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.
Single Crystal Methods
Heat treatment is a process in which the material is heated and cooled to
change the properties of the metal.
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.
Microscopy
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.
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 conjuction with optical microscopy is that of
targetted 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 and through an understanding of these effects and a systematic
experimental process they can be used to determine material composition and
structure.
Introduction
Materials science is an interdisciplinary field involving the properties
of matter and its applications to various areas of science and engineering.
It includes elements of applied physics and chemistry, as well as chemical,
mechanical, civil and electrical engineering. With significant media
attention to nanoscience and nanotechnology in the recent years, materials
science has been propelled to the forefront at many universities, sometimes
controversially.
The choice material of a given era is often its defining point: the stone
age, Bronze Age, and steel age are examples. Materials science is one of the
oldest forms of engineering and applied science. Modern materials science
evolved directly from metallurgy, which itself evolved from mining. A major
breakthrough in the understanding of materials occurred in the late 19th
century, when Willard Gibbs demonstrated that thermodynamic properties
relating to atomic structure in various phases are related to the physical
properties of the material. Important elements of modern materials science
are a product of the space race: the understanding and engineering of the
metallic alloys and other materials that went into the construction of space
vehicles was one of the enablers of space exploration. Materials science has
driven, and been driven by, the development of revolutionary technologies
such as plastics, semiconductors, and biomaterials.
Before the 1960s (and in some cases decades after), many materials
science departments were named metallurgy departments, from a 19th and early
20th century emphasis on metals. The field has since broadened to include
every class of materials, including: ceramics, polymers, semiconductors,
magnetic materials, medical implant materials and biological materials.
In materials science, rather than haphazardly looking for and discovering
materials and exploiting their properties, one instead aims to understand
materials fundamentally so that new materials with the desired properties
can be created.
The basis of all materials science involves relating the desired
properties and relative performance of a material in a certain application
to the structure of the atoms and phases in that material through
characterization. The major determinants of the structure of a material and
thus of its properties are its constituent chemical elements and the way in
which it has been processed into its final form. These, taken together and
related through the laws of thermodynamics, govern the material�s
microstructure, and thus its properties.
An old adage in materials science says: "materials are like people; it is
the defects that make them interesting". The manufacture of a perfect
crystal of a material is physically impossible. Instead materials scientists
manipulate the defects in crystalline materials such as precipitates, grain
boundaries (Hall-Petch relationship), interstitial atoms, vacancies or
substitutional atoms, creating a material with the desired properties.
Not all materials have a regular crystal structure. Polymers display
varying degrees of crystallinity. Glasses, some ceramics, and many natural
materials are amorphous, not possessing any long-range order in their atomic
arrangements. These materials are much harder to engineer than crystalline
materials. Polymers are a mixed case, and their study commonly combines
elements of chemical and statistical thermodynamics to give thermodynamical,
rather than mechanical descriptions of physical properties.
In addition to industrial interest, materials science has gradually
developed into a field which provides tests for condensed matter or solid
state theories. New physics emerges because of the diverse new material
properties needed to be explained.
Radical materials advances can drive the creation of new products or even
new industries, but stable industries also employ materials scientists to
make incremental improvements and troubleshoot issues with currently used
materials. Industrial applications of materials science include materials
design, cost-benefit tradeoffs in industrial production of materials,
processing techniques (casting, rolling, welding, ion implantation, crystal
growth, thin-film deposition, sintering, glassblowing, etc.), and analytical
techniques (characterization techniques such as electron microscopy, x-ray
diffraction, calorimetry, nuclear microscopy (HEFIB), Rutherford
backscattering, neutron diffraction, etc.).
Besides material characterisation, the material scientist/engineer also
deals with the extraction of materials and their conversion into useful
forms. Thus ingot casting, foundry techniques, blast furnace extraction,
electrolytic extraction all are part of the required knowledge of a
materials scientist/engineer. Often the presence, absence or variation of
minute quantities of secondary elements and compounds in a bulk material
will have a great impact on the final properties of the materials produced,
for instance, steels are classified based on 1/10th and 1/100 weight
percentages of the carbon and other alloying elements they contain. Thus,
the extraction and purification techniques employed in the extraction of
iron in the blast furnace will have an impact of the quality of steel that
may be produced.
The overlap between physics and materials science has led to the offshoot
field of materials physics, which is concerned with the physical properties
of materials. The approach is generally more macroscopic and applied than in
condensed matter physics. See the important publications in materials
physics for more details on this field of study.
Alloys of metals is an important and significant part of materials
science. Of all the metallic alloys in use today, the alloys of iron (steel,
stainless steel, cast iron, tool steel, alloy steels) make up the largest
proportion both by quantity and commercial value. Iron alloyed with various
weight percentages of carbon gives low, mid and high carbon steels. For the
steels, the hardness and tensile strength of the steel is directly related
to the amount of carbon present, while increasing carbon levels lead to
lower ductility and toughness. The addition of silicon and graphitization
will produce cast irons (although some cast irons are made precisely with no
graphitization). The addition of chromium, nickel and molybdenum to carbon
steels (more than 10%) gives us stainless steels.
Other significant metallic alloys are those of aluminium, titanium,
copper and magnesium. Copper alloys have been know for a long time (during
the Bronze Age), while the alloys of the other three metals have been
relatively recently developed, due to the chemical reactivity of these
metals and the resultant difficulty in their extraction which wasn't
accomplished (electrolytically) until recently. The alloys of aluminium,
titanium and magnesium are also known and valued for their high strength to
weight ratios and, in the case of magnesium, their ability to provide
electromagnetic shielding. These materials find special applications where
high strength-weight ratios are desired (aero-space industry).
Other than metals, polymers and ceramics are also an important part of
material science. Polymers are the raw materials (the resins) used to make
what we commonly call plastics. Plastics are actually the final product
after many polymers and additives have been processed and shaped into a
final shape and form. Polymers that have been around and are in current
widespread use include polyethylene, polypropylene, polyvinyl-chloride,
polystyrene, nylons, polyesters, acrylics, polyurethane, polycarbonates.
Plastics are generally classified as "commodity", "specialty" and
"engineering" plastics.
PVC is a commodity plastic, it is widely used, low cost and annual
quantities are huge. It lends itself to an incredible array of applications,
from faux leather to electrical insulation to cabling to packaging and
vessels. Its fabrication and processing are simple and well-established. The
versatility of PVC is due to the wide range of additives that it accepts.
Additives in polymer science refers to the chemicals and compounds added to
the polymer base to modify its physical and material properties.
Polycarbon would be normally considered an engineering plastic (other
examples include PEEK, ABS). Engineering plastics are valued for their
superior strengths and other special material properties. They are usually
not used for disposable applications, unlike commodity plastics.
Specialty plastics are really the materials with unique characteristics,
such as ultrahigh strength, electrical conductivity, electro-florescence,
high thermal stability, etc.
It should be noted here that the dividing line between the various types
of plastics is not based on material but rather their properties and
applications. For instance, polypropylene (PP) is a cheap, slippery polymer
commonly used to make disposable shopping bags and trash bags. It is
commodity. But a variety of PP called Ultra-high Molecular Weight
Polypropylene (UHMWPE) is an engineering plastic which is used extensively
as the glide rails for industrial equipment.
Another application of material science in industry is the making of
composite materials. Composite materials are structured materials composed
of at least two different macroscopic phases. An example would be
steel-reinforced concrete. Also, take a look at the plastic casing of your
telly set, cell-phone: these plastic casings are usually a composite made up
of a thermoplastic matrix such as acrylonitrile-butadiene-styrene (ABS)in
which calcium carbonate chalk, talc, glass fibres or carbon fibres have been
added (dispersants) for added strength, bulk, or electro-static dispersion.
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