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)