Strength - To carry design loads without failing.
Stiffness - To not deform excessively under load.
Hardness - To resist wear, scratching or indentation.
Fatigue strength - To resist repeated loading.
Creep resistance - To withstand constant load, especially at high temperatures.
Toughness - To not crack/ fracture suddenly under load.
Low density - To be light weight.
Coefficient of thermal expansion - To withstand wide temperature ranges.
Corrosion resistance - To withstand corrosion.
Non-toxic/ toxicity - To not poison.
Atomic phenomena - Definitions
Atom: Individual structure constituting the basic unit of a chemical element, contains a nucleus around which electrons orbit.
Nucleus: composed of protons and neutrons.
Electrons: Elementary particle orbiting the nucleus in distinct shells/ energy levels.
Atomic number: number of protons in the nucleus.
Isotope: Atoms of the same element with the same atomic number but different atomic masses.
Atomic weight: weighted average of atomic masses of an element's naturally occurring isotopes.
Atomic mass unit: mass of one proton. 1.66 x 10^(-24) g.
Polarity: Charge. Electron has a charge of -1.6 x 10^(-19) C. Proton is equal and opposite.
Avogadro's number: number of protons and neutrons needed for 1g of an element. (6.02 x 10^23).
Mole: amount of a substance containing 6.02 x 10^(23) atoms/ molecules.
Atomic phenomena - Bonding
Metals: outer shells are near empty.
Non-metals: outer shells are near full.
Metalloids: somewhere in between.
Primary bonds (ionic, metallic, covalent) are strong bonds.
Secondary bonds (Van der Waals, Hydrogen bonding, permanent dipoles) are weaker bonds.
Ionic/ metallic bonds are non-directional.
Covalent bonds are directional.
Alloy: Mixture of a metal with one or more other metals or non-metals (binary, ternary etc.)
Component: An element included in an alloy.
Phase: A region of material having uniform physical and chemical characteristics.
Solvent: The element or compound that is present in the greater concentration.
Solute: The element or compound that is present in the lesser concentration.
Solid solution: A single phase region in which solute atoms have mixed with solvent atoms to form a homogeneous composition.
Composition: The mass of each component present in an alloy or phase.
Constitution: The sum of: (the phases, the mass of each phase and the composition)
Phase diagram: The equilibrium constitution of all combinations of temperature and composition.
Eutectic point: Point at which one liquid phase is in equilibrium with two solid phases.
Hume-Rothery rules: Atoms should be: approximately same size (+/- 15%); have the same preferred crystal structure; have same number of valence electrons; have similar electronegativity.
Complete solid solubility means that the solute atoms are completely soluble in the solvent atoms for any given composition of solute and solvent (example is Cu-Ni solid solution.)
Equilibrium phase diagrams imply that we give any phase changes which wish to occur (driven by chemical reactions, not what WE wish to occur) a sufficient amount of time to occur.
Copper alloys: Excellent electrical and thermal conductivity.
70% Cu, 30% Zn: Cartridge Brass, ductile, easily cold worked.
60% Cu, 40% Zn: Common casting alloy, low melting point, brittle.
60-55% Cu, 40-45% Ni: Electrical resistance wire.
80% Cu, 20% Ni: Condenser tubes (corrosion resistance).
95-93% Cu, 5-7% Al: Heat exchangers (thermal conductivity).
Crystal structures and note on toughness.
Hexagonal close pack (HCP): ABAB. 3 close-packed directions. 1 unique close-packed plane. Magnesium, titanium, beryllium all adopt this structure.
Face-centred cubic: ABCABC. 3 close-packed directions. 4 unique close-packed planes. Aluminium, copper, lead and iron (between 910-1400 C) adopt this structure.
Body-centred cubic: 2 close-packed directions. No close-packed planes, but 6 'almost' close-packed planes. Chromium, tungsten and iron (<910 C, >1400 C) adopt this structure.
Miscellaneous note on toughness (don't want to waste space): toughness is the resistance to sudden fracture under load. Ductile, plastic fracture is preferred because a) it is gradual, which allows time to take preventive measures (whereas brittle failure is sudden and catastrophic) and b) ductile materials are generally tougher, so more energy is absorbed during plastic deformation, and not released to cause human harm during fracture.
Toughness requires a combination of ductility and strength and can be quantified as the area under the stress-strain curve.
0.04 - 1.7% Carbon: Steel.
- 0.04 - 0.3% Carbon: Mild steel. Low strength, good ductility, strength, can be welded. General purpose steel. Used in construction.
- 0.3-0.7% Carbon: Medium carbon steel. Intermediate. Nuts, bolts, gears, shafts.
- 0.7-1.7% Carbon: High carbon steel. High hardness, low toughness. Ball bearings, springs.
Low alloys: low C steel can be alloyed with up to 6% Cr, Ni whereas medium/ high carbon steel alloyed with up to 18% Cr to make stainless steel.
>1.7% Carbon: Cast iron.
- White cast iron: only Fe and C.
- Grey cast iron: Si, Mg, and Cerium may also be contained within the alloy. In grey cast iron, graphite flakes form in pearlite matrix. Flakes have little strength, act like internal cracks. In spheroidal cast iron, carbon spheres form in ferrite matrix. These spheres are strong and hard.
Micro-structure of Fe-C alloys
Pure iron := <0.03% Carbon.
- Fe (Ferrite): Exists <910 C. BCC structure. Magnetic. Soft and ductile
- Fe (Austenite): Exists 910 C < T < 1391 C. Non-magnetic. FCC structure.
- Fe: Exists 1391 C < T < 1536 C. Non-magnetic.
, and can also be used to describe the corresponding Fe-C alloys (above are forms of iron). Reason being they retain the crystal structures.
Fe3C (Cementite): 6.7% C. Very hard, very brittle.
Eutectoid point: a solid is in equilibrium with two solid phases (in the Fe-C equilibrium diagram, a eutectoid point exists where is in equilibrium with and Fe3C.
Pearlite: eutectoid structure. Alternating ferrite and cementite lamellar. Alloy containing less than eutectoid C composition is hypo-eutectoid steel. Alloy containing more C is hyper-eutectoid steel.
Metal heat treatments
Coarse/ fine pearlite: formed by slow furnace cooling/ fast air cooling (normalising). Coarse pearlite is ductile and softer, fine pearlite is stronger, harder, but more brittle.
Bainite: transformation of austenite. Consists of two phases: ferrite and cementite. Very fine. Comprised of fine, elongated cementite regions in ferrite. Formed by oil quenching.
Martensite: Quenched (cooled fast in water). No time for atoms to diffuse - literally move by diffusion. Cooled from >910 C to ~105 C. Distorted body-centred tetragonal structure. Hard, strong, brittle.
Tempered Martensite: Martensite re-heated, then held to restore some ductility and and allow diffusion of atoms/ stress relief. Better balance between strength/ ductility.
In general: as the carbon content increases, strength and hardness increase, and ductility decreases.
Non-metallic, inorganic solids. Covalent (SiC, SiO2, Si3N4) or Ionic (Al2O3, MgO, ZrO2).
Glasses have similar properties, but are amorphous.
Young's Modulus: covalent bond is stiff(er than the metallic bond), high Young's Modulus.
Strength: Ionic/ covalent bonds need more energy to break than metallic ones. High strength. Rarely yield in tension; usually fracture first.
Ductility: Coulomb force between atoms makes slip difficult. High slip resistance, low ductility.
Toughness: Low toughness, due to internal defects within material from the formation process.
Other properties: High hardness; higher creep resistance than metals; difficulty in slip makes crack formation difficult ergo high fatigue resistance; low density (equivalent to lighter metals); good thermal/ electrical insulation; higher melting points than metals; excellent corrosion resistance.
Uses: abrasives; cutting tools (cermented carbide); armour; electrical insulation; nanoparticles; fuel cells.
Long chain of monomers all linked together by covalent bonds. Mostly have linear/ branched chains. Melt on heating (due to lack of cross-links between molecule chains - only Van der Waals exist).
Long chain molecules can get tangled (like spaghetti) or form a high degree of order/ crystallinity (HDPE). PE (Polyethylene) is a thermoplastic.
Semi-crystalline: PE; Polypropylene (PP); Polytetrafluoroethylene (PTFE). Advantages: Cheap, easily moulded, tough; higher stiffness than PE, UV/ fatigue resistance; high temperature/ chemical resistance, non-stick. Used for: tubing, film, bottles, pipes; household products, fibres, rope, toys; non-stick pans, chemical containers, bearings.
Amorphous: Polystyrene; Polyvinylchloride; Polymethylmethacrylate (PMMA). Advantages: optically clear, easily moulded, brittle; cheap, stiff, brittle; excellent optical properties (transparency), water resistant. Used for: BIC biros, food containers, packaging, electrical insulation; window frames, artificial leather, fibre, pipes; transparent sheet, mouldings, aircraft domes, windows, surgical instruments.
Polymers: elastomers (rubbers)
Very elastic in tensile deformation. Exhibit no linear region in tensile stress test.
Few cross-links between long poylmer chains; resist melting, thermoset-ic properties.
Covalent and Van der Waals bonding exist between polymer chains.
Examples: Polyisoprene (natural rubber) harvested from sap of Havea tree, Polybutabiene and Polychloroprene.
They exhibit a high degree of covalent bonding/ cross-links between long polymer chains, imparting high strength and stiffness/ rigidity from the covalent bond onto the thermoset. Makes them very brittle.
The high degree of crosslinking also makes thermosets resist melting. Thermosets do not melt (hence the name). They do not soften upon heating.
Once formed, they cannot be reformed (through heating, can still be shredded).
Cross-linking prevents any ordering/ packing of the chains -- they are amorphous.
Examples: Epoxy (composite matrix for fibre composites and as adhesives), phenolic (electrical/ motor housing), polyester (composites, car body components).
Polymers: tensile properties and creep response
Long, polymer chain (in thermoplastics become tangled). Applying a tensile force, this 'untangles' the chain, breaks the weak secondary bonds, allowing the polymer to 'slip.' This is the origin of ductility within thermoplatics. In this region, polymers are very ductile, not very stiff, quite weak.
Once the chain is untangled, strength is dictated by the strength of the covalent bond, imparting much higher strength and stiffness to the polymer.
Crosslinks in thermosets impart stiffness/ strength and embrittle the material. Elastomers have low stiffness, but are elastic up to very large strains.
Amorphous solids exhibit a temperature range T_g where the rate of increase of specific volume is changing, and the material behaves as a viscous liquid. Conversely, crystalline solids melt upon heating. Semi-crystalline materials exhibit both properties.
As a polymer approaches T_g, there is a pronounced reduction in the stiffness of the material. At room temp PE is above T_g => is viscous, PMMA is below T_g => brittle solid.
Thermosets and elastomers also exhibit a T_g.
"A material formed by the combination of two or more phases to achieve superior properties than either constituent acting alone."
Consist of a continuous phase (matrix) which transfers applied loads to the dispersed phase (the reinforcement). Matrix is metal, polymer or ceramic (MMC, PMC, CMC). Reinforcement is often ceramic (high strength).
MMC: Al, Al-Li, Mg, Cu, Co, Ti. Reinforcements: Graphite, Al2O3, SiC. Advantages: high elastic modulus, tough, ductile, high temperature performance. Disadvantages: higher density, expensive to process. Uses: Satellite/ aerospace structures, high-temp engine components.
PMC: Nylon, PP, epoxy, phenolic, polyester. Reinforcement: glass, carbon, boron. Advantages: high specific properties (strength, stiffness), easy to process. Disadvantages: creep at low temperatures. Uses: aircraft/ vehicle structures, marine hulls, sporting equipment.
CMC: SiC, SiNi, Al2O3. Reinforcements: C, Al2O3. Advantages: high temp resistance, corrosion resistance. Disadvantages: expensive to produce, more brittle than MMC. Uses: jet/ automotive engine components, cutting tools, mining equipment (especially deep sea [corrosion]).
Materials for a sustainable world: definitions
Considerations when choosing a material: price/ availability. How will its production/ use contaminate the environment? Recyclable? Biodegradable?
“Sustainable development is such that it meets the needs of the present generation without compromising the ability of future generations to meet their own needs.”
“The current reserve is the known deposits which can be extracted profitably at today’s price using today’s technology. The resource base includes the current reserve, but also includes all deposits that might become available given diligent prospecting…and includes all known and unknown deposits that cannot be mined profitably now, but which-due to higher prices, better technology, transport etc, might reasonably become available in the future.”
A material is biodegradable if, “by interactions with the environment the material deteriorates and returns to virtually the same state in which it existed prior to the initial processing.”
Renewable materials are “substances derived from a living tree, plant, animal or ecosystem which has the ability to regenerate itself.”
Materials for a sustainable world: targets
A material is recyclable if “at the end of its working life in one component it can be recovered and reprocessed for use in a second component.”
The End-Of-Life-Vehicles (ELVs) Directive has set targets that 85% of a vehicle should be recycled.
Metals: can be dismantled, separated, re-melted, reformed. Currently strong case to recycle Al.
Ceramics: No economic case for recycling ceramics e.g. glass which are abundant.
Polymers/ composites: difficult to re-melt and reform. Shredded and used to impregnate virgin material as reinforcement instead.
Kyoto Climate Protocol, 1997: summit meeting between industrialised countries to commit to an overall reduction in greenhouse gas emissions by 5.2% below 1990 levels by 2008-2012. This is far below necessary reduction proposed by intergovernmental committee. The US, the world's biggest polluter, never signed up for the Kyoto protocol. All EU countries did.
True strain: ; True stress:
Steady state creep:
Safety factor in stress: , Safety factor in life:
Volume of atoms in unit cell: (N = no. of atoms in unit cell).
Theoretical density: (M = atomic mass, N_A = Avogadro's number).
Miscellaneous equations 2
Average molecular weight: N_i = number of monomers in a chain.
Secant modulus: On the σ-ε diagram, a secant line (line joining origin to a point on the curve) at a certain strain i.e. a line is drawn between the origin and the point on the curve which corresponds to a certain strain.
Tangent modulus: A tangent line is drawn at the point on the curve at a certain strain (rather than a secant line).
Time-dependent creep of polymers (at a certain temperature):
Critical fibre length:
Composite Young's modulus for iso-strain case:
Miscellaneous equations 3
Composite Young's modulus for iso-stress case:
C = rate of consumption of commodity, t = time, r = percentage growth rate per year: