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Hypereutectic melting point

  1. Apr 2, 2004 #1
    [Originally posted to material science forum but no reply]

    It's no surprise that if I take a particular metal and 'add' atoms of a second metal that has a lower melting point that the Tmelt of the combination is usually lower than that of the pure metal. This is hypoeutectic composition.

    But it seems surprising that if a reverse this, take a low Tmelt metal and add atoms of a higher Tmelt the Tmelt of the combination is even lower. This is hypereutectic composition. In fact there is a minimum of Tmelt at the Eutectic composition.

    Any insight into why adding a few percent of a higher Tmelt metal can lower the Tmelt of the combination?
     
  2. jcsd
  3. Apr 3, 2004 #2

    Bystander

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    Colligative properties of solutions, specifically the melting/freezing point depression.
     
  4. Apr 3, 2004 #3
    Thanks, but this is just another way of stating my question. Solutions of 2 metals have a lower melting point. It doesn't help me understand why the melting point drops.
     
  5. Apr 3, 2004 #4

    Integral

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    I believe that the answer "why" lies in the fact that alloys are not chemically bonded, but more of a mixture, the presence of the different atoms interferes with the crystal structure of both species. Thus the bonds are weaker then if a single species was present.

    I have seen 2 elemental room temperature solids mixed to from a liquid at the same temp. (Indium and Rubidium IIRCC)
     
  6. Apr 4, 2004 #5
    Ok, so let's say we have a silver-Copper solid solution with 20% Cu in the Ag host. This lowers the melting point from 960 C to about 850 C. They are both FCC structures with atomic radii of 0.144 & 0.128. This lets the copper be substitutional in the FCC lattice of the silver host. The presence of the copper weakens the bonds in the surrounding area & strains the lattice (so it starts out with more energy than a lattice without copper). This makes it easier to breakup the solid with thermal energy.

    I like it! Thanks.

    Cool. Physically how were the 2 solids combined? In powdered form? two solid lumps just touching? This would be a fun demo for any physics class.
     
  7. Apr 6, 2004 #6

    Integral

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    As I recall the 2 solids were some what pliable, kinda like a thick putty. When massaged together they just melted into a puddle. I am not sure if body temp was part of the transfromation.
     
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