[en] Work on microstructure in ferritic alloys following irradiation in FFTF is presented: Fe-Cr binary alloys ranging from 3 to 18 Cr, and substitutional ferritic alloys based on Fe-10Cr with minor additions of Si, Mn, V, Ta, and Zr. Irradiations were performed to fluences as high as 30 dpa at 425/degree/C. The simple Fe-Cr binary alloy behavior following irradiation to 15 dpa was similar to that observed following irradiation in EBR-II. All specimens contained radiation damage such as dislocation loops and voids. Swelling was highest, 0.26%, in the Fe-9Cr alloy with a corresponding dislocation network comprising equal parts of a<100> and a/2 <111> Burgers vectors. Less swelling was found in other alloys with a corresponding dislocation structure composed mainly of a<100> loops. A deformation slip band containing elongated voids indicated that channel fracture may occur in ferritic alloys that contain uniform void swelling. The behavior of substantial Fe-10Cr ferritic alloys following irradiation to 30 dpa demonstrated that different solutes affect microstructural development. Void and dislocation development were observed in all conditions but the amount of swelling and the dislocation character (a<100> versus a/2 <111>) and structure (network versus individual loops) varied both with solute type and amount of solute (0.1 or 1.0 wt %). Swelling was lowest in Fe-10Cr-1Si due to delayed void nucleation; the dislocation structures were loop dominated in both Fe-10Cr-1Si and Fe-10Cr-1V. Void shape was found to vary between dodecahedral with /011/ faces and /100/ truncations and cubic with /100/ faces and /011/ truncations. 26 refs., 21 figs., 4 tabs