MATE 202: Introduction to Materials Engineering

Introduction
Chapter 1

The force-distance and energy-distance curves for a covalent bond, and visualization of atomic interaction
between “atoms” of Silly PuttyTM. Atoms become closer and deform under high compressive force
(requiring energy), and a long range interaction under tensile forces remains (also requiring energy).

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Chapter 1: Introduction and Bonding

Interatomic bonding is a major determining factor in how materials are classified and how they behave
in response to various stimuli and service conditions (e.g., mechanical stress, temperature, pressure,
electromagnetic radiation, chemicals, etc.). Bonding is the most basic of foundations for understanding
materials behaviour.

1.1 Definition of materials engineering

Materials engineering encompasses many functions, including:
 Transformation of minerals and recycled materials into usable and valuable forms
 Adjustment of materials processing to elicit different materials properties and performance
 Proper selection and use of materials
 Design of new materials and processes to enhance performance
 Testing of materials
Materials engineers, for the most part, are involved with the design and selection of materials for various
applications, and they use the materials paradigm in design.

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 MATE 202: Introduction to Materials Engineering

The materials paradigm:

STRUCTURE
Atomic scale and up

PERFORMANCE
& COST

PROPERTIES PROCESSING

Figure 1.1 The Interaction of Properties, Processing and Structure on Performance and Cost.

The structure of a material dictates its properties, and the structure is in turn influenced by how the
material was made (processed). Hence, the materials paradigm is an interdependent set of criteria that
must be optimized in order to achieve the desired performance at the specified cost. Many properties are
often considered in materials selection and design.

“THINK LIKE AN ATOM”

If you understand how atoms behave and the implications of the bonds that form between differ-
ent elements then you will be able to predict properties, and then choose suitable processes to take ad-
vantage of materials.

List of all the engineering materials that are present in the classroom.

1.2 Classes of Engineering Materials

All engineering materials can be categorized into one of the 5 classes of materials based on their primary
type of atomic bond, which are listed in green in the above list. In some cases materials can be cross-
listed for the sake of application, but in most cases we classify according to bond type only. There are
also secondary bonds in all materials, these are dipole-dipole interactions of different types known by a
variety of names such as van der Waals, London dispersion forces, Keesom forces etc. The energy of the-
se secondary bonds is typically less than 10% of that of the primary bonds.

In this course you will learn concepts that can be applied to pretty much any situation you may encounter.
I will give you the opportunity to learn concepts and fundamentals, in addition to some discipline specific
declarative knowledge that will allow you to select, use, and be able to work with materials in your engi-
neering career.

1.3 Example of materials engineering

Electric guitar strings.

An electric guitar string is metal – steel in fact, and some are wound with nickel plated steel.

When tuned up to a very high note the “B” string on a guitar will fail (all will for that matter!)
Why ? Think about why the guitar string failed - on an atomic level.

Below is a performance level that you should attain by the end of this course…you should be able to pro-
duce an answer to this problem with at least 75% agreement to what is discussed.

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Chapter 1: Introduction and Bonding

Why does a guitar string break? (from an atomic scale standpoint)

Turn the machine head way

too much!

The string breaks about

here…

… or it breaks down here

by the ball end of the
string… do you know why?

Figure 1.2 The electric guitar and details of the string fracture

First of all we need to know the material, and some of the reasons it was chosen, then the properties of the
material, and what happens to the atomic structure under stress – this is the key to understanding the
result, and to understanding how we apply materials engineering.
Material:
 B string: carbon steel (mostly iron with less < 1wt% C) 0.36mm in diameter, tension at 8.07kg
 Metals are crystalline (formed of many tiny “grains”)
 This steel is processed to be in a very strong metastable phase called martensite
Why steel?
 Very strong, relatively stiff – won’t permanently deform under stress, otherwise it couldn’t be easily
tuned, or hold its note.
 Martensitic steel is magnetically susceptible, so it works with the electronic pickups to allow amplifi-
cation
 Corrosion resistance to human sweat is decent, but not great (which is one of the reasons we need to
replace guitar strings over time). Corrosion products will be ceramics, and metal salts
 Steel is relatively inexpensive (buy a pack of 6 guitar strings for about $8 – which means they cost a
lot less to produce!)
 Steel is easily formed and heat treatable to attain different properties
 While the final form is martensite, the initial structure would have been a much softer structure
(phase) allowing easy formation and wire drawing through a die
 After attaining the proper size the steel would have been heat treated to form metastable marten-
site (austenitized first and then rapidly quenched from high temperature to form martensite)

So, why does the string break?

 Some metals can deform elastically to rather high stresses in tension (up to 2-3GPa).
 Elastic deformation – once the stress is released the dimensions return to their original values.
 Elastic deformation is completely reversible - the atomic bonds are stretched, but no atoms
permanently change position.
 If the metals are stressed past their elastic limit (proportional limit) the deformation is permanent,
called plastic deformation. The stress at which plastic deformation occurs is called the yield stress.
 Plastic deformation is irreversible - the atomic bonds are stretched so much that atomic defects
(dislocations) propagate through the metal crystals causing permanent rearrangement of atoms
through bonds breaking and reforming.
 Martensite, while strong (high yield strength), is not very ductile – which means it cannot handle

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 MATE 202: Introduction to Materials Engineering

much plastic deformation once the yield stress has been reached (not much permanent deformation).
 So, when the guitar string is tensioned, it can handle a great deal of stress before plastic deformation
occurs – but once plastic deformation ensues there is so much stored elastic strain energy that defects
can cause stress concentrations and fracture with relative ease.
 Once plastic deformation begins not much additional stress is required to cause fracture.
 At the yield stress atomic line defects (dislocations) are created and propagate causing shearing
of the metallic crystals. The shear is a localized atomic motion resulting in atomic steps on the
surface of the metal.
 Dislocations move differently in different directions within crystals, and different amounts when
the applied stress is resolved into these directions.
 Dislocations pile up at grain boundaries (regions between crystals) and when they build up
enough a crack can form and travel at the velocity of sound in the material causing fracture.
 As dislocations move they interact and their density increases. These processes lead to a higher
applied stress to cause their motion, and hence the metal becomes stronger as more stress is ap-
plied – this is called strain or work hardening.
 Guitar strings strain harden in certain locations – at the machine head, and by the ball end. The
dislocation density becomes higher in these locations, and the yield stress is higher here… even-
tually, these regions become the localizations of stress concentrations and crack formation –
which is why the strings fail in these locations. You can also notice that the strings have been
permanently deformed because the shape is no longer a straight wire.

A schematic stress-strain curve of two different steels. One that has been heat treated to form martensite,
and another to form spheroidite – a very soft, weak, and ductile form of steel that requires a long time at
high temperature to achieve. We will learn more about stress-strain behaviour in Chapter 2.

Engineering
stress,  Stress-strain curve
approximates a mar-
y X Fracture tensitic steel – (e.g.,
guitar string)

Elastic
Initially Elastic+Plastic at larger Stress-strain curve
stress approximates a sphe-
roidized steel – very
weak but ductile

X Fracture

engineering
strain, 

Figure 1.3 Engineering stress-strain curve for two differently heat treated steels. Notice the high yield
stress, y, of the upper curve, but low ductility (no plastic deformation).

GUITAR STRINGS:

Different strings are made out of different materials. Some wound strings are coated with polymers to
prevent corrosion; some are steel cores wound with phosphorous bronze wire, or nickel plated steel. Oth-
ers are wrapped with silver plated copper wire, or are made out of nylon polymers…the list goes on – all
in the name of tone, longevity, and strength.

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Chapter 1: Introduction and Bonding

Figure 1.4 A close up of some

guitar strings (public domain).

Figure 1.5 A) Schematic view of the investigated samples of electric guitar strings E6, A5 and D4. Sam-
ples consist of two different parts: Sn-plated steel core wire which is hexagonal in cross-section and Ni-
plated steel wrap which is round in cross-section. B) SEM micrograph of the cross-section of the core
and wrap of electric guitar string E6. C) SEM micrograph of the cross-section of the G3 electric guitar
string (same shape also in strings B2 and E1.; difference is only in string diameter. D) Optical micrograph
of cross section of E6 electrical guitar string microstructure, etched with 2% nital (2 mL of HNO3 to 98
mL ethanol)

Materials and Corrosion 2009, 60, No. 9999, Study of microstructure and corrosion kinetic of steel guitar strings in artificial sweat
solution. I. Rezic´*, L. C´urkovic´ and M. Ujevic´

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 MATE 202: Introduction to Materials Engineering

Figure 1.6. Some micrographs of etched cross sections of micrographs showing the structure of the
grains (Source: www.cashenblades.com/info/martensite.htm)

Crystal structure of Martensite:

Figure 1.7. The unit cell for martensite
showing the locations of the C atoms
which affect the c/a ratio in this structure.
Martensite is a metastable structure trans-
formed from a different crystal structure
(FCC austenite) upon cooling a carbon
steel from high temperature (above ~727°
C).
Martensite is very hard and very brittle.
These properties are related to its crystal
structure, and the nature of the defects and
atomistic deformation mechanisms pre-
sent in metals.
Martensite can be transformed by heating
back to high temperature (autenitized).
When it is subsequently recooled it will
form different structures which have dif-
ferent properties depending on the cooling
rate. Martensite only forms with relative-
ly rapid cooling!…
You will learn a lot more about phases and their transformations in Chapters 8 and 9.

Summary of guitar strings

Guitar strings hold a rich study of materials engineering applied to real world common applications. Most
objects in our world have similarly rich discussion with regard to materials engineering - it is all around
you but you may never have known it! or never known why! This course aims to open your mind the fun-
damentals of materials science and engineering in an accessible way.

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Chapter 1: Introduction and Bonding

Units in this course related to guitar strings: ALL OF THEM!

Topic Yes or No?

Introduction, bonding YES
Mechanical Properties YES
Failure YES
Structure YES
Atom movements YES
Plastic Deformation and strengthening YES
Annealing YES
Phase diagrams YES
Transformations YES
Corrosion YES
Polymers YES
Ceramics YES

Why do you think all of these units are required for this course?

The details of the technology around guitar strings are interesting not only as examples showing the range
of materials, but also because we see that the evolution of this technology has been driven by improving
materials. This is a common experience in most areas of technology, progress follows from developments
in materials and processing.

The guitar string example demonstrated that structure, properties, and processing are all important consid-
erations in the performance of materials (the materials paradigm). At the heart of structure is interatomic
(intermolecular) bonding, which will be further explained in Chapter 4, but a brief review is given below.

1.4 Bond characteristics and implications (metals, ceramics, polymers)

Metals & alloys

+ + +
 Metallic bonding
 1-3 donated valence electrons - sea of electrons
+ + +  Non-directional bonds – atoms can move easily without bond
breaking
+ + +
 Variable strength

Ceramics & Glasses

 Covalent bonding
+ +  Shared electrons (denoted by : at left)
 Strong and inflexible bond
 Electronegativity is comparable

+ -  Ionic bonding
 Between + and – ions
 Electron transfer, Coulombic attraction
 Strong and inflexible bond – electrons are tightly bound
 Large electronegativity difference required
Polymers
Side-  Long chains of covalently bound C atoms (usually)
group + + +  Side groups are covalently bound to chain
Chain + + +  Side group interactions are through secondary bonding (i.e., di-
pole-dipole interactions such as van der Waals bonds: London,
+ + + Debye, Hydrogen bonds (Keesom),– denoted as at left)
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 MATE 202: Introduction to Materials Engineering

Implications of bonding
Metals, ceramics, glasses, and polymers differ from each other in a fundamental way with respect to their:
Strength
Formability
Stiffness
Electrical conductivity

Having an understanding of the above primary bond types is critical for applying materials engineering,
because all of the differences in material properties are directly related to atomic bond type, therefore we
cannot easily change a class of materials’ properties. We will talk more about these bond types (and sec-
ondary bonds) in Chapter 4 but the information here is review.

Table 1 summarizes the bond types, their character, occurrence, properties of materials and implications
in materials processing and use.

Table 1

Bond type Bond Seen in materi- Materials Properties Implications

Characteristics als such as*
Metallic Non-directional Metals and alloys Strong Easily formed
(sea of elec- Ductile Moderately strong
trons) Flexible Good for electrical
Electronic conductors current carrier
Covalent Shared Ceramics & Very strong Hard to form
electrons Glasses Brittle Strong in compression
Directional Polymers Stiff Electrically insulating
Semiconductors Poor electronic conduc-
tors
Ionic Electron Ceramics Very strong Hard to form
transfer Brittle Strong in compression
Non-directional Stiff Electrically insulating
Some electronic con-
duction in defected
structures…
Secondary Directional All materials Flexible Easily formed
Inter-chain Polymers Weak
Inter-molecular (Depends mostly on primary
bond type, unless intermolec-
ular interactions are strong,
such as in water, diatomic
gases etc.)

* Composites show a variety of bond type depending on which classes of materials are mixed to form the composite

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Chapter 1: Introduction and Bonding

SYLLABUS REVIEW
Please be familiar with your syllabus, the code of student behaviour, lab schedule, and dates of assign-
ments, exams.

Assignments are group assignments, and 1 assignment per group will be graded with each group member
getting the same grade.

Please understand that the renovations in the CME building are underway and that it will affect the spaces
where some of the labs are held. We don’t have a choice, and we spent most of the summer trying to
make everything as smooth as possible.

The high enrollment, lack of extra lab space, and your other courses with labs all affect the lab schedule.
Labs may not appear to be following the lectures…it is OK for you to have these feelings. You are simply
accustomed to hearing everything in lecture first, then going to the lab. But, you can learn in the lab first -
it may just seem more stressful, and you will have to be more prepared (i.e., read ahead a bit and do the
pre-labs etc.). It turns out that educational research suggests that for deeper and longer lasting learning we
should really be teaching in reverse compared to current practice; ideally you would go to the lab first,
then a seminar, then come to lecture to hear the big concepts of what you have been discovering. Co-op,
or summer engineering jobs set you up for this sort of learning…

Statistics show that students working in groups tend to have higher exam test scores and deeper learning
than those who work alone.

NOTES:

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