- Email: [email protected]
FIB–SEM tomography of 4th generation PWA 1497 superalloy
M A TE RI A L S C HA RACT ER I ZA TI O N 87 ( 20 1 4 ) 1 4 3– 1 4 8
Available online at www.sciencedirect.com
ScienceDirect www.elsevier.com/locate/matchar
FIB–SEM tomography of 4th generation PWA 1497 superalloy Maciej Ziętara, Adam Kruk⁎, Adam Gruszczyński, Aleksandra Czyrska-Filemonowicz International Centre of Electron Microscopy for Materials Science and Faculty of Metals Engineering and Computer Industrial Science, AGH University of Science and Technology (AGH-UST), Al. A. Mickiewicza 30, PL-30 059 Kraków, Poland
AR TIC LE D ATA
ABSTR ACT
Article history:
The effect of creep deformation on the microstructure of the PWA 1497 single crystal
Received 26 August 2013
Ni-base superalloy developed for turbine blade applications was investigated. The aim
Received in revised form 15
of the present study was to characterize quantitatively a superalloy microstructure
November 2013
and subsequent development of rafted γ′ precipitates in the PWA 1497 during creep
Accepted 18 November 2013
deformation at 982 °C and 248 MPa up to rupture. The PWA1497 microstructure was characterized by scanning electron microscopy and FIB–SEM electron tomography. The 3D
Keywords:
reconstruction of the PWA1497 microstructure is presented and discussed.
FIB–SEM tomography
© 2013 Elsevier Inc. All rights reserved.
Ni-base superalloys Rafting
1.
Introduction
Single crystal (SC) nickel-base superalloys have been developed over the past 40 years especially for modern gas turbine applications. This group of alloys has superior mechanical properties such as creep resistance and high temperature strength. They are hardened by a high volume fraction of the ordered γ′ phase, which is coherently precipitated in the γ matrix. These materials are mainly applied as turbine blades and vanes in aero-engines and industrial gas turbines. Turbine blades operate at high temperature under a centrifugal force causing creep deformation of the material, which leads to the so-called rafting (i.e. directional coarsening). A characteristic microstructural property of SC superalloys is the ability of cubed γ′ phase particles to transform, under the influence of stress and temperature, into the plates (rafts). The rafts develop in the early stages of creep at high
temperature (about 1000 °C) and low stress (about 100 MPa). Rafting appears to be an essential factor determining creep strength of nickel-base SC superalloys at high temperature influencing their applications [1–3]. The focused ion beam–scanning electron microscope (FIB–SEM) tomography has been recently widely applied in materials science for studying and modifying material systems at the micro- and nanometer levels. FIB tomography is based on a serial slicing technique employing a FIB–SEM dual beam workstation. Dual-beam FIB–SEM enables the acquisition of serial images with small and reproducible spacing between the single imaging planes because no mechanical stage tilting is necessary between the FIB milling and the electron beam SEM imaging steps. Serial FIB cross-sections are performed through the volume to be investigated and each exposed surface is imaged with an electron microscope. 3D mapping of particles with high Z-resolution by serial FIB slicing and SEM imaging
⁎ Corresponding author. E-mail addresses: [email protected] (M. Ziętara), [email protected] (A. Kruk), [email protected] (A. Gruszczyński), [email protected] (A. Czyrska-Filemonowicz). 1044-5803/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2013.11.003
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was performed. Ga-ion beam was used to perform a precise in-situ milling. Repeated removal of layers as thin as several nm for some hundred times permits to investigate total volume of some μm3 with a voxel size of 10 nm × 10 nm × 10 nm. The SEM images at an accelerating voltage of 1.7 kV were taken utilizing a BSE (back scattered electron) detector. Acquired two-dimensional data are processing with the help of computer algorithms and three-dimensional systems can easily be reconstructed and provide both qualitative and quantitative information [4–6]. To visualize the 3D reconstruction, the Amira 5.4.1 and ImageJA 1.45b software were used. In this work, a quantitative microstructural characterization of the single crystal Ni-base superalloy PWA 1497 after heat treatment and after creep deformation is presented. The 3D reconstructions of a superalloy microstructure are generated to measure the volume fraction of the γ′ phase and the mean thickness of the γ phase in the direction parallel to the tensile axis.
2.
Experimental
2.1.
Description of Materials
A fourth generation Ni-base PWA 1497 superalloy was solidified into single crystal bars and provided by Pratt & Whitney, US. The alloy chemical composition was as follows: Ni–2Cr–5.55Al– 8.25Ta–6W–2Mo–5.95Re–3Ru–16.5Co–0.03C–0.15Hf (wt.%) and 40 ppm B. The solid bars were subjected to a standard heat treatment, involving solution annealing followed by aging.
2.2.
The γ/γ′ Misfit Measurements by X-ray Diffractometry
The XRD measurements of γ/γ′ misfit were performed on selected samples including two samples from the as-received material and a sample after creep rupture, with the highest creep exposure time and strain. The XRD investigation was carried out using a D500 SIEMENS apparatus with Cu X-ray tube and monochromatic radiation obtained by graphite crystal monochromator. The lattice spacing aγ′ and lattice spacing aγ were determined from position of the Kα1 peaks and the γ/γ′lattice misfit was calculated as δ = 2(aγ′ − aγ) / (aγ′ + aγ). The
resulting diffraction patterns were analyzed with MAKRO 50 non-commercial computer software. Fitting was made by the least squares method applied to the Lorentz function. The measured lattice parameters of γ and γ′ phases (aγ and aγ′) and γ/γ′ misfit were as follows: aγ = 0.36086 nm, aγ′ = 0.36003 nm and δ = − 0.26%. The lattice parameters of the γ and γ′ phases (aγ and aγ′, respectively) are higher for the PWA 1497 than for PWA 1984 superalloy where misfit was − 0.12% [7]. The FIB–SEM tomography investigation was performed on two samples of PWA 1497 superalloy; baseline and after creep rupture test (crept at 982 °C/248 MPa, creep strain 25%, duration 870 h) materials.
2.3.
FIB Sectioning
Tomographic data sets were obtained using FIB NEON CrossBeam 40EsB of Zeiss, by serial sectioning. The Pt protective layer was deposited on the surface of both samples (Fig. 1a), in order to prevent the occurrence of the curtaining effect, i.e., the irregular removal of material [8]. The material was removed with a Ga-ion beam (optimum beam current and voltage: 200 pA and 30 kV). Fig. 1b shows the PWA 1497 surface prepared (by milling) for step sectioning and image acquisition. The thickness of each slice was chosen as 10 nm. The parameters of Ga-ion beam milling (i.e. 30 kV, 200 pA) have been optimized in order to obtain time about 1.0 min to remove one layer with a thickness of 10 nm and SEM imaging. Decreasing the Ga-ion beam current can also reduce the thickness of the removed layer up to 5 nm, but substantially increases the acquisition time of one image. Due to the electrical and mechanical apparatus drift increasing acquisition time is probably not advisable in the case of a large number of registration images. The chemical composition of the samples has a significant impact on the single image recording time as well as on the resulting minimum thickness of the removed material layer. In general, the thickness of the layer removed is adjusted according to the pixel size on the 2D image at a given magnification. The SEM micrographs of un-etched γ/γ′ microstructure were recorded using an energy selective backscattered electron (EsB) detector, which provides the best phase contrast (Z-contrast) and uniform gray level for phase visualization.
Fig. 1 – Preparation of the area for repeated removal of layers during acquisition of tomographic series of images. a) Platinum protective layer deposited on the sample surface, b) imaging surface (perpendicular to the sample surface) prepared by ion milling for step sectioning and image acquisition of the PWA 1497 baseline sample.
M A TE RI A L S C HA RACT ER I ZA TI O N 87 ( 20 1 4 ) 1 4 3– 1 4 8
145
Fig. 2 – PWA 1497 superalloy; a) baseline material — regular cuboidal morphology of γ′ precipitates separated by γ channels, b) creep ruptured material (at 982 °C/248 MPa) — final state of coarsening of rafted microstructure, SEM imaging (microstructures after γ dissolving etching).
The working distance was 5 mm and voltage was 1.7 kV. Repeated removal of layers as thin as a 10 nm allowed to explore a total volume of 5.6 × 4.5 × 1.7 μm for baseline and 8.6 × 6.0 × 4.6 μm for creep ruptured materials with the voxel size 10 × 10 × 10 nm (size of raw data stacks were respectively equal: 142 MB for baseline material and 221 MB for creep ruptured material). The 3D visualization of reconstructed space for FIB–SEM tomography was performed using ImageJ 1.44p and Avizo Fire 6.3 software.
2.4.
3D Reconstruction
More than 200 sections were recorded for a baseline sample and around 450 sections for creep ruptured sample. The shift of the electron beam in lateral direction due to slice cutting during recording of the tomogram was calculated and subsequently corrected with the ImageJ 1.44p software. From the series of SEM micrographs, the 3D reconstruction of the investigated alloy volume was generated by Avizo Fire 6.3 software. This program uses the “labeling by reconstruction” technique, based on the “neighborhood-algorithm”. This algorithm comprises neighboring voxels with the same gray scale to local objects. Non-continuous ranges were identified and marked by colors.
3.
Results and Discussion
SEM analysis of the baseline PWA 1497 superalloy sample, revealed regular cuboidal morphology of γ′ particles separated by γ channels, Fig. 2a. During high temperature creep deformation, the initially cubic-shaped microstructure transforms under the influence of stress and temperature into plates (rafts). The rafts develop in the early stages of creep. Simultaneously with the rafting process, a thickening of the γ matrix channels parallel to the rafts occurs. Further evolution leads to an effect called “topological inversion”, the γ′ phase coalesces, coarsens and finally surrounds the γ phase, thereby becoming topologically the matrix. This process was explained in details by Epishin et al. [9]. The γ/γ′ microstructure of ruptured sample deformed at 982 °C/248 MPa for duration of 870 h (creep strain 25%) shows the final state of the coarsening of rafted microstructure, where γ′ phase becomes topologically the matrix and γ phase started to look like precipitates, Fig. 2b. Three dimensional reconstruction of γ/γ′ microstructure of PWA 1497 superalloy for the baseline sample showed similar image like obtained for (2D) SEM investigation, namely regular cuboidal morphology of γ′ particles separated by γ channels
Fig. 3 – PWA 1497 baseline material; 3D reconstruction showing: a) regular cuboidal morphology of γ′ particles, b) γ channels. Reconstructed volume: 5.6 × 4.5 × 1.7 μm.
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Fig. 4 – PWA 1497 baseline material; results of the 3D tomographic reconstruction of spatial distribution of selected (for better visualization) γ′ precipitates in the γ matrix.
(Fig. 3), which is typical for modern SC superalloys. Fig. 4 shows the results of tomographic reconstructed and visualized in 3D the spatial distribution of selected γ′ precipitates in the γ matrix. The microstructure investigation of PWA 1497 alloys using classical testing methods in TEM (BF, DF) and SEM (BSE, SE) does not provide information about the spatial shape of γ′ precipitates and their mutual arrangement in the investigated volume of the sample. Moreover, it does not allow determining whether the separated γ′ phase particles are connected to each other in space. Joining the γ′ phase particles by the vanishing of the γ phase channels indicates the microstructure changes caused by high-temperature heat treatment at the absence of external load for single-crystal nickel superalloys. Typically, the separation is treated as a separating of cuboidal particles of γ′ phases by γ phase channels. Employing the FIB–SEM tomography technique revealed a more complex morphology in terms of the shape of the γ′ particles as well as the spatial distribution. The γ′ particles take the shape of cuboids, or similar to the shape to the letters L and/or Z, occasionally observed cubic particles of γ′ phase were separated by γ phase channels. Due to its complex shape, these particles are interconnected in a reconstructed
volume. Application of numerical procedure Find Connected Region (ImageJ 1.44p) has revealed that in the tomographic reconstructed volume particles of γ phase are interconnected in a space in more than 80% (marked in red, Fig. 5). The other, not connected particles are marked with different colors. Fig. 6 shows the cross-sections of reconstructed volume for: two planes oriented perpendicular to the directions of the <100 > type and one plane arbitrarily oriented in the reconstructed space. The shape and dimensions of the γ and γ′ phases vary significantly on the plane on which the analysis is performed. For baseline material the quantification of the width of γ-phase channel was carried out in planes oriented perpendicular to the < 100 > direction. However, 3D reconstruction of creep ruptured sample shows more complex structure than for 2D SEM micrographs. The evolution of microstructure during creep deformation leads to a final state of the coarsening of rafted microstructure, where γ′ phase becomes topologically the matrix and γ phase started to look like precipitates (Fig. 7). In this case, 3D reconstruction allows for understanding isotropic changes of the microstructure. For 2D SEM analysis it would be more difficult and require analysis of polished section in three different directions.
Fig. 5 – PWA 1497 baseline material; 3D reconstruction showing the spatial distribution of interconnected γ-phase particles in tomographic reconstructed volume. The same color means that the particles are interconnected.
M A TE RI A L S C HA RACT ER I ZA TI O N 87 ( 20 1 4 ) 1 4 3– 1 4 8
147
Fig. 6 – Cross-section of reconstructed volume for two planes oriented perpendicular to the directions of the < 100> type and one plane arbitrarily oriented in the reconstructed space.
The relative area fraction (AA) of γ′ phase for a baseline sample was defined as the average of the successive values determined on the basis of the 15 successive images and is equal to 71.2 ± 0.3%. Based on 3D reconstruction of a baseline sample, measured volume fraction (Vv) of γ′ phase is equal to 68.1%. The relative area fraction for crept sample was measured as 70.7 ± 5.7% and volume fraction as 71.0% (Table 1). The results are in good agreement with our previous TEM investigation of a baseline sample [6] and clearly prove that in the case of the new generation SC superalloys and for sufficiently large numbers of measurements, measurement of area fraction is adequate to volume fraction (AA ≅ Vv). The γ′ volume fraction of superalloys measured by TEM technique was proved to be independent of creep duration [10], while our results of γ′ volume fraction of crept sample were slightly higher than that for baseline sample. The difference of γ′ volume fraction for PWA 1497 superalloy for a baseline and crept sample was most probably caused by a statistic error (i.e. area chosen for investigation) and/or image processing. Microstructure parameters (Vv, AA, thickness of γ phase channels) of the PWA 1497 baseline and creep ruptured samples are shown in Table 1. The mean thickness of γ phase (γ channel width) increased during creep process from 65.5 nm, tripling its value for ruptured specimen (193 nm). Standard deviation for creep deformed sample was higher for all measurements due to the fact that during rafting process the superalloy
microstructure looses their regularity and becomes more wavy and convoluted.
4.
Summary
Quantitative characterization of the 4th generation Ni-base PWA 1497 superalloy microstructure (baseline and creep ruptured samples) was performed by the FIB–SEM tomography technique. The quantitative results of the γ′ area fraction (AA) and volume fraction (Vv) reveal that for baseline sample AA and Vv are not equal and reach respectively around 68 and 71%. For the ruptured specimen, AA and Vv are equal and reach around 71%. Those results clearly prove that in case of modern single crystal superalloys, measurements of area fraction are adequate for volume fraction (AA ≅ Vv). The measurements of a mean γ phase thickness reveal that during creep process γ channel width increases from 65.5 nm, tripling its value to 193 nm for ruptured specimen. The FIB–SEM tomography is a very useful technique for quantitative studies of material microstructure. This technique allows to eliminate errors resulting from thickening of γ channels or γ′ precipitates caused by etching (depended on chemical reagent). This problem usually occurs during standard SEM microstructure investigation of polished and etched cross-sections of analyzed materials. The FIB–SEM tomography is a powerful tool in nanoscale materials science; it
Fig. 7 – PWA 1497 creep ruptured sample; 3D reconstruction showing the final stage of coarsening of rafted microstructure. The γ′ phase becomes topologically the matrix, γ phase looks like the precipitates. Reconstructed volume: 8.6 × 6.0 × 4.6 μm.
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Table 1 – Microstructure parameters of PWA1497 baseline and creep ruptured samples; (Vv, AA, thickness of γ phase channels). PWA 1497 superalloy
Vv of γ′ [%]
AA of γ′ [%]
Thickness of γ channels [nm]
Baseline sample Creep ruptured sample
68.1 71.0
71.2 ± 0.3 70.7 ± 5.7
65.5 ± 20 193 ± 50
creates new possibility for stereological description of material micro- and nanostructures and simplifies to understand complex microstructural changes caused by their deformation.
Acknowledgments This work was supported by the Polish Ministry of Science and Higher Education (project nr 11.11.110.148), and the AGH-UST Statutory Research. We would like to also acknowledge Pratt & Whitney, East Hartford, CT USA for providing the material used in this investigation.
REFERENCES [1] Czyrska-Filemonowicz A, Dubiel B, Ziętara Z, Cetel A. Development of single crystal Ni-based superalloys for advanced aircraft turbine blades. Inżynieria Materiałowa 2007;3–4:128–33.
[2] Caron P, Khan T. Improvement of creep strength in a nickel-base single-crystal superalloy by heat treatment. Mater Sci Eng 1983;61:73–184. [3] Ziętara M, Cetel A, Czyrska-Filemonowicz A. Microstructure stability of 4th generation single crystal superalloy, PWA 1497, during high temperature creep deformation. Mater Trans 2011;52:336–9. [4] Kubis AJ, Shiflet GJ, Dunn DN, Hull R. Focused ion-beam tomography. Metall Mater Trans A 2004;35:935–1943. [5] Uchic MD, Groeber MA, Dimiduka DM, Simmons JP. 3D microstructural characterization of nickel superalloys via serial-sectioning using a dual beam FIB–SEM. Scr Mater 2006;55:23–8. [6] Kruk A, Dubiel B, Czyrska-Filemonowicz A. The 3D imaging and metrology of CMSX-4 superalloy microstructure using FIB–SEM tomography method. Solid State Phenom 2013;197:89–94. [7] Ziętara M. Microstructure stability of second and fourth generation single crystal nickel-base superalloys during high temperature creep deformation. [PhD thesis] Krakow: AGH University of Science and Technology; 2011. [8] Hassomeris O, Schumacher G, Krüger M, Heilmaier M, Banhart J. Phase continuity in high temperature Mo–Si–B alloys: a FIB-tomography study. Intermetallics 2011;19:470–5. [9] Epishin A, Link T, Brückner U, Portella PD. Kinetics of the topological inversion of the γ/γ′ microstructure during creep of a nickel-based superalloy. Acta Mater 2001;49:4017–23. [10] Hemmersmeier U, Feller-Kniepmeier M. Element distribution in the macro- and microstructure of nickel-base superalloy CMSX-4. Mater Sci Eng A 1998;248:87–97.
Available online at www.sciencedirect.com
ScienceDirect www.elsevier.com/locate/matchar
FIB–SEM tomography of 4th generation PWA 1497 superalloy Maciej Ziętara, Adam Kruk⁎, Adam Gruszczyński, Aleksandra Czyrska-Filemonowicz International Centre of Electron Microscopy for Materials Science and Faculty of Metals Engineering and Computer Industrial Science, AGH University of Science and Technology (AGH-UST), Al. A. Mickiewicza 30, PL-30 059 Kraków, Poland
AR TIC LE D ATA
ABSTR ACT
Article history:
The effect of creep deformation on the microstructure of the PWA 1497 single crystal
Received 26 August 2013
Ni-base superalloy developed for turbine blade applications was investigated. The aim
Received in revised form 15
of the present study was to characterize quantitatively a superalloy microstructure
November 2013
and subsequent development of rafted γ′ precipitates in the PWA 1497 during creep
Accepted 18 November 2013
deformation at 982 °C and 248 MPa up to rupture. The PWA1497 microstructure was characterized by scanning electron microscopy and FIB–SEM electron tomography. The 3D
Keywords:
reconstruction of the PWA1497 microstructure is presented and discussed.
FIB–SEM tomography
© 2013 Elsevier Inc. All rights reserved.
Ni-base superalloys Rafting
1.
Introduction
Single crystal (SC) nickel-base superalloys have been developed over the past 40 years especially for modern gas turbine applications. This group of alloys has superior mechanical properties such as creep resistance and high temperature strength. They are hardened by a high volume fraction of the ordered γ′ phase, which is coherently precipitated in the γ matrix. These materials are mainly applied as turbine blades and vanes in aero-engines and industrial gas turbines. Turbine blades operate at high temperature under a centrifugal force causing creep deformation of the material, which leads to the so-called rafting (i.e. directional coarsening). A characteristic microstructural property of SC superalloys is the ability of cubed γ′ phase particles to transform, under the influence of stress and temperature, into the plates (rafts). The rafts develop in the early stages of creep at high
temperature (about 1000 °C) and low stress (about 100 MPa). Rafting appears to be an essential factor determining creep strength of nickel-base SC superalloys at high temperature influencing their applications [1–3]. The focused ion beam–scanning electron microscope (FIB–SEM) tomography has been recently widely applied in materials science for studying and modifying material systems at the micro- and nanometer levels. FIB tomography is based on a serial slicing technique employing a FIB–SEM dual beam workstation. Dual-beam FIB–SEM enables the acquisition of serial images with small and reproducible spacing between the single imaging planes because no mechanical stage tilting is necessary between the FIB milling and the electron beam SEM imaging steps. Serial FIB cross-sections are performed through the volume to be investigated and each exposed surface is imaged with an electron microscope. 3D mapping of particles with high Z-resolution by serial FIB slicing and SEM imaging
⁎ Corresponding author. E-mail addresses: [email protected] (M. Ziętara), [email protected] (A. Kruk), [email protected] (A. Gruszczyński), [email protected] (A. Czyrska-Filemonowicz). 1044-5803/$ – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2013.11.003
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M A TE RI A L S CH A RACT ER IZ A TI O N 87 (2 0 1 4 ) 1 4 3– 1 4 8
was performed. Ga-ion beam was used to perform a precise in-situ milling. Repeated removal of layers as thin as several nm for some hundred times permits to investigate total volume of some μm3 with a voxel size of 10 nm × 10 nm × 10 nm. The SEM images at an accelerating voltage of 1.7 kV were taken utilizing a BSE (back scattered electron) detector. Acquired two-dimensional data are processing with the help of computer algorithms and three-dimensional systems can easily be reconstructed and provide both qualitative and quantitative information [4–6]. To visualize the 3D reconstruction, the Amira 5.4.1 and ImageJA 1.45b software were used. In this work, a quantitative microstructural characterization of the single crystal Ni-base superalloy PWA 1497 after heat treatment and after creep deformation is presented. The 3D reconstructions of a superalloy microstructure are generated to measure the volume fraction of the γ′ phase and the mean thickness of the γ phase in the direction parallel to the tensile axis.
2.
Experimental
2.1.
Description of Materials
A fourth generation Ni-base PWA 1497 superalloy was solidified into single crystal bars and provided by Pratt & Whitney, US. The alloy chemical composition was as follows: Ni–2Cr–5.55Al– 8.25Ta–6W–2Mo–5.95Re–3Ru–16.5Co–0.03C–0.15Hf (wt.%) and 40 ppm B. The solid bars were subjected to a standard heat treatment, involving solution annealing followed by aging.
2.2.
The γ/γ′ Misfit Measurements by X-ray Diffractometry
The XRD measurements of γ/γ′ misfit were performed on selected samples including two samples from the as-received material and a sample after creep rupture, with the highest creep exposure time and strain. The XRD investigation was carried out using a D500 SIEMENS apparatus with Cu X-ray tube and monochromatic radiation obtained by graphite crystal monochromator. The lattice spacing aγ′ and lattice spacing aγ were determined from position of the Kα1 peaks and the γ/γ′lattice misfit was calculated as δ = 2(aγ′ − aγ) / (aγ′ + aγ). The
resulting diffraction patterns were analyzed with MAKRO 50 non-commercial computer software. Fitting was made by the least squares method applied to the Lorentz function. The measured lattice parameters of γ and γ′ phases (aγ and aγ′) and γ/γ′ misfit were as follows: aγ = 0.36086 nm, aγ′ = 0.36003 nm and δ = − 0.26%. The lattice parameters of the γ and γ′ phases (aγ and aγ′, respectively) are higher for the PWA 1497 than for PWA 1984 superalloy where misfit was − 0.12% [7]. The FIB–SEM tomography investigation was performed on two samples of PWA 1497 superalloy; baseline and after creep rupture test (crept at 982 °C/248 MPa, creep strain 25%, duration 870 h) materials.
2.3.
FIB Sectioning
Tomographic data sets were obtained using FIB NEON CrossBeam 40EsB of Zeiss, by serial sectioning. The Pt protective layer was deposited on the surface of both samples (Fig. 1a), in order to prevent the occurrence of the curtaining effect, i.e., the irregular removal of material [8]. The material was removed with a Ga-ion beam (optimum beam current and voltage: 200 pA and 30 kV). Fig. 1b shows the PWA 1497 surface prepared (by milling) for step sectioning and image acquisition. The thickness of each slice was chosen as 10 nm. The parameters of Ga-ion beam milling (i.e. 30 kV, 200 pA) have been optimized in order to obtain time about 1.0 min to remove one layer with a thickness of 10 nm and SEM imaging. Decreasing the Ga-ion beam current can also reduce the thickness of the removed layer up to 5 nm, but substantially increases the acquisition time of one image. Due to the electrical and mechanical apparatus drift increasing acquisition time is probably not advisable in the case of a large number of registration images. The chemical composition of the samples has a significant impact on the single image recording time as well as on the resulting minimum thickness of the removed material layer. In general, the thickness of the layer removed is adjusted according to the pixel size on the 2D image at a given magnification. The SEM micrographs of un-etched γ/γ′ microstructure were recorded using an energy selective backscattered electron (EsB) detector, which provides the best phase contrast (Z-contrast) and uniform gray level for phase visualization.
Fig. 1 – Preparation of the area for repeated removal of layers during acquisition of tomographic series of images. a) Platinum protective layer deposited on the sample surface, b) imaging surface (perpendicular to the sample surface) prepared by ion milling for step sectioning and image acquisition of the PWA 1497 baseline sample.
M A TE RI A L S C HA RACT ER I ZA TI O N 87 ( 20 1 4 ) 1 4 3– 1 4 8
145
Fig. 2 – PWA 1497 superalloy; a) baseline material — regular cuboidal morphology of γ′ precipitates separated by γ channels, b) creep ruptured material (at 982 °C/248 MPa) — final state of coarsening of rafted microstructure, SEM imaging (microstructures after γ dissolving etching).
The working distance was 5 mm and voltage was 1.7 kV. Repeated removal of layers as thin as a 10 nm allowed to explore a total volume of 5.6 × 4.5 × 1.7 μm for baseline and 8.6 × 6.0 × 4.6 μm for creep ruptured materials with the voxel size 10 × 10 × 10 nm (size of raw data stacks were respectively equal: 142 MB for baseline material and 221 MB for creep ruptured material). The 3D visualization of reconstructed space for FIB–SEM tomography was performed using ImageJ 1.44p and Avizo Fire 6.3 software.
2.4.
3D Reconstruction
More than 200 sections were recorded for a baseline sample and around 450 sections for creep ruptured sample. The shift of the electron beam in lateral direction due to slice cutting during recording of the tomogram was calculated and subsequently corrected with the ImageJ 1.44p software. From the series of SEM micrographs, the 3D reconstruction of the investigated alloy volume was generated by Avizo Fire 6.3 software. This program uses the “labeling by reconstruction” technique, based on the “neighborhood-algorithm”. This algorithm comprises neighboring voxels with the same gray scale to local objects. Non-continuous ranges were identified and marked by colors.
3.
Results and Discussion
SEM analysis of the baseline PWA 1497 superalloy sample, revealed regular cuboidal morphology of γ′ particles separated by γ channels, Fig. 2a. During high temperature creep deformation, the initially cubic-shaped microstructure transforms under the influence of stress and temperature into plates (rafts). The rafts develop in the early stages of creep. Simultaneously with the rafting process, a thickening of the γ matrix channels parallel to the rafts occurs. Further evolution leads to an effect called “topological inversion”, the γ′ phase coalesces, coarsens and finally surrounds the γ phase, thereby becoming topologically the matrix. This process was explained in details by Epishin et al. [9]. The γ/γ′ microstructure of ruptured sample deformed at 982 °C/248 MPa for duration of 870 h (creep strain 25%) shows the final state of the coarsening of rafted microstructure, where γ′ phase becomes topologically the matrix and γ phase started to look like precipitates, Fig. 2b. Three dimensional reconstruction of γ/γ′ microstructure of PWA 1497 superalloy for the baseline sample showed similar image like obtained for (2D) SEM investigation, namely regular cuboidal morphology of γ′ particles separated by γ channels
Fig. 3 – PWA 1497 baseline material; 3D reconstruction showing: a) regular cuboidal morphology of γ′ particles, b) γ channels. Reconstructed volume: 5.6 × 4.5 × 1.7 μm.
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Fig. 4 – PWA 1497 baseline material; results of the 3D tomographic reconstruction of spatial distribution of selected (for better visualization) γ′ precipitates in the γ matrix.
(Fig. 3), which is typical for modern SC superalloys. Fig. 4 shows the results of tomographic reconstructed and visualized in 3D the spatial distribution of selected γ′ precipitates in the γ matrix. The microstructure investigation of PWA 1497 alloys using classical testing methods in TEM (BF, DF) and SEM (BSE, SE) does not provide information about the spatial shape of γ′ precipitates and their mutual arrangement in the investigated volume of the sample. Moreover, it does not allow determining whether the separated γ′ phase particles are connected to each other in space. Joining the γ′ phase particles by the vanishing of the γ phase channels indicates the microstructure changes caused by high-temperature heat treatment at the absence of external load for single-crystal nickel superalloys. Typically, the separation is treated as a separating of cuboidal particles of γ′ phases by γ phase channels. Employing the FIB–SEM tomography technique revealed a more complex morphology in terms of the shape of the γ′ particles as well as the spatial distribution. The γ′ particles take the shape of cuboids, or similar to the shape to the letters L and/or Z, occasionally observed cubic particles of γ′ phase were separated by γ phase channels. Due to its complex shape, these particles are interconnected in a reconstructed
volume. Application of numerical procedure Find Connected Region (ImageJ 1.44p) has revealed that in the tomographic reconstructed volume particles of γ phase are interconnected in a space in more than 80% (marked in red, Fig. 5). The other, not connected particles are marked with different colors. Fig. 6 shows the cross-sections of reconstructed volume for: two planes oriented perpendicular to the directions of the <100 > type and one plane arbitrarily oriented in the reconstructed space. The shape and dimensions of the γ and γ′ phases vary significantly on the plane on which the analysis is performed. For baseline material the quantification of the width of γ-phase channel was carried out in planes oriented perpendicular to the < 100 > direction. However, 3D reconstruction of creep ruptured sample shows more complex structure than for 2D SEM micrographs. The evolution of microstructure during creep deformation leads to a final state of the coarsening of rafted microstructure, where γ′ phase becomes topologically the matrix and γ phase started to look like precipitates (Fig. 7). In this case, 3D reconstruction allows for understanding isotropic changes of the microstructure. For 2D SEM analysis it would be more difficult and require analysis of polished section in three different directions.
Fig. 5 – PWA 1497 baseline material; 3D reconstruction showing the spatial distribution of interconnected γ-phase particles in tomographic reconstructed volume. The same color means that the particles are interconnected.
M A TE RI A L S C HA RACT ER I ZA TI O N 87 ( 20 1 4 ) 1 4 3– 1 4 8
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Fig. 6 – Cross-section of reconstructed volume for two planes oriented perpendicular to the directions of the < 100> type and one plane arbitrarily oriented in the reconstructed space.
The relative area fraction (AA) of γ′ phase for a baseline sample was defined as the average of the successive values determined on the basis of the 15 successive images and is equal to 71.2 ± 0.3%. Based on 3D reconstruction of a baseline sample, measured volume fraction (Vv) of γ′ phase is equal to 68.1%. The relative area fraction for crept sample was measured as 70.7 ± 5.7% and volume fraction as 71.0% (Table 1). The results are in good agreement with our previous TEM investigation of a baseline sample [6] and clearly prove that in the case of the new generation SC superalloys and for sufficiently large numbers of measurements, measurement of area fraction is adequate to volume fraction (AA ≅ Vv). The γ′ volume fraction of superalloys measured by TEM technique was proved to be independent of creep duration [10], while our results of γ′ volume fraction of crept sample were slightly higher than that for baseline sample. The difference of γ′ volume fraction for PWA 1497 superalloy for a baseline and crept sample was most probably caused by a statistic error (i.e. area chosen for investigation) and/or image processing. Microstructure parameters (Vv, AA, thickness of γ phase channels) of the PWA 1497 baseline and creep ruptured samples are shown in Table 1. The mean thickness of γ phase (γ channel width) increased during creep process from 65.5 nm, tripling its value for ruptured specimen (193 nm). Standard deviation for creep deformed sample was higher for all measurements due to the fact that during rafting process the superalloy
microstructure looses their regularity and becomes more wavy and convoluted.
4.
Summary
Quantitative characterization of the 4th generation Ni-base PWA 1497 superalloy microstructure (baseline and creep ruptured samples) was performed by the FIB–SEM tomography technique. The quantitative results of the γ′ area fraction (AA) and volume fraction (Vv) reveal that for baseline sample AA and Vv are not equal and reach respectively around 68 and 71%. For the ruptured specimen, AA and Vv are equal and reach around 71%. Those results clearly prove that in case of modern single crystal superalloys, measurements of area fraction are adequate for volume fraction (AA ≅ Vv). The measurements of a mean γ phase thickness reveal that during creep process γ channel width increases from 65.5 nm, tripling its value to 193 nm for ruptured specimen. The FIB–SEM tomography is a very useful technique for quantitative studies of material microstructure. This technique allows to eliminate errors resulting from thickening of γ channels or γ′ precipitates caused by etching (depended on chemical reagent). This problem usually occurs during standard SEM microstructure investigation of polished and etched cross-sections of analyzed materials. The FIB–SEM tomography is a powerful tool in nanoscale materials science; it
Fig. 7 – PWA 1497 creep ruptured sample; 3D reconstruction showing the final stage of coarsening of rafted microstructure. The γ′ phase becomes topologically the matrix, γ phase looks like the precipitates. Reconstructed volume: 8.6 × 6.0 × 4.6 μm.
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M A TE RI A L S CH A RACT ER IZ A TI O N 87 (2 0 1 4 ) 1 4 3– 1 4 8
Table 1 – Microstructure parameters of PWA1497 baseline and creep ruptured samples; (Vv, AA, thickness of γ phase channels). PWA 1497 superalloy
Vv of γ′ [%]
AA of γ′ [%]
Thickness of γ channels [nm]
Baseline sample Creep ruptured sample
68.1 71.0
71.2 ± 0.3 70.7 ± 5.7
65.5 ± 20 193 ± 50
creates new possibility for stereological description of material micro- and nanostructures and simplifies to understand complex microstructural changes caused by their deformation.
Acknowledgments This work was supported by the Polish Ministry of Science and Higher Education (project nr 11.11.110.148), and the AGH-UST Statutory Research. We would like to also acknowledge Pratt & Whitney, East Hartford, CT USA for providing the material used in this investigation.
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