Hypereutectic melting point

[mmwave]

[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?

 

[Bystander]

Colligative properties of solutions, specifically the melting/freezing point depression.

 

[mmwave]

Colligative properties of solutions, specifically the melting/freezing point depression.

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.

 

[Integral]

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)

 

[mmwave]

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.

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 let's 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.

I have seen 2 elemental room temperature solids mixed to from a liquid at the same temp. (Indium and Rubidium IIRCC)

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.

 

[Integral]

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.