Evolution of Lattice Spacing of Gamma Double Prime Precipitates During Aging of Polycrystalline Ni-Base Superalloys: An
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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
R.Y. ZHANG, J. LI, S. PAUL, and H.B. DONG are with the Department of Engineering, University of Leicester, University Road, Leicester LE1 7RH, UK. Contact e-mail: [email protected] H.L. QIN, Z.N. BI, and J. ZHANG are with the High Temperature Materials Research Division, Central Iron and Steel Research Institute, No. 76 Xueyuannanlu, Haidian, Beijing, 100081, China. T.L. LEE is with the ISIS Neutron Source, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Chilton, Oxfordshire, OX11 0QX, UK. S.Y. ZHANG is with the Centre of Excellence for Advanced Materials, No.1 Libin Road, Songshan Lake, Dongguan, 523808, China. Manuscript submitted August 29, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
distribution of the c¢¢ precipitates are key contributors to the mechanical performance of IN718.[1,4–15] A carefully designed heat treatment is necessary to achieve the desired microstructure for optimized properties. 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] During the precipitation process, the size, distribution, and morphology of the c¢¢ precipitates are largely affected by the lattice misfit. Since the lattice misfit between the c¢¢ and the c phases is much greater along the c-axis ([001] direction) than that along the a-axis ([100] direction), the resultant misfit strain is anisotropic. This leads to a disc-like shaped growth of the c¢¢ precipitates with the c-axis normal to the disc-plane.[4] When the c¢¢ precipitates are coherently embedded in the c matrix, the two phases maintain an orientation relationship of {100} c¢¢ // {100} c and [001] c¢¢ // h100i c, and three possible variants of c¢¢ precipitates appear.[6] When no external loads are applied or no residual stresses exist during aging (stress-free aging), the three variants distribute in approximately equal quantities. In the presence of an applied or residual stress (stress aging), different variants undergo different growth rates, resulting in, for example, only one dominant variant if its growth is substantially preferred. Such a stress-induced variant selection (SIVS) is attributed to the
interaction between the applied or residual stresses and the misfit s
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