4.2a Metals and metallic alloys

Conceptual Understanding: Metals and metallic alloys are typically hard and shiny with good electrical and thermal conductivity, metals are a very useful resource for the manufacturing industry. Most pure metals are either too soft, brittle or chemically reactive for practical use and so understanding how to manipulate these materials is vital to the success of any application.

A metal ore is a rock containing a metal, or a metal compound, in a high enough concentration to make it economic to extract the metal. The method used to extract metals from the ore in which they are found depends on their reactivity.

Extracting metal from ore

The Earth's crust contains metals and metal compounds such as gold, iron oxide and aluminium oxide, but when found in the Earth these are often mixed with other substances.

Aluminium Extraction:

Aluminium ore is called Bauxite and is plentiful.

The extraction process is very costly as Bauxite is refined into aluminium oxide trihydrate (alumina) and then electrolytically reduced into metallic aluminium and therefore requires large amounts of electricity.

Steel:

Blast Furnace using oxygen furnace and the electric arc furnace contribute to high rates of steel reusability.

Steel recycling can be done during this process too by remelting steel and forming it back into sheets, tubes, rods, etc.

Modifying physical properties by alloying: A mixture that contains at least one metal. This can be a mixture of metals or a mixture of metals and non-metals.

Examples- Brass, Superalloys

Grain size: Metals are crystalline structures comprised of individual grains. The grain size can vary and be determined by heat treatment, particularly how quickly a metal is cooled.

Modifying physical properties by tempering: A heat treating process designed to increase the toughness of an iron-based metal by heating it and allowing it to cool in air. Tempering decreases the hardness of the material, which usually increases the ductility and decreases the brittleness.

Quick cooling results in small grain (increases density, brittleness, strength and hardness)

Slow cooling results in large grains (increases toughness, ductility and flexibility)

Tempering is a process of heat treating, which is used to increase the toughness of metals containing iron.

Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal for a certain period of time, then allowed to cool in still air.

Tempering reduces brittleness after quenching.

Modifying physical properties by work hardening: Also known as strain hardening or cold working, this is the process of toughening a metal through plastic deformation.

Work hardening or cold working, is the strengthening of a metal by plastic deformation.

As the name suggests the metal becomes harder after the process. The metal is not heated at all. The process involves the metal passing through a set of rollers to reduce its thickness, (compressed) grains are deformed. The shape is changed, but the volume remains constant. The defects of these structures reduce the ability for crystals to move within the metal structure, becoming more resistant to more deformation as they recrystallize.

Processes include -

rolling
bending
shearing
drawing

Design criteria for super alloys: An alloy that exhibits excellent mechanical strength, resistance to thermal creep deformation, good surface stability and resistance to corrosion.

Design criteria for superalloys:

Excellent mechanical strength and creep resistance at high temperatures
Corrosion and oxidation resistance

Creep Resistance:

Creep is the gradual extension of a material under constant force. Dependent on temp. and pressure.
Occurs as a result of thermal vibrations of the lattice. Can result in fracture of superalloy due to development of cavities in the material

Oxidation Resistance:

Presence of other metals such as chromium ensure that a tight oxide film is formed on the surface
This restricts access of oxygen to the metal surface so that the rate of oxidation is heavily reduced.

Applications of Superalloys: Nickel Based Alloy

Jet Engine Components and Exhaust pipes on F1 Cars/High performance cars and motorbikes

Turbine blades operate at high temperature and under extreme stress conditions. In operation they will glow red hot, however they must be creep resistant, fatigue and corrosion resistant.

Recovery and disposal of metals and metallic alloys: Processing ferrous and non-ferrous metal scrap into vital secondary raw material for the smelting of new metals to produce new metal components.

Steel recovery and recycling

Processing ferrous and non-ferrous metal scrap into vital secondary raw material for the smelting of new metals.

Worldwide, over 400 million tonnes of metal is recycled each year.

Aluminium recovery and recycling

Metals recycling protects the environment and saves energy. Using secondary raw materials means less use of natural resources which would otherwise be needed to make new metal compounds – such as iron ore in steelmaking; nickel in stainless steel; or alumina and bauxite in aluminium smelting.