Ribbons of Fe75B25, prepared by quenching from the melt, were mostly amorpbous. After annealing at 773 K, the ribbons were entirely crystalline, with the phases Fe3B, Fe2B and α - Fe. Annealed ribbons were bombarded at RT with 4 MeV argon ions, at a damage rate of 7 × 10−4 dpa sec−1 near the surface. The Fe3B was completely converted to an amorphous form at 0.15 dpa, the lowest damage level at which amorphization of a metal alloy has been reported. The Fe2B disappeared by about 1.5 dpa. The α -Fe disappeared between 17 and 50 dpa, apparently by ion beam mixing rather than direct amorphization. Amorphous films also were prepared by sputtering at low power level, and crystallized at 773 K, which also caused sintering. Bombardment with argon ions produced major changes in resistivity and magnetic coercive force at the same low damage levels that caused amorphization of Fe3B Hc of the bombarded film was the same as in an as-quencheo amorpous ribbon. The relatively high resistivity of as sputtered amorphous film was apparently due to the porosity in the columnar cell walls. With present technology, it should be possible to produce amorphous films or structures of Fe3B at rates approaching 50 m2 hr−1.