Materials

Seite 1

Classification of Materials

Metals

electrical & thermal conductors

ductile

reasonable stiffness, toughness & strength

are either; ferrous (>50% ferrite) or non-ferrous(<50% ferrite)

Polymers

are either thermosoftening (soften with heat) or thermosetting (don't soften with heat)

low electrical & thermal conductivity

softer, less dense & more corrosion resistant

Ceramics

hard, brittle

high compressive, low tensile strengths

low electrical & thermal conductivities

Composites

made by bonding two or more materials together

has a combination of properties

Seite 2

Properties of Materials

Ductility: ability to be drawn into wire by tensile force.

Malleability: ability to be flattened by a compressive force.

Hardness: ability to resist scratching or indentation.

Elasticity: ability to return to original shape.

Toughness: ability to withstand impact force & deform plastically.

Strength: ability to withstand a load or force.

Seite 3

Types of Bonds

Primary Bonds

Ionic Bonding

The transfer of electrons between atoms to create positive & negative ions. The attraction between the two (or more) atoms due to their electronegativity is what bonds the materials and is considerably strong.

Covalent Bonding

The sharing of electrons between different atoms. The common electrons between the atoms attracts their slightly positive sides, which forms the bond and is very strong.

Metallic Bonding

Electrons of the metals form an 'electron cloud' around the positive ions of the metal and can move freely. They are said to be delocalised. The attraction between the electrons and positive ions is what makes the bond strong.

Secondary Bonds

Molecular Bonds

Concentration of electrons on one side of the atom makes one end slightly more negative and the other end slightly more positive, attracting other atoms with the same dipole-like effect.

Hydrogen Bonds

Hydrogen atoms in a molecule create a slightly more positive siude of an atom, creating a molecular dipole, like molecular bonds.

Seite 4

States of Materials

Solids have:

lowest energy

are crystalline

have a fixed volume; rigid

Liquids have:

higher energy

are amorphous

have a fixed volume

Gases have:

highest energy

are amorphous

have no fixed volume

Seite 5

Structure of Metals

Body Centred Cubic (BBC)

atoms in each corner, one in centre

total atom volume in each cell; 2

low ductility

Face Centred Cubic (FCC)

atoms in each corner & centre of each face

tom volume in one cell; 4

high ductility

Close Packed Hexagonal (CPH)

atoms in each corner, centre of each hexagonal end & three in a triangle midway down

lower ductility than FCC metals

A polymorphic/allotropic l can have more than one crystalline structure.

Seite 6

Grain Structure of Metals

Slow cooling of metals produces larger grains and a softer weaker metal.

Metal moulds produce:

chill grains on surface

columnar grains growing towards centre

equiaxed grains in the centre

Seite 7

Deformation of Materials

Elastic Deformation - when atoms do not move permanently from their original position.