Grain boundary engineering of powder processed Ni-base superalloy RR1000: Influence of the deformation parameters

Traditional processing routes for Grain Boundary Engineering (GBE) often require multiple iterations of cold work followed by short annealing cycles. Since each iteration of deformation and annealing imparts a modest increase in the fraction of twin and special grain boundaries, multiple iterations are required to achieve a sufficiently high fraction (~50%) of special grain boundaries that result in the improved properties. This GBE approach therefore is not suitable for the fabrication of large, complex shaped structures and leads to added manufacturing lead time and cost. In this investigation, the effect of deformation parameters on powder processed Ni-base superalloy RR1000 was investigated. By systematically quantifying the effects of deformation temperature (1100–1020 °C), strain rate (0.05–0.001/s) and the post deformation annealing temperature (1115 °C and 1145 °C), a set of optimized processing parameters were identified such that the length fraction of Σ3 boundaries increased from 35% to 52% following a single deformation anneal cycle. Deformation parameters (T>1060 °C and ε ̇ <0.01/s) that resulted in strain accommodation via superplastic flow or grain boundary sliding/rotations did not enhance the formation of Σ3 boundaries upon annealing. Whereas deformation parameters that resulted in a transition in the dominant deformation mechanism from superplastic flow to dislocation plasticity (T<1060 °C and ε ̇ >0.001/s) promoted the formation of annealing twins, which are key for grain boundary engineering and stimulating the formation of Σ3 boundaries during annealing.