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Formation of multiply twinned martensite plates in rapidly solidified Ni3Al-based superalloys
Materials Letters 250 (2019) 147–150
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Formation of multiply twinned martensite plates in rapidly solidified Ni3Al-based superalloys Yefan Li, Chong Li ⇑, Jing Wu, Yuting Wu, Zongqing Ma, Liming Yu, Huijun Li, Yongchang Liu ⇑ State Key Lab of Hydraulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300354, PR China
a r t i c l e
i n f o
Article history: Received 30 April 2019 Accepted 4 May 2019 Available online 4 May 2019 Keywords: Intermetallic alloys and compounds Microstructure Phase transformation Martensitic formation Precipitates
a b s t r a c t Influence of solidification rate on the interdendritic microstructure evolution of a multiphase Ni3Al-based superalloy was investigated. Compared to the original as cast sample, rapid solidification rate leads to high nucleation burst of a-Cr precipitates in spray casting alloy, which promotes the transformation from b.c.c. B2 b phase to f.c.t. L10 martensite plates with high density of stacking faults and microtwins in interdendritic regions. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Ni3Al-based superalloys are widely applied in high temperature structural applications such as the turbine blades and vanes in the aircraft engines owing to their outstanding material properties [1,2]. In decades, to achieve higher microstructural stability and more superior mechanical property, various alloying elements such as Cr, Ti, Mo, Fe and B had been added into Ni3Al-based superalloys [3–6]. However, the complex chemical composition made the alloy phase composition much more complicated, which had a great influence on their properties [7,8]. Lapin [9] observed the presence of different phases (c, c0 , b, b0 and a-Cr) in a directionally solidified Ni-20Al-8Cr-2Fe alloy, and the microstructural characteristics (size, morphology and distribution) significantly influenced the room-temperature yield strength. In our previous research [10], a novelly designed polycrystalline multiphase Ni3Al-based superalloy with iron addition was studied to improve the thermoplasticity and weldability by inducing the formation of interdendritic b phase. Heat treatment and hot compression can lead to the microstructural evolution which significant influence on high-temperature mechanical behavior [10–12]. In addition, the b phase is sensitive to cooling rates, and abundant literatures demonstrate that martensitic transformation from b.c. c. B2 to f.c.t. L10 can occur on cooling because of the structural correlation in Ni-Al alloys [13–16]. ⇑ Corresponding authors. E-mail addresses: [email protected] (C. Li), [email protected] (Y. Liu). https://doi.org/10.1016/j.matlet.2019.05.012 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
In the paper, the influence of solidification rate on the interdendritic b phase in Ni3Al-based superalloys was studied. Rapid solidification rate (spray casting) promotes the transformation from b.c. c. B2 b phase to f.c.t. L10 martensite plates with microtwins and high density of stacking faults. And the transformation mechanism was analyzed and discussed in terms of detail feature of martensite plate structure, which enriches understanding of growth microtwins formation in Ni-Al system superalloys.
2. Materials and experimental procedure The material used in this study is a Ni3Al-based superalloy with chemical composition (wt%): Ni-8.9Al-6.95Cr-11.7Fe-1.18M o-0.45Hf-0.077C, produced by vacuum induction melting (VIM) and electro slag remelting (ESR) techniques. In order to obtain the spray casting samples, the remelting metal, culted from the cast alloy, was yielding ingot with diameter of 4 mm and solidified with the cooling rate about 103 K/s. The cooling rate of original as cast sample was about 10 K/s. A detailed diffraction contrast TEM analysis was performed using a FEI Tecnai G2 F30 (TEM) equipped with energy dispersive spectroscopy, operated at 200 keV. High resolution TEM (HRTEM) images were performed in a probe-corrected FEI Titan field emission gun transmission electron microscope operated at 300 keV. The foils for TEM observation were prepared by grinding using progressively finer SiC paper to a thickness of 40 lm. The final thinning was performed by the method of ion-beam thinning.
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3. Results and discussion As shown in Fig. 1a, the microstructure of the as cast alloy consists of dendrites (dual c + c0 ) and interdendritic regions [10,11]. TEM morphology of the interdendritic region is shown in Fig. 1b. The enlarged micrograph of the region marked by a square in (b) is displayed in Fig. 1c, where a number of approximate spherical particles with the average size about 50 nm are observed in the interdendritic region. The matrix in interdendritic region is the ordered b.c.c. B2-type b phase (Fig. 1e). The EDS results (Fig. 1d) associated with the corresponding FFT pattern (Fig. 1f) indicate that the spherical particle is a-Cr.
For the spray casting sample, the microstructure also consists of dendrites (dual c + c0 ) and interdendritic regions (Fig. 2a). However, the morphology of interdendritic region changes significantly, and typical lamellar microstructure occurs (Fig. 2b). According to the corresponding selected-area electron diffraction (SAED) patterns (Fig. 2c), it indicates that lamellar microstructure is the ordered f.c.t. L10-type martensite phase, and the martensite plates are internally twined, yielding a crystallographic relation h h i i close to 1 1 1 0 1 1 == 1 1 1 0 1 1 . T
M
In order to investigate the detail feature of martensite structure, representative bright-field (BF) TEM images of the interdendritic
Fig. 1. (a) SEM morphology of the original as cast alloy; (b) TEM morphology of the interdendritic region; (c) A higher magnification BF TEM micrograph view of the region marked by a square in (b); (d) EDS result of the sub-spherical phase marked with ‘A’ in (c); (e) HRTEM image of sub-spherical phase in matrix phase, inset showing the corresponding FFT pattern of the matrix region ‘B’; (f) The corresponding FFT pattern of the sub-spherical phase marked with ‘C’ in (e).
Fig. 2. (a) SEM morphology of the spray casting alloy; (b) TEM morphology of the interdendritic region; (c) The corresponding SAED pattern of the region marked ‘A’ in (b).
Y. Li et al. / Materials Letters 250 (2019) 147–150
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Fig. 3. (a) A higher magnification BF TEM micrograph of the interdendritic region of spray casting alloy; (b) Magnified TEM morphology of the martensite plate; (c) and (d) representative stacking faults and multiple microtwins in martensite plates.
region in the spray casting alloy are shown in Fig. 3. It can be seen that fine a-Cr particles (several nanometers) in a high number density are evident in the interdendritic martensite plates (Fig. 3a). Besides a-Cr particles, lots of stacking faults and microtwins are also found in martensite plates (Fig. 3b). A high density of stacking faults on adjacent {1 1 1} planes and multiple twins with different thicknesses can be seen clearly within the martensite plates (Fig. 3c and d). The spacing of the twins are in the range from several {1 1 1} planes distances to tens of atomic planes, as shown in Fig. 3d. Interestingly, there is a high density of stacking faults and microtwins in martensite plates with the existence of lots of a-Cr particles (Region 1), while few microtwins exist in the region with relative few a-Cr particles (Region 2), as shown in Fig. 4a–c. The results suggest that a-Cr phase has a correlation with the formation of stacking faults and multiple microtwin structure. HRTEM image of a-Cr phase in matrix phase (martensite) is shown in Fig. 4d. And it can be seen that lots of dislocations exist around fine a-Cr precipitate (Fig. 4f). Twinnedmicrostructure develops on rapid solidification when the martensitic transformation occurs too rapidly for the structure
to deform and accommodate strain [15,17]. Rapid solidification promotes the high nucleation burst of a-Cr precipitates in the interdendritic region for the spray casting sample (Fig. 3a). The difference of crystal structure between the a-Cr precipitates and the martensite matrix leads to the formation of lattice distortion and mismatch stress. And the fine a-Cr precipitates (stress concentrators) act as sources for dislocations that can relax the high coherency stress at the phase interface (Fig. 4f). When two extended dislocations on the adjacent {1 1 1} planes are close to each other, the intrinsic staking faults of the two dislocations in the overlapping region merge to form an extrinsic stacking fault. When more extended dislocations join the overlapped pair, an extrinsic stacking fault transforms into a microtwin [18–20]. Moreover, the dislocations directly adjacent to the second phases can act as a precursor to microtwinning [21]. The stress at the interface promotes the dislocations merge into stacking faults and microtwins [21]. As a result, martensite with a high density of stacking faults and multiple twins are formed in the spray casting sample, just as shown in Fig. 3. The detail transition from dislocation dissociation and decorrelation to stacking faults or microtwins needs further research through detailed TEM characterization.
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Fig. 4. (a) HRTEM image of another interdendritic region; (b) The corresponding FFT patterns of ‘region 1’ in (a); (c) The corresponding FFT pattern of ‘region 2’ in (a); (d) HRTEM image of a-Cr phase in martensite matrix phase; (e) The corresponding FFT patterns of the area marked by a dashed rectangle in (d); (f) The corresponding inverse FFT image of the area by a dashed rectangle in (d).
4. Conclusions
References
Rapid solidification rate (spray casting) leads to the transformation from b.c.c. B2 b phase to f.c.t. L10 martensite plates in interdendritic regions of Ni3Al-based superalloys. Compared to the original as cast alloy, rapid solidification rate promotes high nucleation burst of a-Cr precipitates acting as sources for dislocations. Dislocations can act as a precursor to stacking faults and microtwinning, which promotes the formation of martensite plates with a high density of stacking faults and multiple microtwins.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Declaration of Competing Interest The authors declare that they have no known conflict of interest that could have appeared to influence the work reported in this paper. Acknowledgment This work was supported by the National Natural Science Foundation of China (Nos. 51774212 and 51674175).
M. Qian, H. Luo, C. Ding, et al., Mater. Charact. 132 (2017) 458–466. P. Zhang, Y. Yuan, B. Li, S.W. Guo, et al., Mater. Sci. Eng. A 655 (2016) 152–159. S. Khan, J.B. Singh, A. Verma, et al., Mater. Sci. Eng. A 686 (2017) 176–183. T.M. Smith, B.D. Esser, N. Antolin, et al., Nat. Commun. 7 (2016) 13434. J. Huo, Q. Shi, Y. Zheng, Q. Feng, Mater. Charact. 124 (2017) 73–82. J. Ding, S. Jiang, Y. Wu, et al., Mater. Lett. 211 (2018) 5–8. S. Milenkovic, I. Sabirov, J. LLorca, Mater. Lett. 73 (2012) 216–219. L. Liu, T.W. Huang, J. Zhang, et al., Mater. Lett. 61 (2007) 227–230. J. Lapin, Intermetallics 5 (1997) 615–624. J. Wu, C. Li, Y.C. Liu, et al., Mater. Sci. Eng. A 743 (2019) 623–635. J. Wu, C. Li, Y. Liu, et al., Intermetallics 109 (2019) 48–59. Y. Wu, Y. Liu, C. Li, et al., J. Alloys Compd. 712 (2017) 687–695. P.L. Potapov, N.A. Poliakova, V.A. Udovenko, Scr. Mater. 35 (1996) 423–427. D. Schryvers, Y. Ma, L. Toth, et al., Acta Metall. Mater. 43 (1995) 4045–4056. S. Chakravorty, C.M. Wayman, Metal. Trans. A 7 (1976) 555–568. D. Schryvers, D. Holland-Moritz, Intermetallics 6 (1998) 427–436. D. Schryvers, D. Holland-Moritz, Mater. Sci. Eng. A 273 (1999) 697–702. Y. Xu, C. Yang, X. Xiao, et al., Mater. Sci. Eng. A 530 (2011) 315–326. P.M. Sarosi, G.B. Viswanathan, et al., Scr. Mater. 55 (2006) 727–730. S. Mahajan, Scr. Mater. 68 (2013) 95–99. R.R. Unocic, N. Zhou, L. Kovarik, et al., Acta Mater. 59 (2011) 7325–7339.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Formation of multiply twinned martensite plates in rapidly solidified Ni3Al-based superalloys Yefan Li, Chong Li ⇑, Jing Wu, Yuting Wu, Zongqing Ma, Liming Yu, Huijun Li, Yongchang Liu ⇑ State Key Lab of Hydraulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300354, PR China
a r t i c l e
i n f o
Article history: Received 30 April 2019 Accepted 4 May 2019 Available online 4 May 2019 Keywords: Intermetallic alloys and compounds Microstructure Phase transformation Martensitic formation Precipitates
a b s t r a c t Influence of solidification rate on the interdendritic microstructure evolution of a multiphase Ni3Al-based superalloy was investigated. Compared to the original as cast sample, rapid solidification rate leads to high nucleation burst of a-Cr precipitates in spray casting alloy, which promotes the transformation from b.c.c. B2 b phase to f.c.t. L10 martensite plates with high density of stacking faults and microtwins in interdendritic regions. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Ni3Al-based superalloys are widely applied in high temperature structural applications such as the turbine blades and vanes in the aircraft engines owing to their outstanding material properties [1,2]. In decades, to achieve higher microstructural stability and more superior mechanical property, various alloying elements such as Cr, Ti, Mo, Fe and B had been added into Ni3Al-based superalloys [3–6]. However, the complex chemical composition made the alloy phase composition much more complicated, which had a great influence on their properties [7,8]. Lapin [9] observed the presence of different phases (c, c0 , b, b0 and a-Cr) in a directionally solidified Ni-20Al-8Cr-2Fe alloy, and the microstructural characteristics (size, morphology and distribution) significantly influenced the room-temperature yield strength. In our previous research [10], a novelly designed polycrystalline multiphase Ni3Al-based superalloy with iron addition was studied to improve the thermoplasticity and weldability by inducing the formation of interdendritic b phase. Heat treatment and hot compression can lead to the microstructural evolution which significant influence on high-temperature mechanical behavior [10–12]. In addition, the b phase is sensitive to cooling rates, and abundant literatures demonstrate that martensitic transformation from b.c. c. B2 to f.c.t. L10 can occur on cooling because of the structural correlation in Ni-Al alloys [13–16]. ⇑ Corresponding authors. E-mail addresses: [email protected] (C. Li), [email protected] (Y. Liu). https://doi.org/10.1016/j.matlet.2019.05.012 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
In the paper, the influence of solidification rate on the interdendritic b phase in Ni3Al-based superalloys was studied. Rapid solidification rate (spray casting) promotes the transformation from b.c. c. B2 b phase to f.c.t. L10 martensite plates with microtwins and high density of stacking faults. And the transformation mechanism was analyzed and discussed in terms of detail feature of martensite plate structure, which enriches understanding of growth microtwins formation in Ni-Al system superalloys.
2. Materials and experimental procedure The material used in this study is a Ni3Al-based superalloy with chemical composition (wt%): Ni-8.9Al-6.95Cr-11.7Fe-1.18M o-0.45Hf-0.077C, produced by vacuum induction melting (VIM) and electro slag remelting (ESR) techniques. In order to obtain the spray casting samples, the remelting metal, culted from the cast alloy, was yielding ingot with diameter of 4 mm and solidified with the cooling rate about 103 K/s. The cooling rate of original as cast sample was about 10 K/s. A detailed diffraction contrast TEM analysis was performed using a FEI Tecnai G2 F30 (TEM) equipped with energy dispersive spectroscopy, operated at 200 keV. High resolution TEM (HRTEM) images were performed in a probe-corrected FEI Titan field emission gun transmission electron microscope operated at 300 keV. The foils for TEM observation were prepared by grinding using progressively finer SiC paper to a thickness of 40 lm. The final thinning was performed by the method of ion-beam thinning.
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3. Results and discussion As shown in Fig. 1a, the microstructure of the as cast alloy consists of dendrites (dual c + c0 ) and interdendritic regions [10,11]. TEM morphology of the interdendritic region is shown in Fig. 1b. The enlarged micrograph of the region marked by a square in (b) is displayed in Fig. 1c, where a number of approximate spherical particles with the average size about 50 nm are observed in the interdendritic region. The matrix in interdendritic region is the ordered b.c.c. B2-type b phase (Fig. 1e). The EDS results (Fig. 1d) associated with the corresponding FFT pattern (Fig. 1f) indicate that the spherical particle is a-Cr.
For the spray casting sample, the microstructure also consists of dendrites (dual c + c0 ) and interdendritic regions (Fig. 2a). However, the morphology of interdendritic region changes significantly, and typical lamellar microstructure occurs (Fig. 2b). According to the corresponding selected-area electron diffraction (SAED) patterns (Fig. 2c), it indicates that lamellar microstructure is the ordered f.c.t. L10-type martensite phase, and the martensite plates are internally twined, yielding a crystallographic relation h h i i close to 1 1 1 0 1 1 == 1 1 1 0 1 1 . T
M
In order to investigate the detail feature of martensite structure, representative bright-field (BF) TEM images of the interdendritic
Fig. 1. (a) SEM morphology of the original as cast alloy; (b) TEM morphology of the interdendritic region; (c) A higher magnification BF TEM micrograph view of the region marked by a square in (b); (d) EDS result of the sub-spherical phase marked with ‘A’ in (c); (e) HRTEM image of sub-spherical phase in matrix phase, inset showing the corresponding FFT pattern of the matrix region ‘B’; (f) The corresponding FFT pattern of the sub-spherical phase marked with ‘C’ in (e).
Fig. 2. (a) SEM morphology of the spray casting alloy; (b) TEM morphology of the interdendritic region; (c) The corresponding SAED pattern of the region marked ‘A’ in (b).
Y. Li et al. / Materials Letters 250 (2019) 147–150
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Fig. 3. (a) A higher magnification BF TEM micrograph of the interdendritic region of spray casting alloy; (b) Magnified TEM morphology of the martensite plate; (c) and (d) representative stacking faults and multiple microtwins in martensite plates.
region in the spray casting alloy are shown in Fig. 3. It can be seen that fine a-Cr particles (several nanometers) in a high number density are evident in the interdendritic martensite plates (Fig. 3a). Besides a-Cr particles, lots of stacking faults and microtwins are also found in martensite plates (Fig. 3b). A high density of stacking faults on adjacent {1 1 1} planes and multiple twins with different thicknesses can be seen clearly within the martensite plates (Fig. 3c and d). The spacing of the twins are in the range from several {1 1 1} planes distances to tens of atomic planes, as shown in Fig. 3d. Interestingly, there is a high density of stacking faults and microtwins in martensite plates with the existence of lots of a-Cr particles (Region 1), while few microtwins exist in the region with relative few a-Cr particles (Region 2), as shown in Fig. 4a–c. The results suggest that a-Cr phase has a correlation with the formation of stacking faults and multiple microtwin structure. HRTEM image of a-Cr phase in matrix phase (martensite) is shown in Fig. 4d. And it can be seen that lots of dislocations exist around fine a-Cr precipitate (Fig. 4f). Twinnedmicrostructure develops on rapid solidification when the martensitic transformation occurs too rapidly for the structure
to deform and accommodate strain [15,17]. Rapid solidification promotes the high nucleation burst of a-Cr precipitates in the interdendritic region for the spray casting sample (Fig. 3a). The difference of crystal structure between the a-Cr precipitates and the martensite matrix leads to the formation of lattice distortion and mismatch stress. And the fine a-Cr precipitates (stress concentrators) act as sources for dislocations that can relax the high coherency stress at the phase interface (Fig. 4f). When two extended dislocations on the adjacent {1 1 1} planes are close to each other, the intrinsic staking faults of the two dislocations in the overlapping region merge to form an extrinsic stacking fault. When more extended dislocations join the overlapped pair, an extrinsic stacking fault transforms into a microtwin [18–20]. Moreover, the dislocations directly adjacent to the second phases can act as a precursor to microtwinning [21]. The stress at the interface promotes the dislocations merge into stacking faults and microtwins [21]. As a result, martensite with a high density of stacking faults and multiple twins are formed in the spray casting sample, just as shown in Fig. 3. The detail transition from dislocation dissociation and decorrelation to stacking faults or microtwins needs further research through detailed TEM characterization.
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Fig. 4. (a) HRTEM image of another interdendritic region; (b) The corresponding FFT patterns of ‘region 1’ in (a); (c) The corresponding FFT pattern of ‘region 2’ in (a); (d) HRTEM image of a-Cr phase in martensite matrix phase; (e) The corresponding FFT patterns of the area marked by a dashed rectangle in (d); (f) The corresponding inverse FFT image of the area by a dashed rectangle in (d).
4. Conclusions
References
Rapid solidification rate (spray casting) leads to the transformation from b.c.c. B2 b phase to f.c.t. L10 martensite plates in interdendritic regions of Ni3Al-based superalloys. Compared to the original as cast alloy, rapid solidification rate promotes high nucleation burst of a-Cr precipitates acting as sources for dislocations. Dislocations can act as a precursor to stacking faults and microtwinning, which promotes the formation of martensite plates with a high density of stacking faults and multiple microtwins.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Declaration of Competing Interest The authors declare that they have no known conflict of interest that could have appeared to influence the work reported in this paper. Acknowledgment This work was supported by the National Natural Science Foundation of China (Nos. 51774212 and 51674175).
M. Qian, H. Luo, C. Ding, et al., Mater. Charact. 132 (2017) 458–466. P. Zhang, Y. Yuan, B. Li, S.W. Guo, et al., Mater. Sci. Eng. A 655 (2016) 152–159. S. Khan, J.B. Singh, A. Verma, et al., Mater. Sci. Eng. A 686 (2017) 176–183. T.M. Smith, B.D. Esser, N. Antolin, et al., Nat. Commun. 7 (2016) 13434. J. Huo, Q. Shi, Y. Zheng, Q. Feng, Mater. Charact. 124 (2017) 73–82. J. Ding, S. Jiang, Y. Wu, et al., Mater. Lett. 211 (2018) 5–8. S. Milenkovic, I. Sabirov, J. LLorca, Mater. Lett. 73 (2012) 216–219. L. Liu, T.W. Huang, J. Zhang, et al., Mater. Lett. 61 (2007) 227–230. J. Lapin, Intermetallics 5 (1997) 615–624. J. Wu, C. Li, Y.C. Liu, et al., Mater. Sci. Eng. A 743 (2019) 623–635. J. Wu, C. Li, Y. Liu, et al., Intermetallics 109 (2019) 48–59. Y. Wu, Y. Liu, C. Li, et al., J. Alloys Compd. 712 (2017) 687–695. P.L. Potapov, N.A. Poliakova, V.A. Udovenko, Scr. Mater. 35 (1996) 423–427. D. Schryvers, Y. Ma, L. Toth, et al., Acta Metall. Mater. 43 (1995) 4045–4056. S. Chakravorty, C.M. Wayman, Metal. Trans. A 7 (1976) 555–568. D. Schryvers, D. Holland-Moritz, Intermetallics 6 (1998) 427–436. D. Schryvers, D. Holland-Moritz, Mater. Sci. Eng. A 273 (1999) 697–702. Y. Xu, C. Yang, X. Xiao, et al., Mater. Sci. Eng. A 530 (2011) 315–326. P.M. Sarosi, G.B. Viswanathan, et al., Scr. Mater. 55 (2006) 727–730. S. Mahajan, Scr. Mater. 68 (2013) 95–99. R.R. Unocic, N. Zhou, L. Kovarik, et al., Acta Mater. 59 (2011) 7325–7339.