<p>The impact of helium ion irradiation on the evolution of microstructure and mechanical properties of a Ni-45%Cr-1.4%Mo (wt%) alloy has been investigated. Analysis via transmission electron microscopy (TEM) of a pristine sample revealed an uneven distribution of alpha-Cr phase precipitates along with a few isolated planar arrangements of dislocations within the fcc γ-Ni matrix phase. A detailed study of irradiated samples established the presence of radiation-induced defect clusters, dislocation loops, network dislocations as perceptible irradiation effects. X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS) studies also confirmed lattice swelling as an effect of irradiation in this alloy, likely caused by helium-stabilized vacancy cluster. A line profile analysis method which takes into account of planar defects, was employed on GIXRD (Grazing Incidence X-ray Diffraction) data to analyze changes in microstructural parameters such as domain size, microstrain, dislocation density and lattice parameter as a function of irradiation dose. All of these irradiation-damage manifestations led to the increase in the hardness of the alloy in nano-indentation experiment. Overall, the findings of this work provide significant insights into the radiation-induced microstructural changes and their effect on the hardening behavior of the studied Ni-Cr-Mo alloy.</p>