METALS AND ALLOYS

METALS AND ALLOYS

1. METAL: opaque, closely packed crystalline structure,

   • Conductors of heat and electricity

2. ALLOYS: Mix of 2 or more metals

   • 2 metals - BINARY
   • 3 metals - TERNARY

3. Composed of many small crystals

   • Form from molten metal
   • As it cools impurities enable metal atoms to deposit upon them
   • These Nucleat crystal growth

4. METALS

5. ALLOYS

6. Crystals termed GRAINS

   • Grow until all melt metal is consumed
   • where crystals abut their neighbour termed = GRAIN BOUNDARIES

7. NUCLEATION may be

   • HETEROGENOUS- many sites of nucleation
   • HOMOGENOUS- single site, metal is pure so requires 4 metal atoms
   • to come together and requires specfial equipment

8. GRAIN SIZE

   • Influences physical properties
   • TEMP OF MOULD - if similar to melt = slow cool and large grain size
   • If different = rapid cooling and small grain size
   • SHAPE OF MOULD - walls of mould intiate and influence crystal growth

9. VISUALISING GRAIN BOUNDARIES

   • Light reflecting microscope
   • Polished metal surface - irregularities scatter light
   • app of suitable etching chemical

10. AMALGAMATION - MIX WITH Hg(mercury)

    • plastic mass, hardening chemical reaction, Crystallisation

11. DUCTILITY - max degree of extensionin response to tensile force

    • MALLEABILITY - max degree of compression in response to an applied compressive force

12. On cooling from molten metals may :

    • remain SOLUBLE in each other=SOLID SOLUTION
    • be completely INSOLUBLE in each other
    • be PARTIALLY SOLUBLE in each other
    • form INTERMETALLIC COMPOUNDS if the metals have an AFFINITY for each other
    • eg. Ag3Sn

13. SOLID SOLUTIONS

    • 3 TYPES - ORDERED SOLID SOLUTION:
      • metal atoms have specific site in common lattice
    • If radii differ distortions of lattice
    • RANDOM SOLID SOLUTION: Random sites in crystal lattice
    • INTERSTITIAL SOLID SOLUTION: atoms of one lie within primarylattice