Why are quasicrystals exceptionally brittle?

Quasicrystals are exceptionally brittle primarily due to the nature of their internal structure and the behavior of dislocations within them.

In typical metals, the ability to bend and deform is facilitated by the creation and movement of dislocations. In contrast, quasicrystals contain very few dislocations, and those that are present exhibit extremely low mobility. Because these dislocations cannot move easily to accommodate stress, the material cannot bend, which results in its characteristic brittleness.1

While this lack of dislocation motion makes the material brittle, it also contributes to the extreme hardness of quasicrystals, as they strongly resist deformation. This combination of properties has made them useful in applications such as high-strength surface coatings.1