[en] Owing to their strong perpendicular magnetic anisotropy, FePd, CoPd, and their Co(Fe)Pt counterparts are candidate materials for ultrahigh density magnetic recording. The stability and magnetic properties of such films are largely dependent on the orientation and local distribution of the L1₀ FePd phase fraction. Therefore, the formation and transformation of the L1₀ phase in such thin films have been the subject of continued interest. Highly ordered epitaxial FePd(001) thin films (with an L1₀ phase fraction of 0.81) were prepared by molecular-beam epitaxy on a MgO(001) substrate. The effect of postgrown room temperature, 130 keV He⁺ irradiation was investigated at fluences up to 14.9x10¹⁵ ions/cm². X-ray diffraction and conversion electron Moessbauer spectroscopy revealed that with increasing fluence, the L1₀ FePd phase decomposes into the face centered cubic phase with random Fe and Pd occupation of the sites. A partially ordered local environment exhibiting a large hyperfine magnetic field also develops. Upon He⁺ irradiation, the lattice parameter c of the FePd L1₀ structure increases and the long range order parameter S steeply decreases. The Fe-Fe nearest-neighbor coordination in the Fe-containing environments increases on average from Fe₄₇Pd₅₃ to Fe₅₄Pd₄₆, indicating a tendency of formation iron-rich clusters. The equilibrium parameters corresponding to the equiatomic L1₀ phase were found at different fluences by conversion electron Moessbauer spectroscopy and by x-ray diffraction a difference, from which a plane-perpendicular compressive stress and a corresponding in-plane tensile stress are conjectured. The steep increase in the interface roughness above 7.4x10¹⁵ ions/cm² is interpreted as a percolation-type behavior related to the high diffusion anisotropy in the L1₀ phase