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Effects of helium ion irradiation on the high temperature oxidation resistance of Inconel 718 alloy
Surface & Coatings Technology 363 (2019) 34–42
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Effects of helium ion irradiation on the high temperature oxidation resistance of Inconel 718 alloy
T
⁎
Hao Wan , Zhan Ding, Jian Wang, Yue Yin, Qinhan Guo, Yuling Gong, Zhenjiang Zhao, Xin Yao School of Naval Architecture and Mechanical-electrical Engineering, Taizhou University, Taizhou 225300, RP, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Superalloy Oxidation Irradiation Nickel Microstructure
The Ni-based Inconel 718 alloy was irradiated using 50 keV helium ion to doses of 0.4, 4 and 40 dpa. The oxidation behaviors of un-irradiated and irradiated samples were studied under isothermal conditions at 900 °C. The phase constitution, oxide layer morphology and microstructure variation were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). It was found that the oxidation kinetics of un-irradiated and irradiated samples obeyed a parabolic law. The oxidation rate was lower in the sample with higher dose irradiation. The oxide layer was comprised of outer oxide scale (mainly Cr2O3) and inner oxide layer (mainly Al2O3 and TiO2). Irradiation raised the surface layer's energy and facilitated the nucleation of fine oxide particles. While fine oxide particles improved the densification of outer oxide scale. Due to the irradiation-induced defects, fast-diffusion paths were provided for Cr to the surface. This accelerated the forming of dense outer Cr2O3 oxide scale, and restricted the inward penetration of O. Therefore, the thickness of oxide layer especially the inner oxide layer significantly decreased. The enhanced oxidation resistance of irradiated samples was essentially attributed to the combination of improved surface layer's energy and irradiation defects.
1. Introduction On account of the excellent high temperature strength, corrosion and oxidation properties, Ni-based alloys are selected as promising structural materials that use in Generation IV advanced nuclear reactors [1–3]. Since irradiation creates abundant Frenkel pairs (interstitials and vacancies in equal numbers), which would form defect clusters and result in microstructural evolution in Ni-based alloys [4–6]. Recently, irradiation induced defects and microstructure changes have been attracting increasing attentions [7–10]. Jin et al. investigated the relation between the density of dislocation structure and irradiation doses in nickel C-276 alloy [7]. In comparison with the microstructure of GH3535 irradiated at room temperature and 600 °C, Huang et al. found that high temperature irradiation reduce the number density of solute clusters in Ni3+ ion irradiated samples [8]. Song et al. studied proton irradiation induced long-range ordered precipitation in Ni-Cr-Mo-Fe alloys and found that the ordering is controlled by thermodynamic driving force [9]. By using in situ heavy ion irradiation in a TEM, Sun et al. confirmed that high angle grain boundaries can effectively absorb irradiation induced dislocation loop and segments in nanocrystalline Ni [10]. Because of irradiation-induced microstructural instability,
⁎
undesired performance degradation such as embrittlement, hardening and swelling are found in irradiated Ni-based alloy [11–13]. Irradiation induced hardening is relate with irradiation induced defects such as dislocation loops, precipitates and bubbles [14,15]. Therefore, an alloy which can suppress the formation of defects under irradiation usually possesses better irradiation hardening resistance [16]. Nevertheless, softening occurred in Ni-based X-750 alloy when the irradiation-induced strengthening phase instability outweighed the hardening effect of irradiation-induced defects [17]. Zhu et al. found that helium ion irradiation induced bubbles and cavities accelerate the intragranular corrosion of Ni-based GH3535 alloy in fluoride molten salt [18]. They identified that the chemical corrosion can be enhanced by the segregation of Si at bubble surface during corrosion [19]. On the other hand, some studies demonstrated that high energy beams irradiation could enhance the oxidation and corrosion resistance of Ni-based alloy [20–23]. Saric et al. found that low energy oxygen ion bombardment leads to an efficient growth of thin films of nickel oxides [20]. Lv et al. reported that high-current pulsed electron beam (HCPEB) irradiation induced nanocrystalline, dislocation and deformation twin have effects in improving the high temperature resistance of Ni-based GH4169 alloy [21]. Similarly, the oxidation resistance of Ni-based GH586 alloy was significantly improved due to the laser shock
Corresponding author. E-mail address: [email protected] (H. Wan).
https://doi.org/10.1016/j.surfcoat.2019.02.021 Received 9 November 2018; Received in revised form 6 February 2019; Accepted 8 February 2019 Available online 10 February 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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occurred. Microstructure changes of un-irradiated and irradiated samples were examined using a JOEL JEM-2100F transmission electron microscope (TEM) operating at 200 keV.
processing (LSP) introduced grain refining and defects [22]. Besides, by producing residual compressive stress and delaying crack initiation in corrosion layer, the corrosion resistance of Ni-based alloy was also improved [23]. As a precipitation hardening Ni-based alloy, a variety of alloying elements Nb, Ti and Al are added in the alloy to precipitate phases such as Ni3(Al, Ti) (γ’), Ni3Nb (γ”) and Ni3Nb (δ) [24,25]. In these alloying elements, Nb, Ti and Al are used to promote precipitation, and Cr is intended to enhance the oxidation resistance of Inconel 718 alloy. Generally, Inconel 718 alloy would form an oxide scale which is primary constituted by Cr2O3 after oxidation [26]. Greene et al. evaluated the oxidation rate of Inconel 718 at temperatures ranging from 973 to 1620 K, and three regimes of oxidation were identified [27]. However, the effects of helium ion irradiation on the oxidation resistance of Inconel 718 are still unknown, and the details of irradiated Inconel 718 oxidized at high temperature should be clarified. The aim of this study is to investigate the oxidation resistance of Inconel 718 alloy with different doses irradiation. In addition, special attention was paid to the correlation between microstructure change and oxide layer morphology variation of un-irradiated and irradiated samples. This enables the mechanistic details of helium ion irradiation on oxidation of Inconel 718 alloy at high temperature to be identified.
3. Results 3.1. Oxidation kinetics Fig. 1(a) shows the mass gain of un-irradiated and 0.4, 4 and 40 dpa irradiated samples during oxidation tests at 900 °C. The mass gain increased rapidly at the initial oxidation stage, and then, the mass gain gradually slow down with increasing oxidation time. The mass gain of un-irradiated sample was the largest while that of 40 dpa irradiated sample was the smallest. After 100 h oxidation, the mass gain of 40 dpa irradiated sample was 0.43 mg·cm−2. Fig. 1(b) shows the relationship between the square of mass gain per unit area and the oxidation time. The oxidation rate is faster at the initial oxidation stage (0−10 h), as shown in Fig. 1(b). Afterwards, the oxidation rate exhibits a constant parabolic rate dependence (20−100 h), as shown in Fig. 1(b) and (c). After the initial oxidation stage, as shown in Fig. 1(c), the oxidation rate κp of un-irradiated and 0.4, 4 and 40 dpa irradiated samples are 1.485 × 10−6, 1.169 × 10−6, 1.041 × 10−6 and 6.616 × 10−7 mg2/ cm4 s, respectively. The results suggest that the oxidation rate of both un-irradiated and irradiated samples is controlled by diffusion mechanism at steady state oxidation, which is consistent with previous investigation on the oxidation kinetics of Inconel 718 alloy [27].
2. Experimental procedures 2.1. Material and sample preparation Table 1 shows the chemical composition of experimental Inconel 718 alloy. The alloy was prepared by vacuum induction melting and vacuum arc re-melting. Afterwards, homogenization treatment was conducted at 1160 °C for 24 h and 1190 °C for 72 h. Samples of 10 × 10 × 3 mm3 and 10 × 10 × 0.5 mm3 were electro-discharge-machine cutted from the as-received Inconel 718 alloy. Before irradiation, samples were ground by a series of SiC sandpapers up to 1000 grit and polished with 0.5 μm diamond spray polishing compounds. Irradiation were performed at 300 °C in a MT3-R ion implanter fabricated by Changzhou Boruiheng Electronic Technology Co., Ltd. Samples were vertically irradiated with 50 keV He+ ion to doses of 0.4, 4 and 40 dpa, respectively. The details of He+ ion irradiation process were described in Ref. [28] previously. Oxidation of un-irradiated and irradiated samples (10 × 10 × 3 mm3) was conducted at 900 °C for 100 h in dry air. In order to avoid weight loss caused by spalled oxides, each sample was placed in an alumina crucible for oxidation test. Before oxidation, the alumina crucibles were preheated for more than 100 h at 900 °C. Mass gains due to the uptake of O were measured by a balance providing a precision of ± 0.1 mg.
3.2. XRD analysis Fig. 2 show the XRD spectrums of un-irradiated and irradiated samples after oxidation at 900 °C for 100 h. The results indicate that the oxide scales are primary composed of Cr2O3. Other oxides such as NiO, NiCr2O4, Cr2Ti7O17, Fe2Ti3O9, NbO2 were also detected. At the initial oxidation stage, Ni and Cr were oxidized into NiO and Cr2O3 at alloy-air interface, respectively. The oxidation reactions are shown as follows:
Ni +
2Cr +
2Cr + 3NiO = 3Ni + Cr2O3
NiO + Cr2O3 = NiCr2O4
Mo
Ti
Al
Mn
Si
Cu
C
Fe
53.74
17.98
5.39
2.87
0.97
0.69
0.08
0.07
0.06
0.02
Bal
(3)
(4)
The affinity of Cr to O is greater than that of Ni to O, only a small amount of NiO and NiCr2O4 existed after oxidation [30]. As the primary oxide, the intensity of Cr2O3-peak that shown in Fig. 2(a)–(d) should be noticed. The strongest Cr2O3-peak appears in the diffraction pattern of un-irradiated sample, as shown in Fig. 2(a). From the diffraction pattern of irradiated sample, the Cr2O3-peak intensity is found to decrease with the increasing of irradiation dose, as shown in Fig. 2(b)–(d). With regard to the oxides such as Cr2Ti7O17, Fe2Ti3O9, NbO2, it may relate with irradiation-induced surface composition alteration.
Table 1 Chemical composition of experimental Inconel 718 alloy (wt%). Nb
(2)
It was reported that the coefficient of lattice diffusion and grain boundary diffusion of O in Cr2O3 oxide scale at 900 °C are 4.4 × 10−17and 1.6 × 10−12 cm2·s−1, respectively [29]. The lattice diffusion coefficient of Cr in Cr2O3 oxide scale is also five orders of magnitude bigger than grain-boundary diffusion coefficient. It was beneficial to the selective oxidation of Cr, which accelerated the formation of Cr2O3. On the other hand, NiO and Cr2O3 oxides can react to form NiCr2O4 spinel as follow:
The surface and cross-section morphologies of oxide layer were characterized by Hitachi S3400 SEM. The chemical composition of some oxides in oxide layer was identified by energy dispersive spectrometry (EDS). The X-ray diffraction spectra were obtained by using D8 Bruker Advance X-ray diffractometer in the 2θ range of 20–100° at room temperature. TEM specimens with a thickness less than 30 μm were prepared from the ion bombarded side of the 10 × 10 × 0.5 mm3 samples. After that, the specimens were thinned in a Gatan-691 precision ion polishing system, from the un-irradiated side to perforation
Cr
3 O2 = Cr2O3 2
(1)
Since the Gibbs energy of Cr2O3 is lower than that of NiO. A reaction between Cr and NiO occurred as follows:
2.2. Characterization
Ni
1 O2 = NiO 2
35
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Fig. 1. (a) The oxidation kinetics of 0, 0.4, 4, and 40 dpa irradiated samples at 900 °C, (b) and (c) the relationship between the square of mass gain per unit area and the oxidation time.
indicate that helium ion irradiation has effects in reducing the size of oxide particles and altering the morphology of oxide scale.
3.3. Morphology of top oxide scale Fig. 3 shows the surface morphologies of oxide scale that formed on un-irradiated and irradiated samples. After 100 h oxidation at the temperature of 900 °C, all samples were covered with oxide scales, as shown in Fig. 3. In the un-irradiated sample, as shown in Fig. 3(a), an oxide scale with raised regions is observed. Additionally, the oxide scale has microcracks distributed on the surface. In 0.4 dpa irradiated sample, the oxide scale is comprised of fine oxide particles, as shown in Fig. 3(b). In the oxidized sample which pre-irradiated to 4 dpa, oxide scale is made of finer oxide particles, as illustrated in Fig.3(c). However, spalling of oxide particles are found at raised regions. Fig. 3(d) shows morphology of oxide scale in 40 dpa irradiated sample. In this case, the size of oxide particles is further reduced and no obvious spalling zones are observed. Actually, fine and homogeneous oxide particle is beneficial for the releasing of inner stress in the oxide scale, which reduces the tendency of cracking and spalling [31]. In comparison with Fig. 3(a)–(d), it can be seen that oxide scales with continuous and raised oxide regions are formed in samples irradiated to 0, 0.4 and 4 dpa. In the case of 40 dpa irradiated sample, the raised oxide region presents with a worm-like morphology, as shown in Fig. 3(d). It has been reported that the activation energy for ion to diffuse through grain-boundary is much less than that required to diffuse through lattice [21]. Oxide particles were easier to nucleate and grew into raised oxide region at grain-boundary. The results here
3.4. Cross-section of oxide layer Fig. 4 shows the cross-sectional SEM images of oxide layers before and after irradiation with different doses of 0.4, 4 and 40 dpa. It is apparent that all samples are covered by continuous outer oxide scales. Underneath the outer oxide scales, inner oxide layers are observed, as shown in Fig. 4. As shown in Fig. 4(a), the oxide layer (including oxide scale and inner oxide layer) that generated on the oxidized un-irradiated sample has the maximum thickness ~50 μm. In this case, the thickness of outer oxide scale and inner oxide layer are ~15 and ~35 μm, respectively. In the 0.4 dpa irradiated sample, the thickness of oxide layer decreases to ~15 μm, as shown in Fig. 4(b). Attributing to the thickness reduction of outer oxide scale and inner oxide layer, the total thickness of oxide layer decreased. Further reduction of inner oxide layer thickness was found in the oxidized 4 and 40 dpa irradiated samples. As shown in Fig. 4(c) and 4(d), the thickness of inner oxide layer in oxidized 4 and 40 dpa irradiated samples are ~10 and ~7 μm, respectively. However, the thickness of outer oxide scale keeps almost unchanged in 4 and 40 dpa irradiated samples. The above results indicate that helium ion irradiation has effects in reducing the thickness of oxide layer. Moreover, the Cr2O3-peak intensity decreasing, as shown in Fig. 2(b)–(d), are related to the reduction of oxide layer's thickness. 36
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Fig. 2. XRD analysis of the oxides in the surface of the un-irradiated and irradiated samples after 100 h oxidation at 900 °C, (a) un-irradiated, (b) 0.4 dpa, (c) 4 dpa, (d) 40 dpa.
Fig. 3. SEM morphology of the top oxide scale of un-irradiated and irradiated samples after 100 h oxidation at the temperature of 900 °C, (a) un-irradiated, (b) 0.4 dpa, (c) 4 dpa, (d) 40 dpa. 37
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Fig. 4. Cross-sectional SEM morphologies of un-irradiated sample and samples irradiated to different doses after oxidation at 900 °C for 100 h, (a) un-irradiated, (b) 0.4 dpa, (c) 4 dpa, (d) 40 dpa.
4. Discussion
Based on the above analysis, it is reasonable to estimate the densification of oxide scale on the basis of the thickness of inner oxide layer. That is, the densification of outer oxide scale formed on the 40 dpa irradiated sample was the best. Fig. 5(a) shows the magnified SEM image of outer oxide scale and inner oxide layer in un-irradiated sample. Some cavities distributed in outer oxide scale and inner oxide layer are observed in Fig. 5(a). Oxide particles formed in the inner oxide layer were analyzed using EDS. The dark gray inner oxide particle is Al2O3 (region A in Fig. 5(a)), as shown in Fig. 5(b). While the gray oxide particle is TiO2 (region B in Fig. 5(a)), as shown in Fig. 5(c). Since the oxide scale formed on un-irradiated Inconel 718 alloy was mainly composed of Cr2O3. More Cr were consumed during oxidation because of the selective oxidation of Cr. The consumption leaded to a gradually reduction of Cr concentration in the surface layer. Driven by the concentration gradient, Cr started to diffuse from inner matrix to the surface. Attributing to the fast-diffusion paths provided by grain-boundary, excessive Cr would be consumed and left a mass of vacancies in the vicinity of these regions. The aggregation and evolution of vacancies resulted in the cavity formation in oxide scale (Fig. 5(a)). Similarly, in the inner oxide layer, cavities formed due to the depletion of Cr. Nevertheless, the occurrence of cavities altered the stress distribution and induces microcracks in the oxide scale, as shown in Fig. 5(a). Surface microcrack provided convenient diffusion path for O to penetrate through outer oxide scale. The penetration of O gave rise to a successive oxidation of alloy underneath the oxide scale. It also increased the partial pressure of O2 in the oxide scale and promoted inner oxide layer formation. Since O were likely to diffuse along crystal defects such as grain-boundary or phase-boundary, island-like or needlelike inner oxide particles formed in the inner oxide layer, as shown in Figs. 4 and 5.
Fig. 6 shows the EDS line analysis of oxide layers in un-irradiated and irradiated samples. The results illustrate that the oxide layer consist of outer oxide scale (mainly Cr2O3) and inner oxide layer (mainly Al2O3 oxides). The detailed formation process of outer oxide scale has been analyzed in Section 3.2. The forming of Al2O3 oxides resulted in the change of the content of Al, as shown in Fig. 6(g)–(h). With regard to the forming of Al2O3 oxide particles in the inner oxide layer, it is related to the kinetics and thermodynamics of oxidation. The selective oxidation of Al or Cr depends on the formation Gibbs energy (ΔG) of Al2O3 and Cr2O3. As ΔGAl2O3 and ΔGCr2O3 can be expressed as follows:
ΔG Al2O3 = ΔGθAl2O3 − RTlnPO2 θ ΔGCr2O3 = ΔGCr − RTlnPO2 2O3 θ
(5) (6) θ
where ΔGAl2O3 is the standard Gibbs energy change of Al2O3, ΔGCr2O3 is the standard Gibbs energy change of Cr2O3, PO2 is the partial pressure of O2, RTlnPO2 is the criteria of partial pressure for forming corresponding oxide. According to the Ellingham diagram, it can be found that ΔGCr2O3θ > ΔGAl2O3θ [21,32]. When samples were oxidized under the same condition, the selective oxidation of Al or Cr elements would depend on the second term (RTlnPO2) in Eq. (5) and (6). Actually, selective oxidation of Cr occurs in near surface layer because of the high enough PO2. In this depth, the effect of RTlnPO2 term is greater than the first term (ΔGθ) in Eqs. (5) and (6), which lead to ΔGCr2O3 < ΔGAl2O3. Hence, Cr2O3 is the primary oxides of outer oxide scale. While for the inner oxidation, the penetration of outer O is the main reason. The effect of ΔGθ is greater than the RTlnPO2 term in Eqs. (5) and (6) due to the lower PO2. As a result, in the case of ΔGCr2O3 > ΔGAl2O3, selective oxidation of Al happens. Moreover, if ΔGAl2O3θ = RTlnPO2, the inner oxidation would stop. When an integral outer oxide scale formed, O and metal ions have to 38
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Fig. 5. SEM and EDS analysis of oxide layer in un-irradiated sample, (a) cross-sectional morphology of oxide layer, (b) EDS result of Al2O3, (c) EDS result of TiO2.
surface and accelerate the forming of dense outer Cr2O3 oxide scale in time. Moreover, in sample with more irradiation, higher surface layer energy for oxides nucleation and more irradiation induced defects for fast-diffusion could be provided. In other words, higher dose irradiation contributes to the formation of oxide scale that be composed of fine oxide particles more quickly in the process of oxidation. Therefore, in spite of the thinnest oxide scale on the surface of 40 dpa irradiated sample, inner oxide layer with the minimum thickness also found in this case due to the best densification of outer oxide scale. On the basis of above analysis, it is easy to understand the lowest mass gain in sample irradiated to 40 dpa.
diffuse through the oxide scale to form new oxide. In fact, intact oxide scales formed on samples after ~10 h oxidation (initial oxidation stage), which can be seen from oxidation kinetic curves (Fig.1(a)). An entire outer oxide scale has effects in restraining further oxidation of Inconel 718 alloy. Therefore, the oxidation rate of sample is related with the densification of outer oxide scale. Due to the thickest oxide layer (Fig. 4(a)), it can be ascertained that the densification of outer oxide scale in un-irradiated sample is the worst. The decrease of oxide layers' thickness (Figs. 4 and 6) indicates that the densification of outer oxide scale can be enhanced by irradiation. Since continuous inner diffusion of O leads to the successive growth of oxide layer. It can be concluded that the oxide scale on unirradiated sample could not prevent the diffusion of O as efficient as the oxide scales on irradiated samples. With regard to the enhanced densification of oxide scale on irradiated samples, the effects of increased surface layer's Gibbs energy and newly produced defects should be studied. Surface layer's energy raise facilitates the generation of oxide scales that consist of fine oxide particles. Fine oxide particles reduce the inner stress and restrict the forming of microcracks and spalling in oxide scales [31]. Furthermore, irradiation produce a mass of dislocation structures in irradiated samples, as shown in Fig. 7. Fig. 7(a) shows the TEM micrograph of γ′ and γ″ precipitates in the un-irradiated sample. As shown in Fig. 7(b), dislocations and dislocation walls were found in sample irradiated to a dose of 0.4 dpa. Increasing the dose to 4 dpa gave rise to dislocations cells, as shown in Fig. 7(c). By 40 dpa, higher density dislocation structure was observed in Fig. 7(d). The dislocationprecipitate interactions were depicted for better understanding of microstructural variation, and peanut-like complex model was proposed as the evolution model [28]. Based on the amount of defects accumulated between δ phase and matrix, the density of defects was found to increase in higher dose irradiated sample [33]. Irradiation induced defects promote the fast-diffusion of Cr to the
5. Conclusions In this study, un-irradiated and 0.4, 4 and 40 dpa helium ion irradiated Inconel 718 samples were oxidized at 900 °C for 100 h in dry air. Continuous oxide scales covered the entire surface of un-irradiated and irradiated samples. Cross-sectional analysis indicated that the oxide layer was made of out oxide scale and inner oxide layer. Irradiation had effects in reducing the thickness of oxide layer. The thinnest oxide layer was found in 40 dpa irradiated sample. In this case, the thickness of outer oxide scale and inner oxide layer were ~2 and ~7 μm, respectively. Irradiation increased the surface layer's energy and facilitated the generation of oxide scale that consist of fine oxide particles. Fine oxide particles reduced the inner stress and restricted the forming of microcrack and spalling in oxide scale. On the other hand, irradiation produced a good deal of defects such as dislocations and dislocation walls in irradiated samples. Furthermore, more defects were produced in samples with higher dose irradiation. Since irradiation-induced defects could promote the fast-diffusion of Cr to the surface and accelerate the forming of dense outer Cr2O3 oxide scale. An oxide scale with the minimum thickness and the best densification generated on the surface 39
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Fig. 6. SEM morphologies and EDS line analysis of cross-sections of un-irradiated sample and samples irradiated to different doses after 100 h oxidation at 900 °C, (a) un-irradiated, (c) 0.4 dpa, (e) 4 dpa, (g) 40 dpa.
of 40 dpa irradiated samples.
(TZXY2017QDJJ014), the Natural Science Research of Jiangsu Higher Education Institutions of China (18KJD430007), the Fifth “311 Highlevel Talents Training Project” of Taizhou City, the college students' innovation and entrepreneurship training project of Jiangsu province (201812917006Y), the National Natural Science Foundation of China (51609164) and the Natural Science Foundation of Jiangsu Province,
Acknowledgments This work was supported by the Scientific Research Foundation of Taizhou University for the Introduction of Talents 40
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Fig. 7. Bright-field TEM micrographs of microstructure variation in samples with different doses irradiation (a) un-irradiated sample, γ′ and γ″ precipitates, (b) 0.4 dpa, dislocations, (c) 4 dpa, dislocation cells (d) 40 dpa, high density dislocation structure.
China (BK20160574).
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alloys (superalloys) at elevated temperatures in air: I, Oxid. Met. 41 (3–4) (1994) 251–269. [31] Y. Hua, Z. Rong, Y. Ye, K. Chen, R. Chen, Q. Xue, H. Liu, Laser shock processing effects on isothermal oxidation resistance of GH586 superalloy, Appl. Surf. Sci. 330 (2015) 439–444. [32] Y.N. Zhang, X. Cao, P. Wanjara, M. Medraj, Oxide films in laser additive manufactured Inconel 718, Acta Mater. 61 (17) (2013) 6562–6576. [33] H. Wan, N. Si, Z. Zhao, J. Wang, Y. Zhang, Study of irradiation induced surface pattern and structural changes in Inconel 718 alloy, Mater. Res. Express 5 (5) (2018) 056503.
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Effects of helium ion irradiation on the high temperature oxidation resistance of Inconel 718 alloy
T
⁎
Hao Wan , Zhan Ding, Jian Wang, Yue Yin, Qinhan Guo, Yuling Gong, Zhenjiang Zhao, Xin Yao School of Naval Architecture and Mechanical-electrical Engineering, Taizhou University, Taizhou 225300, RP, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Superalloy Oxidation Irradiation Nickel Microstructure
The Ni-based Inconel 718 alloy was irradiated using 50 keV helium ion to doses of 0.4, 4 and 40 dpa. The oxidation behaviors of un-irradiated and irradiated samples were studied under isothermal conditions at 900 °C. The phase constitution, oxide layer morphology and microstructure variation were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). It was found that the oxidation kinetics of un-irradiated and irradiated samples obeyed a parabolic law. The oxidation rate was lower in the sample with higher dose irradiation. The oxide layer was comprised of outer oxide scale (mainly Cr2O3) and inner oxide layer (mainly Al2O3 and TiO2). Irradiation raised the surface layer's energy and facilitated the nucleation of fine oxide particles. While fine oxide particles improved the densification of outer oxide scale. Due to the irradiation-induced defects, fast-diffusion paths were provided for Cr to the surface. This accelerated the forming of dense outer Cr2O3 oxide scale, and restricted the inward penetration of O. Therefore, the thickness of oxide layer especially the inner oxide layer significantly decreased. The enhanced oxidation resistance of irradiated samples was essentially attributed to the combination of improved surface layer's energy and irradiation defects.
1. Introduction On account of the excellent high temperature strength, corrosion and oxidation properties, Ni-based alloys are selected as promising structural materials that use in Generation IV advanced nuclear reactors [1–3]. Since irradiation creates abundant Frenkel pairs (interstitials and vacancies in equal numbers), which would form defect clusters and result in microstructural evolution in Ni-based alloys [4–6]. Recently, irradiation induced defects and microstructure changes have been attracting increasing attentions [7–10]. Jin et al. investigated the relation between the density of dislocation structure and irradiation doses in nickel C-276 alloy [7]. In comparison with the microstructure of GH3535 irradiated at room temperature and 600 °C, Huang et al. found that high temperature irradiation reduce the number density of solute clusters in Ni3+ ion irradiated samples [8]. Song et al. studied proton irradiation induced long-range ordered precipitation in Ni-Cr-Mo-Fe alloys and found that the ordering is controlled by thermodynamic driving force [9]. By using in situ heavy ion irradiation in a TEM, Sun et al. confirmed that high angle grain boundaries can effectively absorb irradiation induced dislocation loop and segments in nanocrystalline Ni [10]. Because of irradiation-induced microstructural instability,
⁎
undesired performance degradation such as embrittlement, hardening and swelling are found in irradiated Ni-based alloy [11–13]. Irradiation induced hardening is relate with irradiation induced defects such as dislocation loops, precipitates and bubbles [14,15]. Therefore, an alloy which can suppress the formation of defects under irradiation usually possesses better irradiation hardening resistance [16]. Nevertheless, softening occurred in Ni-based X-750 alloy when the irradiation-induced strengthening phase instability outweighed the hardening effect of irradiation-induced defects [17]. Zhu et al. found that helium ion irradiation induced bubbles and cavities accelerate the intragranular corrosion of Ni-based GH3535 alloy in fluoride molten salt [18]. They identified that the chemical corrosion can be enhanced by the segregation of Si at bubble surface during corrosion [19]. On the other hand, some studies demonstrated that high energy beams irradiation could enhance the oxidation and corrosion resistance of Ni-based alloy [20–23]. Saric et al. found that low energy oxygen ion bombardment leads to an efficient growth of thin films of nickel oxides [20]. Lv et al. reported that high-current pulsed electron beam (HCPEB) irradiation induced nanocrystalline, dislocation and deformation twin have effects in improving the high temperature resistance of Ni-based GH4169 alloy [21]. Similarly, the oxidation resistance of Ni-based GH586 alloy was significantly improved due to the laser shock
Corresponding author. E-mail address: [email protected] (H. Wan).
https://doi.org/10.1016/j.surfcoat.2019.02.021 Received 9 November 2018; Received in revised form 6 February 2019; Accepted 8 February 2019 Available online 10 February 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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occurred. Microstructure changes of un-irradiated and irradiated samples were examined using a JOEL JEM-2100F transmission electron microscope (TEM) operating at 200 keV.
processing (LSP) introduced grain refining and defects [22]. Besides, by producing residual compressive stress and delaying crack initiation in corrosion layer, the corrosion resistance of Ni-based alloy was also improved [23]. As a precipitation hardening Ni-based alloy, a variety of alloying elements Nb, Ti and Al are added in the alloy to precipitate phases such as Ni3(Al, Ti) (γ’), Ni3Nb (γ”) and Ni3Nb (δ) [24,25]. In these alloying elements, Nb, Ti and Al are used to promote precipitation, and Cr is intended to enhance the oxidation resistance of Inconel 718 alloy. Generally, Inconel 718 alloy would form an oxide scale which is primary constituted by Cr2O3 after oxidation [26]. Greene et al. evaluated the oxidation rate of Inconel 718 at temperatures ranging from 973 to 1620 K, and three regimes of oxidation were identified [27]. However, the effects of helium ion irradiation on the oxidation resistance of Inconel 718 are still unknown, and the details of irradiated Inconel 718 oxidized at high temperature should be clarified. The aim of this study is to investigate the oxidation resistance of Inconel 718 alloy with different doses irradiation. In addition, special attention was paid to the correlation between microstructure change and oxide layer morphology variation of un-irradiated and irradiated samples. This enables the mechanistic details of helium ion irradiation on oxidation of Inconel 718 alloy at high temperature to be identified.
3. Results 3.1. Oxidation kinetics Fig. 1(a) shows the mass gain of un-irradiated and 0.4, 4 and 40 dpa irradiated samples during oxidation tests at 900 °C. The mass gain increased rapidly at the initial oxidation stage, and then, the mass gain gradually slow down with increasing oxidation time. The mass gain of un-irradiated sample was the largest while that of 40 dpa irradiated sample was the smallest. After 100 h oxidation, the mass gain of 40 dpa irradiated sample was 0.43 mg·cm−2. Fig. 1(b) shows the relationship between the square of mass gain per unit area and the oxidation time. The oxidation rate is faster at the initial oxidation stage (0−10 h), as shown in Fig. 1(b). Afterwards, the oxidation rate exhibits a constant parabolic rate dependence (20−100 h), as shown in Fig. 1(b) and (c). After the initial oxidation stage, as shown in Fig. 1(c), the oxidation rate κp of un-irradiated and 0.4, 4 and 40 dpa irradiated samples are 1.485 × 10−6, 1.169 × 10−6, 1.041 × 10−6 and 6.616 × 10−7 mg2/ cm4 s, respectively. The results suggest that the oxidation rate of both un-irradiated and irradiated samples is controlled by diffusion mechanism at steady state oxidation, which is consistent with previous investigation on the oxidation kinetics of Inconel 718 alloy [27].
2. Experimental procedures 2.1. Material and sample preparation Table 1 shows the chemical composition of experimental Inconel 718 alloy. The alloy was prepared by vacuum induction melting and vacuum arc re-melting. Afterwards, homogenization treatment was conducted at 1160 °C for 24 h and 1190 °C for 72 h. Samples of 10 × 10 × 3 mm3 and 10 × 10 × 0.5 mm3 were electro-discharge-machine cutted from the as-received Inconel 718 alloy. Before irradiation, samples were ground by a series of SiC sandpapers up to 1000 grit and polished with 0.5 μm diamond spray polishing compounds. Irradiation were performed at 300 °C in a MT3-R ion implanter fabricated by Changzhou Boruiheng Electronic Technology Co., Ltd. Samples were vertically irradiated with 50 keV He+ ion to doses of 0.4, 4 and 40 dpa, respectively. The details of He+ ion irradiation process were described in Ref. [28] previously. Oxidation of un-irradiated and irradiated samples (10 × 10 × 3 mm3) was conducted at 900 °C for 100 h in dry air. In order to avoid weight loss caused by spalled oxides, each sample was placed in an alumina crucible for oxidation test. Before oxidation, the alumina crucibles were preheated for more than 100 h at 900 °C. Mass gains due to the uptake of O were measured by a balance providing a precision of ± 0.1 mg.
3.2. XRD analysis Fig. 2 show the XRD spectrums of un-irradiated and irradiated samples after oxidation at 900 °C for 100 h. The results indicate that the oxide scales are primary composed of Cr2O3. Other oxides such as NiO, NiCr2O4, Cr2Ti7O17, Fe2Ti3O9, NbO2 were also detected. At the initial oxidation stage, Ni and Cr were oxidized into NiO and Cr2O3 at alloy-air interface, respectively. The oxidation reactions are shown as follows:
Ni +
2Cr +
2Cr + 3NiO = 3Ni + Cr2O3
NiO + Cr2O3 = NiCr2O4
Mo
Ti
Al
Mn
Si
Cu
C
Fe
53.74
17.98
5.39
2.87
0.97
0.69
0.08
0.07
0.06
0.02
Bal
(3)
(4)
The affinity of Cr to O is greater than that of Ni to O, only a small amount of NiO and NiCr2O4 existed after oxidation [30]. As the primary oxide, the intensity of Cr2O3-peak that shown in Fig. 2(a)–(d) should be noticed. The strongest Cr2O3-peak appears in the diffraction pattern of un-irradiated sample, as shown in Fig. 2(a). From the diffraction pattern of irradiated sample, the Cr2O3-peak intensity is found to decrease with the increasing of irradiation dose, as shown in Fig. 2(b)–(d). With regard to the oxides such as Cr2Ti7O17, Fe2Ti3O9, NbO2, it may relate with irradiation-induced surface composition alteration.
Table 1 Chemical composition of experimental Inconel 718 alloy (wt%). Nb
(2)
It was reported that the coefficient of lattice diffusion and grain boundary diffusion of O in Cr2O3 oxide scale at 900 °C are 4.4 × 10−17and 1.6 × 10−12 cm2·s−1, respectively [29]. The lattice diffusion coefficient of Cr in Cr2O3 oxide scale is also five orders of magnitude bigger than grain-boundary diffusion coefficient. It was beneficial to the selective oxidation of Cr, which accelerated the formation of Cr2O3. On the other hand, NiO and Cr2O3 oxides can react to form NiCr2O4 spinel as follow:
The surface and cross-section morphologies of oxide layer were characterized by Hitachi S3400 SEM. The chemical composition of some oxides in oxide layer was identified by energy dispersive spectrometry (EDS). The X-ray diffraction spectra were obtained by using D8 Bruker Advance X-ray diffractometer in the 2θ range of 20–100° at room temperature. TEM specimens with a thickness less than 30 μm were prepared from the ion bombarded side of the 10 × 10 × 0.5 mm3 samples. After that, the specimens were thinned in a Gatan-691 precision ion polishing system, from the un-irradiated side to perforation
Cr
3 O2 = Cr2O3 2
(1)
Since the Gibbs energy of Cr2O3 is lower than that of NiO. A reaction between Cr and NiO occurred as follows:
2.2. Characterization
Ni
1 O2 = NiO 2
35
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Fig. 1. (a) The oxidation kinetics of 0, 0.4, 4, and 40 dpa irradiated samples at 900 °C, (b) and (c) the relationship between the square of mass gain per unit area and the oxidation time.
indicate that helium ion irradiation has effects in reducing the size of oxide particles and altering the morphology of oxide scale.
3.3. Morphology of top oxide scale Fig. 3 shows the surface morphologies of oxide scale that formed on un-irradiated and irradiated samples. After 100 h oxidation at the temperature of 900 °C, all samples were covered with oxide scales, as shown in Fig. 3. In the un-irradiated sample, as shown in Fig. 3(a), an oxide scale with raised regions is observed. Additionally, the oxide scale has microcracks distributed on the surface. In 0.4 dpa irradiated sample, the oxide scale is comprised of fine oxide particles, as shown in Fig. 3(b). In the oxidized sample which pre-irradiated to 4 dpa, oxide scale is made of finer oxide particles, as illustrated in Fig.3(c). However, spalling of oxide particles are found at raised regions. Fig. 3(d) shows morphology of oxide scale in 40 dpa irradiated sample. In this case, the size of oxide particles is further reduced and no obvious spalling zones are observed. Actually, fine and homogeneous oxide particle is beneficial for the releasing of inner stress in the oxide scale, which reduces the tendency of cracking and spalling [31]. In comparison with Fig. 3(a)–(d), it can be seen that oxide scales with continuous and raised oxide regions are formed in samples irradiated to 0, 0.4 and 4 dpa. In the case of 40 dpa irradiated sample, the raised oxide region presents with a worm-like morphology, as shown in Fig. 3(d). It has been reported that the activation energy for ion to diffuse through grain-boundary is much less than that required to diffuse through lattice [21]. Oxide particles were easier to nucleate and grew into raised oxide region at grain-boundary. The results here
3.4. Cross-section of oxide layer Fig. 4 shows the cross-sectional SEM images of oxide layers before and after irradiation with different doses of 0.4, 4 and 40 dpa. It is apparent that all samples are covered by continuous outer oxide scales. Underneath the outer oxide scales, inner oxide layers are observed, as shown in Fig. 4. As shown in Fig. 4(a), the oxide layer (including oxide scale and inner oxide layer) that generated on the oxidized un-irradiated sample has the maximum thickness ~50 μm. In this case, the thickness of outer oxide scale and inner oxide layer are ~15 and ~35 μm, respectively. In the 0.4 dpa irradiated sample, the thickness of oxide layer decreases to ~15 μm, as shown in Fig. 4(b). Attributing to the thickness reduction of outer oxide scale and inner oxide layer, the total thickness of oxide layer decreased. Further reduction of inner oxide layer thickness was found in the oxidized 4 and 40 dpa irradiated samples. As shown in Fig. 4(c) and 4(d), the thickness of inner oxide layer in oxidized 4 and 40 dpa irradiated samples are ~10 and ~7 μm, respectively. However, the thickness of outer oxide scale keeps almost unchanged in 4 and 40 dpa irradiated samples. The above results indicate that helium ion irradiation has effects in reducing the thickness of oxide layer. Moreover, the Cr2O3-peak intensity decreasing, as shown in Fig. 2(b)–(d), are related to the reduction of oxide layer's thickness. 36
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Fig. 2. XRD analysis of the oxides in the surface of the un-irradiated and irradiated samples after 100 h oxidation at 900 °C, (a) un-irradiated, (b) 0.4 dpa, (c) 4 dpa, (d) 40 dpa.
Fig. 3. SEM morphology of the top oxide scale of un-irradiated and irradiated samples after 100 h oxidation at the temperature of 900 °C, (a) un-irradiated, (b) 0.4 dpa, (c) 4 dpa, (d) 40 dpa. 37
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Fig. 4. Cross-sectional SEM morphologies of un-irradiated sample and samples irradiated to different doses after oxidation at 900 °C for 100 h, (a) un-irradiated, (b) 0.4 dpa, (c) 4 dpa, (d) 40 dpa.
4. Discussion
Based on the above analysis, it is reasonable to estimate the densification of oxide scale on the basis of the thickness of inner oxide layer. That is, the densification of outer oxide scale formed on the 40 dpa irradiated sample was the best. Fig. 5(a) shows the magnified SEM image of outer oxide scale and inner oxide layer in un-irradiated sample. Some cavities distributed in outer oxide scale and inner oxide layer are observed in Fig. 5(a). Oxide particles formed in the inner oxide layer were analyzed using EDS. The dark gray inner oxide particle is Al2O3 (region A in Fig. 5(a)), as shown in Fig. 5(b). While the gray oxide particle is TiO2 (region B in Fig. 5(a)), as shown in Fig. 5(c). Since the oxide scale formed on un-irradiated Inconel 718 alloy was mainly composed of Cr2O3. More Cr were consumed during oxidation because of the selective oxidation of Cr. The consumption leaded to a gradually reduction of Cr concentration in the surface layer. Driven by the concentration gradient, Cr started to diffuse from inner matrix to the surface. Attributing to the fast-diffusion paths provided by grain-boundary, excessive Cr would be consumed and left a mass of vacancies in the vicinity of these regions. The aggregation and evolution of vacancies resulted in the cavity formation in oxide scale (Fig. 5(a)). Similarly, in the inner oxide layer, cavities formed due to the depletion of Cr. Nevertheless, the occurrence of cavities altered the stress distribution and induces microcracks in the oxide scale, as shown in Fig. 5(a). Surface microcrack provided convenient diffusion path for O to penetrate through outer oxide scale. The penetration of O gave rise to a successive oxidation of alloy underneath the oxide scale. It also increased the partial pressure of O2 in the oxide scale and promoted inner oxide layer formation. Since O were likely to diffuse along crystal defects such as grain-boundary or phase-boundary, island-like or needlelike inner oxide particles formed in the inner oxide layer, as shown in Figs. 4 and 5.
Fig. 6 shows the EDS line analysis of oxide layers in un-irradiated and irradiated samples. The results illustrate that the oxide layer consist of outer oxide scale (mainly Cr2O3) and inner oxide layer (mainly Al2O3 oxides). The detailed formation process of outer oxide scale has been analyzed in Section 3.2. The forming of Al2O3 oxides resulted in the change of the content of Al, as shown in Fig. 6(g)–(h). With regard to the forming of Al2O3 oxide particles in the inner oxide layer, it is related to the kinetics and thermodynamics of oxidation. The selective oxidation of Al or Cr depends on the formation Gibbs energy (ΔG) of Al2O3 and Cr2O3. As ΔGAl2O3 and ΔGCr2O3 can be expressed as follows:
ΔG Al2O3 = ΔGθAl2O3 − RTlnPO2 θ ΔGCr2O3 = ΔGCr − RTlnPO2 2O3 θ
(5) (6) θ
where ΔGAl2O3 is the standard Gibbs energy change of Al2O3, ΔGCr2O3 is the standard Gibbs energy change of Cr2O3, PO2 is the partial pressure of O2, RTlnPO2 is the criteria of partial pressure for forming corresponding oxide. According to the Ellingham diagram, it can be found that ΔGCr2O3θ > ΔGAl2O3θ [21,32]. When samples were oxidized under the same condition, the selective oxidation of Al or Cr elements would depend on the second term (RTlnPO2) in Eq. (5) and (6). Actually, selective oxidation of Cr occurs in near surface layer because of the high enough PO2. In this depth, the effect of RTlnPO2 term is greater than the first term (ΔGθ) in Eqs. (5) and (6), which lead to ΔGCr2O3 < ΔGAl2O3. Hence, Cr2O3 is the primary oxides of outer oxide scale. While for the inner oxidation, the penetration of outer O is the main reason. The effect of ΔGθ is greater than the RTlnPO2 term in Eqs. (5) and (6) due to the lower PO2. As a result, in the case of ΔGCr2O3 > ΔGAl2O3, selective oxidation of Al happens. Moreover, if ΔGAl2O3θ = RTlnPO2, the inner oxidation would stop. When an integral outer oxide scale formed, O and metal ions have to 38
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Fig. 5. SEM and EDS analysis of oxide layer in un-irradiated sample, (a) cross-sectional morphology of oxide layer, (b) EDS result of Al2O3, (c) EDS result of TiO2.
surface and accelerate the forming of dense outer Cr2O3 oxide scale in time. Moreover, in sample with more irradiation, higher surface layer energy for oxides nucleation and more irradiation induced defects for fast-diffusion could be provided. In other words, higher dose irradiation contributes to the formation of oxide scale that be composed of fine oxide particles more quickly in the process of oxidation. Therefore, in spite of the thinnest oxide scale on the surface of 40 dpa irradiated sample, inner oxide layer with the minimum thickness also found in this case due to the best densification of outer oxide scale. On the basis of above analysis, it is easy to understand the lowest mass gain in sample irradiated to 40 dpa.
diffuse through the oxide scale to form new oxide. In fact, intact oxide scales formed on samples after ~10 h oxidation (initial oxidation stage), which can be seen from oxidation kinetic curves (Fig.1(a)). An entire outer oxide scale has effects in restraining further oxidation of Inconel 718 alloy. Therefore, the oxidation rate of sample is related with the densification of outer oxide scale. Due to the thickest oxide layer (Fig. 4(a)), it can be ascertained that the densification of outer oxide scale in un-irradiated sample is the worst. The decrease of oxide layers' thickness (Figs. 4 and 6) indicates that the densification of outer oxide scale can be enhanced by irradiation. Since continuous inner diffusion of O leads to the successive growth of oxide layer. It can be concluded that the oxide scale on unirradiated sample could not prevent the diffusion of O as efficient as the oxide scales on irradiated samples. With regard to the enhanced densification of oxide scale on irradiated samples, the effects of increased surface layer's Gibbs energy and newly produced defects should be studied. Surface layer's energy raise facilitates the generation of oxide scales that consist of fine oxide particles. Fine oxide particles reduce the inner stress and restrict the forming of microcracks and spalling in oxide scales [31]. Furthermore, irradiation produce a mass of dislocation structures in irradiated samples, as shown in Fig. 7. Fig. 7(a) shows the TEM micrograph of γ′ and γ″ precipitates in the un-irradiated sample. As shown in Fig. 7(b), dislocations and dislocation walls were found in sample irradiated to a dose of 0.4 dpa. Increasing the dose to 4 dpa gave rise to dislocations cells, as shown in Fig. 7(c). By 40 dpa, higher density dislocation structure was observed in Fig. 7(d). The dislocationprecipitate interactions were depicted for better understanding of microstructural variation, and peanut-like complex model was proposed as the evolution model [28]. Based on the amount of defects accumulated between δ phase and matrix, the density of defects was found to increase in higher dose irradiated sample [33]. Irradiation induced defects promote the fast-diffusion of Cr to the
5. Conclusions In this study, un-irradiated and 0.4, 4 and 40 dpa helium ion irradiated Inconel 718 samples were oxidized at 900 °C for 100 h in dry air. Continuous oxide scales covered the entire surface of un-irradiated and irradiated samples. Cross-sectional analysis indicated that the oxide layer was made of out oxide scale and inner oxide layer. Irradiation had effects in reducing the thickness of oxide layer. The thinnest oxide layer was found in 40 dpa irradiated sample. In this case, the thickness of outer oxide scale and inner oxide layer were ~2 and ~7 μm, respectively. Irradiation increased the surface layer's energy and facilitated the generation of oxide scale that consist of fine oxide particles. Fine oxide particles reduced the inner stress and restricted the forming of microcrack and spalling in oxide scale. On the other hand, irradiation produced a good deal of defects such as dislocations and dislocation walls in irradiated samples. Furthermore, more defects were produced in samples with higher dose irradiation. Since irradiation-induced defects could promote the fast-diffusion of Cr to the surface and accelerate the forming of dense outer Cr2O3 oxide scale. An oxide scale with the minimum thickness and the best densification generated on the surface 39
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Fig. 6. SEM morphologies and EDS line analysis of cross-sections of un-irradiated sample and samples irradiated to different doses after 100 h oxidation at 900 °C, (a) un-irradiated, (c) 0.4 dpa, (e) 4 dpa, (g) 40 dpa.
of 40 dpa irradiated samples.
(TZXY2017QDJJ014), the Natural Science Research of Jiangsu Higher Education Institutions of China (18KJD430007), the Fifth “311 Highlevel Talents Training Project” of Taizhou City, the college students' innovation and entrepreneurship training project of Jiangsu province (201812917006Y), the National Natural Science Foundation of China (51609164) and the Natural Science Foundation of Jiangsu Province,
Acknowledgments This work was supported by the Scientific Research Foundation of Taizhou University for the Introduction of Talents 40
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Fig. 7. Bright-field TEM micrographs of microstructure variation in samples with different doses irradiation (a) un-irradiated sample, γ′ and γ″ precipitates, (b) 0.4 dpa, dislocations, (c) 4 dpa, dislocation cells (d) 40 dpa, high density dislocation structure.
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