Grain boundary engineering (GBE) has been used to improve the properties of various polycrystalline materials by optimization of their grain... Show moreGrain boundary engineering (GBE) has been used to improve the properties of various polycrystalline materials by optimization of their grain boundary network. Traditional processing routes for GBE often require multiple iterations of cold work followed by short annealing cycles where each iteration imparts a modest increase in the fraction of special grain boundaries. Multiple iterations are then required to achieve sufficiently high fractions (>50%) that result in the improved properties. Thus, this GBE approach is not suitable for the fabrication of large, complex-shaped structures and leads to added manufacturing lead time and cost. In this investigation, the Ni-base superalloy RR1000 used as turbine discs in gas turbine engines manufactured by Rolls-Royce, was considered for GBE using alternative processing routes more suitable to the forging of Ni-base superalloy components. A preliminary study of the e↵ects of hot deformation parameters closer to typical industrial processing revealed that the length fraction of ⌃3 boundaries increased from 35% to 52% following a single deformation/anneal cycle. Deformation parameters that resulted in strain accommodation via superplastic flow did not enhance the formation of ⌃3 boundaries upon annealing. Whereas deformation parameters that resulted in a dominant dislocation-based plasticity flow mechanism promoted the formation of annealing twins. Using misorientation maps and by estimating the stored strain energy from deformation, equations for the length fraction and density of ⌃3 boundaries were generated for high-temperature GBE of RR1000. The grain boundary characters obtained via high-temperature deformation, however, are less ideal than those resulting from traditional cold rolling. The underlying mechanisms responsible for the formation of ⌃3n boundaries during high-temperature GBE were further investigated. A larger starting grain size prior to deformation was found to be unfavorable to the formation of twin boundaries from twin-reorientation and annihilation of preexisting twins. While recrystallization was found to populate the microstructure with grains that contained very few twin boundaries, post-deformation texture was found to promote the formation of ⌃3 boundaries and triple junctions when Goss texture was present. A final consideration of larger scale forgings was used to raise an outlook on the current issues and the potential of high-temperature GBE for turbine engines. Ph.D. in Materials Science and Engineering, May 2016 Show less
