Microstructure and properties of Co-Ni-Al-W γ/γ′ superalloy fabricated via laser fusion of elemental powders

Ball-milled elemental Co, Ni, Al, and W powders were used to fabricate a Co-0.20Ni-0.11Al-0.08W (mole fraction) alloy via laser powder bed fusion (L-PBF). In the as-fused state, microstructural features present within the FCC-γ Co-Ni-Al-W matrix – partially-melted W powders, W-deficient regions, and cracks – are investigated with respect to the laser scanning speed. Tungsten particles appear to block or deflect internal cracks, thus improving cracking resistance due to thermal cycling during laser fusion. However, W-deficient regions, which are also enriched with Ni and Al, can act as crack nucleation sites due to the brittleness of the β-NiAl and eutectic microconstituents. A homogenous FCC-γ Co-0.20Ni-0.11Al-0.08 W (mole fraction) solid solution is obtained after a solutionizing treatment at 1200 °C, as W-rich particles fully dissolve, and W-deficient regions homogenize, into the matrix. A γ/γ′-two-phase microstructure forms upon subsequent aging at 900 °C, whose creep resistance is measured at 850 °C: below 300 MPa, deformation is dominated by diffusional creep (consistent with a relatively fine grain size of 50–100 µm) while above 300 MPa, it is controlled by dislocation creep. Using elemental powder blends is a viable method to create microstructurally-sound, creep-resistant Co-based γ/γ′ superalloys via L-PBF.