Oxide dispersion strengthened (ODS) molybdenum has been characterized using transmission electron microscopy (TEM) to determine the effects of irradiation on material microstructure. This work describes the results-to-date from TEM characterization of unirradiated and irradiated ODS molybdenum. The general microstructure of the unirradiated material consists of fine molybdenum grains (< 5 µm average grain size) with numerous low angle boundaries and isolated dislocation networks. 'Ribbon'-like

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... anthanum oxides are aligned along the working direction of the product form and are frequently associated with grain boundaries, serving to inhibit grain boundary and dislocation movement. In addition to the 'ribbons', discrete lanthanum oxide particles have also been detected. After irradiation, the material is characterized by the presence of nonuniformly distributed large (~ 20 to 100 nm in diameter), multi-faceted voids, while the molybdenum grain size and oxide morphology appear to be unaffected by irradiation. §1. INTRODUCTION Oxide Dispersion Strengthened (ODS) molybdenum is attractive for high-temperature applications due to its excellent high-temperature mechanical properties (Mueller et al. 2000) . It is believed that the dispersed oxide phase in the alloy improves creep resistance by impeding grain boundary sliding and dislocation motion compared to the behavior of unalloyed molybdenum. To optimize the mechanical behavior of these materials, it is necessary to characterize and understand the microstructure in terms of grain size, substructure, and second phase particles, and, in turn, understand how the microstructure affects material behavior. The structure and properties of molybdenum have been well documented, particularly the response of unalloyed molybdenum and molybdenum alloys to neutron irradiation (Abe et al. 1993 , Adda 1972 , Bentley 1974 , Brimhall et al. 1973