METALS . Recap: metallic bonds, metal properties Summary Metal lattice, defects Formation of crystals (crystallisation) Dislocations and Burgersâ€™ vector Poissonâ€™s ratio Case studies: metal whiskers, intergranular corrosion . METALLIC BONDS = A SEA OF ELECTRONS.
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How would motion (i.e, plastic deformation) be possible in metals ?
Whiskers are metal crystals
ideally without defects.
A number of metals can be solidified
so to get whiskers, including tin, zinc, cadmium, silver, iron and nickel.
Limitations of whiskers are their very small dimension (length of up to 10 mm), their brittleness and their cost, due to the high reject rate in the manufacturing process
Tin whisker (diameter 150 µm)
Whiskers are nowadays confined to few applications
(reinforcement in heat exchangers, turbines, catalysts or catalyst carriers),
whilst the formation of whiskers in plated surfaces can create problems
(e.g., short circuits in electromagnetic relays)
Inter-granular corrosion in aluminium
for zinc precipitation
(failed aircraft component)
(dendron is Greek for “tree”)
an extra sheet of atoms
within the lattice
a number of atoms sheets
are transformed in
a helice-like surface
Burgers’ vector represents the deformation produced by a dislocation
(14 atoms per unit cell): aluminium, nickel, iron
Face-centred cubic and hexagonal compact give the maximum possible packing
(nu) is the negative ratio between transverse and longitudinal strain