The answer is probably no. But can you make a metal denser by melting it and make it cool down under high pressure? Or would it just crack under internal stress when it returns to a solid? I'm going to assume that the metal doesn't have air bubbles or other impurities inside it and that we're talking about charted and known densities.

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    $\begingroup$ Why do you think that melting a metal and then cooling it down under high pressure might have the possibility of making it denser? $\endgroup$
    – user93237
    May 11, 2018 at 16:39
  • $\begingroup$ I don't think it will have an effect, hence my opening statement. But I'm curious if it does, that's why I'm asking the community. And surprisingly, one person has already said that it can have an effect. $\endgroup$
    – Pedery
    May 11, 2018 at 17:58
  • $\begingroup$ Why would liquid metal be more compressible than solid metal, or any other liquid (which tend to say "incompressible" right on the tin)? $\endgroup$ May 12, 2018 at 4:03

1 Answer 1


First of all, high pressure (i.e., compressive equitriaxial stress) is simply not going to induce cracks (or failure of any sort) in a uniform solid—there's just nowhere for the material to move to reduce the sum of strain energy and surface energy.

Yes, melting a metal and then freezing it might increase the density in several ways, although the improvement might be negligible relative to simply applying the high pressure. Here are a couple of examples:

First, one might eliminate voids in the material. For example, one can substantially increase the density of a metal foam by melting it and refreezing it as a solid body by applying pressure.

Second, if the material has been thoroughly cold worked, i.e., it contains a large number of dislocations and grain boundaries, then these defects would contribute some slight excess volume that would be removed by melting and refreezing. However, this free volume is estimated to be quite small (think 0.1%) even in cases of severe plastic deformation. The resulting maximum achievable increase in density would be comparable.

Third, as @By_Symmetry notes below, the high temperature might promote a pressure-induced transformation to a higher-density alternative crystalline phase that might have been kinetically limited at lower temperatures.

Note, however, that if you heated the metal and then cooled it down very quickly (i.e., you quenched it), you might actually decrease the density relative to the case of no thermal processing, either by kinetically trapping vacancies (which are much more prevalent at high temperatures), by producing an amorphous structure (by kinetically limiting the material's tendency to freeze into a crystal), or by locking in a relatively low-density alternative crystalline phase.

  • $\begingroup$ It is also possible that a particular material could have a metastable phase with an entirely different structure and correspondingly different spesific volume, and that high pressure could induce a transition between phases. e.g. graphite and diamond. $\endgroup$ May 11, 2018 at 16:47
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    $\begingroup$ It's very rare to have a high-pressure metastable phase which is significantly denser than the ambient pressure phase and which is recoverable from high pressure back to atmospheric pressure. The graphite-to-diamond transition is the only one that I know of offhand. $\endgroup$
    – user93237
    May 11, 2018 at 16:58
  • $\begingroup$ @SamuelWeir I believe it's also theorized that there may be a metallic crystal form of hydrogen which could survive the trip back to roughly normal pressure/temperature. $\endgroup$ May 11, 2018 at 18:21
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    $\begingroup$ @Shufflepants - Yes, it has been theorized that metallic hydrogen may be metastable, but I don't think that there is any scientific consensus on that. BTW, I was a member of the only experimental team ever to have created metallic hydrogen in a laboratory. It didn't survive the trip back to normal pressure and temperature, though. (S.T. Weir, A. C. Mitchell, and W. J. Nellis, PRL v 76, 1860 (1996)). $\endgroup$
    – user93237
    May 11, 2018 at 18:27
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    $\begingroup$ And don't forget Kurt Vonneguts Ice 9, and Joe Satriani's tribute. $\endgroup$ May 11, 2018 at 20:54

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