Metal Alloys

• A substance that posses metallic properties and is

composed of two or more elements of which at least one

is metal, is called alloy.

• The metal present in the alloy in largest proportion is

called the base metal.

• All other elements present in the form of metal (or) non-

metal are called alloying elements.

• Each constituents of an alloy is called a component.

Lecture - 3

Alloys

Steel/cast iron (Alloy of iron and Carbon)

Bronze ( Alloy of copper and tin)

Brass (Alloy of copper and zinc)

Aluminum alloys (Duralumin-Alloy of Al, Mg and Mn)

Nickel base alloys

Magnesium base alloys

Titanium alloys

Examples

Solidification of an Alloys and cooling curves

• Graphical representation of temperature with time, when the material is cooled down from its liquid state, is known as cooling curve.

• It helps in studying the important structural changes that takes place during cooling or solidification of metal and alloys.

Cooling Curve of a Pure Metal

Under equilibrium conditions, all metals

exhibit a definite melting or freezing point. If a

cooling curve is plotted for a pure metal. It will

show a horizontal line at the melting or freezing

temperature.

Cooling Curve of a Solid SolutionA solid solution is a solution in the solid state and consists of two kinds of atoms combined in one type of space lattice.

A solution is composed of two parts: a solute and a solvent.

The solute is the minor part of the solution or the material which is dissolved, while the solvent constitutes the major portion of the solution.

When solidification of the solution starts, the temperature may be higher or lower than the freezing point of the pure solvent. Most solid solutions solidify over a range in temperature.

Fig. Cooling curve for a solid solution.

Cooling Curve of alloy

All pure substances show

similar behavior: with alloys,

there is no single temperature

‘plateau’, and the range over

which solidification occurs

depends on the proportions of

the elements in the alloy.

The points marked ‘a’ and ‘b’ are referred to as ‘arrest

points’, and the temperatures at which they occur as the

‘arrest temperatures’.

A solidified alloy may exist in the following three forms.

i.As a solid solution in which the atoms of the alloying

element may be selectively soluble in the atoms of

base metal.

ii. As a intermetallic compound in which the atoms

of the alloying element may form a compound

with the atoms of base metal.

iii. As a mechanical mixture in which the atoms of

the alloying element may be relatively insoluble in

the atoms of base metal.

Types of alloys

Solid solution alloys

A solid solution occurs when we alloy two metals and they are completely soluble in each other. If a solid solution alloy is viewed under a microscope only one type of crystal can be seen just like a pure metal.

Solid solution alloys have similar properties to pure metals but with greater strength but are not as good as electrical conductors.

The usual forms of solid solution are. 1.Substitutional Solid solution2. Interstitial solid solution

If the atoms of the solvent metal and solute element are of

similar sizes (not more, than 15% difference), they form

substitution solid solution, where part of the solvent atoms

are substituted by atoms of the alloying element.

Substitutional solid solution

A solid solution in which solute

atoms replace some solvent

atoms so that they lie at normal

atom sites of the crystal

structure are as substitutional

solid solution.

Interstitial solid solution

A solid solution in which solute atoms are located in the

interstices (or voids) of the solvent lattice is called

interstitial solid solution.

If the atoms of the alloying elements are considerably

smaller, than the atoms of the matrix metal, interstitial solid

solution forms, where the matrix solute atoms are located in

the spaces between large solvent atoms.

 Intermetallic compounds

An intermetallic compound is one

consisting of two or more metallic

elements present in definite proportions

in alloy.

Mechanical mixture

When a liquid solution is cooled, crystals of all

its components separate from simultaneously

from the melt and form a close mechanical

mixture.

also known as eutectic mixture.

In mechanical mixture, metal retain their

structural identity and properties.

Phase diagram is a chart, which shows the

number of nature of phases that are present in a

given alloy at any temperature and compositions

under equilibrium condition.

It is also known as constitution diagram or

equilibrium diagram.

Phase diagram

Phase: It is a physically and chemically

homogeneous portion of a system, separated

from other portions by a surface, the interface.

                                                                                                

Three parameters required to define the constitution of an alloy: • Phases present • Composition of each phase • Proportion of each phase Equilibrium diagram indicates.i. Temperature at which the solid alloy will

start and finish meltingii. Possible phase change, which will occur

as the result of altering the composition or temperature

Terms in phase diagram1. System

2. Components

3. Phase

4. Lines of equilibrium

5. Solidus

6. Liquidus

System- A system means the whole complex of

phases of one or several components at different

pressures and compositions. It may be composed

of solids, liquids, gases or their combinations. It

may have metals or non metals separately or in any

combination.

Component- Components are the substances

(chemical element or compounds) whose presence

is necessary and sufficient to make up a system.

• Lines of equilibrium or phase boundaries refer to the lines that demarcate where phase transitions occur.

• The solidus is the temperature below which the substance is stable in the solid state.

• The liquidus is the temperature above which the substance is stable in a liquid state.