Mechanism of primary MC carbide decomposition in Ni-base superalloys
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27/4/04
17:40
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Mechanism of Primary MC Carbide Decomposition in Ni-Base Superalloys G. LVOV, V.I. LEVIT, and M.J. KAUFMAN Three industrial gas turbine blades made of conventionally cast (CC) IN-738 and GTD-111 and directionally solidified GTD-111 Ni-base superalloys were examined after long-term exposures in service environments. All three blades exhibit similar, service-induced microstructural changes (MCs) including coarsening and coalescence, excessive secondary M23C6 precipitation, and primary MC degeneration, regardless of the chemical composition and the grain size. Special attention was paid to the primary MC decomposition. It is shown that the primary MC decomposition occurs by carbon diffusion out of the carbide into the matrix, resulting in the formation of Cr-rich M23C6 carbides near the initial carbide/matrix interface. A transition zone is shown to develop between the original MC core and its perimeter, demonstrating the gradual outward diffusion of carbon and a slight inward increase in nickel concentration. The hexagonal Ni3(TiTa) -phase was also found in the MC transition zone and on the MC- / interface. The primary MC decomposition can be expressed by the reaction MC → M23C6 . Finally, it is shown that the grain-boundary (GB) MC decomposes more rapidly than that in the grain interiors. This is consistent with the more rapid GB diffusion that leads to the acceleration of the MC diffusional decomposition processes.
I.
INTRODUCTION
MODERN industrial gas turbine blades, made from Ni-base superalloys, experience high temperatures and stresses during service and undergo various microstructural changes. These microstructural modifications usually lead to a degradation of mechanical properties, such as tensile strength and creep resistance. Extensive studies have shown that prolonged thermal and stress exposure causes “overaging” of the microstructure ( coarsening and coalescence, formation of secondary M23C6 precipitates on the grain boundaries (GB)[1–5] primary MC degeneration,[5,6,7] and topologically close-packed (TCP) phase (e.g., -phase) formation,[1–6]) all of which are detrimental to both tensile strength and creep resistance. Most of these features, such as coarsening and coherency loss, precipitation of GB M23C6, and both intergranular and intragranular TCP phases, are reversible in the sense that they can be resolutioned by thermal processing. This led to the implementation of rejuvenating procedures, such as appropriate heat treatments and hot isostatic pressing (HIP), to restore even severely overaged microstructures and alloy properties to a practically “as-new” condition.[3,8–10] For this reason, such procedures are currently employed successfully throughout the industry. The most comprehensive studies of microstructural change (MC) decomposition in Ni-base superalloys were conducted over 15 years ago.[1,3,5] Since then, little attention has been paid to this important feature of service-induced microstructural degradation of gas turbine blades.
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