4.2a Metalls & Metallic Alloys

Crystals

Metals (pure or alloyed) are crystals. Crystals are regular arrangements of particles (atoms, ions or molecules). When the metal is forming it builds up to create crystals.

Grains

When solidifying each crystal starts at different sections and stacks differently it creates a mismatch which is shown as different grains.

Work Hardening & Heat Treatment

Work Hardening

Bending/changing the metal while it’s cold. Stretch molecules to increase hardness and tensile strength.

Annealing & Normalizing

Cooled slowly.

Quenching

Cooled quickly creates lots of crystals making it hard and brittle. Cooled slowly makes cube structure and thus more flexible.

Tempering

Cooled/heated different temperatures and rates. Heated after work hardened.

Grain Size

Grain Size Modification

The grain size can be controlled and modified by the rate of cooling of the molten metal, or by heat treatment after solidification.
Reheating a solid metal or alloy allows material to diffuse between neighbouring grains and the grain structure to change. Slow cooling allows larger grains to form; rapid cooling produces smaller grains. Directional properties in the structure may be achieved by selectively cooling one area of the solid.

Hot Rolling (Tempered)

Resistance to shock and vibration, increase toughness.

Cold Rolling (Work Hardening)

Increase tensile strength, increase stiffness, increase hardness.

Alloys Definition

This distortion also hinders the movement of dislocations, hence increasing the alloy strength.

Effect on Ductility and Malleability

The presence of “foreign” atoms in the crystalline structure of the metal interferes with the movement of atoms in the structure during plastic deformation.

Superalloys Definition

The strength of most metals decreases as the temperature is increased. Superalloys are metallic alloys that can be used at high temperatures, often in excess of 0.7 of their absolute melting temperature.

Properties

High tensile strength

Lightweight

Withstand extremes most metals couldn’t

Resist corrosion, oxidation

Mostly used in aerospace and energy

Creep

Pull apart of metal over time period, slow movement or deformation under influence of stress.

Oxidation

Interaction with chemicals and oxygen to make rust.

Corrosion

Breakdown of materials from chemical reactions with surroundings.

Applications of superalloys

Superalloys can be based on iron, cobalt or nickel. Nickel-based superalloys are particularly resistant to temperature and are appropriate materials for use in aircraft engines and other applications that require high performance at high temperatures, for example, rocket engines, chemical plants.

Recovery and Disposal

Recovery of Metals

Valuable metals such as gold and copper are being recovered from millions of mobile phones that have gone out of use following the end of product life. Some laptops and mobile phones can be disassembled very quickly without tools to allow materials to be recovered easily.

Disposal of metals

Everything needs to be separated to be disposed of.

Design for Disassembly

Example: Some products (phones, laptops) designed for quick disassembly so materials can be recovered easily.

Example of Work Hardened Metal

screwdrivers, hammered metal, metal rings.

Example of Annealed Metal

copper wire, bullet casings, sheet metals.

Example of Quenched Metal

knives, swords, scissors.

Example of Tempered Metal

washers, cutlery (forks, spoons, knives), tools (picks, hammers).

Example of Alloy

stainless steel (steel + chromium), bronze (copper + tin), red gold (copper + gold).

Example of Superalloy

jet engines, rocket engines, aerospace structures.