Eutectic
Eutectic ( ancient Greek εὐ eu- gut, τήκω teko schmelzen) is a phase equilibrium , which is characterized by the fact that the environmental conditions (degrees of freedom) can only be freely selected within a very small range. The most common representation of a eutectic is in a phase diagram with only two degrees of freedom, namely temperature and concentration of the components involved, see figure.
A similar form, in which all phases involved are already in a solid state, is called a eutectoid .
Eutectic alloys
properties
Eutectic alloys have a clearly definable melting point , the so-called eutectic point , where the solidus and liquidus lines meet . At the eutectic point, all three phases of the system - melt, phase A and B - are in equilibrium. The solidification temperature is also the lowest of all mixtures of the same components. Mixtures with other proportions of the alloying elements, on the other hand, have a melting or solidification range in which, in addition to the melt, a solid phase is also present.
The phase rule for solids at constant pressure is . This gives the value for eutectic alloys with two components and three phases (melt, phase A and phase B) , that is, the complete solidification of the eutectic is only possible in one point.
Because all components of eutectics solidify at the same time and this happens at a much lower temperature than would be the case with the pure components, a fine and uniform structure is created , which usually has a characteristic lamellar structure. The reason for this is the low kinetic energy of the atoms at this temperature, which only allows short paths and thus only the formation of very small crystals (also called crystallites ).
use
A technically frequently used eutectic is z. B. the Ledeburit of the Fe-C system (4.3% C / 1147 ° C ), which is used for casting gray cast iron . A eutectic mixture is also used in the production of aluminum by means of fused salt electrolysis . Here, a mixture with a melting point of 950 ° C is produced from 10.5% (percent by mass) aluminum oxide (melting point 2058 ° C) and 89.5% cryolite (melting point ~ 1000 ° C).
Since the melting point of a eutectic alloy is well below that of pure metals, such alloys are preferred for soldering . This has the advantage that relatively little heat has to be brought in and that the material relationship between the solder and the joining partner can be used when choosing the solder. In addition, the reduced melting point is used to create alloys in which the melting points of the two components are far apart. This is the case, for example, with aluminum (melting point 660 ° C) and tungsten (melting point 3422 ° C). If you tried to produce an aluminum-tungsten alloy directly by simply “throwing both components in a pot” and heating them, the aluminum would have already evaporated (boiling point 2467 ° C) before the tungsten had melted. However, if you first make a master alloy (with a lower melting point) from tungsten and a metal with a high melting point, production is possible. Due to the pretreatment, no pure alloys are created.
Well-known examples of eutectic alloys are
- the system known as " soldering tin " or "seepage solder " made of tin (Sn) and lead (Pb) with a composition of 63% Sn and 37% Pb with a melting temperature of 183 ° C.
- Various silver solders which, in addition to silver (Ag) and copper (Cu ), can also contain zinc (Zn), tin and cadmium (Cd). So contains z. B. the silver solder "L-Ag44" 44% Ag, 30% Cu and 26% Zn.
- Roses metal , an alloy of bismuth (Bi), lead and tin with a melting point of approx. 94 ° C
- Field's metal is a eutectic made up of indium (In), bismuth and tin and melts at around 62 ° C
- Wood's metal (also Wood's alloy ) consists of a eutectic alloy in the bismuth-lead-cadmium-tin system, which melts at around 60 ° C.
- A sodium-potassium alloy is liquid at room temperature.
In addition to their main use as soldering metals, such well-melting alloys are also used in sprinkler systems or in joke articles. Certain quartz porphyries or a solution of 30.9 g of table salt in 100 g of water ( cryohydrate , melting point −21.3 ° C) are also eutectics.
The eutectic alloy known as Galinstan, made of 68 to 69% gallium , 21 to 22% indium and 9.5 to 10.5% tin , has a particularly low melting point , which only crystallizes at −19.5 ° C and is mercury-free analogs Clinical thermometers is used.
In the chemical laboratory, the formation of low-melting eutectics is used to produce cold mixtures .
See also
literature
- Dieter Kohtz: Introduction to materials science for metal welders . In: Der Praktiker: the magazine for welding technology and more . 9/1982 to 1/1985. DVS-Verlag, ISSN 0554-9965 .
