METALS zRecap: metallic bonds, metal properties zSummary yMetal lattice, defects yFormation of crystals (crystallisation) yDislocations and Burgers’ vector yPoisson’s ratio yCase studies: metal whiskers, intergranular corrosion

METALLIC BONDS = A SEA OF ELECTRONS zMetal 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). zElectrons act as a "cement”, holding the kernels in their relatively fixed positions. zThis 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 zMetal crystals are formed through two phases: nucleation i.e., creation of small crystals (nuclei) and growing of nuclei. zSince 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 zAs a consequence, metals are formed with grains, having well defined boundaries zA characteristic which affects mechanical properties of metal is their grain size.

CASE STUDY 2: INTERGRANULAR CORROSION zInter-granular corrosion is localised attack along the grain boundaries or close to them, while the bulk of the grains remain largely unaffected. zThis 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. zThe problem can be addressed e.g., by reheating a welded component, so that chromium is absorbed in the grain. Inter-granular corrosion in aluminium for zinc precipitation (failed aircraft component)

IMPERFECT SOLIDIFICATION: DENDRITES z 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. zIf this is the case, dendrites (tree-like structures) form as the metal solidifies out into the melt, leaving molten metal behind. z Dendrite formation is common: however the better a melt is inoculated, the fewer dendrites. zDendrites 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 zThe theory of dislocations explains how defects in metals can produce plastic deformation. zTwo 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 zBody-centred cubic (b.c.c.) (9 atoms per unit cell): e.g., chromium, iron , tungsten, vanadium zFace-centred cubic (f.c.c.) (14 atoms per unit cell): aluminium, nickel, iron  zHexagonal 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 zLike 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. zG is important to measure the slip between atom sheets in metals, hence the plastic shear deformation zA relation between G and E exists for homogeneous and isotropic materials, which is: (nu) is the negative ratio between transverse and longitudinal strain (Poisson’s ratio)

THE VALUE OF POISSON’S RATIO AND WHAT IT SUGGESTS zPoisson’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. zTypically, metals have Poisson’s ratios around 0.3 zRubbery materials have Poisson’s ratios close to 0.5 zSoft materials with a large amount of porosity(foams) have Poisson’s ratio close to 0 zAs a consequence of these values, most materials are stiffer in the direction they are loaded than in shear