A eutectic material is an alloy with special properties. This can be an
alloy made of 2 elements or 3 or more. We talk about binary eutectic
alloys, ternary, qartenary, quintenary.....eutectic alloys, depending on
the number of elements in the alloy.
As John already said, one of the special properties of a eutectic alloy
is the fact that it solidifies instantaneously below a certain
temperature called the solidus temperature where non eutectic alloys
start to solidify partially at a higher temperature than the solidus
(this temperature is called liquidus because above it the alloy is
totally liquid) and become solid below the solidus.
Thus, the eutectic alloy is the alloy where solidus and liquidus meet.
The eutectic composition can also be explained with thermodynamics.
Every material has 2 kind of thermal energy, entropy and enthalpy. Human
beings can measure enthalpy which is temperature. However, we have
problems in understanding entropy which is a measure of how ordered a
system is. The more entropy the more mess one could say. If we assume
now that atoms are somehow like humans we can say that the bigger the
mess the more the atoms are willing to leave the crowd. This means if we
have a system with a high order such as a pure metal A we need to hit
the atoms pretty hard with enough temperature to leave the crystalline
structure and go into the liquid phase. Adding a second element B
increases the entropy and less enthalpy is needed to melt the material.
At a certain point of mixing A and B the system is in its messiest
state, the entropy is in its maximum and thus the temperature to make
the atoms leaving the crystal is the lowest. Adding more B leads to a
higher order because B takes over as the main constituent of the alloy
and the melting temperature rises again. With other word, the eutectic
alloy is the composition with the highest entropy.
Fascinating isn't it?
Have a nice day
Guenter
_____________________________________________________
EMPA
Swiss Federal Laboratories for Materials Testing and Research
Centre for Reliability
Guenter Grossmann, Senior Engineer
8600 Duebendorf
Switzerland
Phone: xx41 44 823 4279
Fax : xx41 1 823 4054
mail: [log in to unmask]
_____________________________________________________
>>> Lum Wee Mei <[log in to unmask]> 05/23/07 2:08 am >>>
Hello,
What is actually eutectic solder? All along I have this impression that
when we talk of eutectic solder, it is refer to Sn63. However,
yesterday, I attended a course and was told that Sn96 (4% silver) is
also eutectic. Can someone enlightenment me?
Thanks and regards,
Wee Mei
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