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The investigation of the of fatigue crack growth mechanism in powder metallurgy Ni-based superalloy
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ScienceDirect ScienceDirect Available atProcedia www.sciencedirect.com Structuralonline Integrity 00 (2019) 000–000 Structural Integrity Procedia 00 (2019) 000–000
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Procedia Structural Integrity 23 (2019) 251–256
9th Materials Structure 9th International International Conference Conference on on Materials Structure and and Micromechanics Micromechanics of of Fracture Fracture
The The investigation investigation of of the the of of fatigue fatigue crack crack growth growth mechanism mechanism in in powder metallurgy Ni-based superalloy powder metallurgy Ni-based superalloy a b b b,c, M.A. M.A. Artamonov Artamonova*, *, I.N. I.N. Trunkin Trunkinb,, A.V. A.V. Ovcharov Ovcharovb,, A.L. A.L. Vasiliev Vasilievb,c, a A. a
Lyulka Design Bureau PJSC «UEC-Ufa Engine Industrial Association» Subsidiary, Kasatkina st., 13, Moscow 129301, Russia A. Lyulka Design Bureau PJSC «UEC-Ufa Engine Industrial Association» Subsidiary, Kasatkina st., 13, Moscow 129301, Russia b National Research Centre “Kurchatov Institute”, Akademica Kurchatova Sq., Moscow, 1123182, Russia, b National Research Centre “Kurchatov Institute”, Akademica Kurchatova Sq., Moscow, 1123182, Russia, c Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences, c Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences, Leninskiy prospect, 59, 119333, Moscow, Russia Leninskiy prospect, 59, 119333, Moscow, Russia
Abstract Abstract
Nickel alloys with operating temperatures up to 650 °C are used for the manufacture of turbine disks in modern gas Nickel alloys with operating temperatures up to 650 °C are used for the manufacture of turbine disks in modern gas turbine engines. During an operation or testing the high centrifugal forces, which appear in the discs, could initiate turbine engines. During an operation or testing the high centrifugal forces, which appear in the discs, could initiate fatigue cracks formation. If the cracks start from internal defects, there is no air access and their growth occurs in a fatigue cracks formation. If the cracks start from internal defects, there is no air access and their growth occurs in a vacuum conditions. The failure mechanism will correspond to the low-cycle fatigue. Various electron microscopy vacuum conditions. The failure mechanism will correspond to the low-cycle fatigue. Various electron microscopy methods were used in the study of billets formed from the powder nickel alloys. The fractographic analysis methods were used in the study of billets formed from the powder nickel alloys. The fractographic analysis demonstrated that in vacuum conditions a rugged rough surfaces of the crack consisted of spherical particles with demonstrated that in vacuum conditions a rugged rough surfaces of the crack consisted of spherical particles with the sizes in the range between 50 and 150 µm is formed at the beginning. When a crack reaches a billet surface, the the sizes in the range between 50 and 150 µm is formed at the beginning. When a crack reaches a billet surface, the crack surface relief changes to a quasi-faceted one, and that corresponds to the first stage of fatigue fracture crack surface relief changes to a quasi-faceted one, and that corresponds to the first stage of fatigue fracture development. It was found that nano-grains with the size of 100–200 nm are formed in front of the crack tip. The development. It was found that nano-grains with the size of 100–200 nm are formed in front of the crack tip. The crack surfaces are covered with amorphous layers with a thickness of about 20 nm. The destruction occurs mainly crack surfaces are covered with amorphous layers with a thickness of about 20 nm. The destruction occurs mainly along the boundaries of nanocrystals layer. A model is proposed to explain the appearance of such microstructure along the boundaries of nanocrystals layer. A model is proposed to explain the appearance of such microstructure and the key factor is the air access to a crack and hence the oxidation of the crack surfaces. and the key factor is the air access to a crack and hence the oxidation of the crack surfaces. 2019The TheAuthors. Authors. Published by Elsevier © 2019 Published by Elsevier B.V. B.V. © 2019 The Authors. Published by Elsevier B.V. This isisan anopen openaccess access article under the BY-NC-ND CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/4.0/) This article under the CC licenselicense (http://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article under the CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review responsibility of the committee of license the ICMSMF Peer-reviewunder under responsibility of scientific the scientific committee of the ICorganizers MSMF organizers. Peer-review under responsibility of the scientific committee of the IC MSMF organizers.
Keywords: Scanning electron microscope; low-cycle fatigue; transmission electron microscope; powder nickel-base superalloy; EP741NP; Keywords: Scanning electron microscope; low-cycle fatigue; transmission electron microscope; powder nickel-base superalloy; EP741NP; fatigue crack growth mechanism; structural analysis; fatigue crack in vacuum; low-cycle fatigue fatigue crack growth mechanism; structural analysis; fatigue crack in vacuum; low-cycle fatigue
* Corresponding author. Tel.: +7-499-755-08-44; fax: +7-495-683-09-97. * Corresponding author. Tel.: +7-499-755-08-44; fax: +7-495-683-09-97. E-mail address: [email protected] E-mail address: [email protected] 2452-3216 © 2019 The Authors. Published by Elsevier B.V. 2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the IC MSMF organizers. Peer-review under responsibility of the scientific committee of the IC MSMF organizers.
2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the ICMSMF organizers 10.1016/j.prostr.2020.01.095
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M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000–000
1. Introduction Powder metallurgy nickel based superalloys are used for the manufacture of turbine disks (TDs) of modern gas turbine engines (GTE) and power plants. These parts work at temperatures of 650 °C and above. Moreover, the turbine disks are under the influence of the load caused by the rotation of the disk and the centrifugal forces of the turbine blades. During low-cycle fatigue tests in conditions close to real, fatigue cracks from internal defects could appear. The initial stage of growth of fatigue cracks before they reach the surface occurs in a vacuum condition. The study of the development of the fatigue cracks in TDs is interesting from the scientific and practical points of view. Besides, there is a question on the mechanisms of the fatigue cracks development in the presence of the air and without air access. The previous studies demonstrated a significant difference in the crack growth rate for these types of cracks [1,2]. The existing methods of determination of the life between overhauls of TDs are based on monitoring the development of fatigue cracks. In this article we present the results of the microstructural analysis of fatigue cracks, which were originated beneath the sample surface and expanding without air access. Different electron microscopy methods, namely scanning, transmission and scanning-transmission electron microscopy (SEM, TEM and STEM, respectively) together with energy dispersive microanalysis were used. 2. Material and methods The cylindrical billets with a gauge length of 13 mm and a diameter of 4.37 mm, were formed from a high temperature nickel based EP741NP alloy, which is used for the manufacture of the aircraft GTE discs. This material is similar to AF115, AF21DA6, Alloy-10 and LSHR alloys [3]. The powder forging method was standard hydrostatic pressing of the powder, obtained by the plasma rotate electrode process (PREP). The size of the powder was less than 140 µm. The chemical composition of the powder is presented in table 1. Table 1 – The alloy element content (weight %) [3] Ni C Cr Mo W Al Basis 0.04 9.0 3.9 5.5 5.1
Ti 1.8
Co 15.8
Nb 2.6
Hf 0.25
B ≤0.015
Zr <0.015
The alloy after heat treatment exhibited typical microstructure [3]: a γ-Ni solid solution with an average grain size of 40 μm with a hardening intermetallic γ'-phase. The tests were carried out in the soft cycle condition (stress control), the maximum stress level in the cycle was σ max = 980 MPa, the cycle asymmetry parameter R = 0.1. The working conditions of the TDs determines, that if a fatigue crack starts growing inside the billet, the fracture mechanism corresponds to low-cycle fatigue. Two samples with fatigue cracks originated and grew beneath the surface were selected. Sample No.1: the final destruction occurred during the test. The fractured sample after testing was kept in an oven at a temperature of 650 °С. Sample No.2: the fatigue crack was of mixed type. The fatigue crack grew up to the surface and further development occurred with the air access. The microstructural and fractographic analysis were performed for the crack in sample No.1 and only fractographic - for the crack in sample No.2.The fractographic analysis was performed in a scanning electron microscopes (SEM) JSM-IT300LV (JEOL, Japan) and a dual-beam SEM-focus ion beam (FIB) Helios NanoLab 660 (ThermoFisher Scientific, USA). Cross-sections specimens were prepared by standard lift-out technique in a SEM/FIB Helios Nanolab (ThermoFisher Scientific, USA), equipped with a micromanipulator Omniprobe (Omniprobe, USA) for extracting thin lamella specimens. The lamellas were studied in a Titan 80-300 TEM/STEM (FEI, USA) at an accelerating voltage of 300 kV. The device is equipped with an EDX Si(Li) spectrometer (EDAX, USA), High Angle Annular Dark Field (HAADF) electron detector (Fischione, USA) and Gatan Image Filter (GIF) (Gatan, USA).
M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000–000
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3. Results 3.1. Fractographic study of the samples The investigation of specimen No. 1 by SEM demonstrated that crack started from the defects in the alloy (Fig. 1 a), which were formed near HfO2 based precipitates [4]. We found that the crack development occurred in three stages with the formation of a specific surface morphology. The rugged and rough surface covered by round-shaped particles with sizes in the range between 50 nm and 150 nm was formed at the first stage (Fig. 1). It should be noted that the morphology of the particles is determined by their microstructure, not associated with oxidation process. At the second stage, a zone with fatigue striations is formed. The formation of a transition region with the fatigue striations and rounded submicron particles was associated with the change of the crack propagation mechanism. In addition, a third zone was identified and it corresponds to the transition from stable crack growth to the rupture area. In the presented study, the second and third zones were not studied. Unlike sample No. 1, four zones were observed in sample No. 2, and each of them corresponds to four stages of crack development. The first and second zones are indicated on the SEM images shown in Fig. 6a and b. The first zone is similar to the one, found in sample No1. The second zone exhibited quasi-faceted morphology (Fig. 2). Highly likely, it was formed exactly it the moment when the crack had reached the surface of the sample and the admittance of air started with the surface oxidation. The third zone has a distinct appearance with fatigue striations. The fourth zone is similar to the third zone of specimen No. 1 - the transition from stability crack growth to the rupture area. SEM image (Fig. 2) clearly illustrate the moment when the crack reaches the surface. This image was obtained in backscattered electrons (zone 1, Fig. 2 a) and part of it has brighter contrast. That pointed to thinner oxide layer on the crack surface.
3.2. The study of cross-sections. The cross-sections for microstructural analysis were cut out at a distance of 30 μm from the crack origin of sample No. 1 (Fig. 2). The direction of the cut was chosen towards the development of fatigue crack. The HAADF STEM images of the specimens are presented in Fig. 3 a,b. The layer beneath the protective Pt layer exhibits darker contrast and consists of nanoparticles (Fig. 3 a). Several areas with different contrast can be revealed in the enlarged image (Fig. 3 b). The EDX microanalysis indicated that the darker areas between the grains (Fig. 3 b) contains more than 50% C and O. Below this layer, there are large particles, which consist of highly textured sub-grains misoriented relatively each other on 1-2°. The electron diffraction study of the sub-grains unambiguously demonstrate that the crystal structure correspond to face centered cubic Fm 3͞ m space group with the unit cell parameter a = 0.34–0.36 nm and that is completely consistent with γ-phase [5]. The overall orientation of the subgrains in this area is close to [101] cubic unit cell perpendicular to the specimen surface. Apparently, this corresponds to the orientation of the γ’-phase grain before the tests, in which the development of the fatigue crack occurred. It should be noted that, in the volume of sample No. 1, low density of NiCo particles were also detected. The unit cell parameters of this phase with a high content of Ni and Co are close to γ phase [6]. Analysis of high-resolution TEM images (Fig. 4) showed that particles of different phases were formed in the near-surface area of the crack: Ni3C (Space Group R ͞3c, a = 0.45 nm, c = 12.9 nm [7]) and NiO (Space Group Fm ͞3m, a = 0.418 nm [8]). Between Ni, NiO and Ni3C particles thick amorphous layers (up to 20 nm) were found (Fig. 3b and Fig. 4c).
M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000–000
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Fig. 1. Secondary electron SEM images of sample No. 1. (a) – Zone 1. (b) - The enlarged image highlighted by the blue rectangle in (a).
Fig. 2. SEM images of the origin of the fracture area of sample No. 2: (a) – a low magnification backscattered electrons image; (b) - enlarged secondary electrons image of the origin of the fracture area. The fracture area, which corresponds to the crack area developed in vacuum condition, is denoted as “1”, and with the access of the air, denoted as “2 (quasi-faceted fracture). The dotted line shows the boundary between the areas “1” and “2”.
“ Fig. 3. HAADF STEM images of the cross-section of sample No 1: (a) –low magnification TEM image, the arrows indicate the layer containing oxides and carbides; (b) - enlarged image of the oxide layer. The rectangle highlights the area, where EDX microanalysis was performed.
M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000–000
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In some areas nickel carbonates in the form of NiCO3 (Space Group R ͞3c, a = 0.4617 nm, c = 1.4735 nm) [9]) particles embedded in the amorphous layers were found. The formation of these particles could occur after air access. The rounded particles observed in the cross-sections specimens are the ones, which had been found by SEM (Fig. 1b, Fig. 2b).
Fig. 4. (a) Bright field TEM image of nanograins near the fracture surface and the amorphous areas between them; (b)Fast Fourier Transform (FFT) spectrum corresponded to the Ni3C phase; (c) FFT spectrum obtained from the area in the square; (d) FFT corresponding to the NiO phase.
4. Discussion Based on the experimental data the mechanism of an internal fatigue crack development without air access was proposed. At the initial stage (samples No. 1 and No. 2) a plastic deformation zone is formed in front of the crack tip [10]. The formation of nanocrystals observed by SEM (Fig. 1b, 2b), and by TEM (Fig. 3, 4) occurred in this zone. Apparently, the microstructure with the nanocrystals is formed in several loading cycles. The TEM and electron diffraction data pointed to the high degree of misorientation of nanocrystals. The misoriented nanocrystals lead to the appearance of relatively "weak" links at nanocrystal boundaries. As a result, the propagation of a crack occurs through the boundaries of nanocrystals. The formation of "weak" links was also confirmed by the observations of the thick amorphous layers at the boundaries of nanocrystals in specimen No. 1 (Fig. 3b) after high-temperature oxidation of the fracture surface. Thus, the mechanism of failure, discussed above, can be defined as inter nanocrystalline. It is known that the formation of nanocrystals with large misorientation requires an excess of dislocations of the same sign [11]. However, the formation of dislocations ahead of the crack tip is energetically unlikely (Fig. 5a) [11]. To explain the large misorientation of nanocrystals, we proposed the following model. During loading cycles, there are periodic opening and squeezing of the crack. When loading is released and the crack is squeezed, the so-called "cold welding" process can occur [12]. Opposite surfaces of the crack, when they are oxide-free and there is no loading joined and chemical connection forms. After subsequent loading and crack opening dislocations of one sign (see Fig. 5b) ahead of the crack tip are formed due to the stress. A multiple loading cycle arises the misorientation of nanocrystallites (Fig. 5c) followed by further crack propagation along the weak grain boundaries of nanocrystals.
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M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000–000
According to TEM data, the twist of 100 nm blocks by 2° degrees leads to the displacement of atoms by more than 1.7 nm and that significantly exceeds the interatomic distance. Such disorder causes the formation of the amorphous layer, which promotes the crack spread. The proposed model explains the change in the mechanism of crack growth from inter-nanocrystalline to quasi-faceted at the beginning of the air access into the crack space (Fig. 2). After that, the crack surfaces oxidized and the oxide film prevents the "cold welding" process.
Fig. 5. (a) - scheme formation of dislocations in the material under the condition of air access to the crack; (b, c) and in the condition of vacuum.
5. Conclusion It was found that a layer of highly misoriented nanocrystals with a size of 50–200 nm is formed in the first stage of crack formation. It has been established that fatigue cracks propagate along the boundaries of nanocrystals. A model is proposed to explain the mechanism for the formation of the misorientation of nanocrystals during the crack development without air access. The difference in the mechanisms of crack growth with and without air access is found and explained. References 1. J.E. King, 1982. Surface damage and near-threshold fatigue crack growth in a ni-base superalloy in vacuum. Fatigue of Engineering Materials and Structures Vol. 5, No. 2, pp. 177-188,. 2. R.J. Kashinga, L.G. Zhao, V. V.Silberschmidt,1 R. Jiang, and P.A.S. Reed, 2018. A diffusion-based approach for modelling crack tip behaviour under fatigue-oxidation conditions. Int J Fract. 213(2), pp. 157–170 3. J. Radavich, D. Furrer, T. Carneiro, J. Lemsky, 2008. The Microstructure and Mechanical Properties of EP741NP Powder Metallurgy Disc Material. Superalloys 2008, TMS, pp 63-72. 4. Trunkin I.N. Artamonov M.A., Ovcharov A.V., Vasilyev A.L. 2009. Structural study of defects in granulated nickel alloy EP741NP. Crystallography Reports. T. 64. № 4. pp. 539-543 (Rus) 5. Yousuf M., Sahu P.C., Jajoo H.K., Rajagopalan S., Govinda Rajan K., 1986. Effect of magnetic transition on the lattice expansion of nickel. Journal of Physics F., V. 16., №. 5, pp. 373-380. 6. Taylor A., 1950. Lattice parameters of binary nickel-cobalt alloys. Journal of the Institute of Metals, V. 77, pp. 585-594. 7. Nagakura S., 1958. Study of metallic carbides by electron diffraction Part II. Crystal structure analysis of nickel carbide. Journal of the Physical Society of Japan, V. 13, №. 9, pp. 1005-1014. 8. Taylor D., 1984. Thermal expansion data: I. Binary oxides with the sodium chloride and wurtzite structure, MO. Transactions and Journal of the British Ceramic Society, V. 83, №. 1, pp. 5–9. 9. Pertlik F., 1986. Structures of hydrothermally synthesized cobalt (II) carbonate and nickel (II) carbonate. Acta Crystallographica. V. 42, №. 1, pp. 4-5. 10. Broek, D., 1982. Elementary engineering fracture mechanics. Springer Science, pp 469. 11 V.I. Vladimirov, 1984. Physical nature of destruction. M .: Metallurgy, pp. 280. (Rus) 12 Oguma, H.; Nakamura, T., 2013. Fatigue crack propagation properties of Ti-6Al-4V in vacuum environments. International Journal of Fatigue, 50, pp.89-93.
ScienceDirect ScienceDirect Available atProcedia www.sciencedirect.com Structuralonline Integrity 00 (2019) 000–000 Structural Integrity Procedia 00 (2019) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Structural Integrity 23 (2019) 251–256
9th Materials Structure 9th International International Conference Conference on on Materials Structure and and Micromechanics Micromechanics of of Fracture Fracture
The The investigation investigation of of the the of of fatigue fatigue crack crack growth growth mechanism mechanism in in powder metallurgy Ni-based superalloy powder metallurgy Ni-based superalloy a b b b,c, M.A. M.A. Artamonov Artamonova*, *, I.N. I.N. Trunkin Trunkinb,, A.V. A.V. Ovcharov Ovcharovb,, A.L. A.L. Vasiliev Vasilievb,c, a A. a
Lyulka Design Bureau PJSC «UEC-Ufa Engine Industrial Association» Subsidiary, Kasatkina st., 13, Moscow 129301, Russia A. Lyulka Design Bureau PJSC «UEC-Ufa Engine Industrial Association» Subsidiary, Kasatkina st., 13, Moscow 129301, Russia b National Research Centre “Kurchatov Institute”, Akademica Kurchatova Sq., Moscow, 1123182, Russia, b National Research Centre “Kurchatov Institute”, Akademica Kurchatova Sq., Moscow, 1123182, Russia, c Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences, c Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences, Leninskiy prospect, 59, 119333, Moscow, Russia Leninskiy prospect, 59, 119333, Moscow, Russia
Abstract Abstract
Nickel alloys with operating temperatures up to 650 °C are used for the manufacture of turbine disks in modern gas Nickel alloys with operating temperatures up to 650 °C are used for the manufacture of turbine disks in modern gas turbine engines. During an operation or testing the high centrifugal forces, which appear in the discs, could initiate turbine engines. During an operation or testing the high centrifugal forces, which appear in the discs, could initiate fatigue cracks formation. If the cracks start from internal defects, there is no air access and their growth occurs in a fatigue cracks formation. If the cracks start from internal defects, there is no air access and their growth occurs in a vacuum conditions. The failure mechanism will correspond to the low-cycle fatigue. Various electron microscopy vacuum conditions. The failure mechanism will correspond to the low-cycle fatigue. Various electron microscopy methods were used in the study of billets formed from the powder nickel alloys. The fractographic analysis methods were used in the study of billets formed from the powder nickel alloys. The fractographic analysis demonstrated that in vacuum conditions a rugged rough surfaces of the crack consisted of spherical particles with demonstrated that in vacuum conditions a rugged rough surfaces of the crack consisted of spherical particles with the sizes in the range between 50 and 150 µm is formed at the beginning. When a crack reaches a billet surface, the the sizes in the range between 50 and 150 µm is formed at the beginning. When a crack reaches a billet surface, the crack surface relief changes to a quasi-faceted one, and that corresponds to the first stage of fatigue fracture crack surface relief changes to a quasi-faceted one, and that corresponds to the first stage of fatigue fracture development. It was found that nano-grains with the size of 100–200 nm are formed in front of the crack tip. The development. It was found that nano-grains with the size of 100–200 nm are formed in front of the crack tip. The crack surfaces are covered with amorphous layers with a thickness of about 20 nm. The destruction occurs mainly crack surfaces are covered with amorphous layers with a thickness of about 20 nm. The destruction occurs mainly along the boundaries of nanocrystals layer. A model is proposed to explain the appearance of such microstructure along the boundaries of nanocrystals layer. A model is proposed to explain the appearance of such microstructure and the key factor is the air access to a crack and hence the oxidation of the crack surfaces. and the key factor is the air access to a crack and hence the oxidation of the crack surfaces. 2019The TheAuthors. Authors. Published by Elsevier © 2019 Published by Elsevier B.V. B.V. © 2019 The Authors. Published by Elsevier B.V. This isisan anopen openaccess access article under the BY-NC-ND CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/4.0/) This article under the CC licenselicense (http://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article under the CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review responsibility of the committee of license the ICMSMF Peer-reviewunder under responsibility of scientific the scientific committee of the ICorganizers MSMF organizers. Peer-review under responsibility of the scientific committee of the IC MSMF organizers.
Keywords: Scanning electron microscope; low-cycle fatigue; transmission electron microscope; powder nickel-base superalloy; EP741NP; Keywords: Scanning electron microscope; low-cycle fatigue; transmission electron microscope; powder nickel-base superalloy; EP741NP; fatigue crack growth mechanism; structural analysis; fatigue crack in vacuum; low-cycle fatigue fatigue crack growth mechanism; structural analysis; fatigue crack in vacuum; low-cycle fatigue
* Corresponding author. Tel.: +7-499-755-08-44; fax: +7-495-683-09-97. * Corresponding author. Tel.: +7-499-755-08-44; fax: +7-495-683-09-97. E-mail address: [email protected] E-mail address: [email protected] 2452-3216 © 2019 The Authors. Published by Elsevier B.V. 2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the IC MSMF organizers. Peer-review under responsibility of the scientific committee of the IC MSMF organizers.
2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the ICMSMF organizers 10.1016/j.prostr.2020.01.095
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M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000–000
1. Introduction Powder metallurgy nickel based superalloys are used for the manufacture of turbine disks (TDs) of modern gas turbine engines (GTE) and power plants. These parts work at temperatures of 650 °C and above. Moreover, the turbine disks are under the influence of the load caused by the rotation of the disk and the centrifugal forces of the turbine blades. During low-cycle fatigue tests in conditions close to real, fatigue cracks from internal defects could appear. The initial stage of growth of fatigue cracks before they reach the surface occurs in a vacuum condition. The study of the development of the fatigue cracks in TDs is interesting from the scientific and practical points of view. Besides, there is a question on the mechanisms of the fatigue cracks development in the presence of the air and without air access. The previous studies demonstrated a significant difference in the crack growth rate for these types of cracks [1,2]. The existing methods of determination of the life between overhauls of TDs are based on monitoring the development of fatigue cracks. In this article we present the results of the microstructural analysis of fatigue cracks, which were originated beneath the sample surface and expanding without air access. Different electron microscopy methods, namely scanning, transmission and scanning-transmission electron microscopy (SEM, TEM and STEM, respectively) together with energy dispersive microanalysis were used. 2. Material and methods The cylindrical billets with a gauge length of 13 mm and a diameter of 4.37 mm, were formed from a high temperature nickel based EP741NP alloy, which is used for the manufacture of the aircraft GTE discs. This material is similar to AF115, AF21DA6, Alloy-10 and LSHR alloys [3]. The powder forging method was standard hydrostatic pressing of the powder, obtained by the plasma rotate electrode process (PREP). The size of the powder was less than 140 µm. The chemical composition of the powder is presented in table 1. Table 1 – The alloy element content (weight %) [3] Ni C Cr Mo W Al Basis 0.04 9.0 3.9 5.5 5.1
Ti 1.8
Co 15.8
Nb 2.6
Hf 0.25
B ≤0.015
Zr <0.015
The alloy after heat treatment exhibited typical microstructure [3]: a γ-Ni solid solution with an average grain size of 40 μm with a hardening intermetallic γ'-phase. The tests were carried out in the soft cycle condition (stress control), the maximum stress level in the cycle was σ max = 980 MPa, the cycle asymmetry parameter R = 0.1. The working conditions of the TDs determines, that if a fatigue crack starts growing inside the billet, the fracture mechanism corresponds to low-cycle fatigue. Two samples with fatigue cracks originated and grew beneath the surface were selected. Sample No.1: the final destruction occurred during the test. The fractured sample after testing was kept in an oven at a temperature of 650 °С. Sample No.2: the fatigue crack was of mixed type. The fatigue crack grew up to the surface and further development occurred with the air access. The microstructural and fractographic analysis were performed for the crack in sample No.1 and only fractographic - for the crack in sample No.2.The fractographic analysis was performed in a scanning electron microscopes (SEM) JSM-IT300LV (JEOL, Japan) and a dual-beam SEM-focus ion beam (FIB) Helios NanoLab 660 (ThermoFisher Scientific, USA). Cross-sections specimens were prepared by standard lift-out technique in a SEM/FIB Helios Nanolab (ThermoFisher Scientific, USA), equipped with a micromanipulator Omniprobe (Omniprobe, USA) for extracting thin lamella specimens. The lamellas were studied in a Titan 80-300 TEM/STEM (FEI, USA) at an accelerating voltage of 300 kV. The device is equipped with an EDX Si(Li) spectrometer (EDAX, USA), High Angle Annular Dark Field (HAADF) electron detector (Fischione, USA) and Gatan Image Filter (GIF) (Gatan, USA).
M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000–000
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3. Results 3.1. Fractographic study of the samples The investigation of specimen No. 1 by SEM demonstrated that crack started from the defects in the alloy (Fig. 1 a), which were formed near HfO2 based precipitates [4]. We found that the crack development occurred in three stages with the formation of a specific surface morphology. The rugged and rough surface covered by round-shaped particles with sizes in the range between 50 nm and 150 nm was formed at the first stage (Fig. 1). It should be noted that the morphology of the particles is determined by their microstructure, not associated with oxidation process. At the second stage, a zone with fatigue striations is formed. The formation of a transition region with the fatigue striations and rounded submicron particles was associated with the change of the crack propagation mechanism. In addition, a third zone was identified and it corresponds to the transition from stable crack growth to the rupture area. In the presented study, the second and third zones were not studied. Unlike sample No. 1, four zones were observed in sample No. 2, and each of them corresponds to four stages of crack development. The first and second zones are indicated on the SEM images shown in Fig. 6a and b. The first zone is similar to the one, found in sample No1. The second zone exhibited quasi-faceted morphology (Fig. 2). Highly likely, it was formed exactly it the moment when the crack had reached the surface of the sample and the admittance of air started with the surface oxidation. The third zone has a distinct appearance with fatigue striations. The fourth zone is similar to the third zone of specimen No. 1 - the transition from stability crack growth to the rupture area. SEM image (Fig. 2) clearly illustrate the moment when the crack reaches the surface. This image was obtained in backscattered electrons (zone 1, Fig. 2 a) and part of it has brighter contrast. That pointed to thinner oxide layer on the crack surface.
3.2. The study of cross-sections. The cross-sections for microstructural analysis were cut out at a distance of 30 μm from the crack origin of sample No. 1 (Fig. 2). The direction of the cut was chosen towards the development of fatigue crack. The HAADF STEM images of the specimens are presented in Fig. 3 a,b. The layer beneath the protective Pt layer exhibits darker contrast and consists of nanoparticles (Fig. 3 a). Several areas with different contrast can be revealed in the enlarged image (Fig. 3 b). The EDX microanalysis indicated that the darker areas between the grains (Fig. 3 b) contains more than 50% C and O. Below this layer, there are large particles, which consist of highly textured sub-grains misoriented relatively each other on 1-2°. The electron diffraction study of the sub-grains unambiguously demonstrate that the crystal structure correspond to face centered cubic Fm 3͞ m space group with the unit cell parameter a = 0.34–0.36 nm and that is completely consistent with γ-phase [5]. The overall orientation of the subgrains in this area is close to [101] cubic unit cell perpendicular to the specimen surface. Apparently, this corresponds to the orientation of the γ’-phase grain before the tests, in which the development of the fatigue crack occurred. It should be noted that, in the volume of sample No. 1, low density of NiCo particles were also detected. The unit cell parameters of this phase with a high content of Ni and Co are close to γ phase [6]. Analysis of high-resolution TEM images (Fig. 4) showed that particles of different phases were formed in the near-surface area of the crack: Ni3C (Space Group R ͞3c, a = 0.45 nm, c = 12.9 nm [7]) and NiO (Space Group Fm ͞3m, a = 0.418 nm [8]). Between Ni, NiO and Ni3C particles thick amorphous layers (up to 20 nm) were found (Fig. 3b and Fig. 4c).
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Fig. 1. Secondary electron SEM images of sample No. 1. (a) – Zone 1. (b) - The enlarged image highlighted by the blue rectangle in (a).
Fig. 2. SEM images of the origin of the fracture area of sample No. 2: (a) – a low magnification backscattered electrons image; (b) - enlarged secondary electrons image of the origin of the fracture area. The fracture area, which corresponds to the crack area developed in vacuum condition, is denoted as “1”, and with the access of the air, denoted as “2 (quasi-faceted fracture). The dotted line shows the boundary between the areas “1” and “2”.
“ Fig. 3. HAADF STEM images of the cross-section of sample No 1: (a) –low magnification TEM image, the arrows indicate the layer containing oxides and carbides; (b) - enlarged image of the oxide layer. The rectangle highlights the area, where EDX microanalysis was performed.
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In some areas nickel carbonates in the form of NiCO3 (Space Group R ͞3c, a = 0.4617 nm, c = 1.4735 nm) [9]) particles embedded in the amorphous layers were found. The formation of these particles could occur after air access. The rounded particles observed in the cross-sections specimens are the ones, which had been found by SEM (Fig. 1b, Fig. 2b).
Fig. 4. (a) Bright field TEM image of nanograins near the fracture surface and the amorphous areas between them; (b)Fast Fourier Transform (FFT) spectrum corresponded to the Ni3C phase; (c) FFT spectrum obtained from the area in the square; (d) FFT corresponding to the NiO phase.
4. Discussion Based on the experimental data the mechanism of an internal fatigue crack development without air access was proposed. At the initial stage (samples No. 1 and No. 2) a plastic deformation zone is formed in front of the crack tip [10]. The formation of nanocrystals observed by SEM (Fig. 1b, 2b), and by TEM (Fig. 3, 4) occurred in this zone. Apparently, the microstructure with the nanocrystals is formed in several loading cycles. The TEM and electron diffraction data pointed to the high degree of misorientation of nanocrystals. The misoriented nanocrystals lead to the appearance of relatively "weak" links at nanocrystal boundaries. As a result, the propagation of a crack occurs through the boundaries of nanocrystals. The formation of "weak" links was also confirmed by the observations of the thick amorphous layers at the boundaries of nanocrystals in specimen No. 1 (Fig. 3b) after high-temperature oxidation of the fracture surface. Thus, the mechanism of failure, discussed above, can be defined as inter nanocrystalline. It is known that the formation of nanocrystals with large misorientation requires an excess of dislocations of the same sign [11]. However, the formation of dislocations ahead of the crack tip is energetically unlikely (Fig. 5a) [11]. To explain the large misorientation of nanocrystals, we proposed the following model. During loading cycles, there are periodic opening and squeezing of the crack. When loading is released and the crack is squeezed, the so-called "cold welding" process can occur [12]. Opposite surfaces of the crack, when they are oxide-free and there is no loading joined and chemical connection forms. After subsequent loading and crack opening dislocations of one sign (see Fig. 5b) ahead of the crack tip are formed due to the stress. A multiple loading cycle arises the misorientation of nanocrystallites (Fig. 5c) followed by further crack propagation along the weak grain boundaries of nanocrystals.
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According to TEM data, the twist of 100 nm blocks by 2° degrees leads to the displacement of atoms by more than 1.7 nm and that significantly exceeds the interatomic distance. Such disorder causes the formation of the amorphous layer, which promotes the crack spread. The proposed model explains the change in the mechanism of crack growth from inter-nanocrystalline to quasi-faceted at the beginning of the air access into the crack space (Fig. 2). After that, the crack surfaces oxidized and the oxide film prevents the "cold welding" process.
Fig. 5. (a) - scheme formation of dislocations in the material under the condition of air access to the crack; (b, c) and in the condition of vacuum.
5. Conclusion It was found that a layer of highly misoriented nanocrystals with a size of 50–200 nm is formed in the first stage of crack formation. It has been established that fatigue cracks propagate along the boundaries of nanocrystals. A model is proposed to explain the mechanism for the formation of the misorientation of nanocrystals during the crack development without air access. The difference in the mechanisms of crack growth with and without air access is found and explained. References 1. J.E. King, 1982. Surface damage and near-threshold fatigue crack growth in a ni-base superalloy in vacuum. Fatigue of Engineering Materials and Structures Vol. 5, No. 2, pp. 177-188,. 2. R.J. Kashinga, L.G. Zhao, V. V.Silberschmidt,1 R. Jiang, and P.A.S. Reed, 2018. A diffusion-based approach for modelling crack tip behaviour under fatigue-oxidation conditions. Int J Fract. 213(2), pp. 157–170 3. J. Radavich, D. Furrer, T. Carneiro, J. Lemsky, 2008. The Microstructure and Mechanical Properties of EP741NP Powder Metallurgy Disc Material. Superalloys 2008, TMS, pp 63-72. 4. Trunkin I.N. Artamonov M.A., Ovcharov A.V., Vasilyev A.L. 2009. Structural study of defects in granulated nickel alloy EP741NP. Crystallography Reports. T. 64. № 4. pp. 539-543 (Rus) 5. Yousuf M., Sahu P.C., Jajoo H.K., Rajagopalan S., Govinda Rajan K., 1986. Effect of magnetic transition on the lattice expansion of nickel. Journal of Physics F., V. 16., №. 5, pp. 373-380. 6. Taylor A., 1950. Lattice parameters of binary nickel-cobalt alloys. Journal of the Institute of Metals, V. 77, pp. 585-594. 7. Nagakura S., 1958. Study of metallic carbides by electron diffraction Part II. Crystal structure analysis of nickel carbide. Journal of the Physical Society of Japan, V. 13, №. 9, pp. 1005-1014. 8. Taylor D., 1984. Thermal expansion data: I. Binary oxides with the sodium chloride and wurtzite structure, MO. Transactions and Journal of the British Ceramic Society, V. 83, №. 1, pp. 5–9. 9. Pertlik F., 1986. Structures of hydrothermally synthesized cobalt (II) carbonate and nickel (II) carbonate. Acta Crystallographica. V. 42, №. 1, pp. 4-5. 10. Broek, D., 1982. Elementary engineering fracture mechanics. Springer Science, pp 469. 11 V.I. Vladimirov, 1984. Physical nature of destruction. M .: Metallurgy, pp. 280. (Rus) 12 Oguma, H.; Nakamura, T., 2013. Fatigue crack propagation properties of Ti-6Al-4V in vacuum environments. International Journal of Fatigue, 50, pp.89-93.