Alloy design of Ni-based superalloy with high γ′ volume fraction suitable for additive manufacturing and its deformation behavior

Abstract

The alloy design of Ni-based superalloy with high γ′ volume fraction has been studied to be suitable for additive manufacturing (AM) by using thermodynamic calculations. The design approach intended to increase AM processability while maintaining the high-temperature strength of the reference superalloy René 80. In this study, electron beam powder bed fusion also known as a selective electron beam melting (SEBM) process was utilized for alloy fabrication. All René 80 products manufactured by SEBM were cracked despite their small sizes. Using the conventional René 80 composition as a baseline, Ti reduction (5→3 wt%), 2.5 wt% Ta addition, and 1.0 wt% Hf addition successfully prevented crack formation after SEBM fabrication and considerably improved the characteristics of alloy powders, which exhibited more spherical shapes with fewer satellites. The as-SEBM modified René 80 alloy possessed a unidirectional columnar grain microstructure with a primary γ′ area fraction of 40% and size of 382 nm, which were higher than those of the conventional René 80 alloy. Atom probe tomography results indicated that the partitioning of Ti, Al, and Ta atoms into the γ′ phase was more pronounced in AM80 alloy than in René 80 alloy. This facilitated to increase the lattice misfit at the γ/γ′ interface, made it more difficult for dislocations to cut through γ′ particles, which was confirmed by the results of first-principles calculations. The SEBM products, which were subjected to a standard heat treatment demonstrated superior tensile properties at 870 °C as compared with those of the conventional cast René 80 alloy. The primary γ′ phase was sheared by a/3 < 112 > partial dislocation leaving stacking fault behind, and a high density of tangled dislocations was observed as a result of the strong interaction with fine secondary γ′ with sizes smaller than 50 nm.