Effect of Y2O3 dispersoids on microstructure and creep properties of Hastelloy X processed by laser powder-bed fusion

Abstract

• The effect of Y 2 O 3 dispersoids on microstructure and creep properties of Hastelloy X processed by LPBF is investigated. • Grains are found aligned along build direction, with a strong <001>Ni texture along both build and laser scanning directions. • LPBF Hastelloy X exhibits anisotropic creep response, with elongated grains showing higher creep resistance when aligned along loading direction than when aligned perpendicular to loading direction. • Y 2 O 3 dispersoids improve alloy creep resistance at dislocation-creep regime. • Alloy grain structure remains resistant to recrystallization during high-temperature solutionizing and creeping. Laser powder bed fusion (LPBF) was used to consolidate powders of a Ni-Cr-Fe-Mo alloy (Hastelloy X) blended with 1 wt% Y 2 O 3 nanometric powders. The nearly-dense, crack-free specimens, with and without oxide dispersion strengthening (ODS), exhibit high-aspect ratio grains, aligned in the build direction and with a strong texture: 〈001〉 is aligned along both build direction and laser scanning direction. Creep tests performed at 950 °C, with compressive stresses aligned with the elongated grains (and the build direction), reveal two creep regimes: (i) at stresses below ∼50 MPa, diffusional creep dominates, and the ODS alloy is less creep resistant than the non-ODS alloy, consistent with a somewhat smaller grain size resulting from grain-boundary pinning by the oxide dispersoids; (ii) at stresses above ∼50 MPa, dislocation creep dominates and the ODS alloy is more creep resistant, as expected from oxide dispersoids impeding dislocation motion. When tested perpendicular to the build direction, both alloys have the same creep resistance, which is however much lower than when the stress is aligned with the build direction, reflecting a strong effect of texture and grain shape. After ∼10% compressive creep deformation, grain structure and texture remain largely unchanged as compared to pre-creep structure.