The temperature dependence of the yield stress for neutron-irradiated molybdenum Meimei Li * , T.S. Byun, N. Hashimoto, L.L. Snead, S.J. Zinkle Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States Abstract Molybdenum was neutron-irradiated near 80 °C at doses of 7.2 · 10 5 , 7.2 · 10 4 , 7.2 · 10 3 , 0.072 and 0.28 dpa. Irradiated Mo was tensile tested over a temperature range of 50 to 100 °C at a strain rate of 1 · 10 3 s 1 . It was found that the yield stress of irradiated Mo decreased at lower temperatures and increased at higher temperatures, resulting in reduced temperature dependence of yielding at lower doses (<0.001 dpa); the yield stress was increased, and the temper- ature dependence of yielding was nearly unchanged upon further irradiation at higher doses (>0.001 dpa). The temperature dependence of the yield stress for unirradiated and irradiated Mo is consistent with the theoretical expression of the Flei- scher model for interactions of dislocations with tetragonal strain fields. Ó 2007 Published by Elsevier B.V. 1. Introduction Refractory metals have special importance for a number of innovative high technology applications. Molybdenum, in particular, is of great interest for high temperature applications in advanced fission and fusion reactor systems. It is attractive because of its high melting point, excellent high temperature strength, good thermal conductivity, and resistance to irradiation-induced swelling. Molybdenum also exhibits good corrosion resistance in liquid metals and in molten salts [1]. Therefore, it has potential applications in Gen IV systems, particularly in the liquid metal cooled fast reactor concepts and in molten salt coolant concepts. However, Mo, like other body-centered cubic (bcc) metals is susceptible to irradiation embrittlement at low temperatures, leading to an increase in ductile–brittle transition temperature (DBTT). This embrittlement has imposed limitations on its low temperature applica- tions, and has restricted its applications in nuclear energy systems. The temperature dependence of the yield stress at low temperatures is the chief factor that influences the DBTT in bcc metals. Low temperature deforma- tion of bcc metals is characterized by a strong tem- perature dependence of the yield stress as well as the strain rate dependence. This is in marked contrast to face-centered cubic (fcc) metals which exhibit a much weaker dependence of yielding on test temper- ature. The temperature dependence of the yield stress has been studied for both irradiated fcc and bcc metals. The temperature dependence of the flow stress for fcc metals, which is weak in the 0022-3115/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.jnucmat.2007.05.006 * Corresponding author. Fax: +1 865 241 3650. E-mail address: liml@ornl.gov (M. Li). Journal of Nuclear Materials 371 (2007) 53–60 www.elsevier.com/locate/jnucmat