The δ phase precipitation during processing and the influence on grain boundary character distribution and mechanical properties of superalloy 718

The grain boundary character distribution (GBCD) has been studied in low stacking-fault energy (SFE) alloys, due to the formation of coherent and incoherent annealing twins through a combination of cold deformation and annealing, inducing the formation of low energy Sigma 3n boundaries, as well as the break of the connectivity of the random boundaries. This mechanism is based on grain boundary mobility, leading to grain growth. This poses a problem when a refined microstructure is required. The delta phase can be induced for grain refining by a controlled precipitation during solution annealing. Therefore, the objective of this work is to investigate the effect of the delta phase precipitation on the development of the GBCD and its influence on grain size. Four processing routes were used, combining cold rolling with sub-delta-solvus solution annealing. The results show that a high proportion of Sigma 3n boundaries was achieved, concomitantly with a refined microstructure. The delta phase had an important effect on grain refinement, but not affecting markedly the formation of the Sigma 3n boundaries during processing. Likewise, this phase also had influence on alloy strength, while the formation of the Sigma 3n boundaries had a strong influence on the elongation-to-failure.