Evaluation of the Stability of Raft Structure in Nickel Base Superalloys Throughout their Lifetime
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0980-II08-08
Evaluation of the Stability of Raft Structure in Nickel Base Superalloys Throughout their Lifetime Katsushi Tanaka, Toru Inoue, Tetsu Ichitsubo, Kyosuke Kishida, and Haruyuki Inui Materials Science and Engineering, Kyoto University, Yoshidahon-machi, Sakyo-ku, Kyoto, 606-8501, Japan ABSTRACT Stability of raft structure in nickel base superalloys has been examined by using elastic energy calculations based on a microelasticity theory. The numerical calculation method for a structurally heterogeneous system is applied. The results indicate that the raft structure is significantly stabilized by introductions of creep deformations till the critical creep deformation at which the lattice misfit between γ and γ' phases is completely compensated by creep dislocations. When the magnitude of creep deformations exceed the critical value, the (001) lamellar interfaces become elastically unstable and a tilted lamellar interface become the most stable one. This instability of the 001 raft structure leads a tilted or wavy lamellar interfaces for reducing the internal strain energy, that is a precursor to collapse the raft structure. INTRODUCTION Nickel-base single-crystal superalloys, which are currently used in turbine blades of aero engines or gas turbines, are key materials in modern high-temperature engineering. Modern superalloys are hardened by cuboidal L12-ordered γ' precipitates which are coherently formed in the fcc (face centered cubic) disordered γ matrix. The precipitates are arranged on a simple cubic array with their axes parallel to the axes of the underlying cubic crystal structure. When superalloys are subjected to a tensile creep test at high temperatures of about 1373 K under a relatively low tensile stress of about 137 MPa along the [001] direction, γ' precipitates are directionally coarsened normal to the stress direction to form the "001 raft structure" that is a lamellar structure consisting of γ and γ' plates. The excellent creep strength of superalloys is owing to the microstructure containing such a raft structure in which the lateral γ / γ' interfaces block the dislocation motion [1-3]. At the later stage of creep deformation, the raft structure unfortunately collapses, accompanied by the acceleration of creep deformation (see Fig. 1). Hence, understanding the mechanisms behind the formation and collapse of the raft structure is important for further extension of lifetime of newly developed superalloys. The formation mechanism has extensively been discussed [4-14] and reported that the 001 raft structure is elastically stabilized by the application of a tensile stress or by the introduction of creep dislocations. On the other hand, few works have been done to discuss the collapse mechanism. According to Fig. 1, the planer raft structure changes into a wavy structure with γ / γ' interfaces tilting from the initial plane of (001) precursory of its collapsing. This implies us that the planer 001 raft structure is destabilized in this stage, which may leads the collapse of the raft structure.
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