Introduction to Materials Science
Introduction
Different materials have different properties. Think of the difference
between the engine of a car and its wheels; the metal in a wire and its
insulator. All these objects can only be made out of materials that have
properties suited to their application.
Materials science is the study of
the properties of materials. It focuses on the factors that make one
material different from another. Understandably, there are many such
factors, some obvious and some subtle. Examples of these factors might
include elemental composition, arrangement, bonding, impurities, surface
structure, length scale and so on. The ability to understand the
relationships between these factors and the properties of a material has
been crucial to most of mankind's technological breakthroughs. Today,
materials science is a multidisciplinary subject. It draws upon just about
every field of science and engineering, providing insights for other
researchers to use in their field.
This book is aimed at those studying materials science at the
undergraduate level in university whether as their major field or as a
single module of a related engineering course.
Structure of Matter
Atomic Structure and Bonding
Fundamentally, two types of bonding exist- bonds between atoms and bonds
between ions. Bonds between atoms of nonmetals are covalent, meaning that
they share a pair of electrons in the space between them. These two atoms
are bound together and cannot be separated by simple physical means. If
these two atoms have similar
electronegativity, neither atom has more pull on the electron pair than
the other. This type of covalent bond is called Non Polar. Examples
of non polar covalent compounds are
methane,
carbon dioxide and
graphite. In graphite, all atoms are identical and so no atom has
stronger pull than any of the others. In methane, the carbon-hydrogen bonds
are very slightly polar, and the polarities are cancelled because the bonds
all point to the same locus.
Crystal Structure
Defects
Defects of materials are subject to intense study. However there are some
methods to determine the source of defects and, if occurred, the size, shape
and position of defects in the materials. There are: destructive testing
methods and Non destructive testing methods (NDT).
Thermodynamics of Material
Phase Diagrams
Phase diagrams provide a graphical means of presenting the results of
experimental studies of complex natural processes, such that at a given
temperature and pressure for a specific system at equilibrium the phase or
phases present can be determined.
SYSTEM - Any portion of the universe which is of interest and can be studied
experimentally.
PHASE- any particular portion of a system, which is physically
homogeneous, has a specific composition, and can be mechanically removed or
separated from any other phase in the system.
* e.g. A system containing a mixture of ol and pl in equilibrium contains two phases - ol and pl.
In petrology we generally deal with primary phases - any crystalline
phase which can coexist with liquid, i.e. it formed/crystallized directly
from the liquid.
EQUILIBRIUM - The condition of minimum energy for the system such that
the state of a reaction will not change with time provided that pressure and
temperature are kept constant.
In experimental petrology there are three practical criteria used to test
for equilibrium.
1. Time - with time the system does not change its physical or chemical makeup.
2. Approach equilibrium from two directions,
e.g. the melting point of Albite.
* begin with a liquid of Ab composition (Na2O-Al2O3-6SiO2) and cool until Ab crystallizes - T=1100�C
* begin with the same mixture of solid Albite and heat it up until liquid forms - T=1120�C
Melting point of albite = 1110�C + 10�C.
3. Attainment of equilibrium by using different reactants and procedures.
To determine the melting temperature of Albite
* grind up a sample of pure albite
* combine powdered oxides to give pure Ab composition
Use both to determine Ab melting point.
One final term to be defined prior to examining phase diagrams.
COMPONENT - the smallest number of independent variable chemical
constituents necessary to define any phase in the system.
* components may be oxides, elements or minerals, dependant on the system being examined.
For example, experiments carried out in the H2O system, show
that the phases which appear over a wide temperature and pressure range are
ice, liquid water and water vapour. The composition of each phase is H2O
and only one chemical parameter or component is required to describe the
composition of each phase. Systems which can be defined by a single
component are called Unary Systems. H2O System In this system
pressures from 0 to 15 kbars seven phases, each with the same composition -
H2O have been recognized:
* Ice I
* Ice II
* Ice III
* Ice IV(actually not exist)
* Ice V
* Water
* Steam
SiO2 System In the one component SiO2 system in the
temperature range from 0 to 2,000�C and a pressure range from 0 to 30 kbars
six phases of SiO2 are recognized. At pressures > 30 kbar a
seventh phase, stishovite, exists. The six phases of SiO2 are:
* coesite
* alpha quartz (Trigonal)
* beta quartz (hexagonal)
* tridymite
* cristobalite
* anhydrous melt
Materials Processing
In order to produce an article for any application out of a particular
material there are several steps that may be required. The first step is
usually to obtain the raw materials from our environment. This may involve
discovering where these raw materials are located (often achieved with
knowledge of geology) and developing processes to extract them from these
locations (e.g. mining the ores, drilling for oil etc.). Otherwise, it may
be possible to find sources of material suitable for recycling or
reprocessing. Once these raw materials have been obtained they may need to
undergo some initial processing to get them into a usable form. This may be
some form of extractive metallurgy, chemical synthesis or some other
chemical process. It may also be necessary to mix different raw materials to
achieve a certain composition (e.g. alloying in metals) that is appropriate
for or has been optimised the application. The application will usually
require that the material be in a particular shape and a suitable shaping
process or combination of process must be employed to achieve this. Often,
it may be possible to produce a shape out of a material with any one of the
many different shaping processes. However, there is usually one particular
process that either results in particular benefits in terms of the
properties of the material or the article that is produced or meets some
other important criteria - such as low cost - that it is selected over the
other options. Finally, it may be necessary or beneficial to process the
article further, once it has been formed, in order to optimise the
properties of the material.
Firstly, this chapter will present the various chemical processes that
may be necessary to produce suitable materials from the raw materials in our
environment. The different methods for shaping these materials will then be
presented. Finally, the processes used to optimise the properties of the
materials will be discussed.
Chemical Processing
Extraction of Raw Materials
Chemical Synthesis of Materials
Shaping Processes
Melt Processing
Casting
Physical Processing
Forging
Rolling
Extrusion
Powder Process
Powder processes are used in the production of metallic and ceramic
parts. The use of metal powders is commonly referred to as Powder Metallurgy
(P/M).
There are 4 main stages to producing products with, they are: Powder
Mixing, Compaction, Sintering and final finishing.
A metal or ceramic powder is prepared, then compacted into a desired
shape. This part is then heated in a furnace causing the powders to weld
together forming a solid part. The part is then final processed by final
shaping, minor smoothing, or drilling.
Using Powders to produce parts is viable when you require a high volume
of simple parts that need to be cost efficient. All though casting can also
do this, P/M offers near net shape products. This means that the part that
comes out of the process needs little or no finishing done to it.
Ceramics lend themselves well to powder processing as they are very hard
and brittle, thus a near net shape is highly beneficial.
Mixing
Mixing is mainly done to add waxes for the compaction, binders to
temporarily strengthen the compacts and sometimes to get the right
chemistry.
As most suppliers recommend lubricant for idea compaction, mixing is a
very important process, so a homogenous mixture is required. Optimum mixing
occurs with turbulent mixing and at low centrifugal forces.
Along with ensuring a homogenous mix, the mixing process also provides
some milling of the powders. As we all know you can put more tennis balls in
an area than beach balls, thus increasing the surface area of the balls. The
same is true with powders, more surface area, the better the final product
is.
Compaction
Compaction is the process of squishing the powders into the desired shape
with enough force so as to hold its shape. This is called a green body, as
it still has moisture in it and needs to be Sintered. Same basic concept as
pottery, the plate or cup is considered "green" until it is fired
There are 2 categories of pressing: Isostatic and Axial.
Sintering
Sintering is simply the furnace heating of a compacted powder object,
also known as a green body to form a solid part. The powders can be either
metallic or ceramic. They can be in elemental form, as an alloy, or mixture
of both. Most sintering processes are done in a protective atmosphere, such
as nitrogen or hydrogen mixed gas, to avoid degradation of the green bodies,
and at a temperature lower than the melting point, approximately 60~90% of
the main elements meting point. The specific atmosphere and temperature is
dependant upon the material being processed.
If the material being sintered is an alloy, it is possible that one or
more of the constitutes has a melting point lower than the sintering
temperature, thus causing a small amount of liquid to form. This is called
Liquid Phase Sintering. Caution needs to be taken when choosing a
temperature as too much liquid will result in the deformation of the part.
This is referred to as slumping.
The mechanism of sintering is the diffusion of the atoms across the
particle boundaries of compacted powders. As the atoms diffuse, all voids
are filled and the material forms one solid part. As the voids between
particles are no longer present, the part increases in density, and
experiences shrinkage. However, due to the nature of this process, only
93%-98% theoretical density can be achieved, thus further mechanical
processing is needed to obtain 100% dense material.
The resultant microscopic structure resembles the starting green compact.
The starting particle boundaries eventually turn into the final grain
boundaries.
As the voids between the powder particles are filled during the sintering
process, the gases need to be expelled from the compact. These gases are;
air trapped between powders and gasses from additives added during the
mixing and/or compaction process. These gases are expelled through
capillaries formed by the particle boundaries. If the compacts are hated to
fast, these capillaries can be �pinched� off and if these gasses are not
expelled, the part will have defects such as warpage, porosity, or even
holes.
A typical industrial sintering process is done on a traveling grate
furnace with a 2 stages of heating. The green bodies are placed on a
conveyor which travels into the furnace which has a positive pressure
protective atmosphere blown onto the conveyor belt. The parts travel into
the first temperature zone to vaporize and wax and degas. The second
temperature zone is to do the actual sintering of the material. After the
appropriate sinter time, the parts travel through a cooling zone to allow
the parts to be handled, or to lock properties for continued processing.
Degas and sinter times vary based on material.
Finishing
Machining
Welding
Materials Optimisation
Heat Treatment:It is defined as combination of heating and cooling cycles
given to a particular material of interest to achieve desired properties.
Surface Engineering
Materials Characterisation
An important aspect of materials science is the characterisation of the
materials that we use or study in order to learn more about them. Today,
there is a vast array of scientific techniques available to the materials
scientist that enables this characterisation. These techniques will be
introduced and explained in this section.
Macroscopic Observation
The first step in any characterisation of a material or an object made of
a material is often a
macroscopic observation. This is simply looking at the material with the
naked eye. This simple process can yield a large amount of information about
the material such as the colour of the material, its
lustre (does it display a metallic lustre), its shape (whether it
displays a regular, crystalline form), its composition (is it made up of
different phases), its structural features (does it contain porosity) etc.
Often, this investigation yields clues as to what other tests could be
performed to fully identify the material or to solve a problem that has been
experienced in use.
Microscopic Observation
Microscopy is a technique that, combined with other scientific techniques
and chemical processes, allows the determination of both the composition and
the structure of a material. It is essentially the process of viewing the
structure on a much finer scale than is possible with the naked eye and is
necessary because many of the properties of materials are dependent on
extremely fine features and defects that are only possible to observe using
one of the following techniques in this field.
Optical Microscopy
Optical microscopes are formed of lenses that magnify and focus light.
This light may have been transmitted through a material or reflected from a
material's surface and can be used to ascertain a great deal of information
about that material under evaluation. This can include whether the material
is dense or contains porosity, what colour the material is, whether the
material is composed of a single phase or contains multiple phases etc.
A common practice performed in conjunction with optical microscopy is
that of targeted and controlled chemical attack of the material using one of
many chemical reagents available. For metallic materials, this technique
combined with optical microscopy is know as optical metallography. The basis
of this combined technique is that regions of different composition within a
material as well as entirely different materials are affected differently
when exposed to certain chemicals. These chemical effects are catalogued in
various works (for example the ASM Metals Handbook or Metallographic Etching
by G. Petzow) and through an understanding of these effects and a systematic
experimental process they can be used to determine material composition and
structure.
There are several limitations to the usefulness of optical microscopy.
The first is that the maximum resolving power is limited by diffraction
effects to approximately 0.2 micrometres at a magnification of around 1500X
(see
reference). Many of the defects and structural features important in
determining material properties, and therefore of interest to materials
scientists, are of atomic scale. (for
reference, the diameter of a helium atom is approximately 100
picometers) The second major limitation in optical microscopy is limited
depth of field. This limitation means that surfaces with features at
different heights - such as the rough surfaces of a fractured specimen for
example - cannot be seen in sharp focus at the same time. This means that
flat or polished surfaces are preferred for this technique. Furthermore, the
chemical techniques required for identifying different phases within a
structure are destructive. Thus, if a only a small amount of a certain
portion of the sample is present then this may be destroyed by the process
by the etching technique.
Electron Microscopy
Scanning Electron Microscopy
Transmission Electron Microscopy
Chemical Analysis in Electron Microscopy
Diffraction Techniques
Principles of Diffraction
X-Ray Diffraction
Neutron Diffraction
Electron Diffraction
Spectroscopic Techniques
Energy Dispersive X-Ray Spectroscopy
Wavelength Dispersive X-Ray Spectroscopy
Electron Energy Loss Spectroscopy
X-Ray Photoelectron Spectroscopy
Auger Electron Spectroscopy
Infra-red and Raman Spectroscopy
Ultra-violet and Visible Spectroscopy
Electrical and Magnetic Techniques
Electrical Resistance
Impedance Spectroscopy
Thermal Techniques
Thermogravimetric Analysis (TGA)
Differential Scanning Calorimetry (DSC)
Mechanical Testing
Strength
Hardness
Hardness is defined as the resistance of a material to penetration by an
indentor. The Mohs scale of hardness has ten level and diamond is the
material with the highest level of hardness ever known. There are several
methods used to determine material's hardness, such as: Brinell,
Rockwell, Vickers and Poldy.
Hardness Brinell (HB)
Is the method used for raw metallic materials. It uses a spherical ball
indentor in order to stamp a print in the material. An external force
transmitted through the indentor over the surface of the material determines
the material's penetration.
Hardness Rockwell (HRB/HRC)
Is the method used for heat treated metallic materials. It has two
variants regarding the indenter shape (ball or cone).
Hardness Vickers (HV)
Is a method used for the determination of hardness of special metallic
materials, such as high alloyed materials, characterized by a very high
degree of hardness.
Non destructive testing (NDT)
Some of the NDT methods available are: ultrasonic method,
radiation penetration method.
Metals
Metals are materials made of elements on the left hand side of the
periodic tables 'stair step' border starting on the left of Boron and going
down and right and finishing at polonium. These elements can be mixed and
combined with other elements (metals or non-metals) to create materials
called alloys. Alloys are just a mix of elements and materials to create a
new material with favorable properties.
Metals can be generally identified by a set of few physical properties
(these a very general and there are plenty of exceptions). The main
definition of a metal is an element that readily loses electrons and forms
positive ions. The general bulk properties that are used to simply identify
metals is that they tend to be lustrous (shiny when not oxidised), they are
malleable (so can be beaten into a shape and not break), they are ductile
(they can be drawn out into a wire) and that they conduct electricity; this
rises from the fact that they readily lose electrons so there is a free
electron 'gas' where the electrons can move around and this means that a
charge can flow when an electric field is placed across the metal.
The metal that has changed the way the whole world functions and takes up
a huge majority of the industry even now after over a century of its
discovery and use (in terms of its modern production and composition). This
metal is steel and is an alloy of mainly iron (Fe) and carbon (C) with many
other constituent elements added depending on the type of steel wanted and
the properties required.
Steel can be produced in a number of ways. Traditional methods utilise
integrated steel processes which use energy intensive blast furnaces (to
produce iron) sand basic oxygen steelmaking (to convert iron to steel). More
modern methods use electric arc furnaces in which scrap steel is melted
using electric currents and then formed into slabs or ingots for further
processing. When the steel slab or ingot has cooled, a variety of forming
operations such as rolling or extrusion are used to form the metal into flat
sheet for use in cars, fridges, filing cabinets or radiators, or into beams,
and heavy plate for use in construction and ship building
Ceramics
inorganic and non-metallic materials bimevox
Polymers
Polymer is a group of substances that has large molecules consisting of
at least five repeated chemical units bonded together with a same type of
linkage, like beads on a string.Polymer usually contains more than five
repeated units and some polymers contain hundreds or thousands of monomers
in each of their polymer chains. Polymer materials can be natural or
synthetic. Polymer material is a large group of materials whereby they can
be further classified specifically into plastics, elastomers and composites!
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