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49 Cards in this Set
- Front
- Back
What do the shaded areas represent in stress/strain graph curves? (Think generally, I’m thinking about thedimensions/units).
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- Energy Stored - Per unit volume |
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Give one reason why we care about each of resilience and toughness.
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[Resilience]: Energy absorbed before material permanently deforms
[Toughness]: Total energy absorbed before failure |
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Explain why group 1 and group 2 metals are electropositive.
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- Filled inner shells shield valence electrons from nuclear charge -So valence electrons only feel a small effective nuclear charge -Therefore, they feel a small electrostatic attraction to other nuclei |
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Explain how electrons behave in metallic bonding, and alsogive two properties of metallic bonding that occur because of this behavior.
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-Electrons are not attached to any particular atom (delocalized) -They reside between atoms (randomly), providing electrons forces - Ductility due to non-directional bonds. Electrical Conductivity due to free mobile electrons |
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Explain why group 16 and group 17 non-metals are electronegative.
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- Large number of valence electrons, not much shielding by inner shells -So valence electrons see a large effective nuclear charge - Therefore, they feel a large electrostatic attraction to the nucleus |
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Explain how electrons behave in covalent bonding, and alsogive some properties of covalent bonding that occur because of this behavior.
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-Electron pairs shared between pairs of atoms -Electrons locked into a small region of space, mostly between atoms that donated the electrons -Brittle due to directional bonds and Insulators since there are no free electrons |
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- As electron clouds get close, they repel each other -This deforms the electron cloud, creating the induced dipole - The two induced dipoles can then attract each other electrostatically -Those are called Vander Waals Forces |
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Explain why methane is gaseous at room temperature while hexane is not
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-Methane CH4 is a small molecule -Each molecule has a small number of Vander Waals bonds thatcan form with neighbouring atoms -Hexane C6H14 can form more bonds and is therefore held even stronger -The Hexane requires greater thermal energy to separate so it has a higher boiling point |
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-When O-H bonds form, there is an unequal sharing of electrons -This creates a permanent dipole with Oxygen being partially negativelycharged and Hydrogen being partially positively charged -These charged species then attract each other |
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Explain why ethane is gaseous at room temperature while ethanol is not
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-Ethane molecules have only Vander Waals Forces creating intermolecular bonds -Ethanol has H-bonds between molecules due to the alcohol group -The ethanol molecules are therefore held more strongly -They also require more thermal energy to vaporize so they have a higher boiling point |
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What is the electron configuration of aluminium?
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[Ne] 3(s^2) 3(p^1)
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Explain the difference between the terms polycrystalline and polymorphic.
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-Polycrystalline refers to different grains with the same structure -But with different orientation -Polymorphic refers to a material with different crystal structures -Under different conditions such as temperature and pressure |
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Explain one reason why an engineer may want to use a polycrystalline material rather thana single crystal.
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-At a grain boundary, slip planes do not align and the boundaries interrupt slip planes, making the crystal less brittle |
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Explain one reason why an engineer may not want to use a polymorphic crystal.
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-If the device is to be used over a large range of conditions it may disintegrate as the crystal structure changes |
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Describe the conditions required for vacancy diffusion to occur.
Hint: think about the conditions required for a single atom to move. |
-There must be a large number of vacancies for atoms to move into -The temperature must be high enough that atoms have enough energy to breakexisting bonds and move into a vacancy. |
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Why does interstitial diffusion usually occur more quickly than vacancy diffusion?
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-Interstitial spaces always exist. It does not rely on a vacancy appearing nearby. -Interstitial impurities are usual small, light, and therefore move quickly.This speeds up the motion of the impurities through the host. |
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What is diffusion flux, 𝐽? Also, state the units of J
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-The mass of material passing through a unit area per unit time. -[J] = kg m^-2s^-1 |
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If we wish to diffuse carbon into the surface of steel (surface hardening), give two simplemethods to increase the rate of diffusion and therefore the final carbon content.
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-Increase partial pressure of carbon in the gas. -Increase the temperature of the systemAlso, decrease partial pressure of carbon in the gas on side 2. |
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Explain how this slip occurs, referring to slip planes and resolved shear stress.
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-Tensile force is applied at an angle to the slip plane. -If the component of the force in the slip plane, and along the slip direction, is largeenough, then slip planes can being to slide over one another |
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What is the main factor in determining which plane is the main slip plane in a crystal?
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-The densest plane will be the hardest to deform, so will be the slip plane. |
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Describe how dislocations can be moved through a material.
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-Shear forces can move both types of dislocations. |
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At the yield point, dislocation motion is one of the first causes of plastic deformation.Why are dislocations (relatively) easy to move?
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-Bonds near the dislocation are already deformed (strained) and therefore weak. -So, it does not take as much force to break these bonds as unstrained bonds. |
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Reducing grain sizes hardens a material by interrupting dislocation motion and slip planes.Explain how grain boundaries do this.
Hint: Be sure to mention direction of motion and force components. |
-If slip is able to occur in one grain, the slip planes in the next grain may not beoriented in the same direction. -The force creating slip in the first grain will have a smaller component lined up withthe slip plane in the second grain. -This component may not be large enough to cause the second grain to slip |
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Explain how the process of strain hardening (cold working) hardens a material
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-Strain hardening creates a large number of dislocations. -The average interaction between dislocations is repulsive,so the dislocations stop each other from being able to move around. |
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What is the third method of hardening a material?Describe how this method works?
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-Solid solution (alloying) -Impurity atoms that are bigger or smaller than the host atomsinterrupt slip planes. |
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What is the major negative affect that hardening has on a material?
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-Reduced ductility. |
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What is recovery, how is it achieved, and why does it weaken a material?
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-Recovery is the reversal of strain hardening(reduction in number of dislocations). -It is achieved by heating the material (keeping below 𝑇m of course). -Removing dislocations allows remaining dislocation to move more easily,reducing the stress required to create plastic deformation. |
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Why might we want to induce recovery in a material?
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-After cold working, a material may be too brittle to perform more work on. -Recovery will allow us to perform more forming processes on the material. |
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What is recrystallization, how is it achieved, and why does it weaken a material?
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-Recrystallization is the growth of many small grains into fewer larger grains. -It is achieved by heating the material (to about 0.4 𝑇m). -Removing grain boundaries means there are less barriers to slip and dislocationmotion. |
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Why is ductile fracture “preferred” over brittle fracture?
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-Ductile materials can absorb some energy and deform before fracturing. -This gives us some warning, rather than having a very sudden, catastrophic failure! |
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What factors affect how much stress is concentrated?
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-Depth of the crack, 𝑎. -Radius of curvature (sharpness) of the tip of the crack 𝜌t. |
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Why is a crack in a brittle material a bigger problem than in a ductile material?
Hint: Think about what a force will do to the crack in a ductile material? |
-A ductile material is able to deform, absorbing energy and increasing 𝜌t. -This reduces the stress concentration. |
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For flat objects, what methods can be used to increase the critical stress?
Hint: These may include increasing 𝐾Ic. or decreasing crack sizes. |
-Polish the surface to reduce crack depths. -Surface harden by diffusing carbon or nitrogen into the surface. |
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For pieces that must have some shape to them, what other method(s) must be used toincrease the critical stress?
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-Make sure any features have smooth, well rounded curves (large 𝜌t) |
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What is the main cause of a brittle ductile transition?
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-Temperature (materials are more brittle at lower temperatures) |
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What test is used to determine where a brittle-ductile transition occurs?Also, state what the test actually measures.
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-Charpy test. -Measures energy required to fracture a standard specimen of the material. |
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When is creep a concern to an engineer?
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-When a material is placed under a high stress (but below the yield point),and at high temperaturesfor a long time. |
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What is the difference between isomorphous and eutectic alloys?
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-In isoomorphous alloys, the two components are completely soluble in each otherat all compositions (there is only one solid phase). -In eutectic solids, there are two solid phases. |
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Describe the development of microstructure when an isomorphous alloy cools quickly froma liquid to a solid.
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-Solid begins to precipitate at the liquidus -During rapid cooling, coring occurs, in which each new layer of solid forming on theparticles has a different composition |
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What is martensite and how is it formed?
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-Phase supersaturated with interstitial carbon atoms. -Formed by rapidly quenching pure austenite. |
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What are the mechanical properties of martensite and one application?
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-It is incredibly hard. - Blades. |
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If we do not want martensite present in a steel, describe one process for removing it.
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-Austenitizing. -Heat the material above the upper critical temperature -Hold there long enough for all of the material to transform austenite. -Cool again, slowly. |
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In order to increase the amount of martensite, we need to increase the severity of aquench. Provide several methods for doing this.
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-Use water instead of oil. -Have the water circulating to move heated water away from the object. -Cool the water to increase the temperature difference between the object and thewater. |
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Provide at least three reasons why Boeing would be investing $30b in Ti for use on theirplanes (as a replacement of steel and aluminum).
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- Light (compared to steel).
- Very good fatigue characteristics (has a fatigue limit, Al does not). - Does not corrode when in contact with composites (Al does). - Does not corrode in air or water (some steels do). |
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Provide at least three reasons why Boeing might be regretting than decision and going backto using Al in some parts of the plane.
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- Expensive.
- High buy-to-fly ratio (a lot of waste makes it even more expensive!) - Difficult to machine (slow, need expensive tools). - Have to buy a lot of material from Russia…always a chance of being shafted! |
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Ceramic properties |
- Hard - Brittle - Good under compression - Bad under tension |
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Fibers (Ceramic) Properties |
- Good under tension - Bond well to ceramics - Bridge cracks |
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Resin Properties |
- Ductile - Able to absorb energy |
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Fibers (Polymers/Resins) Properties |
- Good under tension - Hard - Brittle - Bonds well to matrix |