The microstructural and microchemical evolution of Fe-15Cr-XNi and ISCr-85Ni austenitic alloys irradiated with fast neutrons at 510 and 538°C and heavy ions at 675°C was investigated to determine whether common mechanisms of nickel's influence on microstructural development existed under different irradiation conditions. A surprising commonality for these two types of irradiations was observed involving nickel-dependent void densities, dislocation and loop dominance of dislocation structures, and fluence dependence of swelling, void density, and solute segregation. Quantitative analyses of microstructure and microchemistry data snowed that the variation of dislocation sink strength with nickel addition may provide the major mechanisms by which swelling depends on nickel content. Other processes, such as nickel's influence on the mobility of vacancies and the different segregation behavior at sessile and mobile defects, also cooperate to produce the overall behavior.