Metals Flashcards
What are the crystalline structures in metals called
Grains
What does grain size influence
Physical properties of the metal- extra nucleation sites allow for a small grain size.
What influences a small grain size
Rapid cooling
Rapid solidification
Nucleation can be divided into
Homegenous- one site of nucleation as the metal is so pure, it requires 4 sites to come together
Hetrogenous- Majority of the dental equipments as they have many sites of nucleation. Many sites where crystals can be formed
What affects grain size?
Rapid solidification and provision of extra nucleation sites- both promote a small grain size
Temperature of mold- slow cooling- large grain size and rapid cooling- small grain size
Shape of mould- these intimate growth and therefore influence the direction of crystal growth
What does polishing do to these grain boundaries?
Its causes irregularities that scatter light.
What does etching do to these grain boundaries>
Etching preferentially attacks the areas that are under high stress
What are the properties of the metal
Malleable
ductile- can be drawn out into a wire- laminar structure that can be disrupted if you overwork the wire e.g. orthodontics.
What is ductility
This is the maximum degree of extension in response to an applied tensile force
What is malleability
This is the maximum degree of compression in response to an applied compressive force
What three states can an alloy exist
1) ordered solution- component metals have specific sites in the lattice
2) random solid solution- random sites in the common crystal lattice
3) interstitial solution - atoms of one lie within the primary lattice sites of each other (smaller atoms are lying within the primary sites)
Alloy properties
Harder, stronger and have higher elastic limits
Hardening- Atoms of differing radii form mechanical resistance to dislocation
Risks of electrolytic corrosion- Electrolytic corrosion is. process of accelerated corrosion where the metallic surface is continuously eroded by other metal that it is contact with.
Insolubility of metals- where metal a is soluble and metal b is insoluble
What is the difference between the cooling curves of a pure metal vs an alloy
A pure metal has a stable plateau phase that leads to crystallisation constantly
Alloy- crystallisation occurs over a range of temperature
What is the impact of the crystallisation
Produces a cored structure where there are zones that are rich in metal a and others rich in metal b- this causes an impact on corrosion resistance
Solidus and liquidus
Liquidus- lowest temperature it takes for ice to become water
solidus- highest temperature for water to become ice
if it is greater on one- it means the alloy is more susceptible to corrosion.
