METALS
Recap: metallic bonds, metal properties
Summary
Metal lattice, defects
Formation of crystals (crystallisation)
Dislocations and Burgers’ vector
Poisson’s ratio
Case studies: metal whiskers, intergranular corrosion
METALLIC BONDS =
A SEA OF ELECTRONS
Metal atoms have one or two outer electrons
easily moving around, not "belonging" to any
one atom, but as a part of the whole crystal,
formed by cations (kernels).
Electrons act as a "cement”, holding the
kernels in their relatively fixed positions.
This structure explains metal characteristics:
good conduction, hardness, stiffness, isotropy
How would motion (i.e, plastic deformation) be possible in metals ?
DEFECTS IN METALS
• Defects in metals have a negative
effect, in that they create internal
stresses.
• However, they also allow plastic
deformation, which may reduce
brittleness
• In principle, impurities have also
to be removed, but alloying may
confer useful properties to the
metal (e.g., resistance to corrosion,
higher surface hardness, improved
workability)
CASE STUDY 1: WHISKERS
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)
HOW DEFECTS ARE FORMED:
SOLIDIFICATION OF METALS
Metal crystals are formed through
two phases: nucleation i.e.,
creation of small crystals (nuclei)
and growing of nuclei.
Since a number of nuclei are
formed in the same liquid metal,
when they come into contact,
they are likely not to fit each
other exactly
As a consequence, metals are
formed with grains, having well
defined boundaries
A characteristic which affects
mechanical properties of metal is
their grain size.
CASE STUDY 2: INTERGRANULAR
CORROSION
Inter-granular corrosion is localised
attack along the grain boundaries
or close to them, while the bulk of
the grains remain largely
unaffected.
This happens because some
elements present in the alloy (e.g.,
chromium in stainless steel) are
segregated at the grain boundaries,
so that resistance to corrosion in
the area is reduced.
The problem can be addressed
e.g., by reheating a welded
component, so that chromium is Inter-granular corrosion in aluminium
absorbed in the grain. for zinc precipitation
(failed aircraft component)
IMPERFECT SOLIDIFICATION:
DENDRITES
During metal solidification, if solid does not
grow from the side wall e.g., of the mould
evenly, some of the heat involved in the
process is absorbed again by the metal.
If this is the case, dendrites (tree-like
structures) form as the metal solidifies out into
the melt, leaving molten metal behind.
Dendrite formation is common: however the
better a melt is inoculated, the fewer dendrites.
Dendrites modify metal hardness and
stiffness, allow corrosion in harsh Dendrite
environments, reduce electrical conductivity (dendron is Greek for “tree”)
and make welding difficult.
HOW DEFECTS MOVE AROUND:
DISLOCATIONS
The theory of dislocations explains how defects in metals can
produce plastic deformation.
Two types of dislocations are possible: edge and screw
dislocations. Most observed dislocations are a mix of the two types.
Edge dislocation
Screw dislocation
DISLOCATION CYCLE
(BURGERS’ VECTOR)
Edge dislocation:
an extra sheet of atoms
within the lattice
Screw dislocation:
a number of atoms sheets
are transformed in
a helice-like surface
Burgers’ vector represents the deformation produced by a dislocation
MAIN TYPES OF METAL UNIT CELLS
Body-centred cubic (b.c.c.)
(9 atoms per unit cell):
e.g., chromium, iron ,
tungsten, vanadium
Face-centred cubic (f.c.c.)
(14 atoms per unit cell):
aluminium, nickel, iron
Hexagonal compact (h.cp.)
(17 atoms per unit cell):
magnesium, zinc, titanium
Face-centred cubic and hexagonal compact give the maximum possible packing
SHEAR DEFORMATION:
POISSON’S RATIO
Like Young’s modulus E measures the resistance of materials to
deformation in the longitudinal direction, another modulus G (shear
modulus) measures their resistance to deformation in the
transverse direction.
G is important to measure the slip between atom sheets in metals,
hence the plastic shear deformation
A relation between G and E exists for homogeneous and isotropic
materials, which is:
E
G
2(1 )
(nu) is the negative ratio between transverse and longitudinal strain
(Poisson’s ratio)
THE VALUE OF POISSON’S RATIO
AND WHAT IT SUGGESTS
Poisson’s ratio gives a measure of how much the
material cross-section changes as far as the material is
elongated. The higher is, the more the material cross
section is reduced.
Typically, metals have Poisson’s ratios around 0.3
Rubbery materials have Poisson’s ratios close to 0.5
Soft materials with a large amount of porosity(foams)
have Poisson’s ratio close to 0
As a consequence of these values, most materials are
stiffer in the direction they are loaded than in shear