Printability and Mechanical Properties of LPBF Ni-Based 625 and 244 Alloys

Abstract

The advanced materials and manufacturing technologies (AMMT) program is investigating two Ni-based alloys of interest to the nuclear industry, alloy 625, due to its acceptable high temperature properties and availability in various forms, and Haynes 244, a promising low Cr high strength alloys for molten salt applications. These two alloys were fabricated by laser powder bed fusion (LPBF) and characterized for microstructure and mechanical performance. Low-defect LPBF 625 was creep tested at 650°C and 725°C and exhibited properties approaching those of wrought 625, though creep ductility perpendicular to the build direction was reduced due to the as-printed elongated grain structure. A recrystallization heat treatment (1150°C, 1 h) partially mitigated this anisotropy, although intergranular cracking persisted at grain boundaries oriented at low angles to the applied stress. Interrupted creep testing performed in vacuum at 725°C/200 MPa on a mirror finish specimen showed that cavitation initiated preferentially between grain boundary precipitates and at precipitate-free boundaries. X-ray computed tomography imaging was also used to quantify cavity growth rates during long-term creep testing of standard LPBF 625 specimens. LPBF 244, fabricated crack-free after printing parameters optimization, demonstrated higher strength but lower ductility at 750°C relative to LPBF 625, with similar build-direction-dependent anisotropy in the as-printed state. The same 1h 1150°C recrystallization treatment improved room-temperature tensile ductility in both orientations, but ductility at 750°C after annealing and aging remained limited. These results establish a baseline understanding of LPBF Ni-based alloy creep and deformation behavior and identify heat treatment optimization and grain boundary precipitate control as key directions for improving high-temperature ductility ahead of nuclear deployment.