EFFECT OF NEUTRON-IRRADIATION ON YIELD STRESS AND DUCTILITY OF REDUCED-ACTIVATION MARTENSITIC 9CR-WVTA STEELS

The tensile properties of reduced-activation martensitic 9Cr-1WVTa and 9Cr-3WVTa steels for fusion reactors were investigated over the tempe rature range from room temperature to 873 K after neutron irradiation in the Japan Materials Testing Reactor at 538 K to fast neutron fluenc es of 2 x 10(23) and 3 x 10(24) n/m(2) (E>1 MeV). A conventional 9Cr-1 MoVNb steel was also examined for comparison. The irradiation caused a n increase in yield stress and a decrease in total elongation. Irradia tion-produced defect clusters could not be seen directly by transmissi on electron microscopy. The increase in yield stress caused by irradia tion Delta sigma(gamma) was proportional to the 1/4 power of the neutr on fluence. The Delta sigma(gamma), was smaller in the 9Cr-1WVTa and 9 Cr-3WVTa steels than in the 9Cr-1MoVNb steel and was smaller in the 1% W steel than in the 3% W steel. With increasing test temperature, the Delta sigma(gamma), exhibited a further increase at 573 similar to 67 3 K and then decreased to zero over the range from 673 to 873 K. The i ncrease in irradiation hardening at 573 similar to 673 K was postulate d to be due to the formation of vacancy-carbon complexes, and the decr ease in it at 673 similar to 873 K was due to the annealing out of irr adiation-produced defects. The decrease in total elongation was shown to be accompanied by a decrease in work hardening rate.