NEUTRON-IRRADIATION DAMAGE AND VOID LATTICE FORMATION IN A MOLYBDENUM ALLOY TZM

Molybdenum alloys are candidate materials for high heat flux components of fusion reactors. In order to study the neutron irradiation damage at high fluence levels, disks of the molybdenum alloy TZM that had been stress relieved at 1199 K for 0.9 ks were irradiated in the FFTF/MOTA at 679, 793 and 873 K to a neutron fluence of 9.6 x 10(26) n/m(2) (E(n)>0.1 MeV). Defect microstructures were observed by transmission electron microscopy. Dislocation structures consisted of isolated loops, aggregated loops (rafts) and elongated dislocations. The size of the loops increased with the irradiation temperature. The levels of void swelling were 0.68 and 1.6% at 793 and 873 K, respectively. A void lattice was developed in the body-centered-cubic (b.c.c.) structure with a lattice spacing of 28 nm. The fine grain size (0.5-2 mu m) was retained following high-temperature irradiation, indicating that the stress relief heat treatment may extend the material's resistance to irradiation damage up to high fluence levels. The relationship between the microstructure and irradiation hardening was determined.