Phase Diagrams: Difference between revisions
From DT Online
mNo edit summary |
(Added Martensite) |
||
| Line 1: | Line 1: | ||
[https://en.wikipedia.org/wiki/Phase_diagram#Binary_phase_diagrams '''Phase Diagrams'''] are charts which show how different '''Phases''' can exist together under certain conditions ''(e.g. the temperature range at which water and ice can exist together in slushy conditions)''. Three obvious '''Phases''' are the three states of matter: [https://en.wikipedia.org/wiki/Solid '''Solid'''], [https://en.wikipedia.org/wiki/Liquid '''Liquid'''], and [https://en.wikipedia.org/wiki/Gas '''Gas'''] but different '''Phases''' also exist when two or more liquids or solids come into contact with each other for example. | [https://en.wikipedia.org/wiki/Phase_diagram#Binary_phase_diagrams '''Phase Diagrams'''] are charts which show how different '''Phases''' can exist together under certain conditions ''(e.g. the temperature range at which water and ice can exist together in slushy conditions)''. Three obvious '''Phases''' are the three states of matter: [https://en.wikipedia.org/wiki/Solid '''Solid'''], [https://en.wikipedia.org/wiki/Liquid '''Liquid'''], and [https://en.wikipedia.org/wiki/Gas '''Gas'''] but different '''Phases''' also exist when two or more liquids or solids come into contact with each other and form distinctive '''[[Crystalline Structures]]''' for example. | ||
{{#ev:youtube|KQ5MjJndHp8|300x250|right||frame|loop=1&autoplay=0&playlist=KQ5MjJndHp8}} | |||
__TOC__ | __TOC__ | ||
In [https://en.wikipedia.org/wiki/Metallurgy '''Metallurgy'''] we are normally concerned with just [https://en.wikipedia.org/wiki/Solid '''Solid'''] and [https://en.wikipedia.org/wiki/Liquid '''Liquid'''] phases plus situations where these might exist together. The temperature line above which everything is [https://en.wikipedia.org/wiki/Liquid '''Liquid'''] is known as the [https://en.wikipedia.org/wiki/Liquidus '''Liquidus'''] and the line below which everything is [https://en.wikipedia.org/wiki/Solid '''Solid''' is known as the [https://en.wikipedia.org/wiki/Solidus_(chemistry) '''Solidus''']. | In [https://en.wikipedia.org/wiki/Metallurgy '''Metallurgy'''] we are normally concerned with just [https://en.wikipedia.org/wiki/Solid '''Solid'''] and [https://en.wikipedia.org/wiki/Liquid '''Liquid'''] phases plus situations where these might exist together. The temperature line above which everything is [https://en.wikipedia.org/wiki/Liquid '''Liquid'''] is known as the [https://en.wikipedia.org/wiki/Liquidus '''Liquidus'''] and the line below which everything is [https://en.wikipedia.org/wiki/Solid '''Solid'''] is known as the [https://en.wikipedia.org/wiki/Solidus_(chemistry) '''Solidus''']. | ||
---- | |||
<span style="color: green">'''Note:''' | |||
DT Online makes no attempt to explore the full depths of the fascinating topic of '''Metallurgy''' but hopes that the brief introduction given will stimulate interest and lead to further study. See the full series of lectures on [https://youtu.be/KQ5MjJndHp8?list=PL31C1F16DE82C106A '''YouTube'''] ''(‘A’ Level Chemistry and beyond)''. | |||
</span> | |||
---- | |||
=====Cooling Curves===== | =====Cooling Curves===== | ||
| Line 28: | Line 36: | ||
=====Phase Diagram for Lead/Tin Eutectic Alloy===== | |||
[[File:SolderPhaseDiagram.png|300px|right|Lead Tin Alloy Phase Diagram]] | [[File:SolderPhaseDiagram.png|300px|right|Lead Tin Alloy Phase Diagram]] | ||
[https://en.wikipedia.org/wiki/Phase_diagram#Binary_phase_diagrams '''Phase Diagrams'''] combine the information from the [https://en.wikipedia.org/wiki/Cooling_curve '''Cooling Curves'''] of mixtures of metals with metals and with other elements to create a chart which is used to show the behaviours of '''[[Alloy | [https://en.wikipedia.org/wiki/Phase_diagram#Binary_phase_diagrams '''Phase Diagrams'''] combine the information from the [https://en.wikipedia.org/wiki/Cooling_curve '''Cooling Curves'''] of mixtures of metals with metals and with other elements to create a chart which is used to show the behaviours of different '''[[Alloy]]''' compositions as they change phases between [https://en.wikipedia.org/wiki/Solid '''Solid'''] to [https://en.wikipedia.org/wiki/Liquid '''Liquid'''] states. | ||
| Line 43: | Line 51: | ||
[https://en.wikipedia.org/wiki/Ferrite_(iron) '''Ferrite'''] can be taken as just another name for '''Iron''' and, at room temperatures, it can absorb only a very small amount of '''Carbon''' ''(i.e. about 0.006%)'' within its '''[[Crystalline Structures|BCC]]''' structure | [https://en.wikipedia.org/wiki/Ferrite_(iron) '''Ferrite'''] can be taken as just another name for '''Iron''' and, at room temperatures, it can absorb only a very small amount of '''Carbon''' ''(i.e. about 0.006%)'' within its single phase '''[[Crystalline Structures|BCC]]''' structure. | ||
But immediately above 723°C '''Iron''' changes to a different single phase '''[[Crystalline Structures|FCC]]''' structure called [https://en.wikipedia.org/wiki/Austenite '''Austenite'''] and now can absorb up to 0.83% carbon - and this increases with rising temperature. [https://en.wikipedia.org/wiki/Austenite '''Austenite'''] can only exist at this elevated temperature and so as it cools all the extra '''Carbon''' has to go somewhere. | |||
---- | |||
<span style="color: green">'''Note:''' | |||
For simplicity the small areas of single phase [https://en.wikipedia.org/wiki/Ferrite_(iron) '''Ferrite'''] on the left of the diagram and single phase '''Carbon''' on the right have been omitted. | |||
</span> | |||
---- | |||
The [https://en.wikipedia.org/wiki/Phase_diagram#Binary_phase_diagrams '''Phase Diagram'''] shows that for '''Steels''' with less than 0.76% '''Carbon''' the mix solidifies into a two phase structure containing [https://en.wikipedia.org/wiki/Ferrite_(iron) '''Ferrite'''] which is very '''[[Hardness|Soft]]''' and '''[[Ductility|Ductile]]''', and a layered structure of both [https://en.wikipedia.org/wiki/Ferrite_(iron) '''Ferrite'''] and [https://en.wikipedia.org/wiki/Cementite '''Cementite''' ''(aka '''Iron Carbide''')''] which is very '''[[Hardness|Hard]]''' and '''[[Brittleness|Brittle]]''' - really a [https://en.wikipedia.org/wiki/Ceramic '''Ceramic''']. This layered structure is called [https://en.wikipedia.org/wiki/Pearlite '''Pearlite'''] because of its appearance. | |||
If cooled slowly, the '''Steel''' is stronger because of the added [https://en.wikipedia.org/wiki/Cementite '''Cementite'''] but still quite workable because large grains are given time to form and create space for '''[[Crystalline_Structures#Dislocations|Dislocations]]''' to move. If cooled quickly, or '''Quenched''' a fine grained [https://en.wikipedia.org/wiki/Pearlite '''Pearlite'''] will result | |||
If '''Steel''' with more than about 0.5% of '''Carbon''' there is too much '''Carbon''' to just form [https://en.wikipedia.org/wiki/Pearlite '''Pearlite'''] and also, if it is '''Quenched''' there is no time for it to precipitate out, so the extra remains trapped in a deformed '''[[Crystalline_Structures|BCC]]''' structure called [https://en.wikipedia.org/wiki/Martensite '''Martensite'''] which is very '''[[Hardness|Hard]]''' and '''[[Brittleness|Brittle]]''' indeed such that the '''Quenched''' steel can break like a stick of chalk. | |||
The more '''Carbon''' ''(e.g. up to about 2%)'' which is present then the more [https://en.wikipedia.org/wiki/Martensite '''Martensite'''] is formed. This does not occur in '''Low Carbon''' steel ''(e.g. less than 0.5%)'' because there is insufficient extra '''Carbon''' to form [https://en.wikipedia.org/wiki/Martensite '''Martensite''']. | |||
[[Category:Materials and Components]] | [[Category:Materials and Components]] | ||
Revision as of 14:06, 23 March 2017
Phase Diagrams are charts which show how different Phases can exist together under certain conditions (e.g. the temperature range at which water and ice can exist together in slushy conditions). Three obvious Phases are the three states of matter: Solid, Liquid, and Gas but different Phases also exist when two or more liquids or solids come into contact with each other and form distinctive Crystalline Structures for example.
In Metallurgy we are normally concerned with just Solid and Liquid phases plus situations where these might exist together. The temperature line above which everything is Liquid is known as the Liquidus and the line below which everything is Solid is known as the Solidus.
Note: DT Online makes no attempt to explore the full depths of the fascinating topic of Metallurgy but hopes that the brief introduction given will stimulate interest and lead to further study. See the full series of lectures on YouTube (‘A’ Level Chemistry and beyond).
Cooling Curves
Plotting a graph of Temperature and Time as liquid metal cools will produce a Cooling Curve. Such graphs serve to illustrate the behaviour of metal as it cools and what changes occur at different temperatures.
Typical Cooling Curves for pure metals will be a stepped graph as shown. Heat is being lost throughout the process but at the point of freezing the temperature doesn't fall because the freezing process will liberate heat at exactly the same rate that it is being lost to the surroundings.
This is the result of energy being released when new atomic bonds are formed formed (e.g changing states from Liquid to Solid Lead or Tin).
The Cooling Curve for an Alloy is more complicated and reveals interesting behaviours as solidification takes place.
With this particular Alloy, nothing happens at the normal freezing point of Lead because the addition of Tin has lowered this. Some solidification of Lead starts around 2500C but Tin remains liquid and there is not enough energy released to cause the Cooling Curve to go horizontal - although it does flatten out because cooling slows down a little.
The temperature stops falling at 1830C when both the Tin and Lead are starting to freeze. Once everything has solidified, the temperature continues to fall.
Phase Diagram for Lead/Tin Eutectic Alloy
Phase Diagrams combine the information from the Cooling Curves of mixtures of metals with metals and with other elements to create a chart which is used to show the behaviours of different Alloy compositions as they change phases between Solid to Liquid states.
The Phase Diagram for the Eutectic Alloy of 62% Tin & 38% Lead shows that at 1830C it changes from : Liquid to Solid without going through any ‘pasty’ stage which makes it ideal for Tinman's Solder.
Phase Diagram for Carbon Steel
As Carbon Steels are heated towards their melting point their Crystalline Structures change from a Body Centred Cubic structure to a Face Centred Cubic structure. A consequence of this is that Iron heated above this Critical Point can absorb a lot more Carbon within a FCC structure.
Ferrite can be taken as just another name for Iron and, at room temperatures, it can absorb only a very small amount of Carbon (i.e. about 0.006%) within its single phase BCC structure.
But immediately above 723°C Iron changes to a different single phase FCC structure called Austenite and now can absorb up to 0.83% carbon - and this increases with rising temperature. Austenite can only exist at this elevated temperature and so as it cools all the extra Carbon has to go somewhere.
Note: For simplicity the small areas of single phase Ferrite on the left of the diagram and single phase Carbon on the right have been omitted.
The Phase Diagram shows that for Steels with less than 0.76% Carbon the mix solidifies into a two phase structure containing Ferrite which is very Soft and Ductile, and a layered structure of both Ferrite and Cementite (aka Iron Carbide) which is very Hard and Brittle - really a Ceramic. This layered structure is called Pearlite because of its appearance.
If cooled slowly, the Steel is stronger because of the added Cementite but still quite workable because large grains are given time to form and create space for Dislocations to move. If cooled quickly, or Quenched a fine grained Pearlite will result
If Steel with more than about 0.5% of Carbon there is too much Carbon to just form Pearlite and also, if it is Quenched there is no time for it to precipitate out, so the extra remains trapped in a deformed BCC structure called Martensite which is very Hard and Brittle indeed such that the Quenched steel can break like a stick of chalk.
The more Carbon (e.g. up to about 2%) which is present then the more Martensite is formed. This does not occur in Low Carbon steel (e.g. less than 0.5%) because there is insufficient extra Carbon to form Martensite.