EFFECTS OF 14 MEV NEUTRON-IRRADIATION ON MECHANICAL-PROPERTIES OF HIGH-PURITY MOLYBDENUM SINGLE-CRYSTALS

High-purity molybdenum single crystals with the antitwinning axial ori entation were irradiated with 14 MeV neutrons at room temperature, 473 K and 673 K under low neutron fluences of (2.1 similar to 2.7)x 10(21 ) n/m(2) using the Rotating Target Neutron Sources (RTNS-II), and then were deformed in tension at 290 K. The irradiations led to the inhomo geneous yielding accompanied by large yield drops. The active slip pla ne, which of the unirradiated specimen was the plane ((1) over bar 01) , was shifted more and more towards the maximum shear stress plane ((2 ) over bar 11) with decreasing irradiation temperature. By the isochro nal annealing, the pronounced yield drop disappeared above 973 K. Furt hermore, the flow stress of the specimens irradiated at room temperatu re changed from softening to hardening near 500 K and the strain-rate sensitivity also responded to this change, which was related to the mi gration of vacancies. Especially for the specimens irradiated at 673 K , it was found that the irradiation effects still remained above 973 K .