Microstructural evolution on neutron-irradiated molybdenum alloys

Full text of publication follows: Molybdenum is a refractory metal that has high strength and creep resistance at high temperatures, as well as possessing high thermal conductivity, and measurable tensile ductility, which are desired properties for many advanced applications. One concern associated with the irradiation of molybdenum, regardless of its good behavior in the unirradiated condition, is the loss of ductility resulting from irradiation hardening that could increase susceptibility to brittle fracture. However, microstructural evolution in molybdenum alloys at various irradiation temperatures is not well-known. In order to better understand the influence of composition, irradiation temperature and fluence on microstructural evolution, three kinds of molybdenum alloys, low carbon arc cast (LCAC), TZM, and ODS molybdenum, were prepared and neutron-irradiated up to 2.2x1026 n/m2 in the temperature range between 300 and 1000 deg. C. Irradiation of molybdenum alloys at 300 deg. C results in the higher number density of smaller loops and voids other than that at 600 deg. C, leading to formation of a stable distribution of obstacles that limits dislocation motion at much lower dose than required for the coarser void structure produced by the 600 deg. C irradiations. This may explain large increases in tensile strength at 300 deg. C, which are comparable for all alloys and irradiation temperature, and also saturation of the increase in tensile strength at lower dose for the 300 deg. C irradiation other than that for the 600 deg. C irradiations. In addition, irradiation-induced precipitates were also observed in matrix of LCAC and ODS molybdenum irradiated at higher irradiation temperature (600 and 1000 deg. C). Energy-dispersive spectrometry (EDS) and diffraction analysis indicated that the precipitates had hypothetical structure composed of molybdenum and ruthenium. (authors)