Microstructure optimization for improving creep resistance of additively manufactured Ni-based superalloy IN939 through heat treatment

Examination of microstructural evolution and mechanical properties of the laser powder bed fusion (LPBF)-processed Inconel 939 (IN939) after direct aging (DA) and solutionizing (S) followed by aging (SA) may enable the development of strategies for optimizing the microstructure with excellent mechanical properties. Herein, the samples that were solutionized followed by water quenching and two-step aging showed commendable room temperature mechanical properties with 0.2% yield strength of 930 MPa and 21% elongation to failure. DA at 800 °C retained a preferred {100} texture from the as-printed condition with 20–50 nm semi-coherent γ′ (Ni3Al(Ti)) precipitates, while upon DA at 1000 °C, the microstructure comprised coarsened γ′ along with plate-shaped eta phase (η-Ni3Ti). The {100} texture continued to be present after SA also. The creep behavior of the optimized LPBF IN939 was evaluated, and the activation energy, stress exponent and Larsen–Miller parameter were established. The critical role of primary γ′-precipitates and carbides in determining creep response has been elucidated. The relative merits of DA versus SA in terms of structure–property relationships are highlighted. This study, therefore, paves a path for the development of simple and relevant strategies for improving the overall mechanical behavior of additively manufactured Ni-based superalloys that cannot be processed through conventional thermo-mechanical routes.