Correlative atom-probe tomography and transmission electron ...
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Scripta Materialia 68 (2013) 909–912 www.elsevier.com/locate/scriptamat
Correlative atom-probe tomography and transmission electron microscope study of a chemical transition in a spinel on an oxidized nickel-based superalloy Sung-Il Baik,a Xin Yina and David N. Seidmana,b,⇑ a
Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, USA Northwestern University Center for Atom-Probe Tomography, Materials Research Science and Engineering Center, Evanston, IL 60028-3108, USA
b
Received 3 January 2013; revised 12 February 2013; accepted 13 February 2013 Available online 22 February 2013
Complex thermally grown oxides (TGOs) are formed on a single-crystal Ni-based superalloy due to a large fraction of alloying elements. Combining atom-probe tomography and transmission electron microscopy analyses, the three-dimensional elemental distributions of metal oxides of the spinel and refractory metal-oxide regions are presented. A spinel-type oxide, Ni(Cr,Al)2O4, is formed with a crystallographic orientation relationship with the alloy substrate, which decomposes into other oxides. The sequence of oxides observed in the TGO are NiO ! Cr2O3 ! Ta2O5 ! NiTaO4, NiTa2O6 ! Ni(Cr,Al)2O4 ! Al2O3. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ni-based superalloy; Metal oxides; Spinel; Transmission electron microscopy; Atom-probe tomography
Single-crystal Ni-based superalloys are widely used for high-temperature environments, such as industrial gas turbine blades for power generation and for jet engines in commercial and military aircraft [1–3]. The thermally grown oxide (TGO) scale on a bare nickelbased alloy plays a crucial role as a thermal barrier coating (TBC), preventing further oxidation [4]. However, a thick oxide scale on a Ni-based alloy substrate can lead to failures, such as cracks and separation of the oxide scale due to high residual stresses (3–6 GPa) [5]. Alumina (Al2O3) is the preferred oxide because of its good adherence to the substrate and the low diffusivity of oxygen in it; therefore, continuous Al2O3 prevents oxidation of the Ni-based substrate [6]. To increase the oxidation resistance and solid-solution strengthening in the hightemperature regime of the Ni-based alloy, refractory metal (RM) elements, such as, Hf, Ta, W and Re, are added to the Ni–Al–Cr-based alloy [2,7]. The oxidation characteristics of a Ni-based superalloy have been investigated utilizing X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS), but detailed studies
⇑ Corresponding
author at: Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, USA. Tel.: +1 8474914391; fax: +1 8474917820; e-mail: [email protected]
of the chemical distributions of different elements at the sub-nanoscale in metal oxides are sparse [8,9]. From the XRD and EDS analyses it was demonstrated that NiO and a spinel-type oxide, Ni(Cr,Al)2O4, are initially formed on the Ni–Cr–Al-based alloy substrate and then the spinel decomposes into NiO and Cr2O3 or Al2O3 in the temperature range 1000–1200 °C [10]. The addition of a high fraction of RM (Hf, Ta, W, Re) and Ru alloying elements make accurate detailed chemical analyses of the metal oxides a challenging problem. The analyses of the microstructures and chemical compositions of the metallic alloy oxide phases have been severely limited due to the complexity of the oxides’ compositions. Additionally, it was restricted at the sub-nanometer to nanometer scale because of poor mass resolving power (Dm/m) for light elements (small atomic number, Z) for standard analytical spectroscopic techniques. Atom-probe tomography (APT), which is a spectrometric technique, is a unique method for obtaining sub-nanometer scale composition information with a similar detection efficiency for all elements in the periodic table [11–13]. APT is now widely utilized to obtain chemical compositions in three dimensions of metallic and semiconducting materials. There are, however, only a limited number of APT analyses of metal oxides using voltage pulsing because of their insulating properties [14,15]. The recent implementation of femtosecond
1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.02.025
910
S.-I. Baik et al. / Scripta Materialia 68 (2013) 909–912
green (515 nm) or ultraviolet (343 nm) lasers has made it possible to analyze poorly conducting metal oxides more readily [16–18]. This paper presents results concerning a chemical transition in a spinel structure occurring in a TGO by performing a correlative APT and transmission electron microscope (TEM) study. Currently used Ni-based superalloys contain up to 16 alloying elements and understanding how each element contributes to the oxide layers is crucial for improving their high-temperature oxidation properties. Ò The material studied is Rene´ N’5 Y+ (23.1 Co–13.1 Cr–17.7 Al–4.5 Ta–3.0 W–2.9 Re–0.8 Hf–4.0 Ru–0.4 Zr–1.4 Mo–2.8 Ti–0.7 Nb–0.6 C–0.06 B–0.08 Y, balance Ni, at.%). This alloy was directionally solidified along a h1 0 0i-direction and then cut into coupons with diameters of 2.54 cm. The Rene´ N5 Y+ sample was oxidized at 1100 °C for 100 h and it developed a 3–5 lm thick oxide layer. These oxidation conditions were chosen to mimic the operating temperature of commercial turbine blades. APT and TEM samples were prepared by lifting out specimens utilizing Ga+ ion milling in a FEI Helios dual-beam focused-ion beam (FIB) microscope employing a final energy for the Ga+ ions of 2 kV at 24 pA [19]. APT microtips were lifted out from selected regions of the TGO from the bulk sample and transferred to a 3 mm diameter Cu grid for characterization by both APT and TEM [20]. The microstructure and crystallography were characterized utilizing a JEOL 2100F TEM operating at 200 kV. Selective area diffraction patterns (SADPs) were indexed using the programs CrystalMaker and SingleCrystal [21]. A Cameca LEAP 4000X-Si tomograph (Cameca, Madison, WI) was employed to measure the compositions of APT microtip samples. Picosecond pulses of ultraviolet laser light (355 nm wavelength) were utilized to evaporate individual atoms at a pulse repetition rate of 100 kHz, a laser pulse energy of 40 pJ per pulse and an average detection rate of 0.005 ions per pulse. The specimen tip temperature was maintained at 60 K, and the gauge pressure was 1.90 109 Pa. Data analyses were performed on the three-dimensional (3-D) reconstructions of specimens utilizing the program IVAS 3.6.1. The microstructure of the TGO was confirmed by TEM analysis (Fig. 1). A bright-field (BF) image (Fig. 1a) displays the oxide on the Ni-based alloy. The TGO is divided into three major layers: (1) top layer (denoted 1) with large grains (>500 nm diameter); (2) middle layer (denoted 2) consisting of small grains
Scripta Materialia 68 (2013) 909–912 www.elsevier.com/locate/scriptamat
Correlative atom-probe tomography and transmission electron microscope study of a chemical transition in a spinel on an oxidized nickel-based superalloy Sung-Il Baik,a Xin Yina and David N. Seidmana,b,⇑ a
Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, USA Northwestern University Center for Atom-Probe Tomography, Materials Research Science and Engineering Center, Evanston, IL 60028-3108, USA
b
Received 3 January 2013; revised 12 February 2013; accepted 13 February 2013 Available online 22 February 2013
Complex thermally grown oxides (TGOs) are formed on a single-crystal Ni-based superalloy due to a large fraction of alloying elements. Combining atom-probe tomography and transmission electron microscopy analyses, the three-dimensional elemental distributions of metal oxides of the spinel and refractory metal-oxide regions are presented. A spinel-type oxide, Ni(Cr,Al)2O4, is formed with a crystallographic orientation relationship with the alloy substrate, which decomposes into other oxides. The sequence of oxides observed in the TGO are NiO ! Cr2O3 ! Ta2O5 ! NiTaO4, NiTa2O6 ! Ni(Cr,Al)2O4 ! Al2O3. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ni-based superalloy; Metal oxides; Spinel; Transmission electron microscopy; Atom-probe tomography
Single-crystal Ni-based superalloys are widely used for high-temperature environments, such as industrial gas turbine blades for power generation and for jet engines in commercial and military aircraft [1–3]. The thermally grown oxide (TGO) scale on a bare nickelbased alloy plays a crucial role as a thermal barrier coating (TBC), preventing further oxidation [4]. However, a thick oxide scale on a Ni-based alloy substrate can lead to failures, such as cracks and separation of the oxide scale due to high residual stresses (3–6 GPa) [5]. Alumina (Al2O3) is the preferred oxide because of its good adherence to the substrate and the low diffusivity of oxygen in it; therefore, continuous Al2O3 prevents oxidation of the Ni-based substrate [6]. To increase the oxidation resistance and solid-solution strengthening in the hightemperature regime of the Ni-based alloy, refractory metal (RM) elements, such as, Hf, Ta, W and Re, are added to the Ni–Al–Cr-based alloy [2,7]. The oxidation characteristics of a Ni-based superalloy have been investigated utilizing X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS), but detailed studies
⇑ Corresponding
author at: Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, USA. Tel.: +1 8474914391; fax: +1 8474917820; e-mail: [email protected]
of the chemical distributions of different elements at the sub-nanoscale in metal oxides are sparse [8,9]. From the XRD and EDS analyses it was demonstrated that NiO and a spinel-type oxide, Ni(Cr,Al)2O4, are initially formed on the Ni–Cr–Al-based alloy substrate and then the spinel decomposes into NiO and Cr2O3 or Al2O3 in the temperature range 1000–1200 °C [10]. The addition of a high fraction of RM (Hf, Ta, W, Re) and Ru alloying elements make accurate detailed chemical analyses of the metal oxides a challenging problem. The analyses of the microstructures and chemical compositions of the metallic alloy oxide phases have been severely limited due to the complexity of the oxides’ compositions. Additionally, it was restricted at the sub-nanometer to nanometer scale because of poor mass resolving power (Dm/m) for light elements (small atomic number, Z) for standard analytical spectroscopic techniques. Atom-probe tomography (APT), which is a spectrometric technique, is a unique method for obtaining sub-nanometer scale composition information with a similar detection efficiency for all elements in the periodic table [11–13]. APT is now widely utilized to obtain chemical compositions in three dimensions of metallic and semiconducting materials. There are, however, only a limited number of APT analyses of metal oxides using voltage pulsing because of their insulating properties [14,15]. The recent implementation of femtosecond
1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.02.025
910
S.-I. Baik et al. / Scripta Materialia 68 (2013) 909–912
green (515 nm) or ultraviolet (343 nm) lasers has made it possible to analyze poorly conducting metal oxides more readily [16–18]. This paper presents results concerning a chemical transition in a spinel structure occurring in a TGO by performing a correlative APT and transmission electron microscope (TEM) study. Currently used Ni-based superalloys contain up to 16 alloying elements and understanding how each element contributes to the oxide layers is crucial for improving their high-temperature oxidation properties. Ò The material studied is Rene´ N’5 Y+ (23.1 Co–13.1 Cr–17.7 Al–4.5 Ta–3.0 W–2.9 Re–0.8 Hf–4.0 Ru–0.4 Zr–1.4 Mo–2.8 Ti–0.7 Nb–0.6 C–0.06 B–0.08 Y, balance Ni, at.%). This alloy was directionally solidified along a h1 0 0i-direction and then cut into coupons with diameters of 2.54 cm. The Rene´ N5 Y+ sample was oxidized at 1100 °C for 100 h and it developed a 3–5 lm thick oxide layer. These oxidation conditions were chosen to mimic the operating temperature of commercial turbine blades. APT and TEM samples were prepared by lifting out specimens utilizing Ga+ ion milling in a FEI Helios dual-beam focused-ion beam (FIB) microscope employing a final energy for the Ga+ ions of 2 kV at 24 pA [19]. APT microtips were lifted out from selected regions of the TGO from the bulk sample and transferred to a 3 mm diameter Cu grid for characterization by both APT and TEM [20]. The microstructure and crystallography were characterized utilizing a JEOL 2100F TEM operating at 200 kV. Selective area diffraction patterns (SADPs) were indexed using the programs CrystalMaker and SingleCrystal [21]. A Cameca LEAP 4000X-Si tomograph (Cameca, Madison, WI) was employed to measure the compositions of APT microtip samples. Picosecond pulses of ultraviolet laser light (355 nm wavelength) were utilized to evaporate individual atoms at a pulse repetition rate of 100 kHz, a laser pulse energy of 40 pJ per pulse and an average detection rate of 0.005 ions per pulse. The specimen tip temperature was maintained at 60 K, and the gauge pressure was 1.90 109 Pa. Data analyses were performed on the three-dimensional (3-D) reconstructions of specimens utilizing the program IVAS 3.6.1. The microstructure of the TGO was confirmed by TEM analysis (Fig. 1). A bright-field (BF) image (Fig. 1a) displays the oxide on the Ni-based alloy. The TGO is divided into three major layers: (1) top layer (denoted 1) with large grains (>500 nm diameter); (2) middle layer (denoted 2) consisting of small grains
