CES Information Guide - Materials Science Engineering

M A T E R I A L S C I E N C E & E N G I N E E R I N G

Background
Material Classes

Metals

Ceramics

Polymers

Composites

Material Structure

Physical Properties

Material Selection

Material Processing

Example Case Studies

Material Classes

Metals
Metals are opaque and lustrous (or shiny) elements that are good conductors of heat and electricity. Most metals are malleable and ductile, and are generally more dense than the other pure solid material. When something is malleable, it means that they can be molded. Ductile means that the material can be stretched into a thin wire.

Metals account for about two thirds of all the elements and about 24% of the mass of the planet. They are all around us in diverse forms such as steel structures, copper wires, aluminum foil, and gold jewelry. Metals are widely used because of their properties: strength, ductility, high melting point, thermal and electrical conductivity, and toughness (a character of durability).

What is their structure? As with all elements, metals are composed of atoms. Let us investigate what each property tells us in the atomic level:

Strong, tough, and high melting point: atoms are held together by strong bonds
Ductile: these bonds must also allow atoms to move
Thermal and electrically conductive: metal atoms are free to vibrate without damaging its original shape and free to transmit electrons through with the least amount of hindrances (impurities, faults, not pure crystalline structure...)

A reasonable model would be one in which atoms are held together by strong, but delocalized, bonds. Please see metallic bonding.

When small loads (stresses) are applied to metals they deform, and they return to their original shape when the load is released. Bending a sheet of steel is an example where the bonds are bent or stretched only a small percentage. This is called elastic deformation and involves temporary stretching or bending of bonds between atoms.

When higher stresses are applied, permanent (plastic) deformation occurs. For example, when a paper clip is bent a large amount and then released, it will remain partially bent. This plastic deformation involves the breaking of bonds, often by the motion of dislocations (See the above figure) Dislocations move easily in metals, due to the de-localized bonding, but do not move easily in ceramics. This largely explains why metals are ductile, while ceramics are brittle.

Aluminum metal soda cans are formed from processed sheet metal the is drawn into shape (i.e. manufactured by a deformation process)