Most engineering materials are polycrystalline solids that are composed of many grains with grain boundaries. The grain boundaries are normally stronger than individual grains because the disruption of grain boundaries provides a source of strengthening by pinning the movement of dislocations.

However, in some cases, the grain boundaries are weaker than individual grains. For instance, researchers have proposed a couple of mechanisms for the low grain boundary strength of intermetallics:
         i) Grain boundaries can be preferential regions of segregation of embrittling impurities. In metals, highly electronegative elements such as P and S can segregate at grain boundaries and pull bonding electrons to themselves, which weakens the metallic bond across the grain boundaries.
         ii) Hydrogen (H) induces embrittlement. Many intermetallics decompose H₂O vapor at the material's surfaces, resulting in rapid H diffusion along the grain boundaries, and thus embrittling the metals and alloys such as aluminides and silicides.
         iii) Nucleation of dislocations can be difficult at grain boundaries due to high energy for dislocation nucleation near the boundaries.
         iv) Electronic charges deplete at grain boundaries. The metallic elements in such materials have unequal electronegativities so that their lattices have lower electron densities than metals. Electron depletion at the boundaries would reduce the cohesive strength of the grain boundaries.