DMS-Metals and alloys Flashcards
What is ductility?
Ability to undergo plastic deformation before fracture.
What is an alloy?
A mix of 2 or more metals in a crystalline structure.
How does the crystalline structure form?
- Nuceli of crystalisation.
- Dendrites are produced
- the crystals grow until they impinge on other crystals.
What a grain?
A single crystal lattice
What is the grain boundary?
Where grains come into contact with each other
Compare fast cooling and slow cooling?
Fast cooling- many nuclei/ small fine grains
Slow cooling- few nuclei- large coarse grains
Which type of grains are desirable and why?
Small fine grains.
Increases
- Tensile strength
- hardness
- elastic limit.
What are dislocations?
These are defects and imperfections in the crystaline structure due to misalignment of the atoms.
How do we deal with dislocations and why?
We put pressure on the lattice in order to move the dislocations to the grain boundary and remove them.
What factors affect dislocation movement?
Grain boundaries (more grain boundaries = more places to push the defects)
Alloys- as there are different sizes of atoms (can prevent dislocation)
Cold working- you build up the dislocations at the grain boundaries.
What is cold working?
When you put pressure on the metal at a lower temperature in order to move the dislocations to the grain boundary. (improving properties
What are the difficulties with cold working and how do we solve them?
This increases the residual stress which causes an instability in the lattice that would result in distortion over time.
This is solved by Stress relief annealing.
What is stress relief annealing?
Heating up the metal so that the atoms can re-arrange and eliminate stress.
What is a solid solution?
When 2 metals are soluble in each other and form a lattice structure.
Compare the types of solid solution?
You can have random/ ordered and interstitial (small particles between large particles)
Compare cooling metals and alloys?
Metals crystalise at the one temperature.
Alloys crystalise over a series of temperatures.
What lines do you find on a phase diagram?
Liquidus - temp at which the alloy starts to crystalises
Solidus- temp at which the alloy completes crystalisation
Tie lines - used to estimate composition
what is coring?
When a structure’s composition varies throughout the grain.
How do we fix coring?
We use homogenous annealing.
The alloy is reheated allowing the atoms to move around and eliminate the cored structure.
What is solution hardening?
When alloys have a distorted grain structure causing improved mechanical properties by impeding dislocation movement.
A greater force is needed to move the disloctions making the alloys more fracture resistant.
What is an Eutectic alloy?
An alloy where the melting point is lower than that of the original metals.
Compare eutectic and non-eutectic composition?
Eutectic composition- the alloy crystalises at once.
Non eutectic composition- The excess metal crystalises first, then the liquid reaches euteric compostion & crystalises.
What is a partially soluble alloy ?
The metals in the alloy are not soluble enough to produce a full series of solid solutions (only certain alloy compositions can exist)
What is precipitation hardening?
When the defects are pushed to the grain boundary in a partially soluble alloy- improving properties.
Why do we want a low density denture material?
As a low density material is lighter- this would make the denture more comfortable for the patient.
Discuss the ideal properties of a base and the clasp?
You want the base to be rigid.
High YM - maintaining shape
High elastic limit- avoid deformation
You want the clasp to be flexible
Low YM - to flex over the tooth.
High elastic limit -maintain elasticity over a wide range of movements
Discuss the components of Type 4 gold?
Cu- causes corrosion
Au- makes partially soluble alloy with copper
Pt- coring can occur.
Pd- grains are bigger & absorbs gas when moutlen (causing a weakened denture frame)
Discuss the components of Co-Cr?
Co- chance of coring
Cr- forms passive oxide layer to reduce corrosion
Ni- improves ductility but reduces strength
Mo-makes particles smaller
C- undesirable- carbide makes it brittle.
Compare Co-Cr and Type 4 Gold
Co-Cr is harder.
Co-Cr greater rigidity
Co-cr greater shrinkage (-ve)
Gold- greater elongation
Gold- Greater strength
Gold- more dense.
Discuss Titanium as partial denture alloy?
- Greater biocompatability
- Components can be welded together
- more expensive
- Good corrosion resistance.
