metal for paedo

Outline  What is a metal ?  General properties  Structure and bonding  Phases and phase transformations  Structure – property relationships  Chemical properties H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Lr Rf Db Sg Bh Hs Mt Uuu Uub Uub What is a Metal ? Non-metals Metals La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No  Opaque  Lustrous  High melting point  Good conductors of heat  Good conductors of electricity  Dense  Strong  Malleable  Ductile Properties of Metals Bonding e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - e - Metallic Bonding Other Types of Bonding Covalent Bonding e.g. salt e - e - CC Ionic Bonding e.g. polymers Na + Cl - 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) Melting Points 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 A B A B A B Hexagonal close-packed (hcp) Each unit cell here contains 2 atoms 8 x 1/8 at each corner 1 x 1 in the centre [...]... 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, an “intermetallic compound”, with a structure... 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 would be, on gradually changing the composition of an alloy from pure metal A (forming the α−phase) to pure metal B (forming the β−phase): •single-phase α solid-solution;... made up of iron atoms α-Fe has the bcc structure γ-Fe has the fcc structure Phase Transformations As iron is heated from room-temperature to above its melting point, the following changes in phase occur: • α transforms to γ; • γ transforms to δ; • δ transforms to liquid iron These changes are examples of phase transformations Phase Equilibria and Phase Diagrams A phase diagram is a chart which shows... 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 the solvent and solute Bonding is more ionic than metallic 3 At certain ratios of the number of valence electrons to the number of atoms in a structure Solid Solutions Even when intermetallic formation... stable under which conditions Stable Metastable Unstable Phase Diagrams For a single element material, the variables that influence phase stability are temperature and pressure Extremely high pressures are generally required to 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... Crystal Metals Single crystal: lattice extends the edges of the material, e.g a diamond Metal single crystals are possible: e.g Ni alloy turbine blades used in aero 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... 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... Metals can usually crystallise even at very 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... will substitute for some of the solvent atoms and the result is called a “substitutional solid-solution” In both substitutional and interstitial solid-solutions the sites occupied by specific atoms are random Adding the solute does not change the crystal structure In contrast, an intermetallic compound often has a different crystal structure to that of the parent metals Binary Alloys For alloys, composition... rules for the formation of intermetallics Since copper and nickel are FCC with almost the same lattice parameter a two phase mixture is not expected The Ni-Cu binary phase diagram Liquid (L) T L+S Solid (S) 0 % Cu 100 More Complex Phase Diagrams T L α+L β+L α β α+β A B Composition Structure – Property Relationships In a “tensile test” a sample is gradually elongated to failure and the tensile force . to significantly change phase equilibria for solid metals Under normal conditions, the phase diagram for a pure metal generally needs only a temperature. Transformations As iron is heated from room-temperature to above its melting point, the following changes in phase occur: • α transforms to γ; • γ transforms