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4.4.1 Draw and describe a metallic bond.
Metals are often described as positively charged nuclei in a sea of electrons. The outer electrons of the metal atom nuclei are free and can flow through the crystalline structure. The bonding is caused by attraction between the positively charged metallic atom nuclei and the negatively charged cloud of free electrons. Specific arrangements of metal atoms are not required.
4.4.2 Explain how the movement of free electrons makes metals very good electrical and thermal conductors.
4.4.3 State that metals (pure or alloyed) exist as crystals.
Crystals are regular arrangements of particles (atoms, ions or molecules). Details of types of crystals are not required.
4.4.4 Draw and describe what is meant by grain size.
4.4.5 Explain how 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.
The permanent deformation of a solid subjected to a stress.
More on the def'n of plastic deformation.
4.4.7 Explain how metals work-harden after being plastically deformed.
Beyond the yield stress metals and alloys harden when plastically deformed.
4.4.8 Describe how the tensile strength of a metal is increased by alloying
The increased strength and hardness, and reduced malleability and ductility, of alloys compared to pure metals is due to the presence of “foreign” atoms which interfere with the movements of atoms in the crystals during plastic deformation (see above diagram on plastic deformation)
4.4.9 Explain the effect of alloying on malleability and ductility.
The presence of “foreign” atoms in the crystalline structure of the metal interferes with the movement of atoms in the structure during plastic deformation.
4.4.10 Describe a superalloy.
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.
4.4.11 List two design criteria for superalloys.
Consider creep and oxidation resistance.
4.4.12 Identify applications for 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.
Superalloys are used where there is a need for high temperature strength and corrosion/oxidation resistance.
Other uses of superalloys are: aircraft and industrial gas turbines, space vehicles, submarines, nuclear reactors, military electric motors and heat exchanger tubing.
Bulleted list and italicised paragraphs are excerpted from Design Technology: guide. Cardiff Wales, UK: International Baccalaureate Organization, 2007.
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