Correlative Microscopy—Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys

JOM, Mar 2018

Nanoscale solute segregation to or near lattice defects is a coupled diffusion and trapping phenomenon that occurs in superalloys at high temperatures during service. Understanding the mechanisms underpinning this crucial process will open pathways to tuning the alloy composition for improving the high-temperature performance and lifetime. Here, we introduce an approach combining atom probe tomography with high-end scanning electron microscopy techniques, in transmission and backscattering modes, to enable direct investigation of solute segregation to defects generated during high-temperature deformation such as dislocations in a heat-treated Ni-based superalloy and planar faults in a CoNi-based superalloy. Three protocols were elaborated to capture the complete structural and compositional nature of the targeted defect in the alloy.

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Correlative Microscopy—Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys

Correlative Microscopy-Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys 0 1.-Max-Planck-Institut fu ̈ r Eisenforschung GmbH , 40237 Du ̈ sseldorf, Germany. 2. -Institute of Microand Nanostructure Research and Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander-Universita ̈t Erlangen-N u ̈rnberg , Cauerstraße 6, 91058 Erlangen , Germany. 3.-Department of Materials Science, Institute for General Materials Properties, Friedrich- Alexander University Erlangen-Nu ̈ rnberg , Erlangen, Germany. 4.- Nanoscale solute segregation to or near lattice defects is a coupled diffusion and trapping phenomenon that occurs in superalloys at high temperatures during service. Understanding the mechanisms underpinning this crucial process will open pathways to tuning the alloy composition for improving the high-temperature performance and lifetime. Here, we introduce an approach combining atom probe tomography with high-end scanning electron microscopy techniques, in transmission and backscattering modes, to enable direct investigation of solute segregation to defects generated during high-temperature deformation such as dislocations in a heat-treated Ni-based superalloy and planar faults in a CoNi-based superalloy. Three protocols were elaborated to capture the complete structural and compositional nature of the targeted defect in the alloy. INTRODUCTION Correlative microscopy, i.e., the combined utilization of a range of microscopy techniques on a single specimen, is increasingly deployed to understand fundamental aspects in material science. The combination of transmission electron microscopy (TEM) and atom probe tomography (APT) enables researchers to relate the atomic structure and composition of nanoscale features of interest and has been gaining influence over the past decades. Typically, correlative TEM/APT analyses have been carried out from distinct specimens from the same alloy,1–7 which fails in cases where the microstructure is inhomogeneous and/or when targeting rare microstructural features for both TEM and APT investigations. As recently reviewed by Herbig,8 full electron microscopy characterization of specific features can be carried out on needle-shaped specimens prior to APT analysis by utilizing specially designed holders. This approach has, for example, led to a better understanding of diffusional mechanisms resulting in local solute segregation at crystalline imperfections9–14 that critically impact the macroscopic material behavior. APT had previously revealed details of the composition of structural imperfections,15 the presence of which were confirmed by field ion microscopy16 or TEM.11,17,18 Here, we introduce methodologies to aid target features of interest for correlative TEM/APT investigation by exploiting advanced scanning electron microscopy (SEM) techniques in deformed Ni- and CoNi-based superalloys, one of the most important classes of engineering materials for temperatures above 1000 C. Their high temperature stability is attributed to the uniform distribution of L12-ordered c¢ precipitates coherently embedded in an fcc solid solution c matrix. Three protocols for site-specific correlative investigation are presented. MATERIALS AND METHODS Materials The polycrystalline Ni-based superalloy IN792, commonly used in land-based gas turbines, was studied. Its chemical composition is Ni–13.9Cr– 8.8Co–1.1Mo–1.3W–7.6Al–4.9Ti–1.3Ta–0.4C–0.1B– 0.012Zr (at.%). It was provided as 20-mm-diameter bars by Howmet. A hot isostatic press (HIP) process at 1195 C and 150 MPa for 2 h was followed by a solution heat treatment stage at 1121 C for 2 h and a final stage of aging at 850 C for 24 h. After heat treatment, specimens were ground and polished with abrasive media to a 1-lm finish and were isothermally exposed at 750 C for 50 h. A single-crystal CoNi-based superalloy with the composition Co–32Ni–8Al–5W–6Cr–2.5Ti–1.5Ta– 0.1Hf–0.4Si (at.%) was prepared by the Bridgman process. The heat treatment steps were 1280 C/ 8 h + 1050 C/5 h + 900 C/16 h to obtain a uniform c/c¢ microstructure. Tensile specimens were cut and crept at 850 C with an applied stress of 400 MPa along the [001]-direction up to 4.6% (380 h) and 0.3% (40 h) plastic strain. Cross sections close to the {100} and {110} habit planes were cut and mechanically polished for further microstructural characterization. Controlled Electron Channeling Contrast Imaging (cECCI) A Zeiss Merlin scanning electron microscope (Carl Zeiss SMT AG, Germany) with a Gemini-type field emission gun electron column and a Bruker e-Flash HR EBSD detector (Bruker Corp., USA) was used. ECCI (electron channeling contrast imaging) under controlled diffraction conditions was employed to rotate and tilt the crystal into two-beam diffraction conditions using the TOCA (Tools for Orientation Determination and Crystallographic Analysis computer program; for details, see (...truncated)


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S. K. Makineni, M. Lenz, P. Kontis, Z. Li, A. Kumar, P. J. Felfer, S. Neumeier, M. Herbig, E. Spiecker, D. Raabe, B. Gault. Correlative Microscopy—Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys, JOM, 2018, pp. 1-8, DOI: 10.1007/s11837-018-2802-7