Metals and Alloys 1 Flashcards
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
