Metals and Alloys 1 Flashcards
Metals and alloys are widely used in dentistry. Give examples of this? (5 points)
- Partial denture framework (CoCr)
- Crowns (stainless steel)
- Denture base (Stainless steel)
- Orthodontic appliance (NiTi)
- Restorations (amalgam)
Why are metals used in dentistry?
- They have superior mechanical properties e.g. strength, rigidity etc
- BUT poor aesthetics
What is the definition of a metal?
Aggregate of atoms in crystalline structure
What is the definition of an alloy?
Combination of metal atoms in a crystalline structure
What are the building blocks of alloys?
Metals
What is meant by the term malleability?
Able to be hammered or pressed into shape without breaking or cracking
What is meant by the term ‘ductility’?
The ability of a metal to be easily bent or stretched
What is the elastic limit?
The maximum stress without plastic deformation
What is plastic limit?
A process in which enough stress is placed on metal or plastic to cause the object to change its size or shape in a way that is not reversible
What is the definition of ductility in relation to plastic deformation?
- Amount of plastic deformation prior to fracture
e. g. measure of the extent that a material can be shaped/manipulated
What does the crystalline structure of metals depend on?
- History (method of production)
- Shaping (crucial for dental applications (cold working, swaging)
What are the 3 factors that affect the mechanical propertied of metals?
- Crystalline structure
- Grain size (single crystal)
- Grain imperfections
What are 3 examples of some simple CRYSTAL or LATTICE structures?
- Cubic
- Face-centred cubic
- Body centred cubic
What is the process of crystal growth? (3 points)
- Atoms act as nuclei of crystallisation
- Crystals grow to form dendrites
- Crystals (or grains) grow until they impinge on other crystals
What is the region where grains make contact with each other called?
The GRAIN BOUNDARY
If crystal growth is of equal dimension in each direction, what do we call the grains?
Equi-axed grains
How is a radical grain structure produced?
Molten metalal is cooled quickly in cylindrical mould
How is a fibrous grain produced?
Wire pulled through die (cold worked metal/alloy)
What type of crystals does fast cooling (quenching) cause? (2 points)
- more nuclei
- small fine grains
What type of crystals does slow cooling produce? (2 points)
- Few nuclei
- Large coarse grains
What are nucleating agents?
Impurities or additives that act as foci for crystal growth
What is a grain?
Each grain is a single crystal (lattice) with atoms orientated in given directions (dendrites)
What is a grain boundary?
- Change in orientation of the crystal planes
- (impurities concentrate here)
What are the advantages of small, fine grains? (3 points)
- High elastic limit
- Increased UTS (fracture stress), hardness
What is a disadvantage of small, fine grains?
Decreased ductility
What are factors for rapid cooling (quenching)? (4 points)
- Small bulk
- Heat metal/alloy just about Tm
- Mould - high thermal conduction
- Quench (take bucket of molten metal and put it in a contained of cold water)
What does a defect in the lattice represent?
A misalignment of the atoms in the lattice network
What is another name for defect?
Dislocation
If you apply a force to a defect what happens?
You have pushed the defect to the grain boundary so they grain is no longer there and you have removed the defect
What are dislocations?
Imperfections/defects in the crystal lattice
What is slip due to in relation to dislocations?
- Due to propagation of dislocations and involves rupture of only a few bonds at a time
What does impede movement of dislocations increase? (3 points)
- Elastic limit
- UTS
- Hardness
What dies impede movements of dislocations decrease? (2 points)
- Ductility
- Impact resistance (so if drop it would be more likely to break)
What are 3 factors that can impede dislocation movement?
- Grain boundaries
- Alloys: different atom sizes
- Cold working - dislocations build up at grain boundaries
What is cold working?
- Dislocations build up at grain boundaries
- Pushing all of the defects to the grain boundary - take a metal and hammer it
What is cold working?
- Work done on metal/alloy
- Done at low temperatures (below recrystallisation temperature)
- Causes slip - so dislocations collect at grain boundaries
- Hence stronger, harder material
What is another term for cold work?
Work or strain hardening
Cold work modifies grain structure. What does it increase?
- Elastic limit
- UTS (ultimate tensile strength)
- Hardness
Cold work modifies grain structure. What does it decrease? (3 points)
- Ductility
- Impact strength
- Lower corrosion resistance
Residual stress causes instability in the lattice which results in distortion over time. This is undesirable, what can it be relived by ?
Can be relieved by the annealing process
What is annealing?
Heating a metal or alloy so that greater thermal vibrations allow migration of atoms (i.e. re-arrangement of atoms)
What can cold work lead to in the metal?
Can result in internal stresses - may lead to distortion of the appliance over time
What does stress relief annealing do?
- Eliminates stresses by allowing atoms to re-arrange within grains
- Grain structure and mechanical properties unchanged
- Some further cold work possible (final shaping)
Recrystallisation occurs when a metal/alloy is heated. What can this cause? (3 points)
- New smaller equiaxed grains
- Lower EL, UTS, hardness
- Increased ductility
What does recrystallisation do in relation to cold work? (3 points)
- Spoils benefits of cold work
- Allows further cold work
- Cold work/recrystallisation repeated until correct shape obtained
What does recrystallisation temperature depend on?
- Depends on about of cold work
- Greater the amount of cold work the lower the recrystallisation temperature
What does excessive temperature rise cause in relation to grain growth?
- Large grains to replace smaller coarse grains yielding poorer mechanical properties -> careful when annealing
What is an alloy?
A co,mbination (or mixture) of two or more metals OR metal(s) with a metalloid (Fe, C)
