Spike in phase change diagrams?

[Fishteacher73]

I was speaking to a friend of mine yesterday and he was discussing a phenomenon that had been brought up in one of his material science labs.

 

It seems that pure metals exhibit a predictable and measureable spike in their phase diagrams just as the metal begains the energy plateau of the phase change from liquid to solid.

 

He was not given an explanation of this phenomenon and we just had to speculate. Anyone familiar with this phenomenon that can explain it?

 

We posed the idea that durring the shift from liquid state, with the atoms at a higher kinetic energy (Liquid state), began to lock into their latice structure. This transion shifted the kinetic energy into thermal energy, thus producing the spike on the graph.

 

Any ideas?

[Tim_Lou]

ah.... metal phase diagram... i have no knowledge on such topic, i hope some experts here can help you with this.

[UncleAl]

Latent enthalpy of a first-order transition.

[Fishteacher73]

Enthalpy is defined to be the sum of the internal energy. Correct? We assumed that the spike came from the internal energy of the system, we we just not sure exactly how or why this energy was suddely expelled from the system in terms a of heat.

[UncleAl]

Specific heat of water, 1 cal/g.

Latent heat of fusion of water, 80 cal/g.

 

When you melt ice or freeze water, the first order phase transition montored by DSC will show a hump or a slump depending on which direction you are proceeding. If you heat a filament of spring steel red hot it will cool black, then spontaneously reheat red hot, then cool black. Know your metallurgy. Al+Cu will cast into a cold mold and cool without surprises. If you reheat you get a fat exotherm as needles of copper aluminide precipitate.

 

If you open one of those security stick-ons at your supermarket you wll get three strips of metal. One is dull and a permanent magnet. The other two are bright with one surface curiously pebbled. If you do a hight temp DSC on a bright strip you get a big exotherm on the way up - but only once. No endotherm on cooling. Try an x-ray diffraction pattern before and after.