THE EFFECTS OF STRAIN ANNEALING ON GRAIN-BOUNDARIES AND SECURE TRIPLEJUNCTIONS IN NICKEL-200

Changes in the microstructure of 99.5% pure nickel as a function of st rain annealing have been investigated. The application of moderate amo unts of compressive strain (6-7%), and subsequent thermal treatment at 750 degrees C under vacuum is shown to increase the density of Sigma 3 grain boundaries, where the coincident-site lattice rationale is ado pted to characterize the boundaries. A preliminary, post-strain surfac e oxidation treatment to 500 degrees C is shown to drastically increas e the proportion of twin boundaries, a majority of which are identifie d as being coherent in nature. In addition, a high level of connectivi ty for these twins is observed, along with greatly increased numbers o f secure triple junctions. It is postulated that the mechanism by whic h grain boundary energy minimization is effected involves grain rotati ons when treatment is conducted under vacuum, but that annealing twin formation is the predominant process when an oxygen-rich environment e xists.