The influence of γ′ morphology and size on stress rupture properties in Ni-base superalloy IN738LC

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This study investigates the influence of γ′ precipitate morphology and size on the creep deformation of the cast superalloy IN738LC under conditions of 760–982 °C and 78–674 MPa. Two microstructures were obtained by different heat treatments. The analysis of the experiments show that, according to the applied stress at 760 °C and 871 °C, the S1A material, with a bimodal distribution with large cuboidal γ′ and small spherical γ′, exhibited the longer creep life compared to those with a monomodal distribution with spherical γ′ precipitates. In the creep test at 760 °C, the S1A material exhibited a stacking fault within the primary cuboidal-shaped γ′ phase. The main deformation mechanism observed was isolated stacking fault (ISF) formation. Conversely, for the S2A material, an extended stacking fault (ESF) was observed, where dislocations continuously traversed several γ′ and γ phases. At 871 °C in low-stress creep tests, the S1A alloy displayed a dislocation network resulting from thermal activation, with localized stacking faults. In the S2A alloy, three-dimensional dislocations generated by climb were primarily observed around spherical γ′ precipitates, accompanied by localized stacking faults. At 982 °C, both S1A and S2A alloys exhibited rafting under lower stress, displaying similar lifetimes. The formation of a regular dislocation network at the interface between γ′ and γ matrix and rafting of γ′ precipitate proved beneficial in improved creep resistance of the alloy.