Abstract
Polycrystalline Ni-base superalloy Inconel 718 derives the strengthening mainly from Ni3Nb γ″ phase. To investigate the evolution of lattice spacing of γ″ precipitates, in situ neutron diffraction experiments were performed during aging heat treatment of two Inconel 718 bar samples at 780 °C for 8 hours. One sample was aged with a negligible applied stress (5 MPa) and the other with an applied tensile stress of 300 MPa. The diffraction data demonstrate three stages of lattice spacing evolution due to compositional and morphological changes during aging. (1) Stage I, the γ lattice spacing decreases isotropically in the first hour of aging as the composition changes. (2) Stage II, the γ lattice spacing decreases and the γ″ lattice spacing increases between 1 and 5 hours of aging, at this stage compositional change on the lattice evolution weakens with aging time and the morphological change becomes comparable. (3) Stage III, as aging proceeds, compositional change is negligible, while morphological change is dominant; the γ lattice spacing decreases in the longitudinal direction but increases in the transverse direction; in contrast, the converse occurs in γ″ lattice spacing evolution.
Highlights
INCONEL 718 (IN718) Ni-base superalloy, used widely in aero-engines and power plants for decades, derives its excellent high-temperature properties from the fine dispersion of Ni3Nb body-centered tetragonal (BCT) c¢¢ precipitates and the Ni3Al/Ti simple cubic c¢ precipitates.[1]
Large lattice misfit arises between the c¢¢ and c phases accounting for the coherent strain strengthening in IN718.[4]. Extensive studies have shown that the volume fraction, size, morphology, and
The c¢¢ variant distributions in differently oriented grains after tensile-stress aging agree with our previous observation on the same material after aging heat treatment with a 300 MPa tensile stress applied for 5 hours.[17]
Summary
INCONEL 718 (IN718) Ni-base superalloy, used widely in aero-engines and power plants for decades, derives its excellent high-temperature properties from the fine dispersion of Ni3Nb body-centered tetragonal (BCT) c¢¢ precipitates and the Ni3Al/Ti simple cubic c¢ precipitates.[1] The c¢¢ phase has a volume fraction three times greater than the c¢ phase and provides the major strengthening in this alloy.[2] In addition, the c¢¢ phase has larger lattice parameters compared to the solid-solution FCC c matrix phase.[3] Large lattice misfit arises between the c¢¢ and c phases accounting for the coherent strain strengthening in IN718.[4] Extensive studies have shown that the volume fraction, size, morphology, and Manuscript submitted August 29, 2019. A typical heat treatment for IN718 consists of a solid-solution heat treatment to dissolve the equilibrium Ni3Nb d phase followed by quenching, and a subsequent aging heat treatment for the precipitation of the c¢¢ phase.[16]
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