e.high melting/boiling points

Examples of ionic compound: Magnesium Oxide MgO

Sodium Chloride NaCl





Why do Ionic compounds have high

melting/boiling point?

When opposite charges attracted, it will

form a strong ionic bond. Thus a lot of

heat/energy is needed to separate the

ions.





When do Ionic compounds conduct

electricity?

When it is in solid form, ionic

compound do not conduct electricity,

because the ions are tighly held in

place and unable to carry charges.

When molten, it can conduct electricity,

because the crystal lattice has broken

down and the ions can move freely.




ii. Covalent Bond

It is always found in compounds that are composed of two or more non-

metallic elements.

Stable electron configurations are assumed by the sharing of electrons between adjacent atoms.

Two atoms that are covalently bonded will each contribute electrons to the bond, and shared the valence electrons.

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Example: each chloride atom would like to gain an electron to form a stable electron configuration. This is done by sharing two of the valence electrons

between pair of chloride atoms, producing stable di-atomic molecules.

2:8:7 2:8:7




Two chloride atoms with outer shell only.

2:8:7 2:8:7




Chloride molecule Cl2

2:8:8 2:8:8




Properties of covalent compound: a. poor heat/electrical conductivity

b. bonds between atoms in the molecules very strong

c. forces between molecules very weak

d. covalent compounds are usually gases or liquids they have low melting/ boiling points

Examples of covalent compound: Water H2O

Methane CH4 Ammonia NH3 Hydrogen H2 Hydrogen Chloride HCl Oxygen O2





Why do Covalent compounds have

low melting/boiling point?

The forces of attraction between the

molecules in covalent compound are

much weaker because of its sharing

electrons.Thus less heat/energy is

needed to separate the molecules of

covalent compound.





When do Covalent compounds

conduct electricity? Conditions for conducting electricity is free

electrons that can move throughout the

structure and carry charge. In covalent

compounds, there are no free electrons.

Most of the covalent compounds that

dissolve in water form solutions have

uncaharged molecules. For example, sugar

is a covalent compound that dissolves in

water but it does not form ions. However,

acids form ions when they dissolve in water.

Thus they conduct electricity.

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iii. Metallic Bond

It is always found in metals and their alloys. It carries small numbers of valence electrons (probably one to three), which

are loosely held, so that they can easily be released to the cloud of electrons.

The remaining non-valence electrons and atomic nuclei form ion cores, which posses positive charge equal to the total valence electron negative charge per

atom.

Fig 1.5 Cloud of electrons




The valence electrons are free to move in cloud of electrons, thus conduct heat and electricity easily. Also, the delocalized sharing of free electrons make

it possible for the atoms to slide past each other when the metal is deformed

instead of fracturing like glass or brittle materials.

Properties of covalent compound:

Conduct heat/electricity Ductile/malleability High melting/boiling point

Fig 1.6 Block of Aluminium atoms





Why do Metallic compounds have

high melting/boiling point? Metals have high melting/boiling points

because of the strength of the metallic

bond. The strength of the metallic bond

depends on the number of electrons in

the delocalised sea of electrons. (More delocalised electrons results in a stronger

bond and a higher melting point) and

packing arrangement of the metal

atoms. (The more closely packed the

atoms are the stronger the bond is and the

higher the melting point.)




Metal is made up of only one element (in the

Periodic Table) which is

orderly arranged and

closely packed together.

For example iron and

steel.

Alloy is a mixture of metals, or metal and non-

metal. For example brass

which is made up of

copper and zinc, two different elements.

Fig 1.7 Types of metals and alloys




Solidification is a process where liquid metals solidify into solid metals. It can

be divided into two stages:

a. Nucleation

b. Growth

Fig 1.8 Cooling curve for the solidification of pure metal




Liquid metal cools from A to B.

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From B to C, the melt metal liberates latent heat

of fusion. Temperature remains constant.




The metal starts solidifying at B and at C the metal

is purely solid.




From C to D, the solid metal

cools and tends to reach room

temperature.




Nucleation is the beginning of a phase transformation. It is marked by the

appearance in the molten metal of tiny regions called nuclei which grow to solid

crystals until the transformation is completed.

Growth follows nucleation. It is a process to determine the final crystallographic

structure of the solid. Growth may be defined as the increase of the nucleus in

size.





How do the grain structures form?

Nucleus

formation Dendrite

formation

dendrite

formation growth

and the arm met

to form grain

boundaries

solidification end

by the existent of

the grain




Solid solution may be defined as a solution in the solid state which consists of

two kinds of atoms combined in one type of space lattice. Some examples of

solid solutions are:

a. Cu-Zn alloys (Brass)

b. Fe-C alloys (Steel)

c. Fe-Cr-Ni alloys (Stainless Steel)

Solid solutions occur in two distinct types, namely:

a. Substitutional solid solution

b. Interstitial solid solution

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Differentiate between substitutional solid

solution and interstitial solid solution.

Substitution of solute atoms enter the lattice

structure to take the positions normally occupied by

solvent atoms.

Black spot: solute atoms

White spot: solvent atoms

Solute atoms of small atomic radius fit into the

empty spaces or interstices of the lattice structure of

the solvent atoms.




Binary phase diagram is a phase diagram in which there are only two

components and a mixture of two metals (a binary alloy).

Fig 1.9 Binary phase diagram




Terminologies In Phase Diagram

a. Phase is s a region that differs in structure or composition from another

region.

b. Equilibrium Phase Diagram is a graphical representations of what phases

are present in a materials system at various temperatures, pressures and

compositions.

c. Composition is the percentage of certain materials contains purposely or

not added to another material. With this it can cause changes in phases, the

properties and the shape of the microstructures.

d. Liquidus is the temperature at which liquid starts to solidify under

equilibrium conditions.

e. Solidus is the temperature which all liquid has completely solidified.




1. What is the main differences between ionic, covalent and metallic bonding?

2. Offer an explanation as to why covalently bonded materials are generally less

dense than ionically or metallically bonded ones.

3. What is the solidification of an alloy?

4. Schematically describe substitutional and interstitial solid solution.

5. Define these terms as applied to equilibrium diagram: system, phase, component

and structural constituents.

chapter one 1 jf302

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