METALS AND ALLOYS

METALS AND ALLOYS

METAL: opaque, closely packed crystalline structure,
1. Conductors of heat and electricity
ALLOYS: Mix of 2 or more metals
1. 2 metals - BINARY
2. 3 metals - TERNARY
Composed of many small crystals
1. Form from molten metal
2. As it cools impurities enable metal atoms to deposit upon them
3. These Nucleat crystal growth
METALS
ALLOYS
Crystals termed GRAINS
1. Grow until all melt metal is consumed
2. where crystals abut their neighbour termed = GRAIN BOUNDARIES
NUCLEATION may be
1. HETEROGENOUS- many sites of nucleation
2. HOMOGENOUS- single site, metal is pure so requires 4 metal atoms
3. to come together and requires specfial equipment
GRAIN SIZE
1. Influences physical properties
2. TEMP OF MOULD - if similar to melt = slow cool and large grain size
3. If different = rapid cooling and small grain size
4. SHAPE OF MOULD - walls of mould intiate and influence crystal growth
VISUALISING GRAIN BOUNDARIES
1. Light reflecting microscope
2. Polished metal surface - irregularities scatter light
3. app of suitable etching chemical
AMALGAMATION - MIX WITH Hg(mercury)
1. plastic mass, hardening chemical reaction, Crystallisation
DUCTILITY - max degree of extensionin response to tensile force
1. MALLEABILITY - max degree of compression in response to an applied compressive force
On cooling from molten metals may :
1. remain SOLUBLE in each other=SOLID SOLUTION
2. be completely INSOLUBLE in each other
3. be PARTIALLY SOLUBLE in each other
4. form INTERMETALLIC COMPOUNDS if the metals have an AFFINITY for each other
5. eg. Ag3Sn
SOLID SOLUTIONS
1. 3 TYPES - ORDERED SOLID SOLUTION:
  1. metal atoms have specific site in common lattice
2. If radii differ distortions of lattice
3. RANDOM SOLID SOLUTION: Random sites in crystal lattice
4. INTERSTITIAL SOLID SOLUTION: atoms of one lie within primarylattice

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