- Email: [email protected]
Microstructure evolution of FeNiCr alloy induced by stress-oxidation coupling using high temperature nanoindentation
Corrosion Science xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Short Communication
Microstructure evolution of FeNiCr alloy induced by stress-oxidation coupling using high temperature nanoindentation Yan Lia,b, Xufei Fanga,b,
⁎,1
, Siyuan Zhangc, Xue Fenga,b,
⁎
a
AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China Center for Mechanics and Materials, Tsinghua University, Beijing 100084, China c Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany b
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Alloy B. STEM C. High temperature corrosion C. Oxidation C. Stress corrosion
The microstructure evolution of a FeNiCr alloy oxidized at 600 °C by simultaneously applying stress via high temperature nanoindentation is reported. Analysis using transmission electron microscopy shows that a sharp crack was induced beneath the indentation area under the stress-oxidation coupling condition. Nanotwins beneath the indentation area were also observed, which acted as a barrier that ceased the crack propagation beneath the indenter by altering the path of the crack. Results reveal a transformation from inter-granular crack propagation along the oxide grain boundaries to intra-granular crack propagation through the nanotwin structure with a zig-zag pattern.
1. Introduction Oxidation of metals/alloys with applied mechanical loading at elevated temperature is a long standing issue that is of great importance for both fundamental understanding of oxidation process and engineering application [1–3]. The advancing of nanotechnology and experimental instruments has made it possible to investigate the oxidation of various materials at small scale. For instance, Viskari et al. [4] used high-resolution analytical techniques to investigate the oxidation at inter-granular crack tip in a Ni-base superalloy and showed that oxidation took place at and immediately ahead of the tip of an open crack. Kitaguchi et al. [5] investigated the oxide growth ahead of the inter-granular crack tip in an advanced Ni-base superalloy using (scanning) transmission electron microscopy and observed different oxide intrusion lengths and oxide formation ahead of the crack tip under various loading conditions. However, in their experiments the cracks in the samples were pre-fabricated using fatigue pre-cracking method at macroscale, and the analysis was mainly focused on the nature of the formation of layered structure oxide ahead of the crack tips, while further understanding of the stress effect on stress-oxidation interaction and microstructure evolution including crack nucleation and propagation is lacking. In addition, microstructures such as twin boundaries, grain boundaries, second phase particles as well as other interfaces all have strong interaction with cracks when locally the cracks encounter such
⁎
1
microstructures [6–8]. For instance, when a crack meets a grain boundary in a material that exhibits preferred crack paths within a grain, the orientation change of such a preferred path in the adjacent grain could retard the crack propagation and thus enhance cleavagecracking resistance [6]. The crack may also deflect in a manner of intragranular propagation to enhance the fracture toughness [7,8]. Both crack nucleation and crack growth depend closely on the microstructure at sub-micro scale (length scales of ∼100 nm–∼1000 nm) [6]. When microstructure evolution and crack formation encounter oxidation at elevated temperature (which is common since oxidation degrades the material properties and results in microstructure evolution as well as crack formation/propagation), the challenges remain to probe into structures at such small scales to investigate the interaction between the cracks and local structures under oxidation conditions with applied mechanical loading. Conventionally, it is difficult to apply the mechanical loading with precise control in a region of interest (e.g. grain boundary, etc.) at such a small scale to study the crack nucleation and propagation during oxidation at high temperature. In order to understand the stress-oxidation coupling effect [9–11] on the microstructure evolution, as well as to investigate this coupling effect on the failure of materials at high temperature in oxidation environment, here in this work the high temperature nanoindnentation is adopted for the design of small scale experiments. High temperature nanoindentaion has been attracting more attention in recent years to measure material properties at high temperatures [12–14], for instance,
Corresponding authors at: AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China. E-mail addresses: [email protected] (X. Fang), [email protected] (X. Feng). Current address: Max-Planck-Institut für Eisenforschung, Max-Planck-Str. 1, 40237 Düsseldorf, Germany.
https://doi.org/10.1016/j.corsci.2018.02.043 Received 22 November 2017; Received in revised form 16 February 2018; Accepted 20 February 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Li, Y., Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.02.043
Corrosion Science xxx (xxxx) xxx–xxx
Y. Li et al.
Fig. 1. Schematic illustration of the experimental flow of the three comparative tests.
Fig. 2. TEM images showing the morphologies in the vicinity of the indentation areas and the white arrows indicate the tip area of each indent: (a) Sample A with crack; (b) Sample B with no crack; (c) Sample C with no crack.
bath and dried afterwards.
on Ni-base superalloys [15,16]. Nanoindentation instrument equipped with high temperature stage can be used as high temperature scanning probe microscope (SPM) to first scan and pinpoint specific locations at small scale and then apply load as normal high temperature indentation. The combination of high temperature SPM and nanoindentation makes it a useful tool to study the oxidation effect on the evolution of microstructures [17–19] at small scale with the ability of precise targeting and loading. Precise targeting at positions of interest also facilitates the post-mortem characterization of the targeted position to better understand the coupled process of stress-oxidation-structure evolution.
2.2. Mechanical loading using nanoindentation The TI 950 Tribo-indenter (Hysitron Inc., USA) was adopted to conduct the experiments. The equipment has a displacement resolution of 0.02 nm and load resolution 1 nN. A Berkovich diamond indenter, which is brazed to a Macor shaft, is adopted to apply the mechanical loading. The thermal conductivity of Macor has a very small value of about 1.5 W m−1 K−1 [20]. The indenter is designed for tests at high temperature by reducing thermal conduction from the surface of the specimen to the transducer as well as preventing the standard probe holder from being oxidized or melted due to the high temperature effect [17]. In the present experiment, the operation stage for carrying out nanoindentation test was heated first to maintain the sample surface temperature at 600 °C within a fluctuation of ± 0.5 °C. Then the Berkovich indenter was brought in contact with the specimen under a small contact load (2 μN). It is noted that such a contact process would introduce a temperature fluctuation, thus the indentation test was not conducted until the monitored temperature of the specimen became thermally stable again (e.g. the temperature fluctuation is within ± 0.5 °C). The indentation was then performed with a linearly increasing period of 5 s to the maximum load 11,000 μN and the indenter was maintained in contact with the specimen for 60 min at this maximum
2. Experiments 2.1. Sample preparation A Fe-20Ni-33Cr (wt.%) alloy (Central Iron & Steel Research Institute, China) was used in the present experiment. The surface of the specimens cut from the bulk was first grinded using an automatic grinding machine with 200–1200-grit silicon carbide papers, followed by polishing with diamond paste with particle size of 2.0 μm and 0.5 μm. Finally vibration polishing using solutions with nanosized silica was carried to remove the mechanically deformed surface layer. The surface of the specimens was then cleaned with ethanol in ultrasonic 2
Corrosion Science xxx (xxxx) xxx–xxx
Y. Li et al.
Fig. 3. (a) Crack initiated in the vicinity of the nanovoid at the oxide grain boundaries in the upper oxide layer and penetrated through the nanotwins beneath the oxide layer; (b) Oxygen concentration map from Cliff-Lorimer quantification (O-K edge is deconvoluted from the overlapping Cr-L edge by multivariate statistical analysis) [24]; (c–d) Confirmation of the nanotwin structure in the vicinity of the crack. The positions of sub-images (b) and (c) are indicated in (a) with blue and red squares, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the composition of the local structure were observed and analyzed using transmission electron microscope (TEM, Tecnai G2 F20 and Titan, FEI Company). Atomically resolved images and energy dispersive X-ray (EDX) mapping were taken on the Titan TEM operated at 300 kV in the scanning TEM (STEM) mode. The aberration corrected probe has a size of ∼1 Å and a convergence semiangle of 24 mrad. The annular bright field (ABF) detector has a collection semiangle of 8–16 mrad. Energy dispersive X-ray (EDX) spectrum imaging was taken using a windowless, four quadrant silicon-drift EDX detector, FEI Super-X, with a solid angle of > 0.7 sr.
load, during which the sample surface including the indented area was oxidized. All the above operations were carried out in a N2 gas (purity 99.999%) flow at a constant rate of 1.7 L/min to avoid aggressive oxidation on the sample surface as well as to minimize the moisture effect. The flow rate of the gas was kept constant to reduce the noise effect of the gas flow within the specifically small and confined tested area for the indentation process. After the experiment, the tip was retracted from the surface and the temperature was cooled down gradually, which took about 30 min for the specimen to cool down to room temperature (RT). The process of the above test is briefly illustrated in Fig. 1(a). For further comparison, two other tests were also conducted, i.e., (i) first simple indentation on the sample surface at room temperature followed by oxidation at 600 °C for 60 min without mechanical loading as shown in Fig. 1(b), and (ii) simple indentation on the sample surface that has been first oxidized at 600 °C for 60 min as shown in Fig. 1(c). Note that in all the three tests, the maximum load of the indentation and the N2 gas flow were kept the same. In the following section, the specimen tested in oxidation environment with mechanical loading for 60 min is denoted as Sample A. The specimens in comparative tests (i) and (ii) are Sample B and Sample C, respectively.
3. Results and analyses By designing the above experiment, the stress at microscale was applied in the local region on the sample surface to study the stressoxidation interaction on the structure evolution beneath the indentation area. In Fig. 2(a) the TEM image shows that a crack was induced beneath the indentation zone in Sample A. In Sample B in Fig. 2(b) and Sample C in Fig. 2(c), no crack was observed. The comparison of the experiments suggests that the continuous interaction of stress and oxidation process results in the crack beneath the indentation zone in Sample A. Thus, the following analysis mainly focuses on Sample A. Further experimental observation of Sample A reveals the nucleation site of the crack as well as two types of mechanisms for the crack propagation in the oxide scale beneath the indentation area: (i) the high stress concentration at the oxide grain boundaries during indentation
2.3. Characterization After the test, the samples were milled using focused ion beam (FIB, FEI Company, Helios 600i) to extract the cross-sections of the indented area. Then cross-sectional morphology of the indented specimens and 3
Corrosion Science xxx (xxxx) xxx–xxx
Y. Li et al.
Fig. 4. (a–b) ABF-STEM images of crack and nanotwin structures ahead of the crack tip; (c) the white contrast indicated by the black arrows ahead of the apparent crack tip is caused by the material thinning due to potential crack propagation; (d) high magnification of the nanotwin strucutre. The red dashed lines indicate the twin boundaries. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
using EDX mapping in Fig. 3(b). Such a crack bifurcation indicates a transformation of crack propagation mechanisms, namely, from intergranular crack propagation along the oxide grain boundaries to intragranular crack propagation within the nanotwin structure in the substrate. This experimental result agrees with the molecular dynamics (MD) simulation conducted on nanocrystalline nickle by Qu et al. [7]. Their simulation result showed that by altering the crack path, nanoscale twinning boundaries (TBs) could be effective in resisting crack propagation [7]. The crack propagation exhibited a zig-zag pattern through the twin planes, as has also been revealed in the recent investigation of nanotwinned copper [25,26]. It is thus believed that TBs are effective in improving the fracture toughness of the materials. In our case, a closer examination of the crack path using ABF-STEM in Fig. 4 provides further proof for the above mentioned zig-zag crack propagation pattern within the twin structure as well. The TEM image in Fig. 4(a) shows that the position immediately ahead of the apparent crack tip exhibits a crack-like feature with white contrast. Such a contrast is believed to be caused by the local thinning of the material due to the potential crack propagation. As indicated by the black arrows in Fig. 4(c), the potential crack propagates in a zig-zag way, same as has been discussed above. Higher magnification of the local region near the crack path revealed that the thickness of the nanotwins could be several nanometers, as shown in Fig. 4(d). Furthermore, it is worth mentioning that in the present work, the orientation of the crack is approximately perpendicular to the twin boundaries and exhibits a feature of a thorough crack through the twin structure. This could be due to the fact that the thickness of each layer
process facilitated the nucleation of the nanovoid [7,21,22], which is located in the triple junction (indicated by the red arrow in Fig. 3(a)) of the oxide grain boundaries. This nanovoid could contribute to the nucleation of the crack. The crack thereafter propagated along the grain boundaries in the oxide scale. In addition, the crack exhibits a brittle feature, which on the one hand is believed to be caused by the crack nucleation within the brittle oxide formed beneath the indent tip. On the other hand, the presence of impurities such as oxide in the material is effective in pinning dislocations and promoting brittle crack growth [23]. (ii) The crack further penetrated through the oxide/substrate interface (the interface is indicated by the white dashed line in Fig. 3(a)) and ceased within the layered nanotwin structure beneath. The nanotwin structure is confirmed in Fig. 3(c and d). Both the crack initiation and propagation are closely related to the change of the local microstructure, including the oxide grain boundary, nanovoid, oxide/ substrate interface, and the nanotwin structure. The exact mechanism of the twin structure formation in this work is not yet fully understood. However, the comparison of the experiments illustrated in Fig. 1 suggests that it is related to the coupling effect of continuous mechanical loading and high temperature. Deeper understanding of this phenomenon requires more comprehensive design of experiments (for instance, a comparative experiment in vacuum system with applied stress at high temperature to exclude the oxidation effect) and will be investigated in the future. Notice that the crack has a branch indicated by the blue square in Fig. 3(a). The location of this crack branch is on the interface of the top oxide layer and the substrate nanotwin structure, as has been confirmed 4
Corrosion Science xxx (xxxx) xxx–xxx
Y. Li et al.
of the twin structure is much smaller (at the order of several nanometers as shown in Fig. 4(d)) compared to the crack length that is about one micrometer. This micro-crack opening initiated from the oxide layer facilitates the crack to penetrate through the nanotwins with the crack propagation energy being consumed at a less pronounced extent.
210–215. [2] C.H. Zhou, H.T. Ma, L. Wang, Effect of mechanical loading on the oxidation kinetics and oxide-scale failure of pure Ni, Oxid. Met. 70 (2008) 287–294. [3] S. Lozano-Perez, K. Kruska, I. Iyengar, T. Terachi, T. Yamada, The role of cold work and applied stress on surface oxidation of 304 stainless steel, Corros. Sci. 56 (2012) 78–85. [4] L. Viskari, M. Hörnqvist, K.L. Moore, Y. Cao, K. Stiller, Intergranular crack tip oxidation in a Ni-base superalloy, Acta Mater. 61 (2013) 3630–3639. [5] H.S. Kitaguchi, H.Y. Li, H.E. Evans, R.G. Ding, I.P. Jones, G. Baxter, et al., Oxidation ahead of a crack tip in an advanced Ni-based superalloy, Acta Mater. 61 (2013) 1968–1981. [6] S. Kumar, W.A. Curtin, Crack interaction with microstructure, Mater. Today 10 (2007) 34–44. [7] H. Zhou, S. Qu, The effect of nanoscale twin boundaries on fracture toughness in nanocrystalline Ni, Nanotechnology 21 (2010) 035706. [8] H. Zhou, S. Qu, W. Yang, Toughening by nano-scaled twin boundaries in nanocrystals, Modell. Simul. Mater. Sci. Eng. 18 (2010). [9] X. Dong, X. Fang, X. Feng, K.-C. Hwang, Diffusion and stress coupling effect during oxidation at high temperature, J. Am. Ceram. Soc. 96 (2013) 44–46. [10] Y. Suo, S. Shen, General approach on chemistry and stress coupling effects during oxidation, J. Appl. Phys. 114 (2013) 164905. [11] H. Wang, S. Shen, A chemomechanical coupling model for oxidation and stress evolution in ZrB2–SiC, J. Mater. Res. (2017) 1–12. [12] J.M. Wheeler, D.E.J. Armstrong, W. Heinz, R. Schwaiger, High temperature nanoindentation: the state of the art and future challenges, Curr. Opin. Solid State Mater. Sci. 19 (2015) 354–366. [13] C.A. Schuh, C.E. Packard, A.C. Lund, Nanoindentation and contact-mode imaging at high temperatures, J. Mater. Res. 21 (2006) 725–736. [14] J.C. Trenkle, C.E. Packard, C.A. Schuh, Hot nanoindentation in inert environments, Rev. Sci. Instrum. 81 (2010) 073901. [15] A. Sawant, S. Tin, High temperature nanoindentation of a Re-bearing single crystal Ni-base superalloy, Scr. Mater. 58 (2008) 275–278. [16] Y. Li, X. Fang, S. Lu, Q. Yu, G. Hou, X. Feng, Effects of creep and oxidation on reduced modulus in high-temperature nanoindentation, Mater. Sci. Eng.: A 678 (2016) 65–71. [17] Y. Li, X. Fang, B. Xia, X. Feng, In situ measurement of oxidation evolution at elevated temperature by nanoindentation, Scr. Mater. 103 (2015) 61–64. [18] X. Fang, Y. Li, X. Feng, Curvature effect on the surface topography evolution during oxidation at small scale, J. Appl. Phys. 121 (2017) 125301. [19] X. Fang, Y. Li, D. Wang, S. Lu, X. Feng, Surface evolution at nanoscale during oxidation: a competing mechanism between local curvature effect and stress effect, J. Appl. Phys. 119 (2016) 1–10. [20] CORNING, Technical Data Sheet, Macor, Corning, Inc., Corning, N.Y, 2018. [21] J. Zhang, High Temperature Deformation and Fracture of Materials, Woodhead Publishing Limited, Cambridge, UK, 2010. [22] M. Ryan, Peering below the surface, Nat. Mater. 3 (2004) 663–665. [23] H. Vehoff, P. Ochmann, M. Goeken, M.G. Gehling, Deformation processes at crack tips in NiAl single- and bicrystals, Mater. Sci. Eng. A A239–A240 (1997) 378–385. [24] S. Zhang, C. Scheu, Evaluation of EELS spectrum imaging data by spectral components and factors from multivariate analysis, Microscopy (2017) 1–9, http://dx. doi.org/10.1093/jmicro/dfx091. [25] Z. Zeng, X. Li, L. Lu, T. Zhu, Fracture in a thin film of nanotwinned copper, Acta Mater. 98 (2015) 313–317. [26] Z.W. Shan, L. Lu, A.M. Minor, E.A. Stach, S.X. Mao, The effect of twin plane spacing on the deformation of copper containing a high density of growth twins, JOM 60 (2008) 71–74.
4. Conclusion In situ oxidation-stress coupling at microscale is realized by using nanoindentation to apply mechanical loading in a specific position of a FeNiCr-base alloy in oxidation environment at 600 °C. Experimental result reveals that oxidation process coupled with continuously applied stress results in drastic microstructure evolution of the material. The nucleation and propagation of micro-crack is identified, and formation of nanotwin structure beneath the indentation area is observed. This work serves as a preliminary step to understand the structural evolution under chemo-mechanical coupling effects at small scale. Future experiment in vacuum with applied stress at high temperature will be carried out to exclude the oxidation effect for comparison, and to shed light on the mechanism of the twin structure formation. Author contributions statement Y. Li, X. Fang, and S. Zhang conducted the experiments. Y. Li and X. Fang drafted the manuscript. X. Feng supervised the research. All authors were involved in the analysis of the results and contributed to writing the manuscript. Competing financial interests The authors declare that they have no competing financial interests. Acknowledgments We gratefully acknowledge the support from National Natural Science Foundation of China (Grant Nos. 11625207, 11320101001, 11227801) and the National Basic Research Program of China (Grant No. 2015CB351900). References [1] C.H. Zhou, H.T. Ma, L. Wang, Comparative study of oxidation kinetics for pure nickel oxidized under tensile and compressive stress, Corros. Sci. 52 (2010)
5
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Short Communication
Microstructure evolution of FeNiCr alloy induced by stress-oxidation coupling using high temperature nanoindentation Yan Lia,b, Xufei Fanga,b,
⁎,1
, Siyuan Zhangc, Xue Fenga,b,
⁎
a
AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China Center for Mechanics and Materials, Tsinghua University, Beijing 100084, China c Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany b
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Alloy B. STEM C. High temperature corrosion C. Oxidation C. Stress corrosion
The microstructure evolution of a FeNiCr alloy oxidized at 600 °C by simultaneously applying stress via high temperature nanoindentation is reported. Analysis using transmission electron microscopy shows that a sharp crack was induced beneath the indentation area under the stress-oxidation coupling condition. Nanotwins beneath the indentation area were also observed, which acted as a barrier that ceased the crack propagation beneath the indenter by altering the path of the crack. Results reveal a transformation from inter-granular crack propagation along the oxide grain boundaries to intra-granular crack propagation through the nanotwin structure with a zig-zag pattern.
1. Introduction Oxidation of metals/alloys with applied mechanical loading at elevated temperature is a long standing issue that is of great importance for both fundamental understanding of oxidation process and engineering application [1–3]. The advancing of nanotechnology and experimental instruments has made it possible to investigate the oxidation of various materials at small scale. For instance, Viskari et al. [4] used high-resolution analytical techniques to investigate the oxidation at inter-granular crack tip in a Ni-base superalloy and showed that oxidation took place at and immediately ahead of the tip of an open crack. Kitaguchi et al. [5] investigated the oxide growth ahead of the inter-granular crack tip in an advanced Ni-base superalloy using (scanning) transmission electron microscopy and observed different oxide intrusion lengths and oxide formation ahead of the crack tip under various loading conditions. However, in their experiments the cracks in the samples were pre-fabricated using fatigue pre-cracking method at macroscale, and the analysis was mainly focused on the nature of the formation of layered structure oxide ahead of the crack tips, while further understanding of the stress effect on stress-oxidation interaction and microstructure evolution including crack nucleation and propagation is lacking. In addition, microstructures such as twin boundaries, grain boundaries, second phase particles as well as other interfaces all have strong interaction with cracks when locally the cracks encounter such
⁎
1
microstructures [6–8]. For instance, when a crack meets a grain boundary in a material that exhibits preferred crack paths within a grain, the orientation change of such a preferred path in the adjacent grain could retard the crack propagation and thus enhance cleavagecracking resistance [6]. The crack may also deflect in a manner of intragranular propagation to enhance the fracture toughness [7,8]. Both crack nucleation and crack growth depend closely on the microstructure at sub-micro scale (length scales of ∼100 nm–∼1000 nm) [6]. When microstructure evolution and crack formation encounter oxidation at elevated temperature (which is common since oxidation degrades the material properties and results in microstructure evolution as well as crack formation/propagation), the challenges remain to probe into structures at such small scales to investigate the interaction between the cracks and local structures under oxidation conditions with applied mechanical loading. Conventionally, it is difficult to apply the mechanical loading with precise control in a region of interest (e.g. grain boundary, etc.) at such a small scale to study the crack nucleation and propagation during oxidation at high temperature. In order to understand the stress-oxidation coupling effect [9–11] on the microstructure evolution, as well as to investigate this coupling effect on the failure of materials at high temperature in oxidation environment, here in this work the high temperature nanoindnentation is adopted for the design of small scale experiments. High temperature nanoindentaion has been attracting more attention in recent years to measure material properties at high temperatures [12–14], for instance,
Corresponding authors at: AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China. E-mail addresses: [email protected] (X. Fang), [email protected] (X. Feng). Current address: Max-Planck-Institut für Eisenforschung, Max-Planck-Str. 1, 40237 Düsseldorf, Germany.
https://doi.org/10.1016/j.corsci.2018.02.043 Received 22 November 2017; Received in revised form 16 February 2018; Accepted 20 February 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Li, Y., Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.02.043
Corrosion Science xxx (xxxx) xxx–xxx
Y. Li et al.
Fig. 1. Schematic illustration of the experimental flow of the three comparative tests.
Fig. 2. TEM images showing the morphologies in the vicinity of the indentation areas and the white arrows indicate the tip area of each indent: (a) Sample A with crack; (b) Sample B with no crack; (c) Sample C with no crack.
bath and dried afterwards.
on Ni-base superalloys [15,16]. Nanoindentation instrument equipped with high temperature stage can be used as high temperature scanning probe microscope (SPM) to first scan and pinpoint specific locations at small scale and then apply load as normal high temperature indentation. The combination of high temperature SPM and nanoindentation makes it a useful tool to study the oxidation effect on the evolution of microstructures [17–19] at small scale with the ability of precise targeting and loading. Precise targeting at positions of interest also facilitates the post-mortem characterization of the targeted position to better understand the coupled process of stress-oxidation-structure evolution.
2.2. Mechanical loading using nanoindentation The TI 950 Tribo-indenter (Hysitron Inc., USA) was adopted to conduct the experiments. The equipment has a displacement resolution of 0.02 nm and load resolution 1 nN. A Berkovich diamond indenter, which is brazed to a Macor shaft, is adopted to apply the mechanical loading. The thermal conductivity of Macor has a very small value of about 1.5 W m−1 K−1 [20]. The indenter is designed for tests at high temperature by reducing thermal conduction from the surface of the specimen to the transducer as well as preventing the standard probe holder from being oxidized or melted due to the high temperature effect [17]. In the present experiment, the operation stage for carrying out nanoindentation test was heated first to maintain the sample surface temperature at 600 °C within a fluctuation of ± 0.5 °C. Then the Berkovich indenter was brought in contact with the specimen under a small contact load (2 μN). It is noted that such a contact process would introduce a temperature fluctuation, thus the indentation test was not conducted until the monitored temperature of the specimen became thermally stable again (e.g. the temperature fluctuation is within ± 0.5 °C). The indentation was then performed with a linearly increasing period of 5 s to the maximum load 11,000 μN and the indenter was maintained in contact with the specimen for 60 min at this maximum
2. Experiments 2.1. Sample preparation A Fe-20Ni-33Cr (wt.%) alloy (Central Iron & Steel Research Institute, China) was used in the present experiment. The surface of the specimens cut from the bulk was first grinded using an automatic grinding machine with 200–1200-grit silicon carbide papers, followed by polishing with diamond paste with particle size of 2.0 μm and 0.5 μm. Finally vibration polishing using solutions with nanosized silica was carried to remove the mechanically deformed surface layer. The surface of the specimens was then cleaned with ethanol in ultrasonic 2
Corrosion Science xxx (xxxx) xxx–xxx
Y. Li et al.
Fig. 3. (a) Crack initiated in the vicinity of the nanovoid at the oxide grain boundaries in the upper oxide layer and penetrated through the nanotwins beneath the oxide layer; (b) Oxygen concentration map from Cliff-Lorimer quantification (O-K edge is deconvoluted from the overlapping Cr-L edge by multivariate statistical analysis) [24]; (c–d) Confirmation of the nanotwin structure in the vicinity of the crack. The positions of sub-images (b) and (c) are indicated in (a) with blue and red squares, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the composition of the local structure were observed and analyzed using transmission electron microscope (TEM, Tecnai G2 F20 and Titan, FEI Company). Atomically resolved images and energy dispersive X-ray (EDX) mapping were taken on the Titan TEM operated at 300 kV in the scanning TEM (STEM) mode. The aberration corrected probe has a size of ∼1 Å and a convergence semiangle of 24 mrad. The annular bright field (ABF) detector has a collection semiangle of 8–16 mrad. Energy dispersive X-ray (EDX) spectrum imaging was taken using a windowless, four quadrant silicon-drift EDX detector, FEI Super-X, with a solid angle of > 0.7 sr.
load, during which the sample surface including the indented area was oxidized. All the above operations were carried out in a N2 gas (purity 99.999%) flow at a constant rate of 1.7 L/min to avoid aggressive oxidation on the sample surface as well as to minimize the moisture effect. The flow rate of the gas was kept constant to reduce the noise effect of the gas flow within the specifically small and confined tested area for the indentation process. After the experiment, the tip was retracted from the surface and the temperature was cooled down gradually, which took about 30 min for the specimen to cool down to room temperature (RT). The process of the above test is briefly illustrated in Fig. 1(a). For further comparison, two other tests were also conducted, i.e., (i) first simple indentation on the sample surface at room temperature followed by oxidation at 600 °C for 60 min without mechanical loading as shown in Fig. 1(b), and (ii) simple indentation on the sample surface that has been first oxidized at 600 °C for 60 min as shown in Fig. 1(c). Note that in all the three tests, the maximum load of the indentation and the N2 gas flow were kept the same. In the following section, the specimen tested in oxidation environment with mechanical loading for 60 min is denoted as Sample A. The specimens in comparative tests (i) and (ii) are Sample B and Sample C, respectively.
3. Results and analyses By designing the above experiment, the stress at microscale was applied in the local region on the sample surface to study the stressoxidation interaction on the structure evolution beneath the indentation area. In Fig. 2(a) the TEM image shows that a crack was induced beneath the indentation zone in Sample A. In Sample B in Fig. 2(b) and Sample C in Fig. 2(c), no crack was observed. The comparison of the experiments suggests that the continuous interaction of stress and oxidation process results in the crack beneath the indentation zone in Sample A. Thus, the following analysis mainly focuses on Sample A. Further experimental observation of Sample A reveals the nucleation site of the crack as well as two types of mechanisms for the crack propagation in the oxide scale beneath the indentation area: (i) the high stress concentration at the oxide grain boundaries during indentation
2.3. Characterization After the test, the samples were milled using focused ion beam (FIB, FEI Company, Helios 600i) to extract the cross-sections of the indented area. Then cross-sectional morphology of the indented specimens and 3
Corrosion Science xxx (xxxx) xxx–xxx
Y. Li et al.
Fig. 4. (a–b) ABF-STEM images of crack and nanotwin structures ahead of the crack tip; (c) the white contrast indicated by the black arrows ahead of the apparent crack tip is caused by the material thinning due to potential crack propagation; (d) high magnification of the nanotwin strucutre. The red dashed lines indicate the twin boundaries. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
using EDX mapping in Fig. 3(b). Such a crack bifurcation indicates a transformation of crack propagation mechanisms, namely, from intergranular crack propagation along the oxide grain boundaries to intragranular crack propagation within the nanotwin structure in the substrate. This experimental result agrees with the molecular dynamics (MD) simulation conducted on nanocrystalline nickle by Qu et al. [7]. Their simulation result showed that by altering the crack path, nanoscale twinning boundaries (TBs) could be effective in resisting crack propagation [7]. The crack propagation exhibited a zig-zag pattern through the twin planes, as has also been revealed in the recent investigation of nanotwinned copper [25,26]. It is thus believed that TBs are effective in improving the fracture toughness of the materials. In our case, a closer examination of the crack path using ABF-STEM in Fig. 4 provides further proof for the above mentioned zig-zag crack propagation pattern within the twin structure as well. The TEM image in Fig. 4(a) shows that the position immediately ahead of the apparent crack tip exhibits a crack-like feature with white contrast. Such a contrast is believed to be caused by the local thinning of the material due to the potential crack propagation. As indicated by the black arrows in Fig. 4(c), the potential crack propagates in a zig-zag way, same as has been discussed above. Higher magnification of the local region near the crack path revealed that the thickness of the nanotwins could be several nanometers, as shown in Fig. 4(d). Furthermore, it is worth mentioning that in the present work, the orientation of the crack is approximately perpendicular to the twin boundaries and exhibits a feature of a thorough crack through the twin structure. This could be due to the fact that the thickness of each layer
process facilitated the nucleation of the nanovoid [7,21,22], which is located in the triple junction (indicated by the red arrow in Fig. 3(a)) of the oxide grain boundaries. This nanovoid could contribute to the nucleation of the crack. The crack thereafter propagated along the grain boundaries in the oxide scale. In addition, the crack exhibits a brittle feature, which on the one hand is believed to be caused by the crack nucleation within the brittle oxide formed beneath the indent tip. On the other hand, the presence of impurities such as oxide in the material is effective in pinning dislocations and promoting brittle crack growth [23]. (ii) The crack further penetrated through the oxide/substrate interface (the interface is indicated by the white dashed line in Fig. 3(a)) and ceased within the layered nanotwin structure beneath. The nanotwin structure is confirmed in Fig. 3(c and d). Both the crack initiation and propagation are closely related to the change of the local microstructure, including the oxide grain boundary, nanovoid, oxide/ substrate interface, and the nanotwin structure. The exact mechanism of the twin structure formation in this work is not yet fully understood. However, the comparison of the experiments illustrated in Fig. 1 suggests that it is related to the coupling effect of continuous mechanical loading and high temperature. Deeper understanding of this phenomenon requires more comprehensive design of experiments (for instance, a comparative experiment in vacuum system with applied stress at high temperature to exclude the oxidation effect) and will be investigated in the future. Notice that the crack has a branch indicated by the blue square in Fig. 3(a). The location of this crack branch is on the interface of the top oxide layer and the substrate nanotwin structure, as has been confirmed 4
Corrosion Science xxx (xxxx) xxx–xxx
Y. Li et al.
of the twin structure is much smaller (at the order of several nanometers as shown in Fig. 4(d)) compared to the crack length that is about one micrometer. This micro-crack opening initiated from the oxide layer facilitates the crack to penetrate through the nanotwins with the crack propagation energy being consumed at a less pronounced extent.
210–215. [2] C.H. Zhou, H.T. Ma, L. Wang, Effect of mechanical loading on the oxidation kinetics and oxide-scale failure of pure Ni, Oxid. Met. 70 (2008) 287–294. [3] S. Lozano-Perez, K. Kruska, I. Iyengar, T. Terachi, T. Yamada, The role of cold work and applied stress on surface oxidation of 304 stainless steel, Corros. Sci. 56 (2012) 78–85. [4] L. Viskari, M. Hörnqvist, K.L. Moore, Y. Cao, K. Stiller, Intergranular crack tip oxidation in a Ni-base superalloy, Acta Mater. 61 (2013) 3630–3639. [5] H.S. Kitaguchi, H.Y. Li, H.E. Evans, R.G. Ding, I.P. Jones, G. Baxter, et al., Oxidation ahead of a crack tip in an advanced Ni-based superalloy, Acta Mater. 61 (2013) 1968–1981. [6] S. Kumar, W.A. Curtin, Crack interaction with microstructure, Mater. Today 10 (2007) 34–44. [7] H. Zhou, S. Qu, The effect of nanoscale twin boundaries on fracture toughness in nanocrystalline Ni, Nanotechnology 21 (2010) 035706. [8] H. Zhou, S. Qu, W. Yang, Toughening by nano-scaled twin boundaries in nanocrystals, Modell. Simul. Mater. Sci. Eng. 18 (2010). [9] X. Dong, X. Fang, X. Feng, K.-C. Hwang, Diffusion and stress coupling effect during oxidation at high temperature, J. Am. Ceram. Soc. 96 (2013) 44–46. [10] Y. Suo, S. Shen, General approach on chemistry and stress coupling effects during oxidation, J. Appl. Phys. 114 (2013) 164905. [11] H. Wang, S. Shen, A chemomechanical coupling model for oxidation and stress evolution in ZrB2–SiC, J. Mater. Res. (2017) 1–12. [12] J.M. Wheeler, D.E.J. Armstrong, W. Heinz, R. Schwaiger, High temperature nanoindentation: the state of the art and future challenges, Curr. Opin. Solid State Mater. Sci. 19 (2015) 354–366. [13] C.A. Schuh, C.E. Packard, A.C. Lund, Nanoindentation and contact-mode imaging at high temperatures, J. Mater. Res. 21 (2006) 725–736. [14] J.C. Trenkle, C.E. Packard, C.A. Schuh, Hot nanoindentation in inert environments, Rev. Sci. Instrum. 81 (2010) 073901. [15] A. Sawant, S. Tin, High temperature nanoindentation of a Re-bearing single crystal Ni-base superalloy, Scr. Mater. 58 (2008) 275–278. [16] Y. Li, X. Fang, S. Lu, Q. Yu, G. Hou, X. Feng, Effects of creep and oxidation on reduced modulus in high-temperature nanoindentation, Mater. Sci. Eng.: A 678 (2016) 65–71. [17] Y. Li, X. Fang, B. Xia, X. Feng, In situ measurement of oxidation evolution at elevated temperature by nanoindentation, Scr. Mater. 103 (2015) 61–64. [18] X. Fang, Y. Li, X. Feng, Curvature effect on the surface topography evolution during oxidation at small scale, J. Appl. Phys. 121 (2017) 125301. [19] X. Fang, Y. Li, D. Wang, S. Lu, X. Feng, Surface evolution at nanoscale during oxidation: a competing mechanism between local curvature effect and stress effect, J. Appl. Phys. 119 (2016) 1–10. [20] CORNING, Technical Data Sheet, Macor, Corning, Inc., Corning, N.Y, 2018. [21] J. Zhang, High Temperature Deformation and Fracture of Materials, Woodhead Publishing Limited, Cambridge, UK, 2010. [22] M. Ryan, Peering below the surface, Nat. Mater. 3 (2004) 663–665. [23] H. Vehoff, P. Ochmann, M. Goeken, M.G. Gehling, Deformation processes at crack tips in NiAl single- and bicrystals, Mater. Sci. Eng. A A239–A240 (1997) 378–385. [24] S. Zhang, C. Scheu, Evaluation of EELS spectrum imaging data by spectral components and factors from multivariate analysis, Microscopy (2017) 1–9, http://dx. doi.org/10.1093/jmicro/dfx091. [25] Z. Zeng, X. Li, L. Lu, T. Zhu, Fracture in a thin film of nanotwinned copper, Acta Mater. 98 (2015) 313–317. [26] Z.W. Shan, L. Lu, A.M. Minor, E.A. Stach, S.X. Mao, The effect of twin plane spacing on the deformation of copper containing a high density of growth twins, JOM 60 (2008) 71–74.
4. Conclusion In situ oxidation-stress coupling at microscale is realized by using nanoindentation to apply mechanical loading in a specific position of a FeNiCr-base alloy in oxidation environment at 600 °C. Experimental result reveals that oxidation process coupled with continuously applied stress results in drastic microstructure evolution of the material. The nucleation and propagation of micro-crack is identified, and formation of nanotwin structure beneath the indentation area is observed. This work serves as a preliminary step to understand the structural evolution under chemo-mechanical coupling effects at small scale. Future experiment in vacuum with applied stress at high temperature will be carried out to exclude the oxidation effect for comparison, and to shed light on the mechanism of the twin structure formation. Author contributions statement Y. Li, X. Fang, and S. Zhang conducted the experiments. Y. Li and X. Fang drafted the manuscript. X. Feng supervised the research. All authors were involved in the analysis of the results and contributed to writing the manuscript. Competing financial interests The authors declare that they have no competing financial interests. Acknowledgments We gratefully acknowledge the support from National Natural Science Foundation of China (Grant Nos. 11625207, 11320101001, 11227801) and the National Basic Research Program of China (Grant No. 2015CB351900). References [1] C.H. Zhou, H.T. Ma, L. Wang, Comparative study of oxidation kinetics for pure nickel oxidized under tensile and compressive stress, Corros. Sci. 52 (2010)
5