Eutectic system
Eutectic (Greek εὐ eu -good, τήκω teko melt ) is a phase equilibrium, which is characterized in that it is possible the environmental conditions (degrees of freedom ) choose freely only in a very small area. Most common presentation of a eutectic is in a phase diagram with only two degrees of freedom, namely temperature and concentration of the components involved, see picture. In the eutectic point ( where touch solidus and liquidus ) are three phases of the system in equilibrium ( in this example, melt, A & B), this phenomenon is called the eutectic. There are other forms with similar names, such as when the participating phases are all present in the solid state, is called the eutectoid. The basic principle does not change.
Eutectic alloys
Properties
Eutectic alloys have an easy to determine the melting point. Its melting point is also the lowest of all mixtures of the same components. Other mixing ratios with the same components, however, have a melting or solidification range, present in the addition of the melt and a solid phase.
Due to the fact that all the components at the same time to solidify and this is done at a much lower temperature than would be the case for the pure components, results in a fine and uniform structure which has a generally characteristic lamellar structure. The reason for this is the low at this temperature kinetic energy of the atoms (also called crystallites ) the only short distances and thus only the formation of very small crystals permits.
Use
A technically frequently used eutectic is eg the ledeburite of the Fe -C system (4.3% C/1147 ° C) which is used for the casting of gray cast iron.
Since the melting point of a eutectic alloy, significantly below that of the pure metals, is, such alloys are preferably used for soldering. This has the advantage that you have to bring in relatively little heat and can use the material relationship of solder and joint partners in the choice of the solder. Furthermore, one uses the reduced melting point for creating alloys in which the melting points of the two components are far apart. This is for example the case of aluminum (melting point 660 ° C ) and tungsten ( melting point 3422 ° C) case. If one tried to produce an aluminum -tungsten alloy directly by both components simply " throws in a pot " and heated, the aluminum would have already evaporated ( boiling point 2467 ° C ) before the tungsten is melted. Is prepared, however, only one alloy ( lower melting point ) of tungsten and a metal having a high melting point, the production is possible. Due to the pre-treatment arise so naturally no pure alloys.
Examples of eutectics are the Sn-Pb system ( " solder " ), for example having a composition of 62/38, the system Ag-Cu ( copper - silver alloy ) having a composition of 72 /28, Roses metal Wood's metal, quartz porphyries certain or a solution of 30.9 g of sodium chloride per 100 g of water ( cryohydrate ). Used these well -melting alloys in fire sprinklers, as solder alloys or in jokes.
A particularly low melting point has the known as galinstan eutectic alloy of 68-69 % gallium, 21 to 22 % indium and 9.5 to 10.5 % tin, which crystallizes only at -19.5 ° C and mercury-free, analog fever thermometers will be used.