We present a detailed study of the crystallization induced in an amorphousalloy by cluster ions. The formation of nanocrystalline iron boride phaseswas observed by transmission electron microscopy in an amorphousFe73.5Cu1Nb3Si13.5B9 alloy irradiatedup to fluences of 1 × 109–2 × 1011 clusters cm−2 at roomtemperature with Cn (n = 5, 10, 60) carbon clusters at 20 MeV, but not after irradiation withC60 fullerenes at 30 MeV. The high level of energy deposited in electronic excitations along thepath of carbon clusters induces radial pressure waves in the vicinity of the projectiletrajectories. The formation of nanocrystallites was interpreted as being due to a structuralrelaxation which occurs in the wake of the shock front. The mechanical stresses induced bythe pressure wave destabilize the amorphous structure and drive the nucleation ofnanocrystallites.