Material Science

Professor S. K. Gupta
Department of Applied Mechanics
Indian Institute of Technology Delhi
Lecture No 1
Introduction

(Refer Slide Time: 1:04)

This is the first course on material science and this material science has 2 words materials and
science, what are these 2 words? We shall first try to understand before we proceed further.
What we see around us is everything is a material including the human beings, the flesh, the
bones, the blood, the plants, the buildings even the pen with which we write or this remote
control unit everything is a material. Are we going to be studying every material on the
Earth? Is not possible in a one semester course, we restrict ourselves to inanimate solids
thereby I am eliminating all living things secondly I am leaving out all liquids and gases and I
am restricting the solid inanimate solids to use by the engineer in the practice of his
profession. I am not worried about which is something which is not being used by an
engineer.

(Refer Slide Time: 2:26)

These materials can be classified in number of ways of course these days lot of new materials
have also come up. Basically if I go in a conventional manner one of the classification is
metals and alloys, metals you know copper, iron, aluminium, gold, silver, nickel, cadmium
these are all metals. Alloys, you have studied in school an alloy is a combination of one metal
with one or more elements such that a combination behaves like a metal. A metal like
behaviour as distinct from other materials can be defined with the help of one single property
and that is electrical resistivity of the material, only in this class of material the resistivity
increases with increasing temperature while in all other cases it decreases with increasing
temperature.

Second category is ceramics and glasses, ceramics is basically oxides and silicates like
porcelain, China and glasses, silicate glasses the window paints you see the windshield in
automobiles these are all glasses transparent material. Basically the oxides and silicates, these
are very hard materials at the same time very brittle. Another classification is of organic
polymers which we are using more and more as the time progresses, we have got now in an
engineering polymers with which you can make gears some examples are day-to-day use
polythene, PVC sheeting we put on the cables and tires, rubber tires you used for
automobiles, trucks, tractors use the polypropylene for the bonnet of a car. We use
polypropylene these days for the bumpers, dashboards these are all organic polymers. One
engineering polymer one very good engineering polymer Teflon provides frictionless
bearings.

Then we have next category which puts semiconductors, these computers which we are using
these facilities and the remote-control we have are all based on certain chip that is silicon
chip, silicon crystal in which we have put some impurities and they behave the material is
behaving like a semiconducting material. Elements are silicon and germanium basically
which behaves like semiconductors. The number of compounds made from elements taken
around group 4 like say for example group 4 and group 4 combination of silicon and carbon
silicon carbide also behaves like semiconductor.

One element taken from group 3 one side of the group 4 the other one taken from the right
side of the group 5 like you have aluminium phosphide, these act as semiconducting
materials similarly zinc sulphide, cadmium sulphide, cadmium Telluride some of these
materials which you have in light emitting diodes, the photocells these are all these kind of
compounds. Gallium arsenide is one very good material is far as laser is concerned. You want
to integrate it with silicon chip, we have certain difficulties, we shall see what are those
difficulties in the course?

Then we have materials which are called composites these can be called basically man-made
materials some of these could be natural we will see that metal ceramic composites like you
have the reinforced concrete cement, the building is made of reinforced concrete cement
where steel rods with which we make the cage and in the cage we pour the mortar containing
gravel, the cement, the sand and this is what is concrete because steel bars have been put
there it is called reinforced concrete cement or simply RCC.

Steel is a from the metals and alloys steel is an alloy because the one which we use is mostly
iron carbon alloy, plain carbon steel but we do have certain alloy elements as well like
manganese, silicon, et cetera in the normal steel which we have and this mortar which we
have put the concrete is a ceramic material containing silicates so this is a composite material
made of metallic metal and ceramic.

Then the second category of composites taken from ceramic and polymers, ceramic materials
as I already said oxide and silicate glasses of ceramic materials. When I reinforced these glass
fibres to the matrix of a resin which is a polymeric material, it is a ceramic polymer
composite. Fibre reinforced plastic you use the helmets made of fibre reinforced plastic, some
of the automobiles have come with a body made of fibre reinforced plastic. Fibre reinforced
plastic is used in various other household applications like the body of a washing machine
can make out of these.

Then we have metal polymer composite like vinyl coated steel, vinyl coating is done in order
to save or protect the steel rusting so it does not come in contact with the environment or you
have resin coated steel bars which are used for reinforcement so that again it does not get
rusted inside the concrete, if it gets rusted the strength of the concrete can come down. So
these are specific composites made by us to serve a certain specific purpose and this is also
called a man-made materials, so composite, we shall look into it briefly.

(Refer Slide Time: 10:39)

Then what we use these materials for as engineers, what is the application of these materials?
The different applications we engineers are putting these materials to, one is a structure, by
structure I mean something which is dead body like a building or a bridge or a body of car or
a body of a ship or a body of an aircraft that is structure and for using making these structures
I use these materials which I talked about. Then second category is machines, some things
which are mobile like engines, gears, pistons so something moving you know, so motors
these are all machines and mobile components.

Then we have devices like you have a computer is a device, radio set is a device, television is
a device it is just like some kind of a black box one kind of an input and other kind of an
output, so input is some signal and you get voice, you get the picture from TV that is a
device. It is a small thing but it does a function which is and converse input whatever is the
input into the output which is a different one altogether different from the input, your
transistor sets, the radios, the televisions all these things fall in this category of devices. Now
to make this we make use of materials from all the categories, say for example I just give you
an example of a structure the building, reinforced concrete cement has metal as well as
ceramic, the doors and windows to make the doorframe out of wood because wood is a
natural composite then you have the flooring you can see the tiles it is a polymeric material
your soundproofing done that is another polymeric material.

(Refer Slide Time: 13:06)

So different material used to make one structure, even for making a computer I am using
plastic, I am using metal, I am using semiconductors, so all very different kinds of materials
are used for making one application, okay. Then the second word in the course material
science is the science with when we talk science, chemistry and physics because mathematics
is also a science is usually the connotation we get is chemistry and physics. Since we are
talking about the materials which are solids it should be chemistry and physics of solid in
other words solid state chemistry or solid-state physics.

Both these subjects are very rigorous in their treatment and as engineers our interest is to see
the material does the function, does the work whatever application I put it to whether my
science is working rigorously or it is not working very rigorously, so we are trying to keep in
mind an engineering usefulness of the material that is what the science is all about I am not
very rigorous I may be using lot of approximations but as long as this approximation gives
me the result with which I predict that my material it is going to work or behave like this
should be good enough for us.

However, we do take lot of information from this solid-state chemistry and solid-state physics
as a matter of fact before this material science came in about some 45 years back we were
talking about the metallurgy where metals and alloys are being talked about, we are talking
about the ceramics, physical ceramics we are talking about the ceramics and silicates, oxides,
glasses and polymer science where we are talking about the polymers. In turn the material
science has an interaction with all these 3 subjects but in the science what we are studying
here is the relationship between the structure and property, what I simply call structure
property relationship in materials. When I say the word structure here I do not mean the
structure, application, structure application which are described just now building or a body
of a ship or a body of an automobile or a bridge those of the structures we call but this
structure when I refer to is an internal structure of the material, how is it related to the
property or the behaviour of the material?

(Refer Slide Time: 16:11)

Here I will just give an example, I have taken 2 cylinders of mild steel one as I received from
the market that is on the right-hand side the other one as it is I put this in the furnace for
annealing for half an hour and after it is anneal I tested like the other one this I am
compressing it in a machine, I reduce the length of the cylinder to 50 percent but you see the
deformation in the 2 is very different. One is giving as received gives me rings after rings
after rings it is called (())(16:55). Other one is giving me diamonds making triangles one
triangle another triangle and then 2 triangles making hexagon if you go through the cylinder
hole it will look like a hexagon.

(Refer Slide Time: 17:14)

What is different in the 2 is also seen here in the stress incur of the material, in the case of as
received the deformation goes on very steeply and then comes down rapidly while in case of
annealed one right here it starts to yield and the yielding goes on and then it takes over the
(())(17:43) then deforms a lot before it fails it deforms less before it fails that is received
annealed one deforms a lot before it fails, right? As far as internal structure is concerned the
difference between the 2 internal structures is shown here, the one which is as received is
little smaller in the grain size of the material, distribution of the another micro constituent that
is per light is finer as compared to the one which is annealed where you can see these black
portions which are per light they are coarser in size and the grain size are also much bigger
than the grain size in the as (())(18:32). This change in internal structure has cause this
change in behaviour that is what I mean by structure property relationship. Understand this
relationship then I will be able to use my materials to the maximum advantage as an engineer.

(Refer Slide Time: 19:00)

Now coming to the internal structure first level is macro structure what I see with the naked
eye or at the most I use a magnifying lens which is maybe 5 times, 10 times and I can read
probably in between these lines or I can see the scratches on the surface of the metal, or the
plastic this is what I mean by macro structure and the use of the naked eye. While resolution
of the human eye is 0.11 millimetre as you know, so anything which is closer than 0.11
millimetre I cannot see with the naked eye the 2 objects are placed closer than this I cannot
distinguish them as 2 different objects then second level is the microstructure one I showed
you for the steel just now.

To see this I have to use an optical microscope and the specimen of course has to be prepared,
polished, (())(20:12) before I see it in microscope and the magnification normally used the
400, 600, 800 that can go up to 1500 that is the limit of optical microscope I cannot
magnified beyond that and we are able to see the microstructure as I assure for the steel.
Third is the substructure the material which is at a still low-level of few tens of Angstrom I
want to see what is happening in the material magnification can go up to 1,000,000 and the
device use to see this is called an electron microscope. Electron microscopes are different
kinds scanning electron microscope it simply scan the surface of the material, opaque
material and tells you what is the surface like, depth of focus is very high and I can use the
transmission electron microscope where the material to be used is very thin so that the
electron beam passed through the material.

So solid metal of an alloy if I have to put I have make thin it down to a very small in the
range of fraction of a micron. Usually what we do is make a thin disk in their range of about
30 to 40 micro-meter and then with the help of jets on from both sides attached it with this
acid or mixture of acid so that a hole is formed on this disk, when the hole is formed with the
help of jet you will see the edges of the whole will be very fine and this are the edges which
we try to see through the transmission electron microscope. Means what I am trying to go
down to very small distances that is of the order of 10 Angstrom or so in the electron
microscope.

So if I want to see further maybe I need (())(22:25) where possibly I see the arrangement of
atoms there but generally to see the arrangement of atoms in the solid in the crystalline solid
we have to see indirectly the crystal structure that is what we call through the diffraction most
commonly we use x-ray diffraction we can also use the Newton diffraction and the electron
diffraction. Only thing is the particles which are behaving like a wave or the axis which we
are using their wavelength should be the same order of magnitude as the spacing between the
atoms, so that I get the diffraction and we will see in this course how with the help of axial
diffraction we can try to conclude what are the simple crystal structures in metals like copper,
aluminium, iron, et cetera we should be able to see that.

Then if I go further down I try to see the electronic structure for which I need spectroscopes
that means atom we are seeing the inside of the atom now it is nucleus around that there are
electrons orbiting different orbitals, right you have heard about s orbitals, p orbitals, d
orbitals, 1 S, 2 S, 2 E like that 3 S, 3 P, 3 D and so on and so forth this is what we have seen
in chemistry this electronic structure and you need this spectroscopes to see that. Then inside
the nucleus you have to see what are the neutrons, protons which are present you need the
nuclear magnetic resonance or the Mossbauer spectroscopy to see that but in this course I
shall be spending more time on 2, 3 and 4 that is why I have put them together and 1, 5 and 6
I have kept them apart.

We will be starting 2, 3 and 4 in this course because these are the ones which are mostly
affecting these properties particularly the mechanical behaviour of materials will need
electronic structure to some extent when we talk about electrical properties but not to a great
extent again we will need them something when we try to talk about the magnetic behaviour
of the materials but that information we can draw what you have done in school chemistry.

We shall start in the opposite direction, we start with the crystal structure we go to the
substructure than to the microstructure the material before we try to see how can we change
the structure in the material. How can be processed this change in these materials and then we
shall try to look at the behaviour of the material and related to the structure than once I know
is it possible for me to change the structure then you see the whole spectrum is available with
us. I am given a material I want to put it to a certain application I need that kind of property
its structure is not suitable I convert the structure to a suitable structure so that I get the
property what I need or else I know the property I must have that kind of structure I must find
a material which has that kind of structure I will locate that material and use it.

(Refer Slide Time: 26:08)

So that is all what we will be able to understand with this and among the properties which we
shall talk about is the mechanical behaviour the response in the material which is when a
mechanical forces applied to the material first of all there is a temporary deformation what
we call elastic deformation and there is a change in shape which is permanent we call it
plastic deformation and ultimately the material fails I will call it fracture or failure of the
material and electrical properties we shall look at conductors we use for conducting
electricity like copper and aluminium wires are used mostly then we have superconductors.

We have certain limitation in the use of the superconductors we shall see those then the
semiconductors mostly of course the chips which we are using and then the magnetic
behaviour of materials these of the properties which we shall try to look at in this course.
With this introduction before I start with the crystal structures and materials I shall like to
recap a few concepts on equilibrium and kinetics which you have done in school, okay.

(Refer Slide Time: 28:23)

All right so few concepts on equilibrium and kinetics, why it is important is materials unless
they are in equilibrium, unless they are stable we will not be able to put them to use that is
why and these kind of concepts I shall be using again and again in the course at different
places, so that is why I think it is better for us to revise that. Equilibrium I will try to explain
with the help of a rectangular block probably you have done it already in school, a
rectangular block lying vertically on the floor and then it is tilted, it passes through a stage
where the center gravity again it passes through is within the base and then it is toppled lies
on the longer side. In this case what is happening is if you look at carefully it is a potential
energy of the block, rectangle a block which is changing. It will remain in the first position if
I do not disturb it, it shall remain in the 3 rd position also for a long time if I do not disturb it
but it is very difficult for it to live in the 2 nd position even though it is Centre of gravity passes
through the base.

(Refer Slide Time: 30:00)

This is what I have shown here in another picture where I have on the y-axis the potential
energy of the block and on the x-axis the configuration of the block. The 1 st position is here
and potential energy you can see it is at its minimum, there is a minima here then position 2 is
there is maximum there, position 3 is here again a minimum. Locally this is a minimum,
locally this is also a minimum but overall potential energy of this is lower than the potential
energy of this one, alright.
(Refer Slide Time: 31:03)

This I express in terms that this rather the slope of this energy configuration curve and I say
that at equilibrium which is at position 1, 2 and 3 and infinitesimally small perturbation does
not change the energy or the state of the rectangular block. Infinitesimally small, in
mathematics while talking about calculus toll while finding out the derivatives limit delta
extends to 0. When I say limited Delta extends to 0 that is what I mean by infinitesimally
small, in other words if you look at the potential energy configuration curve the slope there at
the 3 places is 0 but the 2 places I have a local minimum while the 3 rd the middle position I
have the maximum.

Out of these the stable equilibrium is in the position 1 and in position 3 that is what I say if I
disturb this rectangle a block (())(32:08) small am not talking infinitesimal small delta x not
tending to 0 I am talking about delta x now, so I give a delta x disturbance like this one you
can see here, it remains that is what I mean you disturb it comes back but in the position 2 if
you do that it will not come back either it will go to position 1 or it will go to position 3 it
will not stay in position 2. So in these 2 locations where I have local minimum it is in the
stable equilibrium while at the maximum potential energy it is though in equilibrium but is
unstable. A small disturbance will take it either to position 1 or to the position 3, right.
(Refer Slide Time: 33:13)

If this is not so the material is unstable that is if it is not in local minimum, so when there is a
minima energy is a stable equilibrium when it is a maximum energy it is unstable
equilibrium. Well among the stable equilibria I have distinguish the one which is globally
minimum are most stable like among 1, 2 and 3. 1 and 3 are the minimum but 3 is lower than
1 that will be more stable but if they are more than the 2 positions like that 3 rd one if that is
the lower one that is the most stable. So most stable is the one which is the lowest energy and
where I have simply the local minimum which may not be the global minimum, I simply call
it the metastable equilibrium.

Metastable equilibrium is very important for material scientist because by enlarge the
materials we use are in their metastable states they are not in the most stable stage. The most
stable state of iron (())(34:29) oxide Fe2 o that is how it is found in the nature. Most stable
state of aluminium is again is oxide, maybe for copper sulphide, so the stable state most
stable state is not really of much use to engineer. When I want iron in the form of steel it is in
its metastable state, when I use copper wire copper is in its metastable state, aluminium is in
its metastable state.

So the materials which we are using are in their metastable state and for 100 of years they can
continue to function like that for my lifetime materials behave like that for all purpose which
is stable that is the importance of this metastable equilibrium. Getting global minimum
energy which is for the most stable state though material may not be of much use to an
engineer, okay. Is not that we do not use materials like that, we do use them. Now talking
about this energy which is maximum or minimum, what is this energy? You have read
something about this also in thermodynamics and we shall try to look at that today.

(Refer Slide Time: 36:14)

Among the intensive properties in thermodynamics we have studied are the 2, pressure and
temperature. An intensive property is one its value does not change by changing the amount
of the material, if the atmospheric pressure is one atmosphere here whether I have 1 kilogram
of steel with me or I have 10 grams of steel with me pressure is the same that is the meaning
of intensive property. Similarly the temperature it does not change with the mass or the
volume of the material then there are properties with change with the quantity of the material
are called extensive properties, one of them we have studied is internal energy.

(Refer Slide Time: 37:31)

Internal energy, we write as u and this is equal to u 0 plus integral 0 to T Cv dT. Now what is
u 0 here, internal energy at 0 kelvin, physically what does it mean? We have heard of the
Condon Morse Curve, when 2 atoms are at infinity we consider the energy 0 when they have
bought together they form a bond and this is the distance of equilibrium this is the bond
energy and this bond energy is a negative number. This is u 0, okay then maintaining the
volume constant of the material as the temperature made to rise Cv is the specific heat of the
material at constant volume to integrate this and get the internal energy of the material, right.

(Refer Slide Time: 39:45)

Now coming back to next the enthalpy is also an extensive property we normally depict it by
H and this is written as plus integral 0 to T Cp dT this is at constant pressure specific heat at
constant pressure, now what is H 0 it is enthalpy at 0 kelvin, we relate enthalpy and the
internal energy through a term which is called PV term, P is the pressure, V is the volume.
Well in solids PV term is really negligible as compared to the bond energy we are referring to
here, so I can neglect PV completely.

(Refer Slide Time: 41:22)

This I demonstrate in the slide here my basis of calculation this is one mole of copper at one
atmosphere, one atmosphere 0.1,10 to power 6 Newton per square metre and the volume of
one mole of copper is 7.09 10 to the power minus 6 meter cube and that provides neither PV
term equal to 0.716 joule but in one mole of copper if I look at the copper copper bonds I will
show you this in the crystal structure of copper, every copper atom is surrounded by 12
copper atoms, so it forms bond with 12 neighbours because wanders always form between 2
neighbours, it is the sharing of bond half and half by both the neighbours, so therefore they
are in turn if I have one mole of copper atom in solid I shall have 6 mole of copper copper
bonds, right.

So for 6 mole of copper copper bonds, bond energy of copper is 56.4 kilo joule per mole
multiplied by 6 makes it 338.4 kilojoules while as compared to the is bond energy of one
mole the PV term for one mole 0.716 joule is quite negligible and as an engineer I neglect
this as a result I say or use internal energy and enthalpy for all my solids interchanging and I
talk about internal energy I may mean enthalpy I talk about enthalpy internal energy because
the difference is so small it can be neglected, however when I talk about gases I cannot afford
to neglect this PV term, this PV term becomes very large in case of gases that is what you
studied in school what you were told in school was absolutely correct because we are talking
about the gases, okay but when I talk about my solids this is a negligible term.
(Refer Slide Time: 43:54)

Here I show on the x-axis, on the x-axis I have temperature alright let me put this in here. I
have temperature on the x-axis and I have enthalpy on the y-axis whatever your H 0 value at
0 Kelvin as the temperature rises, enthalpy rises Cp dT and it goes on till there is a change in
the solid it becomes liquid latent heat of fusion is added and then it further rises, now that is
the melting point of the solid. Now as I said what is more stable is the one which has lower
energy alright if you look at the enthalpy if I consider that energy as enthalpy well first of all
what is our common experience, below the melting point materials stays as solid above the
melting point material stays as liquid, this is liquid and this is solid.

So (())(46:22) of the solid sorry the enthalpy of the solid is lower in this temperature range as
compared to that of the liquid and that is stable but what happens above the melting point?
Enthalpy of the solid is still smaller than that of the liquid and what is stable is the liquid that
means it is some other energy not the enthalpy which I need to consider for stability in
materials, it is not the enthalpy or the internal energy which I consider because obviously
what my observation is not I am not able to satisfy the…go back to. I think we shall stop
here.