Metals & Alloys 1
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Where are metals and alloys used in dentistry?
• Partial denture framework (CoCr, Type IV gold) • Crowns (stainless steel) • Denture base (stainless steel) • Orthodontic appliance (NiTi) • Restorations (amalgam)
What is a metal and what is an alloy?
metals - aggregate of atoms arranged in a crystalline structure alloys - combinations of different metal atoms in a crystalline structure
What is the crystalline structure made of?
made up of grains, which are regions of atoms arranged in a specific pattern or lattice
What factors in metals affect mechanical properties?
• crystalline structure • grain size • grain imperfections/defect
What are examples of crystal/ lattice structure?
simple cubic face-centered cubic (FCC) body-centered cubic (BCC)
How do metals form? what are regions where grains make contact called?
When molten cools down 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 a dendrite?
3D branched lattice network
What do grains vary in?
Size Shape
fast cooling VS slow cooling
faster cooling (quenching) - smaller, finer grains, more nuclei slower cooling - larger, coarse grains, less nuclei
What are nucleating agents and where do they concentrate?
Impurities or addictives act as Foci for crystal growth at the grain boundaries
What is a grain?
each grain is a single crystal (lattice) with atoms orientated in given directions (dendrites)
What happens at the grain boundary?
• change in orientation of the crystal planes • (impurites concentrate here)
advantages/disadvantages of small fine grains?
advantages - increased FS, high EL, increased hardness, increased UTS disadvantages - decreased ductility (brittle)
What are factors required for rapid cooling?
*small bulk *heat metal/alloy just above melting temperature *mould - high thermal conduction *quench
What are dislocations and what is slip?
Dislocations are imperfections/defects in the crystal lattice Slip is due to Propagation of Dislocations and involves rupture of only a few bonds at a time
Why are moving dislocations bad?
When a metal is subjected to stress or deformation, dislocations can move relatively freely through the crystal lattice, allowing the metal to deform plastically (permanently change shape). This dislocation movement is responsible for the ductility and malleability of metals.
What happens if you impede movement of dislocations in metals/ alloys?
INCREASES *elastic limit *fracture stress /UTS *hardness Brittle DECREASES *ductility *impact resistance
What are factors impeding dislocation movement?
• Grain boundaries (hence fine grains) • Alloys: different atom sizes • Cold working - dislocations stopped at grain boundaries
What is cold work, what does it cause?
deforming the material at low temperatures causes dislocations due to slip to collect at grain boundary, resulting in strain hardening and increased strength and hardness.
What does cold work do to grain structure and what properties does it cause?
modifies grain structure: higher • Elastic Limit • Fracture stress/UTS • hardness lower • ductility • impact strength • lower corrosion resistance
What stress increases with cold work? how is it relieved?
Internal stress (causes instability/brittleness in lattice) annealing process
What is annealing? what does it benefit?
heating the cold-worked metal to a specific temperature range, holding it at that temperature for a certain time, and then cooling it at a controlled rate. (does not cause metal to remelt but causes recrystallisation - formation of new smaller grains) can relieve internal stresses caused by cold working by allowing atoms to rearrange within the grains
What does recrystallisation result in?
reduces strength, elastic limit, and hardness (eliminates dislocations) but increases ductility.
What does recrystallisation temperature depend on?
• depends on amount of cold work • greater the amount of cold work the lower the recrystallisation temperature