The alloying of magnesium is carried out at 720C which the constituent materials have melting point above 800C.
That's called a eutectic system, the Wiki page is not bad:
https://en.wikipedia.org/wiki/Eutectic_system
Basically, you get this if the energy of the two constituents in a solid phase is larger when they are mixed. In this case, separate phases form at low temperature (miscibility gap). When you increase the temperature, the entropy of the two separated phases is lower than that of the liquid phase where both elements mix freely, therefore the entropy term in the free energy F=U-TS becomes larger so that the liquid phase becomes favourable at a lower temperature
That's called a eutectic system, the Wiki page is not bad:
https://en.wikipedia.org/wiki/Eutectic_system
Basically, you get this if the energy of the two constituents in a solid phase is larger when they are mixed. In this case, separate phases form at low temperature (miscibility gap). When you increase the temperature, the entropy of the two separated phases is lower than that of the liquid phase where both elements mix freely, therefore the entropy term in the free energy F=U-TS becomes larger so that the liquid phase becomes favourable at a lower temperature
Total entropy tends to maximization. If an alloy forms at a temperature lower than the melting points of any of the constituent elements, then the formation of this alloy is endothermic. Thus heat must be absorbed, decreasing the thermal entropy. But the very process of mixing of the constituents to form the alloy increases the configurational entropy. The alloy can form if there is a NET increase in entropy, that is, if the configurational entropy increases more than the thermal entropy decreases.
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