Strengthening mechanisms of carbon in modified nickel-based superalloy Nimonic 80A

Nickel-based superalloy Nimonic 80A with various carbon contents has been developed. Various strengthening mechanisms of carbon were analyzed by optical microscope, X-ray diffraction, scanning electron microscope equipped with energy dispersive spectroscopy and transmission electron microscope. Detailed microstructural analysis revealed that with the increase of carbon content from 0.01% to 0.10%, both the lattice parameters of gamma' and gamma phases decreased, and the misfit delta between the coherent gamma' and gamma phases increased slightly from 0.289% to 0.317% after full heat treatment at 1070 degrees C/8 h, A.C. + 700 degrees C/16 h, A.C. The diameter of spherical gamma' phase decreased from 22.24 nm of 0.01%C alloy to less than 15.76 nm of 0.06%C and 0.10%C alloys. Cr(23)C(6) carbide precipitated at the grain boundary in the alloys with carbon content higher than 0.06%, and when carbon content increased to 0.10%, Cr(23)C(6) carbide had an orientation relationship with the gamma matrix: [00 (1) over bar] Cr(23)C(6) parallel to[00 (1) over bar]gamma matrix and(100)Cr(23)C(6)parallel to (1 0 0) gamma matrix, which can enforce the grain boundary strength. The growth of room temperature tensile strength with the increase of carbon content was primarily due to the coherent strain strengthening and the precipitate strengthening of Cr(23)C(6) carbide. After stress-rupture test at 750 degrees C/310 MPa, the misfit 8 increased from 0.182% to 0.237% with the increase of carbon content, but the values were lower than those after full heat treatment. The precipitate of blocky Cr(23)C(6) in 0.10%C alloy tended to become spherical and dislocation lines were found in the gamma matrix channels between gamma' phases, the combination of the dislocation strengthening and precipitate strengthening of Cr(23)C(6) carbide at grain boundary was beneficial to the stress-rupture properties. (C) 2011 Elsevier B.V. All rights reserved.