Low temperature neutron irradiation effects on microstructure and tensile properties of molybdenum Meimei Li a, , M. Eldrup b , T.S. Byun a , N. Hashimoto c , L.L. Snead a , S.J. Zinkle a a Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA b Materials Research Department, Risø National Laboratory, Technical University of Denmark, DK-4000, Roskilde, Denmark c Materials Science Division, Hokkaido University, Sapporo 060-8628, Japan Received 5 June 2007; accepted 12 December 2007 Abstract Polycrystalline molybdenum was irradiated in the hydraulic tube facility at the High Flux Isotope Reactor to doses ranging from 7.2  10 5 to 0.28 dpa at 80 °C. As-irradiated microstructure was characterized by room-temperature electrical resistivity measure- ments, transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS). Tensile tests were carried out between 50 and 100 °C over the strain rate range 1  10 5 to 1  10 2 s 1 . Fractography was performed by scanning electron microscopy (SEM), and the deformation microstructure was examined by TEM after tensile testing. Irradiation-induced defects became visible by TEM at 0.001 dpa. Both their density and mean size increased with increasing dose. Submicroscopic three-dimensional cavities were detected by PAS even at 0.0001 dpa. The cavity density increased with increasing dose, while their mean size and size distribution was relatively insensitive to neutron dose. It is suggested that the formation of visible dislocation loops was predominantly a nucleation and growth process, while in-cascade vacancy clustering may be significant in Mo. Neutron irradiation reduced the temperature and strain rate dependence of the yield stress, leading to radiation softening in Mo at lower doses. Irradiation had practically no influence on the magnitude and the temperature and strain rate dependence of the plastic instability stress. Ó 2008 Published by Elsevier B.V. 1. Introduction Molybdenum is of great interest for high temperature applications in advanced fission and fusion reactor systems because of its high melting point, excellent high tempera- ture strength, good thermal conductivity, resistance to irra- diation-induced swelling and corrosion resistance in liquid metal coolants [1]. However, Mo, like other body-centered cubic (bcc) metals, is susceptible to low temperatures embrittlement and suffers an increase in ductile-brittle tran- sition temperature (DBTT) after neutron exposure [2–11]. The improvement in low temperature ductility of neu- tron-irradiated Mo is of great importance for its applica- tions in advanced nuclear systems. Low temperature embrittlement is associated with irradi- ation hardening that is controlled by the interactions of mobile dislocations and irradiation-induced defects. Whether or not Mo can be engineered to resist irradiation- induced low temperature embrittlement is dependent on the formation process of sessile defect clusters in Mo. It is known that two formation processes are often competing during irradiation, namely in-cascade clustering and diffu- sive nucleation and growth. If the formation of sessile defect clusters is dominated by a nucleation and growth process such as in bcc Fe [12–16], solute additions may have a strong effect on the formation of defect clusters and radiation hardening. The resistance to irradiation embrittlement may, therefore, be improved by metallurgical approaches. On the other hand, if large, sessile defect clusters originate from displacement cascades, e.g. in bcc W [12–14,17], the low temperature irradiation embrittlement is essentially an inherent problem, and may not be overcome. 0022-3115/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.jnucmat.2007.12.001 Corresponding author. Present address: Argonne National Laboratory. Tel.: + 1 630 2525111; fax: + 1 630 2523604. E-mail address: mli@anl.gov (M. Li). www.elsevier.com/locate/jnucmat Available online at www.sciencedirect.com Journal of Nuclear Materials 376 (2008) 11–28