Transcript DT1410 - Materials and Processes in Design

DT1410 - Materials and
Processes in Design
UNIT 1 - PROPERTIES OF MATERIALS
CONSIST OF:
Sub Atomic
Particles

PROTONS

NEUTRONS

ELECTRONS
Protons

Located in the Nucleus of and Atom

Has a positive charge

Number of Protons is the Atomic
Number
Neutrons

Located in the nucleus of an Atom

Has NO electrical charge

Number of Neutrons is usually equal to
the number or protons.
Electrons

Located outside the nucleus of the
atom

Housed in Electrical Clouds, Shells,
Orbits

Has a negative charge

Moves around the nucleus in an orbit

Atoms have a maximum of 8 Valence
Electrons and are considered to be
stable
Electron Arrangement

The electrons in the outermost shell (if
it is not completely full) are available
for bonding

These electrons are called Valence
Electrons

Bonding is the chemical combining of
2 or more atoms.
Valence

The Capacity of an atom to combine
with other atoms to form a molecule.

Valence considers positive and
negative charge of atoms, as
determined by the gain or lose of
electrons.
Bonding

The process when 2 or more atoms
combine chemically is known as
Bonding

The 2 we are learning today are
 Ionic
Bonding
 Covalent
Bonding
Ionic Bonding

Electrons are either gained or lost by
atom

One atom is positive +, One is
negative -

Occurs between Metals and
Nonmetals

Metals lose electrons

Nonmetals gain electrons

Very strong bond – usually solids - with
high melting temperatures
Covalent Bonding

Electrons are shared between atoms
(co)

Neither atom has an electrical charge

Forces between atoms are weak –
have low boiling or melting points

Usually are liquids or gasses

If a solid is usually a crystal
Metallic Bond

The metallic bond is actually a special case of an ionic bond. Think of metallic bonding
as a more or less static arrangement of positive ions within a moving array of mobile
electrons.

This unique arrangement of cations and electrons give metals their characteristic
properties. Metallic properties include:

Heat conductivity (mobile electrons can carry the kinetic energy of heat),

Shiny appearance (the rapidly moving electrons emit energy in the form of light),

Electricity conductors (electricity is the flow of electrons), and

Malleability (ability to be easily shaped into flat sheets or drawn into wires).
Van der Waals Bond

A weak attractive force between atoms or nonpolar molecules caused
by a temporary change in dipole moment arising from a brief shift of
orbital electrons to one side of one atom or molecule, creating a similar
shift in adjacent atoms or molecules.
Allotropy

The ability of a material to exist in
several crystalline forms.
Metals

ARE MATERIALS THAT OCCUPY THE LEFT
SIDE OF THE PERIODIC TABLE AND ARE
CHARACTERIZED BY HAVING ONE, TWO,
OR THREE VALENCE ELECTRONS, AND
BOND WITH THE METALLIC BOND.

MOST OF THE ELEMENTS ON THE
PERIODIC TABLE ARE METALS,
INCLUDING GOLD, SILVER, PLATINUM,
MERCURY, URANIUM, ALUMINUM,
SODIUM AND CALCIUM. ALLOYS, SUCH
AS BRASS AND BRONZE, ALSO ARE
METALS.
Properties of Metals

Metals are shiny solids are room temperature (except
mercury, which is a shiny liquid element), with
characteristically high melting points and densities.

Many of the properties of metals, including large
atomic radius, low ionization energy, and low
electronegativity, that are due to the fact that the
electrons in the valence shell of a metal atoms can
be removed easily.

One characteristic of metals is their ability to be
deformed without breaking.

Malleability is the ability of a metal to be hammered
into shapes.

Ductility is the ability of a metal to be drawn into wire.

Because the valence electrons can move freely,
metals are good heat conductors and electrical
conductors.
Solidification of Metal

To form the strongest metallic bonds, metals are
packed together as closely as possible. Several
packing arrangements are possible. Instead of
atoms, imagine marbles that need to be packed in a
box.

As atoms of melted metal begin to pack together to
form a crystal lattice at the freezing point, groups of
these atoms form tiny crystals. These tiny crystals
increase in size by the progressive addition of atoms.
The resulting solid is not one crystal but actually many
smaller crystals, called grains.

These grains grow until they impinge upon adjacent
growing crystals. The interface formed between them
is called a grain boundary. Grains are sometimes
large enough to be visible under an ordinary light
microscope or even to the unaided eye. The
spangles that are seen on newly galvanized metals
are grains. (See A Particle Model of Metals Activity)
Figure 5 shows a typical view of a metal surface with
many grains, or crystals.
Grain

In metals, a structure containing atoms
of one crystalline orientation.

Grains form during the solidification (or
crystallization) of metal; they may be
re-formed during recrystallization (reheating)
The Grain of Steel under a Microscope
Processing (Changing the Properties of metals)

It has long been known that the properties of some metals could be changed by heat treating.
Grains in metals tend to grow larger as the metal is heated. A grain can grow larger by atoms
migrating from another grain that may eventually disappear. Dislocations cannot cross grain
boundaries easily, so the size of grains determines how easily the dislocations can move. As
expected, metals with small grains are stronger but they are less ductile.
Processing (Changing the Properties of metals)

There are many ways in which metals can be heat
treated. Annealing is a softening process in which
metals are heated and then allowed to cool slowly.
Most steels may be hardened by heating
and quenching (cooling rapidly). This process was
used quite early in the history of processing steel. In
fact, it was believed that biological fluids made the
best quenching liquids and urine was sometimes
used. In some ancient civilizations, the red hot sword
blades were sometimes plunged into the bodies of
hapless prisoners! Today metals are quenched in
water or oil.

Quenching results in a metal that is very hard but also
brittle. Gently heating a hardened metal and
allowing it to cool slowly will produce a metal that is
still hard but also less brittle. This process is known as
tempering. (See Processing Metals Activity). It results
in many small Fe3C precipitates in the steel, which
block dislocation motion which thereby provide the
strengthening.
Processing (Changing the Properties of metals)

Because plastic deformation results from the movement of
dislocations, metals can be strengthened by preventing this motion.
When a metal is bent or shaped, dislocations are generated and
move. As the number of dislocations in the crystal increases, they will
get tangled or pinned and will not be able to move. This will
strengthen the metal, making it harder to deform. This process is
known as cold working. At higher temperatures the dislocations can
rearrange, so little strengthening occurs.

You can try this with a paper clip. Unbend the paper clip and bend
one of the straight sections back and forth several times. Imagine
what is occurring on the atomic level. Notice that it is more difficult to
bend the metal at the same place. Dislocations have formed and
become tangled, increasing the strength. The paper clip will
eventually break at the bend. Cold working obviously only works to a
certain extent! Too much deformation results in a tangle of
dislocations that are unable to move, so the metal breaks instead.

Heating removes the effects of cold-working. When cold worked
metals are heated, recrystallization occurs. New grains form and grow
to consume the cold worked portion. The new grains have fewer
dislocations and the original properties are restored.
Plastics
A PLASTIC MATERIAL IS ANY OF A
WIDE RANGE OF SYNTHETIC OR
SEMI-SYNTHETIC ORGANIC SOLIDS
THAT ARE MOLDABLE. PLASTICS ARE
TYPICALLY ORGANIC POLYMERS OF
HIGH MOLECULAR MASS, BUT THEY
OFTEN CONTAIN OTHER
SUBSTANCES. THEY ARE USUALLY
SYNTHETIC, MOST COMMONLY
DERIVED FROM PETROCHEMICALS,
BUT MANY ARE PARTIALLY NATURAL.
Polymer

All plastics are polymers, but not all
polymers are plastics. The simplified
diagram below shows the relationship
between monomers and polymers.

The monomers that are found in many
plastics include organic compounds like
ethylene, propylene, styrene, phenol,
formaldehyde, ethylene glycol, vinyl
chloride and acetonitrile. Because there
are so many different monomers that
can combine in many different ways, we
can make many kinds of plastics.
Thermosetting

Thermoset, or thermosetting, plastics are synthetic materials
that strengthen during being heated, but cannot be
successfully remolded or reheated after their initial heatforming. This is in contrast to thermoplastics, which soften when
heated and harden and strengthen after cooling.
Thermoplastics can be heated, shaped and cooled as often as
necessary without causing a chemical change, while
thermosetting plastics will burn when heated after the initial
molding. Additionally, thermoplastics tend to be easier to mold
than thermosetting plastics, which also take a longer time to
produce (due to the time it takes to cure the heated material).
Thermosetting plastics, however, have a number of
advantages. Unlike thermoplastics, they retain their strength
and shape even when heated. This makes thermosetting
plastics well-suited to the production of permanent
components and large, solid shapes. Additionally, these
components have excellent strength attributes (although they
are brittle), and will not become weaker when the temperature
increases.
Thermoplastic

Is a material that is capable of softening of fusing
when heated and hardens when cooled again.

A thermoplastic (sometimes written as thermo
plastic) is a type of plastic made from polymer
resins that becomes a homogenized liquid when
heated and hard when cooled. When frozen,
however, a thermoplastic becomes glass-like and
subject to fracture. These characteristics, which
lend the material its name, are reversible. That is,
it can be reheated, reshaped, and frozen
repeatedly. This quality also makes thermoplastics
recyclable.

There are dozens of kinds of thermoplastics, with
each type varying in crystalline organization and
density. Some types that are commonly
produced today are polyurethane,
polypropylene, polycarbonate, and acrylic.

A FAMILY OF MATERIALS THAT ARE
COMPOUNDS, TRADITIONALLY
CONSISTING OF A METAL AND AN
OXIDE, BUT THEY MAY ALSO BE
CARBIDES, SULFIDES, NITRIDES AND
INTERMETALLIC COMPOUNDS

THEY GENERALLY HAVE AN IONIC
BOND, ARE VERY HARD AND BRITTLE,
AND CAN WITHSTAND HIGH
TEMPERATURES.
Ceramics
(or ceramic materials)