Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
Materials 2
Progress test content
| Term | Definition |
|---|---|
| If oxidation energy is positive vs negative | positive: metal is stable negative: metal will oxidise |
| How is oxidation rate measured | Mass changes per unit area |
| What is the mechanism for parabolic oxidation | Controlled by the diffusivities of the metal and oxygen ions Nonporous oxide layer protects the metal |
| Why do most metals oxidise in service? | Metals are thermodynamically unstable relative to their oxide forms |
| In oxidation what is the cathode and anode | Metal = anode (loses electrons) Oxygen = cathode (gains electrons) |
| What does oxidation rate depend on? | Depends on the nature of the oxide layer, not directly formation energy |
| Why does Al oxidise slower than Fe even though the deltaG for Al2O3 is more negative | Al2O3 forms a dense adherent protective oxide with very low electron diffusivity |
| When would kL be negative? | When oxide evaporates away from the metal surface as soon as its formed |
| Difference between subscript p and subscript L constants | L: constant in linear oxidation p: constant in parabolic oxidation |
| 2 different micro-mechanisms for oxidation | Oxygen ions diffuse through oxide layer so the new layer forms next to the metal OR Metal ions diffuse through the oxide layer so the new oxide layer forms on the outside of the original (not next to the metal) |
| When is the rate of growth controlled by ionic diffusion | When the layer formed is non-porous and adheres to the metal surface |
| 2 factors that determine the rate of oxidation | Diffusion coefficient Electrical resistivity |
| What causes typically linear growth of oxidation rate to stop being constant? | Cracking to relieve stress (vol oxide < vol material) Delamination where the oxidate layer pulls away from the surface (vol oxide > vol material) |
| Why is wet corrosion much faster than dry? | Newly formed precipitate does not deposit a protective layer on the free surface The metal and hydroxide ions diffuse faster in the liquid states than through oxide layers The electrons can move more easily in conductive materials that in oxide layers |
| What is n in the weight change in corrosion equation | n is the number of electrons per metal atom produce or consumed in the process |
| How does crevice corrosion work? | Deposit acts as a shield and creates a stagnant condition Oxygen becomes depleted in the crevice so there is an excess of M+ which is balanced by Cl- MCl + H2O -> MOH + HCl Acidity increases so more corrosion occurs at the site |
| What makes a crevice suitable? | Crevice must be wide enough to allow liquid to penetrate while be narrow enough to maintain a stagnant zone |
| How does pitting corrosion work? | Due to surface roughness or non-uniform chemical composition distribution Damaged spot becomes an anode and everywhere else a cathode Metal ions dissolve in the pit and attract chloride ions making the site acidic MCl + H2O -> MOH + HCl |
| Why does a high concentration of MCl increase the rate of corrosion | It is converted to insoluble MOH and HCl and both H+ and Cl- accelerate corrosion They stop the oxide film from forming |
| Corrosion if 2 dissimilar metals are joined together | Galvanic corrosion will occur on the more reactive metal which becomes the anode. |
| How to reduce the effects of galvanic corrosion | Choose metals close together in the galvanic series Avoid unfavourable anode-to-cathode ratio (use a larger anode area) Electrically insulate dissimilar metals from each other Electrically connect a third anodic metal |
| Galvanising protection | Used to protect the cathodic metal from corrosion Even if a crack forms in the coating, the more anodic metal will continue to corrode over the cathodic metal |
| Sacrificial protection | The metal is physically connected to a 'sacrificial anode', a more anodic metal which will corrode in preference |
| Intergranular corrosion (of alloys with carbon) | Occurs along grain boundaries leading to fracture along them Carbon precipitates out at the boundaries and removes chromium The lowered chromium content means the metal cannot resist attack in corrosive environments |
| How to protect stainless steels from intergranular corrosion | Heat to high temperatures to dissolve all chromium carbide particles Lower the carbon content to minimise carbide formation Alloy with another metal that has a greater tendency to form carbides than chromium |
| Review of crystal structures | FCC: 4 cpp, 3cpd, 12 nn BCC: 0 cpp, 2 cpd, 8 nn HCP: 1 cpp, 3 cpd, 12 nn |
| The Burgers vector, b | magnitude and direction of shear that a dislocation produces |
| What are the 2 angles in the critical resolved shear stress equation and what are they at a maximum | lambda: between slip direction and applied force phi: between the normal to the slip plane and applied force 45 degrees for tauR |
| Polycrystal | Material made of crystals of different orientations |
| Taylor factor | total yielding of polycrystal / dislocation yield strength |
| Deformation by twinning | Mechanism where part of the crystal lattice reorients itself into a mirror image of the parent lattice across a specific plane called the twin plane |
| Difference between slip and twinning | In slip, crystal orientation above and below the plane stays the same, while twinning causes reorientation. Slip occurs in discrete atomic spacing multiples, whereas twinning displacements are smaller than interatomic spacing. |
| Intrinsic lattice resistance, fi | Caused by bonds between the atoms which have to be broken and reformed as dislocations move High = dislocation slip difficult |
| Solid solution hardening | Impurities in a crystal increase the resistance to dislocation motion |
| Substitutional solid solution hardening | When impurities are equivalent in size to the parent atoms, they take up positons which were originally those of the parent atoms in the crystal lattice |
| Interstitial solutes in solution hardening | When impurities are much smaller in size than the parent atoms, they take up interstitial sites These impurities have high diffusivity |
| Precipitation hardening | Hard precipitates act as dislocation movement barriers. In polycrystals, precipitates segregate at grain boundaries. |
| Work hardening | Yield strength increases if the material is unloaded and reloaded after a certain amount of pre-strain. - The interaction of dislocation stress fields where dislocations of the same sign repel - The interpenetration of dislocations cause tangles |
| Advantages and disadvantages of fine grain sizes | Ad: stronger, harder, tougher and most susceptible to work hardening Dis: less resistant to corrosion and creep |
| Grain boundary strengthening | More grain boundaries means: - two grains of different orientations impedes dislocation movement - atomic disorder within a grain boundary results in a discontinuity of slip planes from one grain to another - dislocations 'pile-up' at grain boundaries |
| Recovery | Some of the stored internal strain energy is relieved by dislocation movement and annihilation due to atomic diffusion at an elevated temperature. There is reduction in dislocation density and dislocation configuration. |
| Recrystallisation | Formation of a new set of strain-free equiaxial grains with a low dislocation density. Form as small nuclei and then grow until they consume the parent grains. |
| Recrystallisation temperature and how to reduce it | The temperature when the recrystallisation can complete in 1 hour Increase initial prior cold work to reduce it - there will be a higher stored strain energy |
Created by:
AJZAZA