Effects of Neutron Irradiation on the Microstructure of Niobium and Niobium-Base Alloys

Abstract

The microstructure and microhardness of niobium and commercial prototype niobium-base alloys have been investigated following fast neutron irradiation to a fluence of 1.1 × 1022 neutrons (n)/cm2 (<0.1 MeV), 4 displacements per atom (dpa), at 482°C. The purpose of this work was to determine the effects of molybdenum and zirconium alloy additions on the resistance of niobium-base alloys to neutron irradiation. Neutron irradiation of the niobium and Nb-1Mo alloy produced very small voids whose mean diameters were in the range of 30 to 35 Å. In all other alloys, however, no voids were observed and the principal effect of the neutron irradiation was to form a high density of 30 to 40 Å diameter dislocation loops which are believed to be primarily of interstitial character. The effect of fast neutron irradiation was to increase the microhardness of both niobium and all its alloys, with the largest increase in the Nb-1Zr alloy and the smallest increase in the ternary Nb-5Mo-1Zr alloy. Previous work on niobium and Nb-1Zr alloy has shown that a vacancy trapping mechanism is particularly effective at irradiation temperatures up to 600°C for fast neutron fluences of 2.5 × 1022 n/cm2 (>0.1 MeV). The observation from the present experiments that molybdenum and zirconium additions were effective in the suppression of void formation suggests that a vacancy trapping mechanism was operative in the present alloys. The smaller hardness increase observed for the ternary alloys suggests that these alloys may maintain good engineering properties in addition to their improved resistance to void formation.