The effects of strain annealing on grain boundaries and secure triple junctions in nickel 200

Changes in the microstructure of 99.5% pure nickel as a function of strain annealing have been investigated. The application of moderate amounts of compressive strain (6–7%), and subsequent thermal treatment at 750°C under vacuum is shown to increase the density of Σ3 grain boundaries, where the coincident-site lattice rationale is adopted to characterize the boundaries. A preliminary, post-strain surface oxidation treatment to 500°C is shown to drastically increase the proportion of twin boundaries, a majority of which are identified as being coherent in nature. In addition, a high level of connectivity for these twins is observed, along with greatly increased numbers of secure triple junctions. It is postulated that the mechanism by which grain boundary energy minimization is effected involves grain rotations when treatment is conducted under vacuum, but that annealing twin formation is the predominant process when an oxygen-rich environment exists.