I want to know what is inside a grain boundary?
Is it a gas or vacuum
There could be gases, but it's essentially the same space that is between atoms - or the same space that exists between the nucleus and atomic electrons. At the atomic level, vacuum really doesn't mean anything.
The grain boundary areas are of course highly mis-matched.
Ya vacuum doesn't mean anything at the molecular level. Because free space is every where. In fact even most of an atom is a "vacuum" / free space. So at the atomic level it's more important of thinking about relative energy levels / bonding forces.
A grain boundary is simply the region where two separate nucleation sites meet. The reason for the weakness here is that the lattice structure doesn't match up when the sites meet so there are few to no major chemical bonds in the region. This means that separate nucleation sites are largely kept together by weak electrostatic forces rather than chemical bonds. This is why material strength in practice is generally much weaker than that calculated from that assumed of a perfect lattice structure.
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