Engineering Materials

• A phase is an easily seen part of a microstructure (i.e. different colour, grain shape and grain size to other grains). A phase has the same composition throughout
• the grain.

• An alloy is made up of two or more elements, one of which must be a metal and the alloy
• itself must show metallic characteristics.

• 1.
• Interstitial solid solutions – solute atoms are very small in comparison with the solvent atoms.

• 2.
• Substitutional solid solutions – atoms of the solute material take up positions normally occupied by some of the atoms of the solvent material.

a) Ordered – if the atoms of the solute material occupy similar lattice points within the crystal structure of the solvent material.

b) Disordered – if substitution is random.

Equilibrium means here that the cooling of the alloy is carried out so slowly that at every temperature all possible changes are allowed to go to completion.

Cooling curves show the temperature and time at which metals cool from a liquid state. These diagrams are used to construct thermal equilibrium diagrams.

Undercooling generally occurs if the cooling rate is not slow enough to allow the necessary nuclei to form.

• Left = 'a pure metal.'
• Right = 'an alloy.'

1. Binary alloys forming acontinuous series of solid solutions. (eg. Copper and Nickel)

• 2. Binary alloys forming Eutectic mixtures.
• a. Mutual liquid solubility but solid insolubility. (eg. Cadmium and Bismuth)

b. Mutual liquid solubility, partial solid solubility and a Eutectic mixture. (eg. Lead and Tin)

4. Layer-type diagram. (eg. Aluminium and Lead)

A Eutectic mixture is formed in an alloy system when distinct solid phases separate simultaneously and at constant temperature from a single liquid phase.

• 1. The heating of the metal to
• the pre-determined heat treating temperature.

• 2. The soaking of the metal at
• that temperature until the structure becomes uniform throughout the section.

3. The cooling of the metal at some pre-determined rate such as will cause the formation of, or will maintain desirable structures within the metal.

Hardening is done by heating steel with sufficient carbon (i.e. above about 0.3%) to austenite zone and cooling quickly. This is what happens during rapid cooling.

Annealing is the heating and cooling of a metal to produce the softest possible state.

Tempering consists of heating the hardened steel to some temperature below its lower critical temperature.

Purposes:

1. to relieve internal stresses

2. to improve the ductility and toughness of the steel

Surface hardening of steel produces a product that has a hard surface while its core or centre portion is relatively soft.

1. Carburising – to impregnate the surface of low carbon steel with sufficient carbon to raise its composition to the eutectoid level.

2. Nitriding – steel is heated at 500oC for 40 to 90 hours in a gas-tight container in which ammonia is circulated. Its surface will become very hard due to nitride formation while its core will remain unaffected.

3. Selective hardening – Heating the surface into the austenitic range then quenching rapidly. The surface will become martensitic while the core remains soft.