Metals and Alloys COPY COPY Flashcards
Give some features of metals at ambient temperature:
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Are there any exceptions?
- hard and lustrous
- closely packed crystalline structures
- opaque
- conductors of heat and electricity
One exception is Hg, mercury, used in dental amalgam.
What is an alloy? Give examples of different types:
What is interesting with regards to metals in molten state?
An alloy is a mixture of two or more metals
e.g. 2 metals: binary alloy, 3 metals: ternary alloy
In molten state, metals display mutual solubility.
What are metals of regular crystalline structure composed of and how are they formed?
When do crystals stop growing?
What is a grain boundary?
- composed of many small crystals which form from molten metal
As the metal cools;
- impurities enable metal atoms to deposit upon them
- these nucleate crystal growth
Grains
The grains grow until all the melt of the metal has been consumed
Grain boundary: where crystals join with their neighbours
Name and describe the two types of nucleation:
Heterogeneous:
- many sites of nucleation
- majority of dental applications
Homogeneous:
- single site
- metal is pure so requires 4 metal atoms to come together
- requires specialised equipment
How does grain size affect metals?
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What is the lost wax technique?
List other factors affecting grain size:
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- influences physical properties
- controlled by manufacturer: rapid solidification, provision of ‘extra’ nucleation sites
Lost wax technique: make a replica of thing we wish to make, in wax, then surround wax with investment material, then melt wax out
- Temperature of mould: if similar to the melt, slow cooling and large grain size. If different to the melt, perhaps cooler, rapid cooling and small grain size
- Shape of mould: walls of mould initiate growth, therefore influence direction of crystal growth
List methods to shape metals/alloys:
- hammering, rolling, pressing (e.g. stainless steel denture bases), drawing through a die (e.g. wire), cold working known as wrought working
- casting: pouring molten form into an investment mould
- amalgamation: mix alloy with mercury, leads to a plastic mass, a hardening chemical reaction and crystallisation
Define ductility:
Define malleability:
- the maximum degree of extension in response to an applied tensile force e.g. drawing metal out into a wire)
- the maximum degree of compression in response to an applied compressive force
Do elemental metals, such as pure gold, silver, copper, etc have much practical use?
Not much practical use, severe limitations of their properties, however uses may be extended by mixing metals to form alloys.
On cooling alloys from molten components, metals may:
- remain soluble in each other - solid solution
- be completely insoluble in each other
- be partially soluble in each other
- form intermettalic compounds if the metals have affinity for each other e.g. Ag3Sn
Name the three types of solid solutions:
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- ordered solid solution
- random solid solution
- interstitial solid solution
What type of solid solution is this?
List its features:
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Ordered solid solution:
- component metal atoms have specific sites in common lattice
- this photo is substitutional as atomic radii are similar
- if radii differ, distortions of lattice may occur which may be advantageous or disadvantageous
Which type of solid solution is this?
Random solid solution: random sites in common crystal lattice
What type of solid solution is this?
Interstitial solid solution: atoms of one lie within the primary lattice sites of the other, atomic radii dependent
Solid solutions compared to pure metals are:
What do they have?
- harder, stronger, higher elastic limits
Hardening effect known as solution hardening - as atoms of differing radii form mechanical resistance to dislocations
Is insolubility of metals common or rare?
What issues can arise:
- rare
Consequences:
- areas high in metal A and areas high in pure metal B
- risks electrolytic corrosion and a battery is set up, leading to corrosion and cored
Explain this cooling curve for a pure metal:
Plateau at melting point indicates temperature constant during crystallisation.
For the binary alloy, where A has the highet MP explain the temperature of crystallisation, and when crystallisation is rich in A and B
What are the implications of this?
- Crystallisation occurs over a range of temperatures
- At T1 crystallisation rich in A
- At T2 crystallisation rich in B
Implications:
- a cored structure
- impact on corrosion resistance
Label the liquidus and solidus:
Explain how to interpret this graph to conclude whether risk of corrosion is great or not:
The larger the separation between the solidus and liquidus, the greater the coring, therefore a greater risk of corrosion!
Label this diagram:
