Abstract
Density measurements were made on 0.762 mm W-l wt pct ThO2 wires which had been annealed at temperatures between 1000°C and 2000°C for times up to 120 min. Selected specimens were examined using optical and scanning electron microscopy. The results of this study indicate that the low, as-worked wire density is a result of large cracks associated with the ThO2 particles. Time-temperature annealing regimes, significantly below those necessary for recrystallization of the wire, result in partial healing of the cracks. This crack healing process can be followed qualitatively by observing longitudinal fracture surfaces in the scanning electron microscope and quantitatively by changes in the density of the wire. The apparent activation energy for the crack healing process was found to be 57.8 kcal/mole. The results suggest that crack closure is a result of both shear and grain boundary diffusion transport processes. It is concluded that the presence of cracks can influence the morphology of the recrystallized grains.
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