Metals & Alloys
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Why are metals used in denstistry
Superior mechanical properties eg - strength -rigidity -hardness -elastic limit -ductility
What is a metal
aggregation of atoms in crystalline structure
What is an alloy
combination of metal atoms in a crystalline structure
What are some simple crystal or lattice structures
Simple cubic Face-centred cubic Body centric cubic
What is the cooling curve of pure metal
Decreases gradually then stops for a while and then decreases once more
What is a grain boundary
The region where grains make contact
What are the types of grain structures
Equi-axed -Crystal growth is of equal dimension in each direction Radical -molten metal cooled quickly in cylindrical mould Fibrous -wire pulled through die
What impact does cooling have on crystal growth
Quenching -more nuclei -small fine grains Slow cooling -few nuclei -large coarse grains
Where do impurities concentrate
At the grain boundary
What are the advantages of small fine grains
High elastic limit increased FS and UTS, hardness decreased ductillity
What qualifies a metal for quenching
small bulk heat metal/alloy just above Tm mould - high thermal conduction quench
What are dislocations
imperfections/defects in the crystal lattice
What is SLIP
SLIP is due to Propagation of Dislocations and involves rupture of only a few bonds at a time
What does stopping movement of dislocations do
INCREASES elastic limit fracture stress /UTS hardness DECREASES ductility impact resistance
What natural factors reduce dislocation movement
Grain boundaries (hence fine grains) Alloys: different atom sizes Cold working - dislocations stopped at grain boundaries
What are the effects of cold working
Causes SLIP increase strength and residual stress decreases ductillity
How is residual stress relieved
Annealing process
What are the effects of residual stress
Causes instability in lattice Results in distortion over time Undesirable
What is annealing
heating metal (or alloy) so that greater thermal vibrations allows migration of atoms (ie re-arrangement of atoms)
What does recrystallisation cause and when does it occur
occurs when metal/alloy heated causing :- - new smaller equiaxed grains - lower EL, UTS, hardness - increased ductility
Why would recrytallisation be used
spoils benefits of cold work allows further cold work cold work/recrystallisation repeated until correct shape obtained
What is an alloy
a combination (or mixture) of: two or more metals OR a metal(s) with a metalloid (Si, C)
What is phase
physically distinct homogeneous structure
Which are preferable metals or alloys
Alloys