Manufacturability and performance response of developed Ni-based alloys for reactor environments

Abstract

Under the Advanced Materials and Manufacturing Technologies program, two Ni-based alloys fabricated by laser powder bed fusion (LPBF) at ORNL and laser powder directed energy deposition (LP-DED) at INL have been evaluated: γ′-strengthened Haynes 282 and solution-strengthened Inconel 625. Printing defects were observed in the LPBF 282 alloy fabricated using a Renishaw AM250 machine, likely due to particle spattering during printing. For both the LPBF and LP-DED 282 alloys, annealing at 1,180°C for 1 h resulted in partial recrystallization and a bimodal grain distribution, while subsequent aging for 4 h at 800°C led to the formation of a high density of nano size γ′-strengthening precipitates. Creep testing performed at 750°C revealed lower creep life and ductility for the LPBF and LP-DED 282 compared with wrought 282. X-ray computed tomography combined with optical and scanning electron microscopy microstructural characterization revealed crack formation in the LPBF 282 alloy during creep testing, initiated either from printing defects or from creep cavitation at grain boundaries. Creep cavitation and cracking at grain boundaries were also observed for the LP-DED 282 alloy, and optimization of the printing strategies and post heat treatments will be needed to fabricate high performance 282 components for high temperature nuclear applications. Printing LPBF 625 on an EOS M290 machine using the recommended EOS parameters resulted in a very low defect density. The alloy exhibited superior tensile strength at room temperature and at 725°C as compared with wrought 625, with acceptable ductility. Solution annealing was carried out at 1,150°C for 1 h and led to full recrystallization of the LPBF 625 alloy. While the as printed alloy exhibited higher tensile strength but lower ductility perpendicular to the build direction, isotropic tensile properties were observed for the annealed LPBF 625, with lower strength but greater ductility compared with the as printed alloy. Superior creep strength was observed at 725°C for the as-printed LPBF 625 along the build direction when compared with wrought 625. The specimen tested at 725°C, 200MPa failed after 1191h and 12% strain to failure, while the specimen machined perpendicular to the build direction and tested under similar conditions ruptured after 627h with a strain to rupture of 6%. Small, needle-like δ-phase precipitates were observed after creep testing for 100 h at 725°C and 200 MPa. The rapid formation of the  phase in the LPBF 625 is directly related to the initial Nb and Mo segregation in the cell walls, and its effect on the high-temperature performance of both as printed and annealed LPBF 625 will be evaluated. The as printed LP-DED 625 exhibits a highly directional columnar grain structure with strong crystallographic texture. As for the LPBF 625, a cellular substructure with segregation of Mo and Nb in the cell walls was observed. Full recrystallization and the suppression of interdendritic segregation was achieved for the LP-DED 625 after annealing for 30 min at 1200°C. Future work will evaluate the creep, fatigue and creep fatigue performance of both LPBF and LP-DED 625.