Microstructure Evolution and Analysis of A [011] Orientation, Single-Crystal, Nickel-Based Superalloy During Tensile Cre
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SINGLE-crystal, nickel-based superalloys have superior mechanical properties, such as excellent creep resistance and high-temperature strength, and they are widely used to make the blades in industrial gas turbines, aero engines, etc. The creep behavior of them is closely related to their microstructure.[1–3] One of the most striking characteristics during high-temperature tensile/compressive creep of single-crystal, nickel-based superalloys is the directional coarsening of the cuboidal c¢-Ni3Al precipitates to form preferentially orientated rafts. The directional coarsening of the c¢ phase under the action of an applied stress was first studied in detail by Tien and Copley,[4] and this phenomenon has subsequently been observed in many experimental and commercial nickel-based, single-crystal superalloys.[5–16] According to the results of several experimental observations, two types of rafting behavior of the c¢ phase in the h001i orientation, single-crystal, nickel-based superalloys have been identified: (a) N-type rafts. The formation of them (found for d < 0) in a direction perpendicular to the applied load has been reported[11,14,15] as beneficial for tensile SUGUI TIAN and CHENXI LI, Professors, SHU ZHANG and YONG SU, PhD Students, XINGFU YU, Teacher, and LILI YU, Mater Student, are with the School of Materials Science and Engineering, Economic & Technological Development Zone, Shenyang University of Technology, Shenyang 110870, P.R. China. Contact e-mail: [email protected] HUICHEN YU, Professor, is with the Beijing Institute of Aeronautical Materials, Wenquan Town, Haidian Dist, Beijing 100095, P.R. China. Manuscript submitted May 25, 2011. Article published online April 20, 2012 3880—VOLUME 43A, OCTOBER 2012
creep resistance of these alloys at low stresses when the precipitates are unshearable and the creep rate is controlled by the climb of dislocations around the rafts. (b) P-type rafts. In sharp contrast, the formation of P-type rafts (found for d > 0) growing in the direction parallel to the tensile load axis has been found to impair the creep properties.[11,13] Li and Wahi[16] showed that the magnitude and sign of the lattice misfit (d) play a major role in determining the morphology of the c¢ phase at elevated temperatures under loads. And it is considered that the rafting rate of the c¢ phase is proportional to the applied load, the lattice mismatch of c/c¢ phases, and the change of the strain energy density.[17,18] The proposed rafting mechanisms are roughly separated into the elastic and elastic–plastic regimes. In the framework of elasticity, some publications state that the combination of the elastic strain and lattice misfits determines the choice of the N-type or P-type rafted structures,[15] and the rafting mechanism is discussed by the elastic strain field numerically in terms of the strain energy.[19–21] They explained the rafting mechanism of the N-type structure in terms of the Al diffusion, which is caused by the composition change in the phase equilibrium under the external stress. But withi
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