Changes in the microhardness of an aged Nimonic 90 containing γ′-precipitates are monitored as a function of dose after 50 keV helium ion irradiation at room temperature in the dose range from 3 × 10²⁰ to 3 × 10²² helium ions/m². The microhardness showed an oscillatory behaviour. Transmission and scanning electron microscopy studies were performed to investigate the microstructural changes and surface modifications due to irradiation. An attempt is made to correlate the variations in the microhardness with the microstructural changes and surface damage caused by irradiation.