materials lecture 13

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  1. many of the important properties of materials are due to
    the presence of imperfections
  2. types of imperfections
    • vacancy atoms 
    • interstitial atoms 
    • substitutional atoms 
    • dislocations 
    • grain boundaries 
    • pores
  3. list the types of point defects
    • vacancy atoms
    • interstitial atoms
    • substitutional atoms
  4. list the type of line defects
    dislocations
  5. list the type of area defects
    grain boundaries
  6. list the types of volume defects
    pores
  7. vacancies define
    vacant atomic sites in structure
  8. define self interstitial
    extra atoms positioned between atomic sites
  9. dislocations
    • are line defects 
    • slip between crystal planes result when dislocations move
    • produce permanent plastic deformation
  10. interstitial
    when a smaller different atom is placed in the atomic structure
  11. substitutional
    when a larger different atom added to the atomic structure
  12. linear defects are
    1D defects around which atoms are misaligned
  13. edge dislocations are
    extra half plane of atoms inserted in a crystal structure burges vector is perpendicular to the dislocation line
  14. screw dislocation
    • spiral planar ramp resulting from shear deformation 
    • burgers vector is paralle to the dislocation line
  15. brugers vector b
    • measure of lattice distortion 
    • its the vector necessary to close a stepwise loop around the defect i.e.
    • the displacement of the crystal due to the defect being there
  16. dislocation motion requires
    • the successive bumping of half a plane of atoms 
    • bonds across the slipping planes are broken and remade in succession
  17. the grains can be from nanometers to millimetres in size and the orientations of the atomic planes are rotated with respect to the neighbouring grains. These materials are called
    polycrystals
  18. the individual grains are separated by
    grain boundaries, regions that are less densely and regularly packed as compared to the bulk of the grains
  19. low angle grain boundary
    • arrays of dislocations separated by areas of the strained lattice. 
    • b = lattice translation of the crystal
  20. tilt boundaries are
    edge dislocations
  21. twist boundaries are
    screw dislocations
  22. special or coincident grain boundaries
    • special orientation relation between 2 grains on either side of boundary
    • fraction of the total lattice slides between the 2 grains that coincide
  23. volume 3D defects
    • grain boundaries with thick glass films 
    • pores
    • grains themselves especially when coarsened
  24. the overall aim of characterisation is to
    gain an understanding of the microstructure at different scale lengths
  25. macrostructure
    how it looks to the eye
  26. optical microscopes
    • large pores 
    • grains and grain boundaries 
    • crystal density
  27. SEM
    topographic and atomic number contrast
  28. TEM
    meso and nano structure
  29. optical microscopy can be broadly divided into the following categories
    • reflected light 
    • transmitted light, crossed polars
  30. reflected light
    • grinding and polishing to view porosity 
    • chemical and thermal etching to view grain boundaries
  31. transmitted light, cross polars
    parallel grinding of optical section on glass slide followed by use of a cover slip
  32. ceramics may be sensitive to
    water
  33. porous ceramics must be mounted in
    low viscosity resin to hold the sample together
  34. chemical etching
    • acids or alkalis
    • removal of grain boundary phases
  35. thermal etching
    • heat about 200c below sintering temperature for 10-30 mins 
    • surface mass diffusion away from grain boundary leads to dark contrast in reflected light
  36. reflect light microscopy technique
    • bulk samples
    • bright field 
    • pores are dark 
    • relative reflectivity R from Fresnel formula 
    • R = (n-1)^2 / (n+1)^2 
    • where n = refractive index
  37. brighter phases have a higher
    refractive index
  38. transmitted light microscopy technique
    thin sections that can give info on optical properties and crystal system
  39. polarised light can be used with both
    reflected and transmitted light microscopes
  40. polarisation - when crossed. no light reaches eye. when rotated
    analysers can bring to fully transmission or reflexion
  41. in isotropic crystals or glass
    light moves in all directions with equal velocity and vibrates in all directions perpendicular to the direction of propagation. when placed between crossed polars they remain dark
  42. in uniaxially anisotropic crystals
    light only moves parallel to c axis with vibrations in all directions in basal plane. when placed between crossed polars and rotated some light gets through at particular orientations
  43. in biaxially anisotropic crystals
    there are 2 directions in which light travels through the crystals
  44. its easy to distinguish glass and cubic crystals from any other e.g. when a MgO graphite refractory sample is rotated between crossed polars
    the cubic periclase stays the same while the hexagonal graphite changed intensity and colour

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Author:
ghoran
ID:
329166
Filename:
materials lecture 13
Updated:
2017-03-04 15:29:47
Tags:
introduction materials microstructure
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Description:
uni year 1 materials module
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