Introduction to Engineering Materials

General definition of material

Materials are defined as ‘substances of which something is composed or made’. Materials play
an important role for our existence, for our day to day needs, and even for our survival.
We obtain materials from earth crust and atmosphere. Examples:-
 Silicon and Iron constitute 27.72 and 5.00 percentage of weight of earths crust respectively.
 Nitrogen and Oxygen constitute 78.08 and 20.95 percentage of dry air by volume
respectively.

Definition of materials science

Materials science is an engineering and scientific discipline that involves the study of the
properties and characteristic of all materials. This field relies heavily on the principles of physics,
chemistry and in some cases biology, and is dedicated to improving the properties of existing
engineering materials as well as creating new ones.

Definition: Engineering Material:

Part of inanimate matter, which is useful to engineer in the practice of his/her profession (used to
produce products according to the needs and demand of society)

Why the study of materials is important

 Production and processing of materials constitute a large part of our economy.
 Engineers choose materials to suite design.
 New materials might be needed for some new applications.
Example: - High temperature resistant materials.
- Space station and Mars Rovers should sustain conditions in space.
(High speed, low temperature, strong but light.)
 Modification of properties might be needed for some applications.
Example: - Heat treatment to modify properties.

Over the past several decades numerous traditional building methods have been investigated and
in some cases revived and improved upon by a new visionary design builder. These techniques
are often grouped under the level “natural building” which is a building philosophy that relies on
materials and techniques that are ecologically sound, culturally sensitive, reliant on local
resources and skills, and are within economic reach of local inhabitants.

Natural building has emerged as a response to an increasing concern to our beauty environment.
Natural materials are an alternative to toxic substances which have led to wide spread
environmental illness. As part of natural building technique, materials are being developed in
line with environmental issues; these materials have been dubbed eco-materials. The concept of
eco-materials is meant to encourage the development of materials that are:
 i) Harmless to the environment and human beings
ii) Exert minimal burden on the planet.

STRUCTURE OF MATERIAL
Structure means a description of the arrangements of atoms or ions in a material.
 Subatomic level: Electronic structure of individual atoms that defines interaction among
atoms (interatomic bonding).
 Atomic level: Arrangement of atoms in materials (for the same atoms can have different
properties, e.g. two forms of carbon: graphite and diamond)
 Microscopic structure: Arrangement of small grains of material that can be identified by
microscopy.

Crystallography
Crystallography is the formal study of the arrangements of atoms in solids. Crystallographic
directions are used to indicate a particular orientation of a single crystal or of an oriented
polycrystalline material. Knowing how to describe these can be useful in many applications.
Metals deform more easily, for example, in directions along which atoms are in closest contact.
 Materials may be classified as Crystalline or Non-Crystalline structures
 Crystalline solid can be either Single Crystal Solid (crystal lattice of entire sample is
continuous and unbroken to edges of sample with no grain boundary) or Poly Crystal Solid
(aggregate of many crystals separated by well-defined boundaries)
 Cluster of crystals with identical structure (same crystallographic planes & directions) are
known as Grains separated by Grain Boundaries
 X-ray diffraction analysis shows that atoms in metal crystal are arranged in a regular,
repeated 3-D Pattern known as Crystalline Structure

Bonding forces

Bonding forces are forces of attraction or repulsion which act between neighboring particles such
as atoms, molecules or ions. Bonding force determines the elastic modulus (or Young's modulus)
of a material (how stiff a material is).

The strength of chemical bonds varies considerably; there are "strong bonds" such as covalent or
ionic bonds, and "weak bonds" such as dipole-dipole interactions and hydrogen bonding.

Since interaction energy and bonding force are directly related, the stronger the bond energy, the
harder is to move the atoms, such as to melt the solid or to evaporate its atoms.

Atomic bonding
There are four important mechanisms by which atoms are bonded in engineered materials.
These are
1. Metallic bonds;
2. Covalent bonds;
3. Ionic bonds; and
4. van der Waals bonds.

The first three types of bonds are relatively strong and are known as primary bonds (relatively
strong bonds between adjacent atoms resulting from the transfer or sharing of outer orbital
electrons). The van der Waals bonds are secondary bonds and originate from a different
mechanism and are relatively weaker.
The binding energy is related to the strength of the bonds and is particularly high in ionically and
covalently bonded materials. Materials with a high binding energy often have a high melting
temperature, a high modulus of elasticity, and a low coefficient of thermal expansion.
A metallic bond is formed as a result of atoms of low electronegativity elements donating their
valence electrons and leading to the formation of a “sea” of electrons. Metallic bonds are non-
directional and relatively strong. As a result, most pure metals show a high Young’s modulus
and ductility. They are good conductors of heat and electricity and reflect visible light.

A covalent bond is formed when electrons are shared between two atoms. Covalent bonds are
found in many polymeric and ceramic materials. These bonds are strong and most inorganic
materials with covalent bonds exhibit high levels of strength, hardness, and limited ductility.
Most plastic materials based on carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds show
relatively lower strengths and good levels of ductility. Most covalently bonded materials tend to
be relatively good electrical insulators.

The ionic bonding found in many ceramics is produced when an electron is “donated” from one
electropositive atom to an electronegative atom, creating positively charged cations and
negatively charged anions. As in covalently bonded materials, these materials tend to be
mechanically strong and hard, but brittle. Melting points of ionically bonded materials are
relatively high. These materials are typically electrical insulators

The van der Waals bonds are formed when atoms or groups of atoms have a nonsymmetrical
electrical charge, permitting bonding by an electrostatic attraction.

Six categories that encompass the materials available to practicing engineers:

1) Metals (crystalline materials, metallic bonding)
2) Ceramics (crystalline materials, ionic bonding)
3) Glasses (mainly non-crystalline materials, ionic bond)
4) Polymers (non-crystalline materials, covalent bonding)
5) Semiconductors (unique electrical conducting behavior)
6) Composites (mixture of above materials)

Properties of materials
Each material has a property profile. The properties of engineering materials can be classified
into the following main groups: the physical and chemical. The physical properties can also be
further grouped into categories: mechanical, thermal, electrical, magnetic, optical etc. the
chemical properties include: environmental and chemical stability.

Composition, Bonding, Crystal Structure and Microstructure DEFINE Materials Properties.

Composition

Properties

Bonding Crystal Structure

Modulus of elasticity
The modulus of elasticity, or Young’s modulus (E), is the slope of the stress-strain curve in the
elastic region. This relationship between stress and strain in the elastic region is known as
Hooke’s Law.
The modulus is closely related to the binding energies of the atoms. A steep slope in the force-
distance graph at the equilibrium spacing indicates that high forces are required to separate the
atoms and cause the material to stretch elastically.
Thus, the material has a high modulus of elasticity. Binding forces, and thus the modulus of
elasticity, are typically higher for high melting point materials. In many materials, elastic stress
and elastic strain are linearly related.
Large elastic deformations are observed in elastomers (e.g., natural rubber, silicones), for which
the relationship between elastic strain and stress is non-linear.

Strength: The maximum stress material can take is known as ultimate strength. Ultimate
strength is equal to maximum load divided by original area of cross section.

 Tensile strength: The stress that corresponds to the maximum load in a tensile test.
 Compressive strength

The melting point of a solid is the temperature at which it changes state from solid to liquid at
atmospheric pressure. At the melting point the solid and liquid phases exist in equilibrium. The
melting point of a substance depends on pressure and is usually specified at standard pressure.
Melting point helps to determine materials to be used due to their difference in melting points
e.g. for soldering while joining metals. This also helps to know equipment used in hot working
processes such as furnaces, casting machines and forging dies must be designed to withstand
their high working temperatures.

The boiling point of a substance is the temperature at which the vapor pressure of the liquid
equals the pressure surrounding the liquid and the liquid changes into a vapor. This helps in
understanding of heating and cooling operations of substances.

Defect: A microstructural feature representing a disruption in the perfect periodic arrangement of

atoms ions in a crystalline material.
In actual crystals, imperfection or defects are always present, which are important to understand,
as they influence the properties of material.