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 crystalsideally without defects.A number of metals can be solidifiedso 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 absorbed in the grain.

Inter-granular corrosion in aluminiumfor 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 environments, reduce electrical conductivity and make welding difficult.

Dendrite

(dendron is Greek for “tree”)

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 sheetsare 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:

)1(2

EG

(nu) is the negative ratio between transverse and longitudinal strain (Poisson’s ratio)

THE VALUE OF POISSON’S RATIOAND 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