General Material Classifications

There are thousands of materials available for use in engineering applications. Most

materials fall into one of three classes that are based on the atomic bonding forces of a

particular material. These three classifications are metallic, ceramic and polymeric.

Additionally, different materials can be combined to create a composite material. Within

each of these classifications, materials are often further organized into groups based on

their chemical composition or certain physical or mechanical properties. Composite

materials are often grouped by the types of materials combined or the way the materials are

arranged together. Below is a list of some of the commonly classification of materials within

these four general groups of materials.

MetalsFerrous metals and alloys (irons, carbon steels, alloy steels, stainless steels, tool and die steels) Nonferrous metals and alloys (aluminum, copper, magnesium, nickel, titanium, precious metals, refractory metals, superalloys) PolymericThermoplastics plasticsThermoset plasticsElastomers

CeramicsGlassesGlass ceramics Graphite Diamond  CompositesReinforced plasticsMetal-matrix composites Ceramic-matrix composites Sandwich structures Concrete Each of these general groups will be discussed in more detail in the following pages.

Metals

Metals account for about two thirds of all the elements and about 24% of the mass of the

planet. Metals have useful properties including strength, ductility, high melting points,

thermal and electrical conductivity, and toughness. From the periodic table, it can be seen

that a large number of the elements are classified as being a metal. A few of the common

metals and their typical uses are presented below.

Ferrous and Non-Ferrous Metals

Ferrous

These are metals which contain iron. They may have small amounts of other metals or other elements added, to give the required properties.

All ferrous metals are magnetic and give little resistance to corrosion Non-Ferrous Metals

These are metals which do not contain any iron. They are not magnetic and are usually more resistant to corrosion than ferrous metals.

Examples are aluminium, copper, lead. zinc and tin.

Common Metallic Materials

Iron/Steel - Steel alloys are used for strength critical applications

Aluminum - Aluminum and its alloys are used because they are easy to form, readily

available, inexpensive, and recyclable.

Copper - Copper and copper alloys have a number of properties that make them useful,

including high electrical and thermal conductivity, high ductility, and good corrosion

resistance.

Titanium - Titanium alloys are used for strength in higher temperature (~1000° F)

application, when component weight is a concern, or when good corrosion resistance is

required

Nickel - Nickel alloys are used for still higher temperatures (~1500-2000° F) applications or

when good corrosion resistance is required.

Refractory materials are used for the highest temperature (> 2000° F) applications.

The key feature that distinguishes metals from non-metals is their bonding. Metallic

materials have free electrons that are free to move easily from one atom to the next. The

existence of these free electrons has a number of profound consequences for the properties

of metallic materials. For example, metallic materials tend to be good electrical conductors

because the free electrons can move around within the metal so freely. More on the

structure of metals will be discussed later.

Non-Ferrous Metals Chooser Chart

Name Composition Properties Uses

Aluminium Pure Metal Greyish-White, soft, malleable, conductive to heat and electricity, It is corrosion resistant. It can be welded but this is difficult. Needs special processes.

Aircraft, boats, window frames, saucepans, packaging and insulation, pistons and cranks.

Aluminium alloys-Duraluminium)

Aluminium +4% Copper+1%Manganese

Ductile, Malleable, Work Hardens.

Aircraft and vehicle parts.

Copper Pure metal Red, tough, ductile, High electrical conductor, corrosion resistant, Can work hard or cold. Needs frequent annealing.

Electrical wire, cables and conductors, water and central heating pipes and cylinders. Printed circuit boards, roofs.

Brass 65% copper +35%zinc.

Very corrosive, yellow in colour, tarnishes very easily. Harder than copper. Good electrical conductor.

Castings, ornaments, valves,forgings.

Lead Pure metal The heaviest common metal. Soft, malleable, bright and shiny when new but quickly oxidizes to a dull grey. Resistant to corrosion.

Protection against X-Ray machines. Paints, roof coverings, flashings.

Zinc Pure metal A layer of oxide protects it from corrosion, bluish-white, easily worked.

Makes brass. Coating for steel galvanized corrugated iron roofing, tanks, buckets, rust-proof paints

Tin Pure metal White and soft, corrosion resistant.

Tinplate, making bronze.

Gilding metal85% copper+15% zinc.

Corrosion resistant, golden colour, enamels well.

Beaten metalwork, jewellery.

Ferrous Metals Chooser Chart

Name Composition Properties Uses

Mild Steel 0.15 to0.30% carbon Tough, high tensile strength, ductile. Because of low carbon content it can not be hardened and tempered. It must be case hardened.

girders, Plates, nuts and bolts, general purpose.

High Speed Steel medium carbon, tungsten, chromium and vanadium.

Can be hardened and tempered. Can be brittle. Retains hardness at high temperatures.

Cutting tools for lathes.

Stainless Steel 18% chromium, and 8% nickel added.

Corrosion resistant Kitchen draining boards. Pipes, cutlery, aircraft.

High Tensile SteelLow carbon steel,, nickel,and chromium.

Very strong and very tough.

Gears, shafts, engine parts.

High Carbon Steel

0.70% to 1.40% carbon. The hardest of the carbon steels. Less ductile, tough and malleable.

Chisels, hammers, drills, files, lathe tools, taps and dies.

Medium Carbon Steels

0.30% to 0.70% carbon. Stronger and harder than mild steels. Less ductile, tough and malleable.

Metal ropes, wire, garden tools, springs.

Cast Iron

Remelted pig iron with small amounts of scrap steel.

Hard, brittle, strong, cheap, self-lubricating. Whitecast iron, grey cast iron, malleable cast iron.

Heavy crushing machinery. Car cylinder blocks, vices, machine tool parts, brake drums, machine handle and gear wheels, plumbing fitments.

            

Definitions of properties of Materials.

Hardness The resistance a materials has to cutting and surface indentations.

ToughnessThis describes the amount of energy a material can absorb without breaking. This is the opposite to brittleness. We measure a material's ability to absorb shock.

Tensile StrengthThe maximum force a material can withstand in tension(pulling) compression(squashing) , torque(twisting) and shearing(sideways pressure).

MalleabilityThe amount of hammering, pressing and shaping a material can take without breaking.

Ductility The length that a material can be stretched without breaking.

ElasticityThe length that a material can be stretched and return to its original length when released.

Heat and Electrical Conductivity

The measure of how well a material can conduct heat or electricity.

HeavinessThe denseness of materials. A dense material will be heavy in relation to its size.

Strength The measure of how a material withstands a heavy load without breaking.

Pure Metals

A pure metal only consists of a single element. This means that it only has one type of atom

in it.

The common pure metals are:-aluminium, copper, iron, lead, zinc, tin, silver and gold.

Alloys

An alloy is a mixture of two or more metals. When a material is needed which requires

certain properties and this does not exist in a pure metal we combine metals .

Pure white aluminium is very soft and ductile. Other elements can be added to create an

aluminium alloy. This can produce a metal which is stronger than Mild Steel has improved

hardness and is resistant to corrosion while still remaining light in weight.

Corrosion is a process that takes place when essential properties within a given material

begin to deteriorate, after exposure to elements that recur within the environment. Most

often, this deterioration is noticed in metals and referred to as rust. What happens in this

case is the chemical reactions that are set up by an exposure of the electrons in the metal

to the presence of water and oxygen. As an example, a tin roof is exposed to the wind and

the rain.

Over time, the basic workings of that exposure will allow the creation of acids that begin to

alter the surface of the tin. The top layer becomes encrusted with corrosion in the form of

a red-brown substance that lacks the cohesive nature of the tin. Continued development of

the corrosion will eventually weaken the entire roof and the tin will eventually become so

weak that it will no longer provide adequate protection as a roof material.

One of the ways to fight corrosion is to apply a protective layer to any metal surface that

must come in contact with water and oxygen. For example, some forms of enamel are

ideal for protecting metal surfaces

CORROSION

CORROSION

DEFINITIONA STATE OF DETERIORATION IN METALS CAUSED BY OXIDATION OR CHEMICAL ACTION

EROSION BY CHEMICAL ACTION

It is convenient to classify corrosion by the forms in which it manifests itself, the basis for

this classification being the appearance of the corroded metal. Each form can be identified

by mere visual observation. In most cases the naked eye is sufficient, but sometimes

magnification is helpful or required. Valuable information for the solution of a corrosion

problem can often be obtained through careful observation of the corroded test specimens

or failed equipment. Examination before cleaning is particularly desirable. Some of the

eight forms of corrosion are unique, but all of them are more or less interrelated. The eight

forms are: (1) uniform, or general attack, (2) galvanic, or two-metal corrosion, (3) crevice

corrosion, (4) pitting, (5) intergranular corrosion, (6) selective leaching, or parting, (7)

erosion corrosion, and (8) stress corrosion.

Uniform Attack

Uniform attack is the most common form of corrosion. It is normally characterized by a

chemical or electrochemical reaction which proceeds uniformly over the entire exposed

surface or over a large area. The metal becomes thinner and eventually fails. For example, a

piece of steel or zinc immersed in dilute sulfuric acid will normally dissolve at a uniform rate

over its entire surface. A sheet iron roof will show essentially the same degree of rusting over

its entire outside surface.

Galvanic or Two-Metal Corrosion

A potential difference usually exists between two dissimilar metals when they are immersed

in a corrosive or conductive solution. If these metals are placed in contact (or otherwise

electrically connected), this potential difference produces electron flow between them.

Corrosion of the less corrosion-resistant metal is usually increased and attack of the more

resistant material is decreased, as compared with the behavior of these metals when they

are not in contact. The less resistant metal becomes anodic and the more resistant metal

cathodic. Usually the- cathode or cathodic metal corrodes very little or not at all in this type

of couple. Because of the electric currents and dissimilar metals involved, this form of

corrosion is called galvanic, or two-metal, corrosion. It is electrochemical corrosion, but we

shall restrict the term galvanic to dissimilar-metal effects for purposes of clarity

Crevice Corrosion

Intense localized corrosion frequently occurs within crevices and other shielded areas on

metal surfaces exposed to corrosives. This type of attack is usually associated with small

volumes of stagnant solution caused by holes, gasket surfaces, lap joints, surface deposits,

and crevices under bolt and rivet heads. As a result, this form of corrosion is called crevice

corrosion or, sometimes, deposit or gasket corrosion.

Pitting

Pitting is a form of extremely localized attack that results in holes in the metal. These holes

may be small or large in diameter, but in most cases they are relatively small. Pits are

sometimes isolated or so close together that they look like a rough surface. Generally a pit

may be described as a cavity or hole with the surface diameter about the same as or less

than the depth.

Intergranular Corrosion

Grain boundary effects are of little or no consequence in most applications or uses of metals.

If a metal corrodes, uniform attack results since grain boundaries are usually only slightly

more reactive than the matrix. However, under certain conditions, grain interfaces are very

reactive and intergranular corrosion results. Localized attack at and adjacent to grain

boundaries, with relatively little corrosion of the grains, is intergranular corrosion. The alloy

disintegrates (grains fall out) and/or loses its strength.

Selective leaching

Selective leaching is the removal of one element from a solid alloy by corrosion processes.

The most common example is the selective removal of zinc in brass alloys (dezincification).

Similar processes occur in other alloy systems in which aluminum; iron, cobalt, chromium,

and other elements are removed. Selective leaching is the general term to describe these

processes, and its use precludes the creation of terms such as dealuminumification,

decobaltification, etc. Parting is a metallurgical term that is sometimes applied, but selective

leaching is preferred.

Erosion Corrosion

Erosion corrosion is the acceleration or increase in rate of deterioration or attack on a metal

because of relative movement between a corrosive fluid and the metal surface. Generally,

this movement is quite rapid, and mechanical wear effects or abrasion are involved. Metal is

removed from the surface as dissolved ions, or it forms solid corrosion products which are

mechanically swept from the metal surface. Sometimes, movement of the environment

decreases corrosion, particularly when localized attack occurs under stagnant conditions,

but this is not erosion corrosion because deterioration is not increased.

Stress-corrosion cracking

Stress-corrosion cracking refers to cracking caused by the simultaneous presence of tensile

stress and a specific corrosive medium. Many investigators have classified all cracking

failures occurring in corrosive mediums as stress-corrosion cracking, including failures due

to hydrogen embrittlement.