IB Design Technology - Topic 4: Metals and Glass

IB Design technology - Topic 4: Metals and Glass

Metals 

• When molten metals cool, their particles reform into a crystal lattice structure, there are three types:

• Close Packaged Hexagonal - weak, poor strength to weight ratio
• Body-Centered Cubic - ductile, good electrical conductor
• Face-Centered Cubic - hard and tough

• Crystals grow into dendrites (think neurons) and form a skeletal structure
• Grains - dendritic structures, meet at surfaces called the grain boundary
• Grain size and shape can be controlled by the rate of cooling of a metal
• Grain size can also be controlled via hardening, tempering, normalizing, and annealing

• Not that annealing and normalizing are extremely similar processes but annealing involves a non-ferrous metal and normalizing involves a ferrous metal

• Reheating allows grains to diffuse and structures to change
• Slower cooling grows larger grains as they have more time to grow, vice versa
• Alloys - A mixture of a metal and another metal or a metal and a non-metal
• Alloys can harden a metal by disrupting its structure with differently sized atoms and not allowing grains to slide past each other as easily
• Alloying usually increases tensile and compressive strength but reduces ductility and malleability
• Pure iron is too soft and ductile but many alloys can be made with it
• Work Hardening - Causes permanent distortion in metal while working it cold

• Usually performed by striking metal flatter with a hammer
• The striking flattens grains and makes them harder
• Can cause internal stresses and compromise internal stability

• Heating - Reverses internal stresses from work hardening by heating a metal to its crystal growth temperature, which would allow the grains to relax
• Tempering - Hardens metal by heating it and quenching it in a pool of water or oil
• Superalloy - Extremely durable alloy used at high temperatures, usually at or above 75% of its melting point

• Creep and oxidation resistance are important criteria for superalloys

• Creep - Gradual extension of a material under constant force, creep accelerates at higher temperatures
• Oxidation - Occurs when oxygen gas is present or when exposed to an extremely high/low pH level
• Recovery - The collection of used metal for reuse 

• Metals are infinitely recyclable and recycling them reduces the energy required to make new products

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Glass

• Glass is made of melted silica (sand) + an alkali that lowers the melting point
• Characteristics - Hard, brittle, transparent, high compressive strength, high tensile strength, thermal insulator 
• Metal oxides are often added to glass to add color
• GLASS IS AMORPHOUS - Has no definite structure
• Laminated Glass - Layers of glass with a layer of thin plastic sandwiched between layers of glass. The plastic layers can absorb more energy because they are flexible, so the glass is stronger
• Tempered Glass - A process that heats the glass and cools the outside faster than the inside, which causes internal stresses within the glass that place the inside under compression and the outside under tension.
• Soda Glass - Poor glass in general, poor resistance to thermal shock
• Pyrex - Borosilicate glass, less susceptible to thermal shock, used for cooking
• Pharmaceutical - Glass’s non-porous nature makes it easy to clean and more chemically resistant
• Recycling - Glass is infinitely recyclable 

• The process starts with crushing glass to a very fine powder called cullet
• Reusing cullet in glass manufacturing reduces the energy cost and raw material usage

• Note the difference between something green and biodegradable
• Green - Simply helpful for the environment
• Biodegradeable - Breaks down easily and is not harmful to the environment
• Glass is green in that its use and reuse helps the environment by using less raw materials
• Glass is not biodegradable as it takes a very long time (predicted to be >1,000,000 years) to break down, not easy