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I have seen youtube videos of people treating some aluminium object with liquid gallium and after letting it sit for a while, they are able to break up the aluminium into tiny little pieces with their bare hands.

Can someone tell me what is exactly going? I haven't been able to find a satisfactory explanation as of yet.

Does the gallium somehow incorporate itself into the metal lattice, disrupting the bonds? or is it forming an amalgam of sorts and actually dissolving the aluminium?

Is it a combination of these two, or are my ideas off completely? Also, what would be the exact mechanics of a process like this?

EDIT: How exactly does gallium "unzip" the grain boundaries? Also, besides Ga or Hg, what other metals would produce a similar effect? What mechanism/property allows them to behave in such a manner?

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    $\begingroup$ It moves down the grain boundaries and 'unzips' them efficiently. $\endgroup$ – Jon Custer Jun 5 '15 at 1:06
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As you state, it forms an amalgam, just as mercury does. @Jon Custer's explanation is correct, and furthermore, the alloy does not form a protective oxide, so the aluminum oxidizes to $\ce{Al2O3}$ and quickly crumbles. This happens even faster with an Hg-Ga alloy!

BTW, gallium should be safe enough, though expensive, but I would not like to work with mercury without a hood, and without careful containment.

Thanks for the question; this encouraged me to view that amazing demo on Vimeo!

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