Transmission electron microscopy study of the microstructural evolution during high-temperature and low-stress (011) $\l
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lcan Göbenlib) and Gunther Eggelerc) Institut für Werkstoffe, Lehrstuhl Werkstoffwissenschaft, Ruhr-Universität Bochum, Bochum 44801, Germany (Received 20 April 2017; accepted 26 July 2017)
The present work describes the shear creep behavior of the superalloy LEK 94 at temperatures between 980 and 1050 °C and shear stresses between 50 and 140 MPa for loading on the macroscopic crystallographic shear system (MCSS) (011)½011. The strain rate versus strain curves show short primary and extended secondary creep regimes. We find an apparent activation energy for creep of Qapp 5 466 kJ/mol and a Norton-law stress exponent of n 5 6. With scanning transmission electron microscopy, we characterize three material states that differ in temperature, applied stress, and accumulated strain/time. Rafting develops perpendicular to the maximum principal stress direction, c channels fill with dislocations, superdislocations cut c9 particles, and dislocation networks form at c/c9 interfaces. Our findings are in agreement with previous results for high-temperature and low-stress [001] and [110] tensile creep testing, and for shear creep testing of the superalloys CMSX-4 and CMSX-6 on the MCSSs (111)½011 and (001)[100]. The parameters that characterize the evolving c/c9 microstructure and the evolving dislocation substructures depend on creep temperature, stress, strain, and time.
I. INTRODUCTION
Ni-base superalloy single crystals (SXs) are cast materials that are utilized to make the first-stage blades for gas turbines in power plants and jet engines.1–5 Their microstructure consists of small c9 cuboids (L12 ordered phase, volume fraction 70%, edge length 0.5 lm) separated by thin c channels (fcc crystal lattice, volume fraction 30%, width: 50 nm). SXs operate in the creep range where they must withstand mechanical loads at elevated temperatures. Creep, the time dependent accumulation of irreversible plastic strain, shows a strong stress and temperature dependence.6–12 In the present study, we investigate specimens that are subjected to shear creep loading. The stress and temperature dependence of the minimum or secondary creep rate in a shear experiment can be described by the equation Contributing Editor: Mathias Göken a) Address all correspondence to this author. e-mail: [email protected] b) This author contributed equally to this work. c) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/editor-manuscripts/. DOI: 10.1557/jmr.2017.336
Qapp c_ s ¼ cn s exp ; RT n
ð1Þ
where c_ s is the secondary creep rate, cn is a constant, s is the shear stress, n is the stress exponent, Qapp is the apparent activation energy for creep, and R and T have their usual meanings. Most published creep data represent results from uniaxial creep experiments (r1 . 0, r2 5 r3 5 0) [e.g., Refs. 13–18]. Shear creep testing has received limited attention in the literature, even though it also re
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