Metals materials properties

METALS  What is a metal ?  General properties  Structure and bonding  Phases and phase transformations  Structure – property relationships  Chemical properties What is a Metal ? Non-metals H Li Be Metals Na Mg K Ca Sc Ti Rb Sr Y Cs Ba Fr Ra V He B C N O F Ne Al Si P S Cl Ar Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Lr Rf Db Sg Bh Hs Mt Uuu Uub Uub La Ce Pr Nd Pm Sm Eu Ac Th Pa U Np Gd Tb Dy Ho Er Tm Yb Pu Am Cm Bk Cf Es Fm Md No Properties of Metals  Opaque  Lustrous  High melting point  Good conductors of heat  Good conductors of electricity  Dense  Strong  Malleable  Ductile Bonding Metallic Bonding e- e- e- e- e- e- e- e- e ee- - e- ee- ee- - e e e- e- - e- ee ee- ee- Other Types of Bonding Covalent Bonding e.g polymers Ionic Bonding e.g salt C e- e- Na+ Cl- C Melting Points Ionic bonding is often stronger than metallic bonding Ceramics tend to have higher melting points than metals Covalent bonds can also be extremely strong Covalently bonded materials may also have higher melting points than metals, e.g diamond However, many covalently bonded materials have very low melting points due to the existence of molecules Inter-molecular bonds can be rather weak (e.g thermoplastic polymers) Electrical Conductivity Freely moving electrons can conduct electricity Metallic materials tend to be good electrical conductors Some metals are better conductors of electricity than others, e.g copper is a better electrical conductor than tin Ceramics and polymers tend to be good electrical insulators Crystallinity In a crystalline solid, the “lattice” is made up of repeating units of atoms A repeating array of atoms in a lattice is called the “unit cell” The smallest repeating array of atoms is called the “primitive” unit cell The lengths of the sides of the unit cell are called the lattice parameters Packing of Atoms Hexagonal close-packed (hcp) ABABAB Recovery Unlike plastic deformation, work hardening is reversible By heating to a sufficiently high temperature (“annealing”) the dislocations are able to re-orient themselves into networks, in a process called “recovery” Further heating actually allows the growth of new grains with a low dislocation density This is called “recrystallisation” Both recovery and especially recrystallisation remove dislocation pile-ups and hence reduce the hardness of the material Grain Growth If a material is cooled immediately after recrystallisation, the new grains will be quite small With prolonged heating, grain growth occurs A grain can grow larger by atoms migrating from another grain that may eventually disappear Grain size is very important in plastic deformation, because there is an inverse relationship (the Hall - Petch relationship) between grain size (δ) and yield stress: σy = σ0 + κy (δ−0.5) where κy and σ0 are constants For a single crystal δ is large and so σy = σ0 Grain Size Effects Dislocations cannot cross grain boundaries easily, so the size of grains determines how easily the dislocations can move As expected, metals with small grains are stronger but they are less ductile A fine grain size is not only makes the material stronger, but also enhances toughness (unlike other methods of strengthening that reduce ductility) The only time that a fine grain size is not desirable is for high-temperature service, because the presence of grain boundaries enhances creep Oxidation Metals Polymers Unstable in O2 Ceramics Be ∆E 2MO If ∆E < 0, oxidation will occur Al , Zr Ti -1000 Energy / kJ mol-1 2M + O2 Silicon -500 500 Si3N4 SiC Diamond / Graphite MgO/SiO2/ Al2O3 Mo W Fe Pt Ag Au LiCl NaCl 1000 Ta, Nb, Cr KCl Most polymers PTFE Foamed polymers Stable in O2 -1500 Rate of Oxidation An oxide layer forms at the surface of the metal The amount of oxidised material formed, ∆m, varies with time t Weight gain, ∆m linear oxidation ∆m = kLt (kL is constant) parabolic oxidation (∆m)2 = kPt t (kP is constant) Rate of Oxidation Oxidation rates follow Arrhenius’ Law kP=APe-QP/RT ∆m Ln kL, ln kP kL = ALe-QL/RT 1/T T increasing t The oxidation rate also increases with partial pressure of O Parabolic Oxide Growth - Mechanism Metal +M Oxide Air O2- 2e- + O MO +2e- O2- diffuses in through oxide Oxide grows at metal-oxide interface e.g Ti, Zr, U Mechanism Metal Oxide M Vacancies M2+ +2e- Air +O MO M2+ diffuses out through oxide Oxide grows at air-oxide interface e.g Cu, Fe, Cr, Co Mechanism Metal Oxide Air +2e- Electrons move only very slowly Formation of oxide at metal-oxide or air-oxide interface depends on whether Mn+ diffuses faster than O2e.g Al Protection The parabolic rate constant kP depends on the diffusion coefficient D KP ∝ cD0e-Q/RT where c is the concentration of oxygen Protective films are those with low diffusion coefficients Films which are electrically insulating are also more highly resistant to attack These mechanisms not explain the case of linear weight gain Linear Oxide Growth Such behaviour depends on the formation of cracks in the oxide film as oxidation proceeds Vox > Vmet If the volume of the oxide formed is much less or much greater than that of the metal on which it is forming, it will crack or lift off the metal to relieve the strain Only if the volumes are comparable will protective parabolic growth occur Wet Corrosion Metals may also oxidise in water or aqueous solution Abraded Iron Aerated Water O2 + 2H2O + 4e- 2Fe - 4e- 2Fe2+ 2Fe2+ + 4OH- Oxidation (loss of electrons) 4OH- 2Fe(OH)2 Reduction (gain of electrons) Surface Degradation Attack on the metal is much faster than for dry oxidation  The species formed may be soluble in water, diffusing away into the liquid rapidly  If the oxide is insoluble, deposition may occur away from the surface  If deposition on the surface does occur, the layer formed may be relatively loose, giving little protection to further oxidation Corrosion Potentials E/V Mg2+ +2e Mg The voltages shown are those which would just stop the metal from oxidising in aerated water -20 Al3+ +3e Ti2+ +2e Al Ti -10 2H+ + 2e H2 Zn2+ +2e Zn Cr3+ +3e Fe2+ +2e Co2+ +2e Ni2+ +2e Sn2+ +2e Cr Fe Co Ni Sn Cu2+ +2e Cu O + 2H O + 4e 2 Ag+ +e Ag Pt2+ +2e Pt Au3+ +3e Au 10 Note: these values assume unit activity concentrations In more dilute solutions, corrosion can occur more rapidly 4OH- If two dissimilar metals are joined, the one with the more negative potential will be attacked, while the other remains unattacked Corrosion Cracking Wet corrosion often attacks metals selectively as well as, or instead of, uniformly This can lead to more rapid failure Stress corrosion cracking under stress, corrosion may enhance the growth rate of cracks Corrosion fatigue corrosion enhances the rate of growth of fatigue cracks in most metals and alloys Intergranular attack if grain boundaries corrode preferentially, cracks may be formed and propagate Pitting preferential attack can also occur at breaks in the oxide film or at precipitates [...]... grains and grain boundaries, determine many of the mechanical properties of metals Alloys An alloy is a mixture of a pure metal and one or more other elements Often, these other elements are metals For example, brass is an alloy of copper and zinc Metals can also be alloyed with non -metals In many cases, metals are quite soluble in other metals In other cases, instead of a solid-solution a new phase,... fill the space completely Amorphous Metals If a molten metal is cooled very rapidly, the atoms do not have time to rearrange to form an orderly crystalline lattice Instead, a random “amorphous” arrangement is produced and the result is a non-crystalline material The best known amorphous material is window glass: amorphous materials are often referred to as glasses Metals can usually crystallise even... significantly change phase equilibria for solid metals Under normal conditions, the phase diagram for a pure metal generally needs only a temperature axis T The pure iron phase diagram at constant pressure (not to scale) The phase diagram consists of single phase regions Two phases are only found together at a point Liquid δ-Fe γ-Fe α-Fe Single Crystal Metals Single crystal: lattice extends the edges... gas-turbine engines (“jet engines”) can be produced as single crystals Above their melting points, metals are liquids The atoms are randomly arranged and relatively free to move On cooling to below the melting point, the atoms rearrange forming the ordered, crystalline solid structure Polycrystalline Metals In most cases, solidification begins from multiple sites, each of which can produce a different... centred cubic (bcc) A third common packing arrangement in metals is body centred cubic The BCC unit cell has atoms at each of the eight corners of a cube plus one atom in the center of the cube The unit cell contains a total of 2 atoms 1 x 1 in the centre 8 x 1/8 at the corners Iron (Fe), Vanadium (V), Chromium (Cr) - bcc Crystal structures of some metals (rt) Aluminum FCC Nickel FCC Cadmium HCP Niobium... structure of a metal can determine some of its mechanical properties E.g ductility Phases The most familiar phases are solid, liquid and gas Within the solid-state, a metal may exist in several different solid phases Pure iron has three different solid phases (α, γ and δ-Fe) at different temperatures Each of these phases has its own distinctive structure and properties, although all three are made up of iron... high cooling rates, but under extreme conditions metallic glasses can be produced in some alloys NB metallic glasses are not transparent The lack of long range order in metallic glasses produces unusual properties which may have specialist applications Crystal Defects Metallic crystals are not perfect Perfect Vacancies Interstitials Dislocations Dislocations are a localised imperfection in the alignment... 1,500,000 x or more Microstructure features are in the range ~100 pm (i.e 1 x 10-10 m, 0.1 nm, or 1 Å) to ~100 µm (i.e 1 x 10-4 m, or 0.1 mm) Feature such as grains count as microstructure Both microstructure properties of a metal and crystallography influence the Grain Shapes In 3D, the grains in a polycrystal are usually polygonal The interfaces between the grains (“grain boundaries”) have an interfacial... many cases, metals are quite soluble in other metals In other cases, instead of a solid-solution a new phase, an “intermetallic compound”, with a structure different from that of any of its constituent metals can be produced Intermetallic Compounds Hume-Rothery rules 1 “Size factor” compounds Only a limited amount of the solute can be dissolved in the solvent 2 Large difference in electronegativity between... certain ratios of the number of valence electrons to the number of atoms in a structure Solid Solutions Even when intermetallic formation does not occur, there may not be perfect solid-solubility If two metals have different crystal structures then at some intermediate composition there will have to be a change from the crystal structure of the one metal to that of the other In such a case the result