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CONSTRUCTION MATERIALS 0670214

INTRODUCTION

The importance of the study of construction materials:

It is known that construction materials are the principal entiry

for various engineering of buildings and machines and

equipment etc. so the engineer with different specialization and

different currency should be dealing with engineering materials

in all its steps to create different engineering.

We believe that the work of a major architect directly with the

material, so the engineer to perform its work successfully and

efficiently requires a sound knowledge and expertise in material,

characteristics and resistance and the impact of external factors

on these materials. this knowledge requires civil study theory

and practice.

Materials used by civil engineer in his works such as metal,

stones, cement, brick, glass, plastic and insulation materials of

petroleum and energy article and many others.These materials

it’s the units of building and responsible for life of the building

To understand and study the properties of civil engineering

materials,we have to study the structure and properties of

matter.

STRUCRURE AND PROPERTIES 0F MATTER

Look around the classroom. Everything, from the clothes you are wearing

to the air you breath is matter. Matter is very important. Matter makes

up everything including living things like plants and people. It also makes

non-living things such as tables and chairs. Things as big as an elephant or

as tiny as a grain of sand on a beach are matter.

Everything is matter and matter comes in three

different states: solid, liquid and gas. That means that everything is

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either a solid, a liquid, or a gas. Each state has properties.

2. What does property mean?

Each state has properties, but what does that mean?

A property describes how an object looks, feels, or acts. So that means

that liquids look, act, or feel differently than solids or gases.

One property of all matter, whether it's a solid, liquid, or gas, is

that it takes up space and has mass.

To help you decide if something is a solid, a liquid or a gas, you need to

know the properties, (how it looks, acts or feels) of these three states.

3. What are the properties of a solid?

1. Solids don't change shape easily.

Think of a piece of paper, you can change its shape by crumpling it, but it

doesn't change its shape by itself. You have to use your energy to make

the shape change.

If you put a solid in a container it won't change its shape... No matter how

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much you move or slide it around. Think of an ice cube inside a cup. The

cube is solid and it stays the same shape.

2. Solid particles don't move around.

3. Solid particles are in an aligned

array. Look at the pictures. Notice

the circles (particles) are lined up in

tight rows. They are so tight they

can't move, they just wiggle.

4. What are the properties of liquids?

1. Liquids take the shape of their container.

If you pour milk into a glass it will take the shape of the glass. If

you pour the milk into a bowl, it takes the shape of the bowl.

2. Liquids have surface tension. The particles hold on to each other,

like holding hands with a friend. The skin or surface of a glass filled with

water holds together because the particles hold one to each other. That

is called surface tension.

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3. Liquids move around. The particles

in liquids are farther apart than those

of solids, so they can move around

more. That's why liquids take the

shape of their container.

5. What are the properties of gas?

1. Gas is invisible. That means you can't see it. The particles are so far

apart they are invisible, but they are still there! Think about oxygen. You

can't see it, but you know it's there because you breath it.

2. Gas particles move around

freely. They are spread out move fast,

like when you are running on the

playground at recess.

PROPERTIES OF ATOMS

Proton (p+) is positively charged particle of the atomic nucleus. The atomic

number of an element represents the number of protons in the nucleus.

Neutron is an uncharged particle of the nucleus of all atoms EXCEPT hydrogen.

For a given element, the mass number is the number of

protons and neutrons (nucleons) in the nucleus.

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Electron (e-) is negatively charged particle that can occupy a volume of space

(orbital) around an atomic nucleus. All atoms of an element have the same number of

electrons (i.e. any Chlorine atom is going to have 17 electrons). Electrons can be

shared or transferred among atoms.

Atoms have an equal number of protons and electrons; therefore, they have a no

net charge.

Ion is an atom that has gained or lost one or more electrons, thus becoming positively

or negatively charged..

Isotope is one of two or more forms of atoms of an element that differ in their

number of neutrons. They have the same atomic number (same number of protons and

electrons), but a different mass number due to more or fewer neutrons.

Particle Charge RelativeCharge** Mass Relative

mass** Location

Proton +1.60 x 10-19

C +1

1.672

x 10-

24 g

1 amu nucleus

Electron -1.60 x 10-19

C -1

9.05

x 10-

28 g

0 amu

~(1/1840

amu)

electron cloud

(orbital)

Neutron neutral 0

1.674

x 10-

24 g

1 amu nucleus

Number of electrons in each shell

Shell name

Subshell name

Subshell max

electrons

Shell max

electrons

K 1s 2 2

L 2s 2 2 + 6 = 8

6

2p 6

M

3s 2 2 + 6 + 10

= 18 3p 6

3d 10

N

4s 2 2 + 6 +

+ 10 + 14 = 32

4p 6

4d 10

4f 14

Each subshell is constrained to hold 4ℓ + 2 electrons at most, namely:

Each s subshell holds at most 2 electrons

Each p subshell holds at most 6 electrons

Each d subshell holds at most 10 electrons

Each f subshell holds at most 14 electrons

Each g subshell holds at most 18 electrons

Therefore, the K shell, which contains only an s subshell, can hold up to 2 electrons; the L shell, which contains an s and a p, can hold up to 2 + 6 = 8 electrons, and so forth; that's why nth shell can hold up to 2n

2 electrons.

[1]

Although that formula gives the maximum in principle, in fact that maximum is only achieved (by known elements) for the first four shells (K, L, M, N). No known element has more than 32 electrons in any one shell.

[6][7] This is

because the subshells are filled according to the Aufbau principle. The first elements to have more than 32 electrons in one shell would belong to the g-block of period 8 of the periodic table. These elements would have some electrons in their 5g subshell and thus have more than 32 electrons in the O shell (fifth principal shell).

STRUCTURE A PHYSICAL PROPERTIES

Electrons, Levels and Energy

Electrons

Electrons are one of the three subatomic

particles. They have a negative charge and

about 1/2000 the mass of a proton. They are

attracted to the positively charged protons in the

atomic center, but they are repelled by one

another. It is impossible to absolutely define the

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position (momentum and location) of an

electron in space and time. Instead electrons are

described as having different probabilities of

distribution around the atomic center. These

volumes of space (where an electron is found

more often) are called atomic orbitals

Atomic Orbitals

Atomic orbitals have different shapes. All s are

spherical. Electrons in s orbitals have the same

probability of being found in any direction and

at a given distance from the atomic center.

Electrons in an s orbital may even be found

right at the atomic center!

In all other types of orbitals occupying electrons

have no probability of being found at the center.

All p orbitals are shaped somewhat like a

dumbbell, with the thin, pinched region of zero

probability lying right over the center. No

matter what its shape, an orbital can only hold a

maximum of two electrons at any time.

Energy Levels

Orbitals are grouped in zones at different

distances from the atomic center. Electrons in

zones close to the center are lower in energy

than electrons in zones at greater distances from

the center. According to Bohr, the amount of

energy needed to move an electron from one

zone to another is a fixed, finite amount. These

zones are known as energy levels (or sometimes

calledelectron shells).

At the lowest energy level, the one closest to the

atomic center, there is a single 1s orbital that

can hold 2 electrons.

At the next energy level, there are four orbitals;

a 2s, 2p1, 2p2, and a 2p3. Each of these orbitals

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can hold 2 electrons, so a total of 8 electrons can

be found at this level of energy.

In larger and larger atoms, electrons can be

found at higher and higher energy levels

(e.g. 3s and 3p).

Moving between

Levels

As Neils Bohr showed, it is possible for

electrons to move between energy levels. Light

contains energy. If a photon of light strikes an

atom, it is possible for the energy in the light ray

to be transferred to one of the low energy

electrons moving around the atomic center. The

electron with its extra packet of energy

becomes excited, and promptly moves out of its

lower energy level and takes up a position in a

higher energy level.

This situation is unstable, however. Almost

immediately the excited electron gives up the

extra energy it holds, usually in the form of

light, and falls back down to the lower energy

level again.

Florescence is a phenomenon of moving

electrons. Ultra violet ("black") light has a short

wavelength and high energy. When these rays

hit certain atoms this energy is absorbed as

described above. But the electrons cannot hold

this energy for long, and when they fall back to

the lower energy levels they give off the

yellowish "glow" of longer wavelength, lower

energy light that we can see with our eyes.

Internal energy

In thermodynamics, the internal energy of a thermodynamic system, or a body with well-

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Chemical Bond Types

defined boundaries, denoted by U, or sometimes E, is the total of thekinetic energy due to the motion of molecules (translational, rotational, vibrational) and the potential energy associated with the vibrational and electric energy ofatoms within molecules or crystals. It includes the energy in all the chemical bonds, and the energy of the free, conduction electrons in metals.

The internal energy is a thermodynamic potential and for a closed thermodynamic system held at constant entropy, it will be minimized.

One can also calculate the internal energy of electromagnetic or blackbody radiation. It is a state function of a system, an extensive quantity. The SI unit of energy is the joule although other historical, conventional units are still in use, such as the (small and large) calorie for heat.

..

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Ionic Bonds

An ionic bond is formed by the attraction of

oppositely charged atoms or groups of atoms.

When an atom (or group of atoms) gains or loses

one or more electrons, it forms an ion. Ions have

either a net positive or net negative charge.

Positively charged ions are attracted to the

negatively charged 'cathode' in an electric field and

are called cations. Anions are negatively charged

ions named as a result of their attraction to the positive 'anode' in an electric field.

Every ionic chemical bond is made up of at least one cation and one anion.

Ionic bonding is typically described to students as

being the outcome of the transfer of electron(s)

between two dissimilar atoms. The Lewis structure below illustrates this concept.

For binary atomic systems, ionic bonding typically

occurs between one metallic atom and one

nonmetallic atom. The electronegativity difference

between the highly electronegative nonmetal atom

and the metal atom indicates the potential for electron transfer.

Sodium chloride (NaCl) is the classic example of

ionic bonding. Ionic bonding is not isolated to

simple binary systems, however. An ionic bond can

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occur at the center of a large covalently bonded

organic molecule such as an enzyme. In this case,

a metal atom, like iron, is both covalently bonded

to large carbon groups and ionically bonded to

other simpler inorganic compounds (like oxygen).

Organic functional groups, like the carboxylic acid

group depicted below, contain covalent bonding in

the carboxyl portion of the group (HCOO) which

itself serves as the anion to the acidic hydrogen ion (cation).

Covalent

A covalent chemical bond results from the sharing

of electrons between two atoms with similar

electronegativities A single covalent bond

represent the sharing of two valence electrons

(usually from two different atoms). The Lewis

structure below represents the covalent

bond between two hydrogen atoms in a H2 molecule.

Dot Structure Line Structure

Multiple covalent bonds are common for certain

atoms depending upon their valence configuration.

For example, a double covalent bond, which occurs

in ethylene (C2H4), results from the sharing of two

sets of valence electrons. Atomic nitrogen (N2) is an example of a triple covalent bond.

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Double Covalent Bond

Triple Covalent Bond

The polarity of a covalent bond is defined by any

difference in electronegativity the two atoms

participating. Bond polarity describes the

distribution of electron density around two bonded

atoms. For two bonded atoms with similar

electronegativities, the electron density of the

bond is equally distributed between the two atom

is This is anonpolar covalent bond. The electron

density of a covalent bond is shifted towards the

atom with the largest electronegativity. This

results in a net negative charge within the bond

favoring the more electronegative atom and a net

positive charge for the least electronegative atom. This is a polar covalent bond.

Coordinate Covalent

A coordinate covalent bond (also called a dative

bond) is formed when one atom donates both of

the electrons to form a single covalent bond. These

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electrons originate from the donor atom as an unshared pair.

Both the ammonium ion and hydronium ion

contain one coordinate covalent bond each. A lone

pair on the oxygen atom in water contributes two

electrons to form a coordinate covalent bond with

a hydrogen ion to form the hydronium ion.

Similarly, a lone pair on nitrogen contributes 2

electrons to form the ammonium ion. All of the

bonds in these ions are indistinguishable once formed, however.

Ammonium (NH4+) Hydronium (H3O

+)

Network Covalent

Some elements form very large molecules by

forming covalent bonds. When these molecules

repeat the same structure over and over in the

entire piece of material, the bonding of the

substance is called network covalent. Diamond is

an example of carbon bonded to itself. Each carbon

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forms 4 covalent bonds to 4 other carbon atoms

forming one large molecule the size of each crystal of diamond.

Silicates, [SiO2]x also form these network covalent

bonds. Silicates are found in sand, quartz, and many minerals.

Metallic

The valence electrons of pure metals are not

strongly associated with particular atoms. This is a

function of their low ionization energy. Electrons in

metals are said to be delocalized (not found in

one specific region, such as between two particular

atoms).

Since they are not confined to a specific area,

electrons act like a flowing “sea”, moving about the positively charged cores of the metal atoms.

Delocalization can be used to explain

conductivity, malleability, and ductility.

Because no one atom in a metal sample has a

strong hold on its electrons and shares them

with its neighbors, we say that they are

bonded.

In general, the greater the number of

electrons per atom that participate in metallic bonding, the stronger the metallic bond.

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Hydrogen Bonding

Hydrogen bonding differs from other uses of the word

"bond" since it is a force of attraction between a hydrogen

atom in one molecule and a small atom of

high electronegativity in another molecule. That is, it is an

intermolecular force, not an intramolecular force as in the

common use of the word bond.

When hydrogen atoms are joined in a polar covalent

bondwith a small atom of high electronegativity such as

O, F or N, the partial positive charge on the hydrogen is

highly concentrated because of its small size. If the

hydrogen is close to another oxygen, fluorine or nitrogen

in another molecule, then there is a force of attraction

termed a dipole-dipole interaction. This attraction or

"hydrogen bond" can have about 5% to 10% of the

strength of a covalent bond.

Hydrogen bonding has a very important effect on the

properties of water and ice. Hydrogen bonding is also very

important in proteins and nucleic acids and therefore in

life processes. The "unzipping" of DNA is a breaking of

hydrogen bonds which help hold the two strands of the

double helix together.

Radioactivity? What is Radiation?

Unstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability. The decomposition process is called radioactivity. The energy and particles which are released during the decomposition process are

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called radiation. When unstable nuclei decompose in nature, the process is referred to as natural radioactivity. When the unstable nuclei are prepared in the laboratory, the decomposition is called induced radioactivity.

There are three major types of natural radioactivity:

1. Alpha Radiation Alpha radiation consists of a stream of positively charged particles, called alpha particles, which have an atomic mass of 4 and a charge of +2 (a helium nucleus). When an alpha particle is ejected from a nucleus, the mass number of the nucleus decreases by four units and the atomic number decreases by two units. For example: 23892U → 42He + 23490Th The helium nucleus is the alpha particle.

2. Beta Radiation Beta radiation is a stream of electrons, called beta particles. When a beta particle is ejected, a neutron in the nucleus is converted to a proton, so the mass number of the nucleus is unchanged, but the atomic number increases by one unit. For example:

23490 → 0-1e + 23491Pa The electron is the beta particle.

3. Gamma Radiation Gamma rays are high-energy photons with a very short wavelength (0.0005 to 0.1 nm). The emission of gamma radiation results from an energy change within the atomic nucleus. Gamma emission changes neither the atomic number nor the atomic mass. Alpha and beta emission are often accompanied by gamma emission, as an excited nucleus drops to a lower and more stable energy state.

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Radioactivity

An atom is made up of three fundamental subatomic

particles protons, neutrons and electrons. Out of these

three particles, protons and neutrons located at the center

of the atom as a hard and dense part known as nucleus.

The rest of the part of atom contains negatively charged

particles called as electron which balance out the charge

of the protons and make the atom electrically neutral.

The total mass of an atom accumulate at the center of

atom in the form of nucleus as the mass of electrons is

negligible. Hence, the sum of total number of protons and

neutrons is called as mass number.

There must be some nuclear force which maintains the

existence of nucleus, because there is a repulsion force

between positively charged proton which are collected in a

small region of nucleus. If the number of proton is less

in an atom, other forces can hold the protons together

and atom becomes stables. But as the ratio of protons to

neutrons is increases, protons cannot be held firmly

together and hence form an unstable nucleus.

Radioactivity Unit The unit of radioactivity called as curie (ci). One curie

defined as "the amount of a radioactive substance

which has a decay rate of 3.7 x 1010

disintegration's

per second." This large number is based on the

observation that one gram of radium disintegration at the

rate of 3.7 x 1010

disintegration per second.

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Types of Radioactivity

The radioactive decay or transmutation can occur naturally or

by artificial means. On this basis, transmutation classified as

two types.

1. Natural radioactivity

2. Artificial radioactivity

Natural Radioactivity The atoms of radioactive elements on the emission of

alpha particles and beta particles would change into

atoms of another element. This change is spontaneous

and occurs due to instability of heavy nuclei. Such type of

radioactive decay termed as natural radioactive decay and

phenomenon called as natural radioactivity.

Rutherford was first to observed the decay of radon by the

loss of alpha and beta particles by the preceding

elements. Natural radioactive elements decay naturally

without any external effect until the convert in stable

nuclei.

For example; Uranium-238 disintegrated in to Thorium-

234 by the emission of alpha particle which further

changes in Protactinium-234 by the loss of a beta particle

and anti-neutrino.

92U238

→ 90Th234

+ 2He4

90Th234

→ 91Pa234

+ -1e0+ Ï…

The emission of an alpha particle results in the formation

of an element which lie two place to the left in periodic

table and the emission of beta particle results in the

formation of an element which lies one place to the right.

This is called as group displacement law. Natural

radioactivity series.

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What is half-life?

Radioactive substances will give out radiation all the time, regardless of what happens to them physically or chemically. As they decay the atoms change to daughter atoms, until eventually there won't be any of the original atoms left.

Different substances decay at different rates and so will last for different lengths of time. We use the half-life of a substance to tell us which substances decay the quickest.

Half-life - is the time it takes for half of the radioactive particles to decay.

It is also the time it takes for the count-rate of a substance to reduce to half of the original value.

We cannot predict exactly which atom will decay at a certain time but we can estimate, using the half-life, how many will decay over a period of time.

The half-life of a substance can be found by measuring the count-rate of the substance with a Geiger-Muller tube over a period of time. By plotting a graph of count-rate against time the half-life can be seen on the graph.

Using radioactivity

Different radioactive substances can be used for different purposes. The type of radiation they emit and the half-life are the two things that help us decide what jobs a substance will be best for. Here are the main uses you will be expected to know about:

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1. Uses in medicine to kill cancer - radiation damages or kills cells, which can cause cancer, but it can also be used to kill cancerous cells inside the body.

2. Uses in industry - one of the main uses for radioactivity in industry is to detect the thickness of materials. The thicker a material is the less the amount of radiation that will be able to penetrate it.

3. Alpha particles would not be able to go through metal at all, gamma waves would go straight through regardless of the thickness. Beta particles should be used, as any change in thickness would change the amount of particles that could go through the metal.

They can even use this idea to detect when toothpaste tubes are full of toothpaste!

4. Photographic radiation detectors - these make use of the fact that radiation can change the colour of photographic film

5. Dating materials - The older a radioactive substance is the less radiation it will release. This can be used to find out how old things are. The half-life of the radioactive substance can be used to find the age of an object containing that substance.

There are three main examples of this:

i) Carbon dating - many natural substances contain two isotopes of Carbon. Carbon-12 is stable and doesn't disintegrate. Carbon-14 is radioactive. Over time Carbon-14 will slowly decay. As the half-life is very long for Carbon-14, objects that are thousands of years old can be compared to new substances and the change in the amount of Carbon-14 can date the object.

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ii) Uranium decays by a series of disintegrations that eventually produces a stable isotope of lead. Types of rock (igneous) contain this type of uranium so can be dated, by comparing the amount of uranium and lead in the rock sample.

iii) Igneous rocks also contain potassium-40, which decays to a stable form of Argon. Argon is a gas but if it can't escape from the rock then the amount of trapped argon can be used to date the rock

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.

Metals

Ferrous metals and

alloys (irons, carbon

steels, alloy steels,

stainless steels, tool

and die steels)

Nonferrous metals

and alloys

(aluminum, copper,

Polymeric

Thermoplastics

plastics

Thermoset

plastics

Elastomers

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magnesium, nickel,

titanium, precious

metals, refractory

metals, superalloys)

Ceramics

Glasses

Glass ceramics

Graphite

Diamond

Composites

Reinforced

plastics

Metal-matrix

composites

Ceramic-matrix

composites

Sandwich

structures

Concrete

What are the properties of metals?

The versatility of metals attests to the very wide range of properties of the more than 70 metals on the periodic table. A description of all of these properties and the applications in which they are used is well beyond the scope of this section. The following therefore provides an introduction to some of the more prominent properties Chemical Properties: Metals combine with other metals and some non-metallic elements to form a vast number of alloys that enhance the properties of metals in specific applications, e.g., the combination of iron, nickel and chromium provides a series of stainless steel alloys that are in common use. Metals such as nickel, vanadium, molybdenum, cobalt, rare earths and the platinum group metals enable the catalytic reactions for the synthesis of many organic chemicals from petroleum. A wide variety of metal compounds and salts impart beneficial properties to products like plastics in terms of colour,

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brightness, flame resistance and resistance to degradation. Photography has been made possible by the effect of light on metal salts. Mechanical Properties: The properties of strength and ductility enable the extensive use of metals in structures and machinery. Metals and alloys exhibit ductility, malleability and the ability to be deformed plastically (that is, without breaking), making them easy to shape into beams (steel beams for construction), extrusions (aluminum frames for doors and windows), coins, metal cans and a variety of fasteners (nails and paper-clips). The strength of metals under pressure (compression), stretching (tensile) and sheer forces makes them ideal for structural purposes in buildings, automobiles, aircraft frames, gas pipelines, bridges, cables, and some sports equipment. Conductivity: Metals are excellent conductors of both heat and electricity. In general, conductivity increases with decreasing temperature, so that, at absolute zero (-273°C), conductivity is infinite; in other words, metals become superconductors. Thermal conductivity is harnessed in automobile radiators and cooking utensils. Electrical conductivity provides society with the ability to transmit electricity over long distances to provide lights and power in cities remote from electrical generating stations. The circuitry in household appliances, television sets and computers relies on electrical conductivity. Resistance to Wear, Corrosion, Fatigue and Temperature: Metals are hard and durable. They are used in applications sensitive to corrosion such as chemical plants, food preparation, medical applications, plumbing and lead in storage batteries. Wear resistance is critical in bearings for all modes of transportation and in machine tools. Fatigue resistance - the ability to resist breaking after repeated deformation such as bending - enables the use of metals in springs, levers and gears. Temperature resistance makes metals suitable for jet engines and filaments in light-bulbs. Optical Characteristics: Metals are uniformly lustrous and, except for copper and gold, are silvery or greyish. This is because all metals absorb light at all frequencies and immediately radiate it. Metals impart mirrors with their reflective surface. The lustre of metals gives them the attractive appearance that is so important in jewellery and coins. (Interestingly, metals also provide the intangible, distinctive "metallic ring" that is associated with coins.)

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Magnetic Properties: Ferromagnetism is exhibited by iron and several other metals. In addition, other metals and alloys can be magnetized in an electrical field to exhibit paramagnetism. Magnetic properties are employed in electric motors, generators, and speaker systems for audio equipment. Emission Properties: Metals emit electrons when exposed to radiation (e.g. light) of a short wavelength or when heated to sufficiently high temperatures. These phenomena are exploited in television screens, using rare earth oxides and in a variety of electronic devices and instruments. Conversely, the ability of metals such as lead to absorb radiation is employed in shielding, for example in the apron provided by dentists during an X-ray examination.

STRUCTURE OF METAL AND CRYSTAL DEFECTS

Metals account for about two thirds of all the elements and

about 24% of the mass of the planet. They are all around us in

such forms as steel structures, copper wires, aluminum foil, and

gold jewelry. Metals are widely used because of their

properties: strength, ductility, high melting point, thermal and

electrical conductivity, and toughness.

These properties also offer clues as to the structure of metals. As

with all elements, metals are composed of atoms. The strength

of metals suggests that these atoms are held together by strong

bonds. These bonds must also allow atoms to move; otherwise

how could metals be hammered into sheets or drawn into wires?

A reasonable model would be one in which atoms are held

together by strong, but delocalized, bonds.

Bonding

Such bonds could be formed between metal atoms that have low

electronegativities and do not attract their valence electrons

strongly. This would allow the outermost electrons to be shared

by all the surrounding atoms, resulting in positive ions (cations)

surrounded by a sea of electrons (sometimes referred to as an

electron cloud).

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Figure 1: Metallic Bonding.

Because these valence electrons are shared by all the atoms,

they are not considered to be associated with any one atom. This

is very different from ionic or covalent bonds, where electrons

are held by one or two atoms. The metallic bond is therefore

strong and uniform. Since electrons are attracted to many atoms,

they have considerable mobility that allows for the good heat

and electrical conductivity seen in metals.

Above their melting point, metals are liquids, and their atoms

are randomly arranged and relatively free to move. However,

when cooled below their melting point, metals rearrange to form

ordered, crystalline structures.

Figure 2: Arrangement of atoms in a liquid and a solid.

Crystals

To form the strongest metallic bonds, metals are packed together

as closely as possible. Several packing arrangements are

possible. Instead of atoms, imagine marbles that need to be

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packed in a box. The marbles would be placed on the bottom of

the box in neat orderly rows and then a second layer begun. The

second layer of marbles cannot be placed directly on top of the

other marbles and so the rows of marbles in this layer move into

the spaces between marbles in the first layer. The first layer of

marbles can be designated as A and the second layer as B giving

the two layers a designation of AB.

Layer "A" Layer "B" AB packing

Figure 3: AB packing of spheres. Notice that layer B spheres fit

in the holes in the A layer.

Packing marbles in the third layer requires a decision. Again

rows of atoms will nest in the hollows between atoms in the

second layer but two possibilities exist. If the rows of marbles

are packed so they are directly over the first layer (A) then the

arrangement could be described as ABA. Such a packing

arrangement with alternating layers would be designated as

ABABAB. This ABAB arrangement is called hexagonal close

packing (HCP).

If the rows of atoms are packed in this third layer so that they do

not lie over atoms in either the A or B layer, then the third layer

is called C. This packing sequence would be designated

ABCABC, and is also known as face-centered cubic (FCC).

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Both arrangements give the closest possible packing of spheres

leaving only about a fourth of the available space empty.

The smallest repeating array of atoms in a crystal is called a unit

cell. A third common packing arrangement in metals, the body-

centered cubic (BCC) unit cell has atoms at each of the eight

corners of a cube plus one atom in the center of the cube.

Because each of the corner atoms is the corner of another cube,

the corner atoms in each unit cell will be shared among eight

unit cells. The BCC unit cell consists of a net total of two atoms,

the one in the center and eight eighths from the corners.

In the FCC arrangement, again there are eight atoms at corners

of the unit cell and one atom centered in each of the faces. The

atom in the face is shared with the adjacent cell. FCC unit cells

consist of four atoms, eight eighths at the corners and six halves

in the faces. Table 1 shows the stable room temperature crystal

structures for several elemental metals.

Table 1: Crystal Structure for some Metals (at room

temperature)

Aluminum FCC Nickel FCC

Cadmium HCP Niobium BCC

Chromium BCC Platinum FCC

Cobalt HCP Silver FCC

Copper FCC Titanium HCP

Gold FCC Vanadium BCC

Iron BCC Zinc HCP

Lead FCC Zirconium HCP

28

Magnesium HCP

Unit cell structures determine some of the properties of metals.

For example, FCC structures are more likely to be ductile than

BCC, (body centered cubic) or HCP (hexagonal close packed).

Figure 4 shows the FCC and BCC unit cells. (See Crystal

Structure Activity)

Body Centered Cubic Face Centered Cubic

Figure 4: Unit cells for BCC and FCC.

As atoms of melted metal begin to pack together to form a

crystal lattice at the freezing point, groups of these atoms form

tiny crystals. These tiny crystals increase in size by the

progressive addition of atoms. The resulting solid is not one

crystal but actually many smaller crystals, called grains. These

grains grow until they impinge upon adjacent growing crystals.

The interface formed between them is called a grain boundary.

Grains are sometimes large enough to be visible under an

ordinary light microscope or even to the unaided eye. The

spangles that are seen on newly galvanized metals are grains.

(See A Particle Model of Metals Activity) Figure 5 shows a

typical view of a metal surface with many grains, or crystals.

29

Figure 5: Grains and Grain Boundaries for a Metal.

Mild and low-carbon steel[

Mild steel[ also known as plain-carbon steel, is the most common

form of steel because its price is relatively low while it provides

material properties that are acceptable for many applications, more

so than iron. Low-carbon steel contains approximately 0.05–

0.320% carbon[1] making it malleable and ductile. Mild steel has a

relatively low tensile strength, but it is cheap and malleable;

surface hardness can be increased through carburizing.[3]

It is often used when large quantities of steel are needed, for

example as structural steel. The density of mild steel is

approximately 7.85 g/cm3 (7850 kg/m3 or 0.284 lb/in3)[4] and

the Young's modulus is 210 GPa (30,000,000 psi).[5]

Low-carbon steels suffer from yield-point runout where the material

has two yield points. The first yield point (or upper yield point) is

higher than the second and the yield drops dramatically after the

upper yield point. If a low-carbon steel is only stressed to some

point between the upper and lower yield point then the surface

may develop Lüder bands.[6] Low-carbon steels contain less

carbon than other steels and are easier to cold-form, making them

easier to handle.[7]

Higher carbon steels[edit]

Carbon steels which can successfully undergo heat-treatment

have a carbon content in the range of 0.30–1.70% by weight.

Trace impurities of various otherelements can have a significant

effect on the quality of the resulting steel. Trace amounts

of sulfur in particular make the steel red-short, that is, brittle and

crumbly at working temperatures. Low-alloy carbon steel, such

as A36 grade, contains about 0.05% sulfur and melts around

1,426–1,538 °C (2,599–2,800 °F).[8]Manganese is often added to

improve the hardenability of low-carbon steels. These additions

turn the material into a low-alloy steel by some definitions,

but AISI's definition of carbon steel allows up to 1.65% manganese

30

by weight.

Low carbon steel

<0.3% carbon content, see above.

Medium carbon steel

Approximately 0.30–0.59% carbon content.[1] Balances ductility

and strength and has good wear resistance; used for large parts,

forging and automotive components.[9][10]

High-carbon steel (ASTM 304)

Approximately 0.6–0.99% carbon content.[1] Very strong, used for

springs and high-strength wires.[11]

Ultra-high-carbon steel

Approximately 1.0–2.0% carbon content.[1] Steels that can be

tempered to great hardness. Used for special purposes like (non-

industrial-purpose) knives, axles orpunches. Most steels with more

than 1.2% carbon content are made using powder metallurgy.

Note that steel with a carbon content above 2.14% is

considered cast iron.

Crystal Defects:

Metallic crystals are not perfect. Sometimes there are empty

spaces called vacancies, where an atom is missing. Another

common defect in metals aredislocations, which are lines of

defective bonding. Figure 6 shows one type of dislocation.

31

Figure 6: Cross Section of an Edge Dislocation, which extends

into the page. Note how the plane in the center ends within the

crystal.

These and other imperfections, as well as the existence of grains

and grain boundaries, determine many of the mechanical

properties of metals. When a stress is applied to a metal,

dislocations are generated and move, allowing the metal to

deform.

Polymer Structure

Engineering polymers include natural materials such as rubber

and synthetic materials such as plastics and elastomers.

Polymers are very useful materials because their structures can

be altered and tailored to produce materials 1) with a range of

mechanical properties 2) in a wide spectrum of colors and 3)

with different transparent properties.

Mers

A polymer is composed

of many simple

molecules that are

repeating structural units

called monomers. A

single polymer molecule

may consist of hundreds

to a million monomers

and may have a linear,

branched, or network

structure. Covalent

Mer – The repeating unit in a polymer

chain

Monomer –

A single mer unit (n=1)

Polymer –

Many mer-units along a chain

(n=103 or more)

Degree of Polymerization –

The average number of mer-units in

a chain.

32

bonds hold the atoms in the polymer molecules together and

secondary bonds then hold groups of polymer chains together to

form the polymeric material. Copolymers are polymers

composed of two or more different types of monomers.

Polymer Chains (Thermoplastics and Thermosets) A polymer is an organic material and the backbone of every

organic material is a chain of carbon atoms. The carbon atom

has four electrons in the outer shell. Each of these valence

electrons can form a covalent bond to another carbon atom or to

a foreign atom. The key to the polymer structure is that two

carbon atoms can have up to three common bonds and still bond

with other atoms. The elements found most frequently in

polymers and their valence numbers are: H, F, Cl, Bf, and I with

1 valence electron; O and S with 2 valence electrons; n with 3

valence electrons and C and Si with 4 valence electrons.

The ability for molecules to form long chains is a vital to

producing polymers. Consider the material polyethylene, which

is made from ethane gas, C2H6. Ethane gas has a two carbon

atoms in the chain and each of the two carbon atoms share two

valence electrons with the other. If two molecules of ethane are

brought together, one of the carbon bonds in each molecule can

be broken and the two molecules can be joined with a carbon to

carbon bond. After the two mers are joined, there are still two

33

free valence electrons at each end of the chain for joining other

mers or polymer chains. The process can continue liking more

mers and polymers together until it is stopped by the addition of

anther chemical (a terminator), that fills the available bond at

each end of the molecule. This is called a linear polymer and is

building block for thermoplastic polymers.

The polymer chain is often shown in two dimensions, but it

should be noted that they have a three dimensional structure.

Each bond is at 109° to the next and, therefore, the carbon

backbone extends through space like a twisted chain of

TinkerToys. When stress is applied, these chains stretch and the

elongation of polymers can be thousands of times greater than it

is in crystalline structures.

The length of the polymer chain is very important. As the

number of carbon atoms in the chain is increased to beyond

several hundred, the material will pass through the liquid state

and become a waxy solid. When the number of carbon atoms in

the chain is over 1,000, the solid material polyethylene, with its

characteristics of strength, flexibility and toughness, is obtained.

The change in state occurs because as the length of the

molecules increases, the total binding forces between molecules

also increases.

It should also be noted that the molecules are not generally

straight but are a tangled mass. Thermoplastic materials, such as

polyethylene, can be pictured as a mass of intertwined worms

randomly thrown into a pail. The binding forces are the result of

van der Waals forces between molecules and mechanical

entanglement between the chains. When thermoplastics are

34

heated, there is more molecular movement and the bonds

between molecules can be easily broken. This is why

thermoplastic materials can be remelted.

There is another group of polymers in which a single large

network, instead of many molecules is formed during

polymerization. Since polymerization is initially accomplished

by heating the raw materials and brining them together, this

group is called thermosetting polymers or plastics. For this type

of network structure to form, the mers must have more than two

places for boning to occur; otherwise, only a linear structure is

possible. These chains form jointed structures and rings, and

may fold back and forth to take on a partially crystalline

structure.

Since these materials are essentially comprised of one giant

molecule, there is no movement between molecules once the

mass has set. Thermosetting polymers are more rigid and

generally have higher strength than thermoplastic polymers.

Also, since there is no opportunity for motion between

molecules in a thermosetting polymer, they will not become

plastic when heated.

35

Types of polymers

o Commodity plastics

PE = Polyethylene

PS = Polystyrene

PP = Polypropylene

PVC = Poly(vinyl

chloride)

PET = Poly(ethylene

terephthalate)

o Specialty or Engineering

Plastics

Teflon (PTFE) =

Poly(tetrafluoroethyl

ene)

PC = Polycarbonate

(Lexan)

Polyesters and

Polyamides (Nylon)

Elastic/Plastic Deformation

When a sufficient load is applied to a

metal or other structural material, it

will cause the material to change shape. This change in shape is

called deformation. A temporary shape change that is self-

reversing after the force is removed, so that the object returns to

its original shape, is called elastic deformation. In other words,

elastic deformation is a change in shape of a material at low

stress that is recoverable after the stress is removed. This type of

deformation involves stretching of the bonds, but the atoms do

not slip past each other.

When the stress is sufficient to permanently deform the metal, it

is called plastic deformation. As discussed in the section on

crystal defects, plastic deformation involves the breaking of a

limited number of atomic bonds by the movement of

dislocations. Recall that the force needed to break the bonds of

all the atoms in a crystal plane all at once is very great.

36

However, the movement of dislocations allows atoms in crystal

planes to slip past one another at a much lower stress levels.

Since the energy required to move is lowest along the densest

planes of atoms, dislocations have a preferred direction of travel

within a grain of the material. This results in slip that occurs

along parallel planes within the grain. These parallel slip planes

group together to form slip bands, which can be seen with an

optical microscope. A slip band appears as a single line under

the microscope, but it is in fact made up of closely spaced

parallel slip planes as shown in the image.

Creep and Stress Rupture Properties

Creep Properties

37

Creep is a time-dependent deformation of a material while under

an applied load that is below its yield strength. It is most often

occurs at elevated temperature, but some materials creep at

room temperature. Creep terminates in rupture if steps are not

taken to bring to a halt.

Creep data for general design use are usually obtained under

conditions of constant uniaxial loading and constant

temperature. Results of tests are usually plotted as strain versus

time up to rupture. As indicated in the image, creep often takes

place in three stages. In the initial stage, strain occurs at a

relatively rapid rate but the rate gradually decreases until it

becomes approximately constant during the second stage. This

constant creep rate is called the minimum creep rate or steady-

state creep rate since it is the slowest creep rate during the test.

In the third stage, the strain rate increases until failure occurs.

Creep in service is usually affected by changing conditions of

loading and temperature and the number of possible stress-

temperature-time combinations is infinite. While most materials

are subject to creep, the creep mechanisms is often different

between metals, plastics, rubber, concrete.

Stress Rupture Properties

Stress rupture testing is similar to creep testing except that the

stresses are higher than those used in a creep testing. Stress

rupture tests are used to determine the time necessary to produce

failure so stress rupture testing is always done until failure.

Data is plotted log-log as in the chart above. A straight line or

best fit curve is usually obtained at each temperature of interest.

This information can then be used to extrapolate time to failure

for longer times. A typical set of stress rupture curves is shown

below.

38

Toughness

The ability of a metal to deform plastically and to absorb energy

in the process before fracture is termed toughness. The emphasis

of this definition should be placed on the ability to absorb

energy before fracture. Recall that ductility is a measure of how

much something deforms plastically before fracture, but just

because a material is ductile does not make it tough. The key to

toughness is a good combination of strength and ductility. A

material with high strength and high ductility will have more

toughness than a material with low strength and high ductility.

Therefore, one way to measure toughness is by calculating the

area under the stress strain curve from a tensile test. This value

is simply called “material toughness” and it has units of energy

per volume. Material toughness equates to a slow absorption of

energy by the material.

39

There are several variables that have a profound influence on the

toughness of a material. These variables are:

Strain rate (rate of loading)

Temperature

Notch effect

A metal may possess satisfactory toughness under static loads

but may fail under dynamic loads or impact. As a rule ductility

and, therefore, toughness decrease as the rate of loading

increases. Temperature is the second variable to have a major

influence on its toughness. As temperature is lowered, the

ductility and toughness also decrease. The third variable is

termed notch effect, has to due with the distribution of stress. A

material might display good toughness when the applied stress

is uniaxial; but when a multiaxial stress state is produced due to

the presence of a notch, the material might not withstand the

simultaneous elastic and plastic deformation in the various

directions.

40

There are several standard types of toughness test that generate

data for specific loading conditions and/or component design

approaches. Three of the toughness properties that will be

discussed in more detail are 1) impact toughness, 2) notch

toughness and 3) fracture toughness.

Fatigue Crack Growth Rate Properties

For some components the crack propagation life is neglected in

design because stress levels are high, and/or the critical flaw

size small. For other components the crack growth life might be

a substantial portion of the total life of the assembly. Moreover,

preexisting flaws or sharp design features may significantly

reduce or nearly eliminate the crack initiation portion of the

fatigue life of a component. The useful life of these components

may be governed by the rate of subcritical crack propagation.

Aircraft fuselage structure is a good example of structure that is

based largely on a slow crack growth rate design. Many years

ago, the USAF reviewed a great number of malfunction reports

from a variety of aircraft. The reports showed that the

preponderance of structural failures occurred from 1) built-in

preload stresses, 2) material flaws and 3) flaw caused by in-

service usage. These facts led to a design approach that required

the damage tolerance analysis to assume a material flaw exists

in the worst orientation and at the most undesirable location.

The analysis helps to ensure that structures are designed that

will support slow stable crack growth until the crack reaches a

length where it can reliable be detected

using NDT methods.

41

The rate of fatigue crack propagation is determined by

subjecting fatigue-cracked specimens, like the compact

specimen used in fracture toughness testing, to constant-

amplitude cyclic loading. The incremental increase in crack

length is recorded along with the corresponding number of

elapsed load cycles acquire stress intensity (K), crack length (a),

and cycle count (N) data during the test. The data is presented in

an “a versus N” curve as shown in the image to the right.

Various a versus N curves can be generated by varying the

magnitude of the cyclic loading and/or the size of the initial

crack.

The data can be reduced to a single curve by presenting the data

in terms of crack growth rate per cycle of loading (Da/ DN or

da/dN) versus the fluctuation of the stress-intensity factor at the

tip of the crack (DKI). DKI is representative of the mechanical

driving force, and it incorporates the effect of changing crack

length and the magnitude of the cyclic loading. (See the page on

fracture toughness for more information on the stress-intensity

factor.) The most common form of presenting fatigue crack

growth data is a log-log plot of da/dN versus DKI.

The fatigue crack propagation behavior of many materials can

be divided into three regions as shown in the image. Region I is

the fatigue threshold region where the Dk is too low to

propagate a crack. Region II encompasses data where the rate of

crack growth changes roughly linearly with a change in stress

intensity fluctuation. In region III, small increases in the stress

intensity amplitude, produce relatively large increases in crack

growth rate since the material is nearing the point of unstable

fracture.

General Material Classifications

42

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.

Metals

Ferrous 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)

Polymeric

Thermoplastics

plastics

Thermoset plastics

Elastomers

Ceramics

Glasses

Glass ceramics

Graphite

Diamond

Composites

Reinforced

plastics

Metal-matrix

composites

Ceramic-matrix

composites

Sandwich

structures

Concrete

Metal

Metallic elements

43

Alkali metals

lithium

sodium

potassium

rubidium

caesium

francium

Alkaline earth metals

beryllium

magnesium

calcium

strontium

barium

radium

Transition metals

scandium

titanium

vanadium

chromium

manganese

iron

cobalt

nickel

copper

zinc

yttrium

zirconium

niobium

molybdenum

technetium

ruthenium

rhodium

palladium

silver

cadmium

hafnium

tantalum

tungsten

rhenium

osmium

iridium

platinum

gold

mercury

rutherfordium

dubnium

seaborgium

bohrium

hassium

copernicium

Post-transition metals

aluminium

gallium

44

indium

tin

thallium

lead

bismuth

polonium

flerovium

Lanthanides

lanthanum

cerium

praseodymium

neodymium

promethium

samarium

europium

gadolinium

terbium

dysprosium

holmium

erbium

thulium

ytterbium

lutetium

Actinides

actinium

thorium

protactinium

uranium

neptunium

plutonium

americium

curium

berkelium

californium

einsteinium

fermium

mendelevium

nobelium

lawrencium

Elements which are possibly metals

meitnerium

darmstadtium

roentgenium

ununtrium

ununpentium

livermorium

ununseptium

Elements which are sometimes considered metals

germanium

arsenic

antimony

astatine

45

V

T

E

A metal (from Greek "μέταλλον" – métallon, "mine, quarry, metal"[1][2]) is a material (an element, compound, or alloy) that is typically

hard, opaque, shiny, and has good electrical and thermal conductivity. Metals are generally malleable— that is, they can be

hammered or pressed permanently out of shape without breaking or cracking — as well asfusible (able to be fused or melted)

and ductile (able to be drawn out into a thin wire).[3] About 91 of the 118 elements in the periodic table are metals (some elements

appear in both metallic and non-metallic forms).

The meaning of "metal" differs for various communities. For example, astronomers use the blanket term "metal" for convenience to

collectively describe all elements other than hydrogen and helium (the main components of stars, which in turn comprise most of the

visible matter in the universe). Thus, in astronomy and physical cosmology, the metallicityof an object is the proportion of its matter

made up of chemical elements other than hydrogen and helium.[4] In addition, many elements and compounds that are not normally

classified as metals become metallic under high pressures; these are known as metallic allotropes of non-metals.

Contents

[hide]

1 Structure and bonding

2 Properties

o 2.1 Chemical

o 2.2 Physical

o 2.3 Electrical

o 2.4 Mechanical

3 Alloys

4 Categories

o 4.1 Base metal

o 4.2 Ferrous metal

o 4.3 Noble metal

o 4.4 Precious metal

5 Extraction

6 Recycling of metals

7 Metallurgy

8 Applications

9 Trade

10 History

11 See also

12 References

13 External links

Structure and bonding

46

hcp and fcc close-packing of spheres

The atoms of metallic substances are closely positioned to neighboring atoms in one of two common arrangements. The first

arrangement is known as body-centered cubic. In this arrangement, each atom is positioned at the center of eight others. The other is

known as face-centered cubic. In this arrangement, each atom is positioned in the center of six others. The ongoing arrangement of

atoms in these structures forms a crystal. Some metals adopt both structures depending on the temperature.[5]

Atoms of metals readily lose their outer shell electrons, resulting in a free flowing cloud of electrons within their otherwise solid

arrangement. This provides the ability of metallic substances to easily transmit heat and electricity. While this flow of electrons occurs,

the solid characteristic of the metal is produced by electrostatic interactions between each atom and the electron cloud. This type of

bond is called a metallic bond.[6]

Properties[edit] Chemical[edit]

Metals are usually inclined to form cations through electron loss,[6] reacting with oxygen in the air to form oxides over various

timescales (iron rusts over years, while potassium burns in seconds). Examples:

4 Na + O2 → 2 Na2O (sodium oxide)

2 Ca + O2 → 2 CaO (calcium oxide)

4 Al + 3 O2 → 2 Al2O3 (aluminium oxide).

The transition metals (such as iron, copper, zinc, and nickel) are slower to oxidize because they form a passivating

layer of oxide that protects the interior. Others, like palladium, platinum and gold, do not react with the atmosphere

at all. Some metals form a barrier layer of oxide on their surface which cannot be penetrated by further oxygen

molecules and thus retain their shiny appearance and good conductivity for many decades (like aluminium,

magnesium, some steels, and titanium). The oxides of metals are generally basic, as opposed to those

of nonmetals, which are acidic. Blatant exceptions are largely oxides with very high oxidation states such as CrO3,

Mn2O7, and OsO4, which have strictly acidic reactions.

47

Painting, anodizing or plating metals are good ways to prevent their corrosion. However, a more reactive metal in

the electrochemical series must be chosen for coating, especially when chipping of the coating is expected. Water

and the two metals form an electrochemical cell, and if the coating is less reactive than the coatee, the coating

actually promotes corrosion.

Physical[edit]

Gallium crystals

Metals in general have high electrical conductivity, high thermal conductivity, and high density. Typically they are

malleable and ductile, deforming under stress without cleaving.[6] In terms of optical properties, metals are shiny

and lustrous. Sheets of metal beyond a few micrometres in thickness appear opaque, but gold leaf transmits green

light.

Although most metals have higher densities than most nonmetals,[6] there is wide variation in their

densities, Lithium being the least dense solid element and osmium the densest. The alkali and alkaline earth metals

in groups I A and II A are referred to as the light metals because they have low density, low hardness, and low

melting points.[6] The high density of most metals is due to the tightly packed crystal lattice of the metallic structure.

The strength of metallic bonds for different metals reaches a maximum around the center of the transition

metal series, as those elements have large amounts of delocalized electrons in tight binding type metallic bonds.

However, other factors (such as atomic radius, nuclear charge, number of bonds orbitals, overlap of orbital

energies and crystal form) are involved as well.[6]

Electrical[edit]

Filling of the electronic Density of states in various types of materials at equilibrium. Here the vertical axis is energy while the

horizontal axis is the Density of states for a particular band in the material listed. Inmetals and semimetals the Fermi level EF lies

inside at least one band. In insulators and semiconductorsthe Fermi level is inside a band gap; however, in semiconductors the

bands are near enough to the Fermi level to be thermally populated with electrons orholes.

edit

The electrical and thermal conductivities of metals originate from the fact that their outer electrons aredelocalized.

This situation can be visualized by seeing the atomic structure of a metal as a collection of atoms embedded in a

sea of highly mobile electrons. The electrical conductivity, as well as the electrons' contribution to the heat capacity

and heat conductivity of metals can be calculated from the free electron model, which does not take into account

the detailed structure of the ion lattice.

When considering the electronic band structure and binding energy of a metal, it is necessary to take into account

the positive potential caused by the specific arrangement of the ion cores – which is periodic in crystals. The most

48

important consequence of the periodic potential is the formation of a small band gap at the boundary of the Brillouin

zone. Mathematically, the potential of the ion cores can be treated by various models, the simplest being the nearly

free electron model.

Mechanical[edit]

Mechanical properties of metals include ductility, i.e. their capacity for plastic deformation. Reversible elastic

deformation in metals can be described by Hooke's Law for restoring forces, where the stress is linearly

proportional to the strain. Forces larger than the elastic limit, or heat, may cause a permanent (irreversible)

deformation of the object, known as plastic deformation or plasticity. This irreversible change in atomic

arrangement may occur as a result of:

The action of an applied force (or work). An applied force may be tensile (pulling) force, compressive (pushing)

force, shear, bending or torsion (twisting) forces.

A change in temperature (heat). A temperature change may affect the mobility of the structural defects such

as grain boundaries, point vacancies, line and screw dislocations, stacking faults and twins in

both crystalline and non-crystalline solids. The movement or displacement of such mobile defects is thermally

activated, and thus limited by the rate of atomic diffusion.

Hot metal work from a blacksmith.

Viscous flow near grain boundaries, for example, can give rise to internal slip, creep and fatigue in metals. It can

also contribute to significant changes in the microstructure like grain growth and localized densification due to the

elimination of intergranular porosity. Screw dislocations may slip in the direction of any lattice plane containing the

dislocation, while the principal driving force for "dislocation climb" is the movement or diffusion of vacancies through

a crystal lattice.

In addition, the nondirectional nature of metallic bonding is also thought to contribute significantly to the ductility of

most metallic solids. When the planes of an ionic bond slide past one another, the resultant change in location

shifts ions of the same charge into close proximity, resulting in the cleavage of the crystal; such shift is not

observed in covalently bondedcrystals where fracture and crystal fragmentation occurs.[7]

Alloys[edit] Main article: Alloy

An alloy is a mixture of two or more elements in which the main component is a metal. Most pure metals are either

too soft, brittle or chemically reactive for practical use. Combining different ratios of metals as alloys modifies the

properties of pure metals to produce desirable characteristics. The aim of making alloys is generally to make them

less brittle, harder, resistant to corrosion, or have a more desirable color and luster. Of all the metallic alloys in use

today, the alloys of iron (steel,stainless steel, cast iron, tool steel, alloy steel) make up the largest proportion both

by quantity and commercial value. Iron alloyed with various proportions of carbon gives low, mid and high carbon

steels, with increasing carbon levels reducing ductility and toughness. The addition of silicon will produce cast

irons, while the addition of chromium, nickel and molybdenum to carbon steels (more than 10%) results in stainless

steels.

Other significant metallic alloys are those of aluminium, titanium, copper and magnesium. Copper alloys have been

known since prehistory—bronze gave theBronze Age its name—and have many applications today, most

importantly in electrical wiring. The alloys of the other three metals have been developed relatively recently; due to

their chemical reactivity they require electrolytic extraction processes. The alloys of aluminium, titanium and

magnesium are valued for their high strength-to-weight ratios; magnesium can also provide electromagnetic

49

shielding[citation needed]. These materials are ideal for situations where high strength-to-weight ratio is more important

than material cost, such as in aerospace and some automotive applications.

Alloys specially designed for highly demanding applications, such as jet engines, may contain more than ten

elements.

Categories[edit] Base metal[edit] Main article: Base metal

Zinc, a base metal, reacting with an acid

In chemistry, the term base metal is used informally to refer to a metal that oxidizes or corrodes relatively easily,

and reacts variably with dilute hydrochloric acid (HCl) to form hydrogen. Examples include iron, nickel, lead and

zinc. Copper is considered a base metal as it oxidizes relatively easily, although it does not react with HCl. It is

commonly used in opposition to noble metal.

In alchemy, a base metal was a common and inexpensive metal, as opposed to precious metals, mainly gold and

silver. A longtime goal of the alchemists was the transmutation of base metals into precious metals.

In numismatics, coins in the past derived their value primarily from the precious metal content. Most modern

currencies arefiat currency, allowing the coins to be made of base metal.

Ferrous metal[edit] Main article: Ferrous and non-ferrous metals

The term "ferrous" is derived from the Latin word meaning "containing iron". This can include pure iron, such

as wrought iron, or an alloy such as steel. Ferrous metals are often magnetic, but not exclusively.

Noble metal[edit] Main article: Noble metal

Noble metals are metals that are resistant to corrosion or oxidation, unlike most base metals. They tend to be

precious metals, often due to perceived rarity. Examples include gold, platinum, silver and rhodium.

Precious metal[edit]

A gold nugget

Main article: Precious metal

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A precious metal is a rare metallic chemical element of high economic value.

Chemically, the precious metals are less reactive than most elements, have high luster and high electrical

conductivity. Historically, precious metals were important as currency, but are now regarded mainly as investment

and industrialcommodities. Gold, silver, platinum and palladium each have an ISO 4217 currency code. The best-

known precious metals are gold and silver. While both have industrial uses, they are better known for their uses

in art, jewelry, and coinage. Other precious metals include the platinum group metals: ruthenium, rhodium,

palladium, osmium, iridium, and platinum, of which platinum is the most widely traded.

The demand for precious metals is driven not only by their practical use, but also by their role as investments and

a store of value. Palladium was, as of summer 2006, valued at a little under half the price of gold, and platinum at

around twice that of gold. Silver is substantially less expensive than these metals, but is often traditionally

considered a precious metal for its role in coinage and jewelry.

Extraction[edit]

Crystal Defects

A perfect crystal, with every atom of the same type in the correct position,

does not exist. All crystals have some defects. Defects contribute to the

mechanical properties of metals. In fact, using the term “defect” is sort of a

misnomer since these features are commonly intentionally used to

manipulate the mechanical properties of a material. Adding alloying

elements to a metal is one way of introducing a crystal defect. Nevertheless,

the term “defect” will be used, just keep in mind that crystalline defects are

not always bad. There are basic classes of crystal defects:

point defects, which are places where an atom is missing or

irregularly placed in the lattice structure. Point defects include lattice

vacancies, self-interstitial atoms, substitution impurity atoms, and

interstitial impurity atoms

linear defects, which are groups of atoms in irregular positions.

Linear defects are commonly called dislocations.

planar defects, which are interfaces between homogeneous regions of

the material. Planar defects include grain boundaries, stacking faults

and external surfaces.

It is important to note at this point that plastic deformation in a material

occurs due to the movement of dislocations (linear defects). Millions of

dislocations result for plastic forming operations such as rolling and

extruding. It is also important to note that any defect in the regular lattice

structure disrupts the motion of dislocation, which makes slip or plastic

deformation more difficult. These defects not only include the point and

planer defects mentioned above, and also other dislocations. Dislocation

movement produces additional dislocations, and when dislocations run into

each other it often impedes movement of the dislocations. This drives up the

force needed to move the dislocation or, in other words, strengthens the

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material. Each of the crystal defects will be discussed

in more detail in the following pages.

Point Defects

Point defects are where an atom is missing or is in an

irregular place in the lattice structure. Point defects

include self interstitial atoms, interstitial impurity

atoms, substitutional atoms and vacancies. A self

interstitial atom is an extra atom that has crowded its

way into an interstitial void in the crystal structure.

Self interstitial atoms occur only in low concentrations

in metals because they distort and highly stress the

tightly packed lattice structure.

A substitutional impurity atom is an atom of a different type than the bulk

atoms, which has replaced one of the bulk atoms in the lattice. Substitutional

impurity atoms are usually close in size (within approximately 15%) to the

bulk atom. An example of substitutional impurity atoms is the zinc atoms in

brass. In brass, zinc atoms with a radius of 0.133 nm have replaced some of

the copper atoms, which have a radius of 0.128 nm.

Interstitial impurity atoms are much smaller than the atoms in the bulk

matrix. Interstitial impurity atoms fit into the open space between the bulk

atoms of the lattice structure. An example of interstitial impurity atoms is

the carbon atoms that are added to iron to make steel. Carbon atoms, with a

radius of 0.071 nm, fit nicely in the open spaces between the larger (0.124

nm) iron atoms.

Vacancies are empty spaces where an atom should be, but is missing. They

are common, especially at high temperatures when atoms are frequently and

randomly change their positions leaving behind empty lattice sites. In most

cases diffusion (mass transport by atomic motion) can only occur because of

vaca

Linear Defects - Dislocations

Dislocations are another type of defect in crystals. Dislocations are areas

were the atoms are out of position in the crystal structure. Dislocations are

generated and move when a stress is applied. The motion of dislocations

allows slip – plastic deformation to occur.

Before the discovery of the dislocation by Taylor, Orowan and Polyani in

1934, no one could figure out how the plastic deformation properties of a

metal could be greatly changed by solely by forming (without changing the

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chemical composition). This became even bigger mystery when in the early

1900’s scientists estimated that metals undergo plastic deformation at forces

much smaller than the theoretical strength of the forces that are holding the

metal atoms together. Many metallurgists remained skeptical of the

dislocation theory until the development of the transmission electron

microscope in the late 1950’s. The TEM allowed experimental evidence to

be collected that showed that the strength and ductility of metals are

controlled by dislocations.

There are two basic types of dislocations, the edge dislocation and the screw

dislocation. Actually, edge and screw dislocations are just extreme forms of

the possible dislocation structures that can occur. Most dislocations are

probably a hybrid of the edge and screw forms but this discussion will be

limited to these two types.

Edge Dislocations The edge defect can be easily visualized as an extra half-plane of atoms in a

lattice. The dislocation is called a line defect because the locus of defective

points produced in the lattice by the dislocation lie along a line. This line

runs along the top of the extra half-plane. The inter-atomic bonds are

significantly distorted only in the immediate vicinity of the dislocation line.

Understanding the movement of a dislocation is key to understanding why

dislocations allow deformation to occur at much lower stress than in a

perfect crystal. Dislocation motion is analogous to movement of a

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caterpillar. The caterpillar would have to exert a

large force to move its entire body at once. Instead

it moves the rear portion of its body forward a

small amount and creates a hump. The hump then

moves forward and eventual moves all of the body

forward by a small amount.

As shown in the set of images above, the

dislocation moves similarly moves a small amount

at a time. The dislocation in the top half of the

crystal is slipping one plane at a time as it moves

to the right from its position in image (a) to its

position in image (b) and finally image (c). In the

process of slipping one plane at a time the dislocation propagates across the

crystal. The movement of the dislocation across the plane eventually causes

the top half of the crystal to move with respect to the bottom half. However,

only a small fraction of the bonds are broken at any given time. Movement

in this manner requires a much smaller force than breaking all the bonds

across the middle plane simultaneously.

Screw Dislocations There is a second basic type of dislocation, called screw dislocation. The

screw dislocation is slightly more difficult to visualize. The motion of a

screw dislocation is also a result of shear stress, but the defect line

movement is perpendicular to direction of the stress and the atom

displacement, rather than parallel. To visualize a screw dislocation, imagine

a block of metal with a shear stress applied across one end so that the metal

begins to rip. This is shown in the upper right image. The lower right image

shows the plane of atoms just above the rip. The atoms represented by the

blue circles have not yet moved from their original position. The atoms

represented by the red circles have moved to their new position in the lattice

and have reestablished metallic bonds. The atoms represented by the green

circles are in the process of moving. It can be seen that only a portion of the

bonds are broke at any given time. As was the case with the edge

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dislocation, movement in this manner requires a much smaller force than

breaking all the bonds across the middle plane simultaneously.

If the shear force is increased, the atoms will continue to slip to the right. A

row of the green atoms will find there way back into a proper spot in the

lattice (and become red) and a row of the blue atoms will slip out of position

(and become green). In this way, the screw dislocation will move upward in

the image, which is perpendicular to direction of the stress. Recall that the

edge dislocation moves parallel to the direction of stress. As shown in the

image below, the net plastic deformation of both edge and screw

dislocations is the same, however.

The dislocations move along the densest planes of atoms in a material,

because the stress needed to move the dislocation increases with the spacing

between the planes. FCC and BCC metals have many dense planes, so

dislocations move relatively easy and these materials have high ductility.

Metals are strengthened by making it more difficult for dislocations to

move. This may involve the introduction of obstacles, such as interstitial

atoms or grain boundaries, to “pin” the dislocations. Also, as a material

plastically deforms, more dislocations are produced and they will get into

each others way and impede movement. This is why strain or work

hardening occurs.

In ionically bonded materials, the ion must move past an area with a

repulsive charge in order to get to the next location of the same charge.

Therefore, slip is difficult and the materials are brittle. Likewise, the low

density packing of covalent materials makes them generally more brittle

than metals.

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Bulk Defects

Bulk defects occur on a much bigger scale than the rest of the crystal defects

discussed in this section. However, for the sake of completeness and since

they do affect the movement of dislocations, a few of the more common

bulk defects will be mentioned. Voids are regions where there are a large

number of atoms missing from the lattice. The image to the right is a void in

a piece of metal The image was acquired using a Scanning Electron

Microscope (SEM). Voids can occur for a number of reasons. When voids

occur due to air bubbles becoming trapped when a material solidifies, it is

commonly called porosity. When a void occurs due to the shrinkage of a

material as it solidifies, it is called cavitation.

Another type of bulk defect occurs when impurity atoms cluster together to

form small regions of a different phase. The term ‘phase’ refers to that

region of space occupied by a physically homogeneous material. These

regions are often called precipitates. Phases and precipitates will be

discussed in more detail latter.