Theory Of alloys

Alloys• Alloys are combinations or mixtures of elements.

• Metals are alloyed to improve on properties of pure

metals such as hardness, strength, corrosion resistance,

etc.

• Ex.:- Instead of pure aluminum an alloy of aluminum

having combination of Al –Zn –Mg – Cu – Mn(A five

element alloy ) is used to construct the air craft body.

• Alloys may behave differently when combined, some mix

easily while others will only be soluble to a limited extent.

Alloys An alloy system contains all the alloys that can be

formed by several elements combined in all possible proportions.

Binary Alloy System- If the system is made up of two elements it is called binary alloy system.

Ternary Alloy System- If the system is made up of three elements it is called ternary alloy system.

In each system large number of different alloys –Series of alloys possible.

Ex:- Carbon, Silicon, manganese, nickel, chromium, molybdenum, tungsten, vanadium, copper and Aluminum

Mixture Of Two Liquids:-When Alloy are formed solid

or liquid solutions can form.

Ex:Sugar added into water form an

aqueous Sugar solutionPhosphorus added into single crystal silicon they

form a solid solution.When two liquid metal are mixed in varying

proportion;I. Complete (unlimited)SolubilityII. Partial (limited) SolubilityIII. Complete insolubility

Base Metal

Alloying Element

Alloy

Complete (unlimited)Solubility

We add water + ethyl alcohol and stir them upon standstill only one phase appear.

This solution has unique properties and compositions. It appears like one phase homogeneous solutions.

Other examples :- Cu-Ni, Pt- Au, Ge-Si, and Ag-Au.

A solution is not a mixture. It contains solute and solvent dissolved into each other.

Solute:- Major portion Solvent:- Minor portion

Partial(limited)Solubility Here, each liquid is partially soluble in the other. Up to that

limit the solution which is formed is homogeneous solution.

When more Solute is added limit of solubility reached. Now two layer formedI. More Denser one at the bottom.II. Less Denser one at the top.

Ex:-Phenol + Water Solution. At top water with small amount of phenol dissolved into

it. At bottom Phenol With small amount of Water dissolved

into it.

Silicon and Germanium Doped with Phosphorus , Boron, Arsenic, Etc. To produce semi conducting materials.

Complete Insolubility Each Liquid is completely insoluble in the other. Mixture of two liquids always separate into two layers.

each liquid can not be dissolved into one another they can be dispersed into one another.

To complete insolubility,More dissimilar are the components, both chemically and atomic in size.

When oil and water are mixed together upper one is oil and lower one is water, according to their densities.

Ex:- A mixture of liquid Lead and liquid aluminum. A mixture of liquid Lead and liquid copper.

Mixture Of Two Liquids

SolidifyCrystalline separately solid

solutions

In the Solid solutions , The two components may be completely or partly soluble in

each other in solid state.

In such solid solutions the solute atoms distributes themselves throughout the solvent crystals randomly.

The crystal structure of the solvent being maintained.

Classification of Alloys. Alloys may be homogeneous (uniform) or mixtures.

If the alloy is homogeneous, it will consist of single phase, and if it is a mixture it will be a combination of several phases.

A phase is anything which is homogeneous and physically distinct.

Thus a phase is a region of space, throughout which all physical properties (density, tensile strength, etc.) of a material, its chemical composition and structure are uniform.

Classification of Alloys. In the solid state there are three possible phases:I. Pure metal II. Compound or intermediate alloy phase and III. Solid solution

Thus if an alloy is homogeneous (composed of a single phase) in the solid state, it can be only a solid solution or a compound.

If the alloy is a mixture, it is then composed of any combination of the phases available in the solid state. It may be mixture of two pure metals, or two solid solutions, or two compounds, or a pure metal and a solid solution, and so on.

A summary of possible alloy structures is shown in the figure given below.

Pure metal Characteristics of a pure metal are discussed in the

crystallization. Under equilibrium conditions, all metals show 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 point as shown in above figure.

Intermediate alloy phase or Compounds

In many binary alloy systems, when the chemical affinity of elements is great, their mutual solubility becomes limited and compounds (also called intermediate phases) are formed (rather than solid solutions).

Most ordinary chemical compounds are combination of +ve & -ve valence elements.

Expressed by chemical formula Ex:- H2O, NaCl, H2SO4 etc….

When compound is formed, the elements loose their individual identity and characteristic properties to a large extent.

Intermediate alloy phase or Compounds Example:-

Water (H2O) is composed of elements that are normally gases at room temperature, yet the compound is liquid at room temperature.

In NaCl, sodium is very active metal that oxidize very rapidly & so

it is stored in kerosene ,Chlorine is poisonous gas . But the combination of these two elements gives the element used as food.

Intermediate alloy phase or Compounds The most common intermediate alloy phases are:-I. Intermetallic Compounds or Valence Compounds II. Interstitial Compounds III. Electron Compounds

Intermetallic Compounds or Valence Compounds

They are generally formed between chemically dissimilar metals.

They usually show poor ductility and poor electrical conductivity and may have a complex crystal structure.

Examples:-Mg2Pb, Cu2Se, etc.

Intermediate alloy phase or Compounds Interstitial Compounds The word interstitial means between the spaces. They are formed between the transition metals such as Sc,

Ti, Ta, W and Fe with hydrogen, oxygen, carbon, boron and nitrogen.

Examples are Fe4N, Fe3C, W2C, CrN, etc.

Electron Compounds In alloys of copper, gold, silver, iron and nickel with the

metals cadmium, magnesium, tin, zinc, and aluminium, a number of compounds are formed.

Example – AgCd, Ag5Cd8 and AgCd3

They have a definite ratio of valence electrons to atoms and are therefore called electron compounds.

Intermediate alloy phase or Compounds Electron Compounds

Compound Electron-Atom Ratio

Crystal Structure

AgCd 3:2 Body centered cubic

Ag5Cd8 21:13 Complex cubic

AgCd3 7:4 Close packed hexagonal

Solid solution Solubility is the property of a solid, liquid or gaseous

chemical substance where solute (minor part) dissolves in a solvent (major part) to form a homogeneous solution.

The solvent is a chemical substance and can be in a solid, liquid or gaseous state. Thus solution can exist in a gaseous, liquid or solid state.

There are three possible conditions for a solution: unsaturated, saturated and supersaturated.

Solid solution figure shows the cooling curve for a solid solution alloy

containing 50 % Sb (antimony) and 50 % Bi (bismuth). It may be noted that this alloy begins to solidify at temperature lower than the freezing point of pure antimony (1170° F) and higher than the freezing point of pure bismuth (520° F).

Solid solution A solid solution is simply a solution in the solid state and

consists of two kinds of atoms combined in one type of

space lattice

Depending on the atom size and solute and solvent

elements, two types of solid solutions may be formed –

1. substitutional

2. interstitial.

Solid solution Substitutional solid solutions The solute atoms substitute the solvent atoms in the

lattice of solvent. Ex. Silver atoms may substitute for gold atoms without

loosing the FCC structure of gold and gold atom may substitutes for silver atoms on regular normal sites in the FCC lattice of silver.

Here all alloy of silver and gold binary system are formed by random distribution of these two types of atoms in the FCC lattice of structure.

Solid solution Substitutional solid solutions

As shown in the above figure, in this type of

solution, some of the solvent atoms are

substituted by atoms of the solute (alloying

element) atoms.

Solid solution Hume - Rothery Rules.

The solid solubility in alloy system by substitutions is

controlled by several factors known as Hume - Rothery

Rules.

Crystal Structure Factor:- Complete solid solubility of two elements is never obtained

unless the elements have the same types of the crystal structure.

Ex. Copper-Nickel(FCC), Silver-gold-platinum(FCC) complete solubility.

Copper-Zinc(FCC-HCP) partial solubility with 35% solubility of zinc in copper.

Solid solution Hume - Rothery Rules.

Relative Size Factor:- Fore extensive solubility atomic diameter shall be similar.

Greater size atom can not be fit in the same structure as a substitutional solid solution without producing excessive strain and corresponding instability.

Extensive solid solubility is encountered only when the two different atom differs in size by less than 15% called a favourable size factor.

Ex. Cu-Ni, Au-Pt

Solid solution Hume - Rothery Rules.

Chemical affinity factor:-

The greater the chemical affinity of two metals the more

restricted is their solid solubility.

When their chemical affinity is great the tendency

towards compound formations or intermediate phase is

more.

Generally the further apart the elements are in periodic

table the greater is their chemical affinity.

Solid solution Hume - Rothery Rules.

Relative valence factor:-

If the solute metal atom has different valance from that of

the solvent atom the number of valence electron per atom

is called the electron ratio, will be changed.

o Ex. Al-Ni relative size factor is 14%. (Ni is lower in valance than Al). The Ni dissolve 5% Al while Al dissolve 0.04% Ni.

Solid solution Interstitial solid solutions

These are formed when atoms of small atomic

radii fit into the spaces or interstices of the

lattice structure of the larger solvent atoms as

shown in figure given below.

Solid solution Interstitial solid solutions

Since the spaces of the lattice structure are restricted in

size, only atoms with radii less than one angstrom are likely

to form interstitial solid solutions. These are hydrogen,

boron, carbon, nitrogen and oxygen.

Interstitial solid solutions normally have very limited

solubility and generally are of little importance. Carbon in

iron is a notable exception and forms the basis for

hardening steel.

In both type of solid solutions, distortion of the

lattice structure will exist in the region of the

solute atoms.

Solid solution Interstitial solid solutions

This distortion will interfere with the movement of

dislocations on slip planes and will therefore increase

the strength of the alloy. This is the primary basis for

the strengthening of a metal by alloying.

The properties of an alloy can be manipulated by

varying its composition. For example steel formed

from iron and carbon can vary substantially in

hardness depending on the amount of carbon added

and the way in which it was processed.