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Thermal Equilibrium Diagrams The
Solidification of Metals A pure metal solidifies at a fixed temperature, a fact which can be checked by plotting a cooling curve. A cooling curve may be obtained by melting a small amount of a metal and recording the temperature drop at suitable time intervals as this metal solidifies (the metal must be allowed to cool very slowly i.e. under equilibrium conditions) . We can then plot a graph of temperature against time to give us the cooling curve for that particular metal.
At temperatures above and below line "ab" the
curve falls smoothly without "kinks". When the solidification
temperature is reached, the temperature remains CONSTANT
for some time thus giving rise to the step "ab"
in the curve. The
solidification of Alloys Unlike
pure metals alloys solidify over a range of temperatures. Below the temperature
at which the alloy begins to solidify and the temperature when it is completely
solidified the alloy is in a "pasty" state gradually becoming
stiffer as the lower limit of the solidification range is approached.
Note that all these alloys possess two arrest points with the exception of the 62% TIN alloy (tin mans solder). This alloy has only one single arrest point as the alloy does not go through a pasty state (like a pure metal) it goes directly from a liquid to a solid state . This is called the EUTECTIC alloy.
Instead of dealing with several different cooling curves for any alloy a quicker graph has been created using the various arrest points of all the alloys. When these points are marked on a graph and joined up we get a thermal equilibrium diagram (T.E.D.). Let us take a closer look at this diagram.
As
you can see the area in green
is the liquid state
while the area in black is the solid state while the area in
yellow is the pasty
state which consists of a solid phase and a liquid phase. A very important point
to note is that the line joining all the points where the liquid begins to
solidify is known as the Liquidus
line while
the line joining all the points where solidification is just complete is known
as the Solidus
line. Now
that you know how the diagram is created lets see where it is used. For
example: if we want to find at what temperature 60% Copper fully solidifies at
in an alloy of Copper and Nickel. Firstly we must graph the thermal equilibrium
diagram for the alloy of copper and tin.
This diagram shows the thermal equilibrium diagram for
the alloy of copper (Cu) and nickel (Ni). In order to find what temperature 60%
copper solidifies at we simply draw a vertical line from 60% copper until it
hits the solidus line and at this is the point where 60% copper has fully
solidified. Note: Thermal
equilibrium diagrams are the metallurgist's maps. Just as a person considering
climbing or walking in a new area would obtain and study an Ordnance Survey map
in order to fully plan the outdoor pursuit, so also should a student of
engineering be able to comprehend and appreciate thermal equilibrium diagrams. Original source and acknowledgement to: Michael Walshe For further information follow: http://www.ul.ie/~walshem/fyp/mainmenu.htm http://www.matter.org.uk/matscicdrom/manual/pd.html |
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