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
what role do grain boundaries play in the movement of dislocations? why is this important?
- grain boundaries stop the propagation of defects spreading to another grain
- therefore in metals/alloys made up of many fine grains… the movement of dislocations are impeded leading to SPECIFIC PROPERTIES
what specific properties does impeding movement of dislocations in metals/alloys cause?
INCREASES:
- elastic limit
- fracture stress
- hardness
DECREASES:
- ductility
- impact resistance
what is meant by cold work?
- applying forces to metals/alloys alongside a LOW TEMP
what is the purpose of cold work?
- causes SLIP
- therefor dislocations collect at grain boundaries
- hence stronger, harder material
what is a negative aspect of cold work?
- lower ductility
- lower impact strength
- lower corrosion resistance**
what is a residual stress?
stresses that form within the metal/alloy itself (UNDESIRABLE)
what does residual stress lead to?
- instability in lattice
- distortion over time
how do residual stresses occur?
residual stresses increase as cold work increases
how can residual stresses be relieved?
ANNEALING PROCESS
what is meant by annealing?
heating metal (or alloy) so that greater thermal vibrations allow migration of atoms (any instability in the lattice is reversed)
does annealing alter the grain structure and mechanical properties?
NO
what occurs when metal/alloy is heated?
recrystallisation of new smaller equiaxed grains
the greater the amount of cold work….
the lower the recrystallisation temperature
what would occur if there was an excessive temperature rise of a metal?
large grains replace the smaller coarse grains yielding poorer mechanical properties
