Metals and Alloys 1
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what are metals (& alloys) used for in dentistry?
• partial denture framework • crowns • denture base • orthodontic appliance • restorations
why are metals used?
they have superior mechanical properties: • strength • rigidity • elastic limit • ductility • hardness
what is meant by good ductility?
shape is able to be adjusted chairside
what is a negative of metals?
poor aesthetics
what is the definition of a metal?
aggregate of atoms in a crystalline structure
what is meant by an alloy?
combination of metal atoms in a crystalline structure
what metal factors affect its mechanical properties?
• crystalline structure • grain size • grain imperfections
what type of structures can metals take?
• crystal - lattice
when a PURE METAL cools, what occurs (in terms of the cooling curve)?
• molten • temperature drops to melting point • atoms begin to crystallise (straight line on graph) • all atoms are crystallised • solid metal cools to room temp
explain the growth of metal crystals?
• atoms act as nuclei of crystallisation • crystals grow to form dendrites • crystals (or grains) grow until they impinge on other crystals • region where grains make contact is called GRAIN BOUNDARY
what is meant by equi-axed grains?
occurs when crystal growth is of equal dimension in each direct
when would a radical grain structure occur?
molten metal cooled quickly in cylindrical mould
when would a fibrous grain structure occur?
wire pulled through die
in terms of crystal growth, what happens when the metal undergoes fast cooling (quenching)?
• more nuclei - small fine grains
in terms of crystal growth, what occurs when a metal undergoes slow cooling?
• few nuclei - large coarse grains
what are some nucleating agents?
impurities or additives act as foci for crystal growth more impurities = more small grains
what is meant by a ‘grain’ ?
each grain is a single crystal (lattice) with atoms orientated in given directions (dendrites)
what is meant by a ‘grain boundary’?
• where grains collect with one another - impurities concentrate here
what are some advantages of small grains?
• high elastic limit (deformation requires a large stress) - increased FS & hardness
what is a disadvantage of small fine grains?
decreased ductility
what is meant by the dislocation area of a lattice?
where the lattice structure is most likely to fracture
what occurs if a force is applied to the dislocation area?
the forces moves the dislocation along the lattice until it reaches the grain boundary
what is the propagation of dislocations through a lattice known as?
slip
what are dislocations?
imperfections/defects in the crystal lattice