materials lecture 13

the presence of imperfections

• vacancy atoms 
• interstitial atoms 
• substitutional atoms 
• dislocations 
• grain boundaries 
• pores

• vacancy atoms
• interstitial atoms
• substitutional atoms

dislocations

grain boundaries

pores

vacant atomic sites in structure

extra atoms positioned between atomic sites

• are line defects 
• slip between crystal planes result when dislocations move
• produce permanent plastic deformation

when a smaller different atom is placed in the atomic structure

when a larger different atom added to the atomic structure

1D defects around which atoms are misaligned

extra half plane of atoms inserted in a crystal structure burges vector is perpendicular to the dislocation line

• spiral planar ramp resulting from shear deformation 
• burgers vector is paralle to the dislocation line

• 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

• the successive bumping of half a plane of atoms 
• bonds across the slipping planes are broken and remade in succession

polycrystals

grain boundaries, regions that are less densely and regularly packed as compared to the bulk of the grains

• arrays of dislocations separated by areas of the strained lattice. 
• b = lattice translation of the crystal

edge dislocations

screw dislocations

• special orientation relation between 2 grains on either side of boundary
• fraction of the total lattice slides between the 2 grains that coincide

• grain boundaries with thick glass films 
• pores
• grains themselves especially when coarsened

gain an understanding of the microstructure at different scale lengths

how it looks to the eye

• large pores 
• grains and grain boundaries 
• crystal density

topographic and atomic number contrast

meso and nano structure

• reflected light 
• transmitted light, crossed polars

• grinding and polishing to view porosity 
• chemical and thermal etching to view grain boundaries

parallel grinding of optical section on glass slide followed by use of a cover slip

water

low viscosity resin to hold the sample together

• acids or alkalis
• removal of grain boundary phases

• heat about 200c below sintering temperature for 10-30 mins 
• surface mass diffusion away from grain boundary leads to dark contrast in reflected light

• bulk samples
• bright field 
• pores are dark 
• relative reflectivity R from Fresnel formula 
• R = (n-1)^2 / (n+1)^2 
• where n = refractive index

refractive index

thin sections that can give info on optical properties and crystal system

reflected and transmitted light microscopes

analysers can bring to fully transmission or reflexion

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

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

there are 2 directions in which light travels through the crystals

the cubic periclase stays the same while the hexagonal graphite changed intensity and colour