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The magnetic properties of ternary Fe-Ni-Cu invar alloys are affected by ion irradiation, which goes on increasing with increasing ion fluence (Φ), and by increasing Cu content. In the present study, the ions used are He with 2 MeV energy and with 1 × 10 cm, 1 × 10 cm, 5 × 10 cm, 1 × 10 cm and 5 × 10 cm fluence (dose) for irradiation purpose. The face centered cubic structure of the alloy was investigated after ion irradiation using x-ray diffraction (XRD) and found unchanged. However, the peaks become broader with increasing ion dose. Additionally, the lattice fluctuations were observed in XRD study. Curie temperature ( T) is also increased after irradiation. Many factors are considered here for the reason for increasing T, such as the stopping of incident ions, atomic mixing effect at micro scale level owing to ion irradiation, which might change local concentration and ordering already reported in diffuse scattering, and as a result the Fe-Fe interatomic distance and the Fe-Fe coupling are changed. A comparative study shows that the effect of irradiation on T and magnetization with increasing ion fluence is more distinctive than the addition of Cu. [ABSTRACT FROM AUTHOR]