Characterization and annealing behavior of voids in neutron-irradiated nickel

Abstract

Transmission electron microscopy (TEM) and small angle scattering (SAS) techniques were used to determine the size and density of voids in high purity nickel that had been irradiated to 3 × 10 21n/cm 2 (>0.1 MeV) at 450°C and subsequently annealed for 2 hr at 650, 800, 900, 975, 1050 and 1150°C. The as-irradiated material contained 1.8 × 10 14 truncated octahedral voids cm 3 whose average size was 400 Å. TEM revealed that the dissolution of the voids during high temperature anneals was rather inhomogeneous with those voids nearest the grain boundaries disappearing first. This resulted in a grain boundary denuded zone whose width depended on temperature, varying from 1 μ at 650°C to 15 μ at 975°C and complete “denuding” at 1050°C. Both TEM and SAS revealed that the voids in the center of the grains did not coarsen during annealing, and the average size remained at ~400 Å up to the temperature of complete annealing (1050°C). Such behavior has been predicted for voids that are partially gas-filled, and annealing studies at 1050 and 1150°C did indeed reveal gas bubbles at the recrystallized grain boundaries. Microhardness measurements were also made on the samples after various anneals, and a good correlation was found between the hardness increase, ΔH, and the quantity (Nd) 1 2 , where N is the number of voids/cm 3 and d is their diameter. More specifically, ΔH = 6 μb(Nd) 1 2 , where μ is the shear modulus and b is the burgers vector.