Metals and Alloys 2 Flashcards
What is an alloy?
A combination (or mixture) of two or more metals OR metals with a mettalloid
What is a metalloid?
Something that a metal can be replaced with e.g. Si, C
What are 3 advantages of alloys over metals?
They have improved properties:
- Mechanical (EL, UTS, hardness)
- Corrosion resistance
AND
- Lower melting point than an individual metal
What is STEEL used for in dentistry? (2 points)
Burs, instruments
What is AMALGAM used for in dentistry?
Filling material
What is GOLD ALLOY used for in dentistry? (5 points)
- Inlays
- Crowns
- Bridges
- Partial dentures
- Wires
What is NICKEL CHROMIUM used for in dentistry? ( 3 points)
- Crowns
- Bridges
- Wires
What is the definition for ‘phase’?
- Physically distinct homogeneous structure (can have more than one component)
What is the definition for a solution?
- Homogeneous mixture at an atomic scale (lattice arrangement with 2 or more metals that coexist in the same lattice structure)
How many phases are there for grains composed of one metal only?
1 phase
How many phases are these for grains composed of individual grains of metal A and B situated in a lattice network?
- Two phases as the 2 metals don’t coexist in the same grain
How many phases are there for grains composed of Metal A and B in a homogeneous mixture?
- One phase (homogeneous as the 2 metals are coexisting in the same lattice structure)
What is a solid solution?
2 or more types of atoms or molecules that share a crystal lattice
- The grains are of varying size and shape
Are metals usually soluble or insoluble when molten?
Usually soluble
On crystallisation of alloys, what are the 3 possible options that can occur?
- Metals are insoluble with each other so there is no common lattice - 2 phases
- The metals form an intermetallic compound with specific chemical formulation
- The metals are soluble in each other and form a solid solution i.e. form a common lattice (there are 3 types of solid solution)
What is a ‘substitutional’ solid solution?
- Atoms of one metal replace the other metal in the crystal lattice/grain
What is a ‘random’ substitutional solid solution?
- Metals are similar in: size, valency, crystal structure
- They are randomly positioned
What is an ‘ordered’ substitutional solid solution?
- Metal atoms in regular lattice arrangement
- Forms an ordered solid solution where you can predict the position of the different types of metals
What is an ‘interstitial’ solid solution?
- Atoms markedly different in size
- Smaller atoms are located in spaces in lattice/grain structure of larger atoms
What does the plateau on the cooling curve if a pure metal indicate?
- Where the crystallisation begins and the metal is becoming a solid (happens at one temperature)
What is the cooling curve like for an alloy?
- Temp TL is the temp at which crystallisation of the alloy begins
- Crystallisation continues and the temp drops
- TS is when the crystallisation process is complete
- This is different from the cooling curve of a pure metal as the process of crystallisation doesn’t happen over a single temp
For an alloy, are the cooling curves the same or different for alloys with different compositions of metals?
Different
What is the lower line of a phase diagram called?
The solidus
What is the liquidous on a phase diagram?
Line representing the temperatures at which different alloy compositions begin to crystallise
What is the solidus in a phase diagram?
The line representing the temperatures which different alloy compositions have completely crystallised
What does slow cooling of molten alloys allow? (3 point s)
- Allows metal atoms to diffuse through the lattice
- ENSURES grain composition is homogenous
BUT
- This results in large grains (which is not desirable as it causes poor mechanical properties)
What is the tie line on a phase diagram?
Horizontal line from liquidous to solidus
- This is roughly when the first grain is going to form which is going to be a composition of 85% A and 15% B
What does rapid cooling of alloys produce?
Coring
Rapid cooling of a molten alloy causes CORING. What is this?
The composition of metals varies throughout the grain
What are the conditions required for coring? (2 points)
- Fast cooling of liquid state
- Liquidous and solidus must be separated and determines extent of the coring
Is a phase diagram has a liquidous and a solidus that are quite far apart will you get a lot of or little coring?
A lot of coring
What may coring reduce of the solid form of an alloy?
May reduce the corrosion resistance
What does fast cooling of alloys cause? (2 points)
- Generates many small grains which impede dislocation movement, improving its MECHANICAL PROPERTIES
- BUT causes coring, which is undesirable
What is homogenising annealing of alloys?
- Once the solid, cored alloy is formed:
- REHEAT to allow atoms to diffuse and so cause grain composition to become homogenous
- NOTE: keep below recrystallisation temperature, otherwise grains are altered
What is solution hardening? (2 points)
- Alloys forming a SOLID SOLUTION and consisting of metals of different atomic size have a distorted grain structure
- Which IMPEDES dislocation movement and so improves mechanical properties
What is the dislocation movement like in a metal lattice and how much energy is needed for this? (2 points)
- Defects ‘roll’ over the atoms in the lattice plane
- Little energy/force is needed for defect to move along the slip plane
What is the dislocation movement like for a solid solution and how much energy is required for this? (3 points)
- Defect does not ‘roll’ over the lattice plane. Instead it falls into the larger space existing between the large and small atom
- More energy/force is needed for the defect to overcome the different sized atoms, and move along lattice to the grain boundary
- Hence, it requires greater stress to move dislocations in a solid solution - making alloys inherently more fracture resistant (i.e. stronger) than metals
What is order hardening?
- Alloys forming an ORDERED SOLID SOLUTION (atoms distributed at specific lattice sites) have a DISTORTED grain structure
- This IMPEDES dislocation movement and so improves mechanical properties
What is an Eutectic alloy?
2 metals that are not soluble in one another
What are the properties of Eutectic alloys? (4 points)
- Metals are soluble in liquid state
- Metals INSOLUBLE in solid state (2 phases)
- i.e. each metal forms physically distinct grains
- CONTRAST this with Au-Pt alloy
What is the melting point of a eutectic alloy like?
- Les than the MP of metal A&B that are combined to make the alloy
What is eutectic composition?
- Where liquidous and solidus coincide (i.e. where crystallisation process occurs at a single temperature)
- Where grains of individual metals are formed simultaneously
what are 3 properties of eutectic alloys?
- Lowest melting point
- Hard but brittle
- Poor corrosion resistance
What happens with alloys that are non-eutectic composition ? (3 points)
- Excess metal crystallises first
- Then liquid reaches eutectic composition
- And BOTH metals crystallise (forming separate grains)
What does the solubility limit line in a partially soluble alloy phase diagram indicate?
- Indicates that a range of compositions of Ag and Cu (corresponding to the horizontal section of the solidus) ARE NOT POSSIBLE
- Hence molten alloy composition z:
- DOES NOT cool rapidly to produce a 50:50 grain comprising Ag and Cu
- Instead grains of alpha and beta are formed
In partially soluble alloys on annealing, a supersaturated alloy will undergo what?
- Precipitation hardening
Alloys have better mechanical properties than metals, give 3 examples of these and say what it is due to?
- Fracture strength, rigidity, elastic limit surface hardness
- This is due to solution, order and precipitation hardening
BUT
- CORED structure must be removed by annealing
