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Thermal fatigue mechanism of recrystallized tungsten under cyclic heat loads via electron beam facility
Int. Journal of Refractory Metals and Hard Materials 61 (2016) 61–66
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Thermal fatigue mechanism of recrystallized tungsten under cyclic heat loads via electron beam facility Liang Wang, Bo Wang ⁎, Shu-Dan Li, Dong Ma, Yun-Hui Tang, Hui Yan School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e
i n f o
Article history: Received 18 May 2016 Received in revised form 3 July 2016 Accepted 27 July 2016 Available online 05 August 2016 Keywords: Tungsten Cyclic heat loads Thermal fatigue Extruded flake structures
a b s t r a c t Thermal fatigue resistance of plasma facing materials (PFMs) is an inevitable concern for component lifetime and plasma operations, since the temperature fluctuations will always exist in future nuclear fusion facilities and reactors. Accordingly, experiments were performed in the electron beam facility to investigate the thermal fatigue behavior under operational loading conditions. The tungsten is investigated in its stress relieved and fully recrystallized state for a better understanding of the thermal fatigue process when exposed to cyclic heat loads. The heat loads range from 24 to 48 MW/m2 and the number of cycles increases from 100 to 1000 times. The results indicate that the thermal fatigue damage (surface roughening) due to plastic deformation strongly depends on the loading conditions and the cycle index. As the power density and the number of cycles increase, the density of the intragranular shear bands in each grain becomes higher and the swelling of grain boundaries becomes more pronounced. The shear bands are generally parallel to different directions for varying grains, showing strong grain orientation dependence. Additionally, extruded flake structures on shear bands were observed in these damaged areas. It found that the shear bands are generally parallel to the traces of {112} slip planes with the surface. The results suggest that slip plastic deformation represent the predominant mechanism for thermal fatigue and a set of schematic diagram is presented to explain the formation of thermal fatigue damage morphology (extrusion and intrusion structures). © 2016 Elsevier Ltd. All rights reserved.
1. Introduction One of the key concerns for future nuclear fusion devices is the development of plasma facing materials (PFMs). These FPMs should be able to withstand the severe and complex environmental conditions especially in the divertor region. Tungsten (W) has multiple favorable thermal properties, which makes it a most promising plasma facing material [1]. Nevertheless, a PFM will be subjected to steady-state heat loads and several types of short transient events during plasma operation in fusion reactors, which can cause unacceptable material damage, such as cracks, surface melting, evaporation, droplet ejection and fatigue fracture [2–4]. Therefore, different experimental set-ups to simulate these environmental conditions are essential to determine behavior and damage mechanisms of tungsten. Recently, a series of experiments have been conducted using pulsed laser and electron beams to reproduce fusion relevant transient and steady state heat loads. Based on these results, it is concluded that the damage types, such as surface roughening, cracks and melting, evolve as function of transient heat loads, number of pulses and the base temperature of W [5–8]. However, few studies focused on the detailed microscopic analysis of the thermal fatigue damage (surface roughening) caused by cyclic heat loads. W as a ⁎ Corresponding author. E-mail address: [email protected] (B. Wang).
http://dx.doi.org/10.1016/j.ijrmhm.2016.07.022 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
plasma facing material (PFM) would be exposed to a variety of cyclic heat loads and subjected to stress under temperature changes due to different strain rates. Therefore, thermal fatigue resistance of W is an inevitable concern for component lifetime and plasma operations, since the temperature fluctuations will always exist in future nuclear fusion facilities and reactors. Y·Yuan [9] has focused on surface roughening and conjectured that it is mainly caused by twin plastic deformation. Unfortunately, the microscopic images of the Yuan's papers showed only the images of overall deformation features without local details. In this work, W is investigated in its stress relieved and fully recrystallized state to ascertain a better understanding of the thermal fatigue process under cyclic heat loads. Thermal fatigue resistance is assessed by changing the power density and cycle index. Additionally, we study on the potential mechanism of surface deformation (typically the extruded flake structures) observed in loaded areas. 2. Experimental Thermal fatigue tests were performed in the electron beam facility with 8 kV rated voltage. All samples were mounted on carbon crucible placed on a water-cooled Cu block. To ensure a good contact for heat conduction, a small amount of metal gallium was added between the samples and the crucible. The thermal fatigue tests were conducted out with 100, 300, 500 and 1000 pulses at an absorbed power density
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Fig. 1. The maximum temperature evolution of electron beam deposited area simulated by FEM during one cyclic heat loads.
of 48 MW/m2 with a duration of 1 s and followed by a 9 s interval to cool the loaded area to the base temperature. In addition, the thermal fatigue tests were carried out at two other absorbed power densities of 24, 36 MW/m2 with 500 pulses. In the tests, the maximum current was observed to be 107, 160 and 214 mA during a pulsed loading, and the electron beam deposited area is a circle with a diameter of 5 mm. Thereby, the absorbed power densities are 24, 36 and 48 MW/m2, respectively (the electron absorption coefficient of W is 0.55 [9]). The tests were performed on fully recrystallized W plate manufactured by AT&M Co., Ltd. (China). The internal stresses induced by rolling deformation were completely released after full recrystallization [10]. Thereby, for the loaded recrystallized W, the stress distribution largely depends on the thermal stresses induced during loading. All samples were cut from the recrystallized W plate with the dimensions of 10 × 10 × 2 mm3 for thermal fatigue tests. In order to obtain a stress-free and a well-defined reference surface, the surface of the samples was electro-polishing in a 2 wt.% NaOH solution to a low roughness of Ra b 0.1 μm. After the thermal fatigue tests, the surface morphology and microstructure were observed by a scanning electron microscope
Fig. 2. SEM images of the electro-polished recrystallized W after exposure to cyclic heat loads (absorbed power density 48 MW/m2) in different cycle indexes. (a) 100 pulses, (b) 300 pulses, (c) 500 pulses and (d) 1000 pulses. (e and f) is higher magnification of grains A and B shown in (c).
L. Wang et al. / Int. Journal of Refractory Metals and Hard Materials 61 (2016) 61–66
63
Fig. 3. SEM images of the electro-polished recrystallized W after exposure to 500 heat loads shots (a) absorbed power density 24 MW/m2, (b) absorbed power density 36 MW/m2.
(SEM) using a field emission microscope. The subsurface cross-section information of the cyclic heat loaded area was obtained by focused ion beam (FIB) technology and imaging (SEM). In addition, electron backscatter diffraction (EBSD) was used to correlate the crystallographic orientation of grains to the surface morphology induced via cyclic heat loads. 3. Results and discussion 3.1. Microstructural analysis Fig. 1 shows the maximum temperature evolution of the samples simulated by FEM during one cyclic heat loads. The samples loaded at RT with 24, 36 and 48 MW/m2, respectively. The temperature of loaded area increases rapidly during 1 s pulsed loading and rapidly cool down to the initial temperature since the subjacent tungsten remains thermally unaffected. When the power density was increased from 24 to 48 MW/m2, a higher peak temperature and a faster heating rate are induced, which would result a more severe stress state at the loaded areas. Fig. 2 illustrates the surface morphology of the electro-polished recrystallized W after exposure to cyclic heat loads (absorbed power
density 48 MW/m2) in different cycle indexes, (a) 100 pulses, (b) 300 pulses, (c) 500 pulses and (d) 1000 pulses. It can be found that the surface thermal fatigue damage is gradually aggravated with the increase of the number of cycles. The microcracks (shear bands) are generally parallel to different directions for varying grains and are appeared in partial grains under fewer cycle loads, showing strong crystallographic orientation dependence. However, when the number of cycles increases to 1000 pulses, the sample was severely damaged (see Fig. 2d). As the number of cycles increases from 100 to 1000 times, the density of the intragranular shear bands in each grain becomes higher and the swelling of grain boundaries becomes more pronounced. Additionally, the transgranular shear bands and extruded flake structures are observed in these damaged areas (see Fig. 2e and f). The formation of extruded flake structures would cause pronounced surface roughening. Thermal fatigue and accumulated dislocations produced at elevated pulse numbers would create a highly stressed and deformed surface [8]. The surface morphology of the electro-polished recrystallized W exposed to 500 heat loading shots with two different absorbed power densities is shown in Fig. 3. As seen in Figs. 2c and 3, surface damage became more significant since a more severe stress state is induced at the loaded areas when the power density is increased from 24 to 48 MW/
Fig. 4. EBSD maps (a and b) and 70°-tilted oblique view SEM image of extruded flake structure morphology correlating to the orientation maps. The lattices of A, B and C grains embed in orientation map (b).
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Table 1 The detailed crystal orientation information of the different positions in A, B and C grains indicated in Fig. 2. Euler angle
A B
C
Miller indexes
φ1
Ф
φ2
(h k l)
[uvw]
Crystal orientation
a1
269.8
113.1
3.3
f021gh012i
89.9
63.8
357.8
ð021Þ (021)
[012]
a2
½012
Grain No.
b1
284.9
117.4
4.1
ð021Þ
½124
b2
285.1
117.4
4.0
ð021Þ
b3
277.0
27.2
187.2
b4
194.0
92.9
62.2
ð012Þ (210)
½124 [142]
b5
103.8
64.5
265.8
ð201Þ
c1
281.6
101.4
110.7
c2
188.0
109.4
255.9
ð311Þ (311)
m2 for 500 pulses. Therefore, it can be found that the number of cycles had little effect on the surface damage when the power density is relatively small (see Fig. 3a). The reason for the surface modification is that the thermal stress state of the samples changes alternatively between compressive and tensile stresses. The surface damages are ascribed to the swelling of grain boundaries and the irreversible plastic deformation of the heated grains due to compressive stresses during heat loading [9]. When the sample is heated by the transient load, the temperature of the loaded area increases rapidly, crossing the material's ductile to brittle transition temperature (DBTT), see Fig. 1. Thereby, the compressive stresses are induced during loading since the loaded area is subjected to thermal expansion and the surrounding area is colder and expands less. The induced compressive stresses can lead to the swelling of grain boundaries and the irreversible plastic deformation, depending on stress amplitude and temperature [8,11]. The tensile stresses are induced due to the volumetric shrinkage in the following cool down phase during the interval of loading. Due to this mechanism, the cycle alternately of compressive and tensile stresses is induced during cyclic heat loads. 3.2. Mechanism analysis The extruded flake structures on shear bands formed via cyclic heat loads are parallel to different directions for varying grains, strongly depending on crystallographic orientation of the grains. The characteristics of the surface morphology are similar with the results obtained in Ref [12]. The author conjectured that the repetitive anisotropic deformation causes these surface modifications. However, the detailed formation mechanism of this morphology is not concerned. To ascertain the formation of the extruded flake structures and get a precise explanation, EBSD was used to correlate the crystallographic orientation of grains to the surface morphology. Fig. 4 shows EBSD maps (a and
f021gh124i
½241 ½214 [103]
f311gh103i
½103
b) and 70°-tilted oblique view SEM image of extruded flake structure morphology correlating to the orientation maps. The extruded flake structures are induced under 500 cyclic heat loads with 48 MW/m2. The rotation angle of boundary between A and B grains obtained from the EBSD maps is b15°. Obviously, the extrusions (shear bands) can be easily extended to the adjacent grains, when the crystallographic orientation of two adjacent grains is close. However, a large deflection between the extrusions on either side of a grain boundary would be caused, if the misorientation between adjacent is large, see Fig. 4a and c. Moreover, the crystallographic and macroscopic orientations of the shear bands were calculated based on the method [9,13]. Euler angle information of the different positions in A, B and C grains is obtained from the EBSD image (see Fig. 4a), and the corresponding Miller indexes is calculated through the Euler angle. The detailed grain orientation information is illustrated in Table 1. After analysis, it found that the extrusion zone and the substrate of grains have the same crystal orientation although the Euler angle is different. Accordingly, we conjecture that slip deformation is the predominant mechanism for the formation of extrusion surface morphology under cyclic heat loads. Based on body-centered cubic structure, W commonly shear on three different crystallographic slip systems 〈111〉 {110}, 〈 111 〉 {112} and 〈111〉 {123} [14]. Table 2 shows the detailed information of slip systems occurring readily for A, B and C grains in 48 slip systems, Schmidt factor and the inclination angle of the slip bands to the xdirection (ψ1 the calculated angle of the intersection of slip plane and surface to the x-direction and ψ2 the measured angle between the slip bands and the x-direction from the micrograph in Fig. 4a). The slip system listed in Table 2 is most likely to occur (Schmidt factor is relatively large). The Schmidt factor can be calculated using m ¼ cosα cosβ− cosγ cosδ; where α and β are the angles of the slip direction (b)/ the normal
Table 2 The detailed information of slip systems occurring readily for A, B and C grains, respectively, Schmidt factor and the inclination angle of the slip bands to the x-direction.
Grain No.
Miller indexes (hkl) [uvw]
Slip systems n⋅b
A
ð021Þ½012
ð121Þ½111=ð121Þ½111 ð112Þ½111=ð112Þ½111
B
C
ð021Þ½124
ð311Þ½103
ψ1 (°)
ψ2 (°)
0.613
60.8
50–73
0.613
53.3
Schmidt factor (|m|)
ð110Þ½111=ð110Þ½111
0.572
24.1
ð101Þ½111=ð101Þ½111
0.572
41.8
ð231Þ½111=ð231Þ½111=ð213Þ½111 ð213Þ½111
0.617
48.2
ð121Þ½111
0.727
73.4
ð110Þ½111
0.665
36.7
ð132Þ½111
0.701
84.9
ð231Þ½111
0.727
60.8
ð112Þ½111
0.857
73.2
ð101Þ½111
0.772
58.9
ð123Þ½111
0.830
117.8
ð213Þ½111
0.853
92.5
73
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Fig. 5. FIB cross-sectional SEM images of the extruded flake structures induced via 1000 heat loads of absorbed power density 48 MW/m2. (a) Original, (b) after etching.
direction of slip surface (n) to the RD-direction. Accordingly, γ and δ are the angles of the slip direction (b)/ the normal direction of slip surface (n) to the ND-direction. Compared with ψ1 and ψ2, it was found that the slip bands are parallel to the traces of {112} slip planes with the surface. The A grain slip occurred on (121)[111], (121)[111], (112)[111] and (112)[111] slip systems, B grain on (121)[111] slip system and C grain on (112)[111] slip system. Four slip systems simultaneously occurred and then produced cross-slips in A grain, leading to the formation of a curved slip band morphology, see Fig. 4a. However, parallel slip bands are formed in B and C grains because there is only one slip system that occurred, see Fig. 4a. In order to further investigate thermal fatigue mechanism of W under cyclic heat loads, the FIB was implemented for the detailed observation of the cross-section of the extrusions. Fig. 5 shows cross-sectional SEM images of the extruded flake structures within the loaded surface before and after etching. Similar cross section information of extrusions is also presented in Ref [15]. Unfortunately, no useful information obtained from the cross section due to the effect of FIB processing and the formation of the extruded flake structures is not clearly explained. Therefore, the cross-section surface was treated with the etching solution (0.5 g K3[Fe(CN)6]: 0.5 g NaOH: 100 ml H2O) for 20 min in this study, aiming to get meaningful information, see Fig. 6b. After etching, parallel lines arranged in a certain direction appear on the crosssection surface. The parallel lines correspond to the extrusions on surface, and the lines of the two surfaces are connected to form a surface, which is a slip plane. Accordingly, it can be stated that the extruded
flake structures are pressed out material/grains along the crystallographic slip plane during cyclic slip caused by cyclic shear stress. In the case of tests, the surface temperature rapidly increases induced by pulsed loading, and gradually decreases to base temperature when the loading ends. Therefore, the surface temperature of the loaded area periodically altered following with the cyclic heat loads and then the cycle alternate of compressive and tensile stresses is induced on the loaded area. The shear stress induced during cyclic heat loads on crystallographic slip planes is different for varying grains, strongly depending on crystallographic orientation. When the cyclic shear stress arrived critical value on a slip system in some grains at the material surface, cyclic slip would occur. With the shear stress switching-over, irreversible slip will occur in the same slip band, but occur on adjacent parallel slip planes. As a consequence, the extrusion/intrusion structures are pressed out/in grains along the crystallographic slip planes during cyclic plastic deformation, see Fig. 6b and c. Additionally, the coexistent structure morphology of extrusion and intrusion could be created due to this mechanism, see Fig. 6d. 4. Conclusions Thermal fatigue resistance of the recrystallized W under cyclic heat loads has been studied in the electron beam facility with different loading parameters such as absorbed power density and cycle index. As the power density and the number of cycles increase, the density of the intragranular shear bands in each grain becomes higher and the
Fig. 6. The formation schematic diagrams of thermal fatigue damage morphology (extrusion and intrusion structures).
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swelling of grain boundaries becomes more pronounced. However, the number of cycles have little effect on the surface damage when the power density is relatively small. Based on the results, it can be stated that the influence of power density is greater than cycle index because the thermal stress is determined by loading power. Additionally, extruded flake structures on shear bands are observed in these damaged areas, which would cause serious surface roughening. The directions of shear bands are generally parallel to different directions for varying grains, showing strong grain orientation dependence. It found that the extrusion zone and the substrate have the same crystal orientation after calculating the crystallographic indexes of the extrusions and the substrate of grains. Accordingly, it can be stated that slip plastic deformation represents the predominant mechanism for thermal fatigue damage. The directions of the shear bands are found to be generally parallel to the traces of {112} slip planes with the surface, which implies that the extruded flake structures are pressed out material/grains along the crystallographic slip plane during cyclic slip. Acknowledgement This work was supported by the National Magnetic Confinement Fusion Science Program of China under Grant 2013GB109003, and the National Nature Science Foundation of China under contract no. 51571003. References [1] V. Philipps, Tungsten as material for plasma-facing components in fusion devices, J. Nucl. Mater. 415 (1) (2011) S2–S9.
[2] X. Zhang, Q. Yan, The thermal crack characteristics of rolled tungsten in different orientations, J. Nucl. Mater. 444 (1) (2014) 428–434. [3] Z. Zhou, G. Pintsuk, J. Linke, et al., Transient high heat load tests on pure ultra-fine grained tungsten fabricated by resistance sintering under ultra-high pressure, Fusion Eng. Des. 85 (1) (2010) 115–121. [4] M. Roedig, I. Kupriyanov, J. Linke, et al., Simulation of transient heat loads on high heat flux materials and components, J. Nucl. Mater. 417 (1) (2011) 761–764. [5] M. Wirtz, S. Bardin, A. Huber, et al., Impact of combined hydrogen plasma and transient heat loads on the performance of tungsten as plasma facing material, Nucl. Fusion 55 (12) (2015) 123017. [6] T. Loewenhoff, A. Bürger, J. Linke, et al., Evolution of tungsten degradation under combined high cycle edge-localized mode and steady-state heat loads, Phys. Scr. 2011 (T145) (2011) 014057. [7] J.H. Yu, R.P. Doerner, T. Dittmar, et al., ITER-relevant transient heat loads on tungsten exposed to plasma and beryllium[J], Phys. Scr. 2014 (T159) (2014) 014036. [8] T. Loewenhoff, J. Linke, G. Pintsuk, et al., Tungsten and CFC degradation under combined high cycle transient and steady state heat loads, Fusion Eng. Des. 87 (7) (2012) 1201–1205. [9] Y. Yuan, J. Du, M. Wirtz, et al., Surface damage and structure evolution of recrystallized tungsten exposed to ELM-like transient loads, Nucl. Fusion 56 (3) (2016) 036021. [10] A. Rollett, J. Humphreys F, S. Rohrer G, et al., Recrystallization and Related Annealing Phenomena, Elsevier, 2004. [11] J. Linke, T. Loewenhoff, V. Massaut, et al., Performance of different tungsten grades under transient thermal loads, Nucl. Fusion 51 (7) (2011) 073017. [12] K. Tokunaga, H. Kurishita, H. Arakawa, et al., High heat load properties of nanostructured, recrystallized W–1.1 TiC, J. Nucl. Mater. 442 (1) (2013) S297–S301. [13] X. Huang, Q. Liu, Determination of crystallographic and macroscopic orientation of planar structures in TEM, Ultramicroscopy 74 (3) (1998) 123–130. [14] D. Roundy, C.R. Krenn, M.L. Cohen, et al., The ideal strength of tungsten, Philos. Mag. A 81 (7) (2001) 1725–1747. [15] I. Steudel, A. Huber, A. Kreter, et al., Sequential and simultaneous thermal and particle exposure of tungsten, Phys. Scr. T167 (2016) 014053.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Thermal fatigue mechanism of recrystallized tungsten under cyclic heat loads via electron beam facility Liang Wang, Bo Wang ⁎, Shu-Dan Li, Dong Ma, Yun-Hui Tang, Hui Yan School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e
i n f o
Article history: Received 18 May 2016 Received in revised form 3 July 2016 Accepted 27 July 2016 Available online 05 August 2016 Keywords: Tungsten Cyclic heat loads Thermal fatigue Extruded flake structures
a b s t r a c t Thermal fatigue resistance of plasma facing materials (PFMs) is an inevitable concern for component lifetime and plasma operations, since the temperature fluctuations will always exist in future nuclear fusion facilities and reactors. Accordingly, experiments were performed in the electron beam facility to investigate the thermal fatigue behavior under operational loading conditions. The tungsten is investigated in its stress relieved and fully recrystallized state for a better understanding of the thermal fatigue process when exposed to cyclic heat loads. The heat loads range from 24 to 48 MW/m2 and the number of cycles increases from 100 to 1000 times. The results indicate that the thermal fatigue damage (surface roughening) due to plastic deformation strongly depends on the loading conditions and the cycle index. As the power density and the number of cycles increase, the density of the intragranular shear bands in each grain becomes higher and the swelling of grain boundaries becomes more pronounced. The shear bands are generally parallel to different directions for varying grains, showing strong grain orientation dependence. Additionally, extruded flake structures on shear bands were observed in these damaged areas. It found that the shear bands are generally parallel to the traces of {112} slip planes with the surface. The results suggest that slip plastic deformation represent the predominant mechanism for thermal fatigue and a set of schematic diagram is presented to explain the formation of thermal fatigue damage morphology (extrusion and intrusion structures). © 2016 Elsevier Ltd. All rights reserved.
1. Introduction One of the key concerns for future nuclear fusion devices is the development of plasma facing materials (PFMs). These FPMs should be able to withstand the severe and complex environmental conditions especially in the divertor region. Tungsten (W) has multiple favorable thermal properties, which makes it a most promising plasma facing material [1]. Nevertheless, a PFM will be subjected to steady-state heat loads and several types of short transient events during plasma operation in fusion reactors, which can cause unacceptable material damage, such as cracks, surface melting, evaporation, droplet ejection and fatigue fracture [2–4]. Therefore, different experimental set-ups to simulate these environmental conditions are essential to determine behavior and damage mechanisms of tungsten. Recently, a series of experiments have been conducted using pulsed laser and electron beams to reproduce fusion relevant transient and steady state heat loads. Based on these results, it is concluded that the damage types, such as surface roughening, cracks and melting, evolve as function of transient heat loads, number of pulses and the base temperature of W [5–8]. However, few studies focused on the detailed microscopic analysis of the thermal fatigue damage (surface roughening) caused by cyclic heat loads. W as a ⁎ Corresponding author. E-mail address: [email protected] (B. Wang).
http://dx.doi.org/10.1016/j.ijrmhm.2016.07.022 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
plasma facing material (PFM) would be exposed to a variety of cyclic heat loads and subjected to stress under temperature changes due to different strain rates. Therefore, thermal fatigue resistance of W is an inevitable concern for component lifetime and plasma operations, since the temperature fluctuations will always exist in future nuclear fusion facilities and reactors. Y·Yuan [9] has focused on surface roughening and conjectured that it is mainly caused by twin plastic deformation. Unfortunately, the microscopic images of the Yuan's papers showed only the images of overall deformation features without local details. In this work, W is investigated in its stress relieved and fully recrystallized state to ascertain a better understanding of the thermal fatigue process under cyclic heat loads. Thermal fatigue resistance is assessed by changing the power density and cycle index. Additionally, we study on the potential mechanism of surface deformation (typically the extruded flake structures) observed in loaded areas. 2. Experimental Thermal fatigue tests were performed in the electron beam facility with 8 kV rated voltage. All samples were mounted on carbon crucible placed on a water-cooled Cu block. To ensure a good contact for heat conduction, a small amount of metal gallium was added between the samples and the crucible. The thermal fatigue tests were conducted out with 100, 300, 500 and 1000 pulses at an absorbed power density
62
L. Wang et al. / Int. Journal of Refractory Metals and Hard Materials 61 (2016) 61–66
Fig. 1. The maximum temperature evolution of electron beam deposited area simulated by FEM during one cyclic heat loads.
of 48 MW/m2 with a duration of 1 s and followed by a 9 s interval to cool the loaded area to the base temperature. In addition, the thermal fatigue tests were carried out at two other absorbed power densities of 24, 36 MW/m2 with 500 pulses. In the tests, the maximum current was observed to be 107, 160 and 214 mA during a pulsed loading, and the electron beam deposited area is a circle with a diameter of 5 mm. Thereby, the absorbed power densities are 24, 36 and 48 MW/m2, respectively (the electron absorption coefficient of W is 0.55 [9]). The tests were performed on fully recrystallized W plate manufactured by AT&M Co., Ltd. (China). The internal stresses induced by rolling deformation were completely released after full recrystallization [10]. Thereby, for the loaded recrystallized W, the stress distribution largely depends on the thermal stresses induced during loading. All samples were cut from the recrystallized W plate with the dimensions of 10 × 10 × 2 mm3 for thermal fatigue tests. In order to obtain a stress-free and a well-defined reference surface, the surface of the samples was electro-polishing in a 2 wt.% NaOH solution to a low roughness of Ra b 0.1 μm. After the thermal fatigue tests, the surface morphology and microstructure were observed by a scanning electron microscope
Fig. 2. SEM images of the electro-polished recrystallized W after exposure to cyclic heat loads (absorbed power density 48 MW/m2) in different cycle indexes. (a) 100 pulses, (b) 300 pulses, (c) 500 pulses and (d) 1000 pulses. (e and f) is higher magnification of grains A and B shown in (c).
L. Wang et al. / Int. Journal of Refractory Metals and Hard Materials 61 (2016) 61–66
63
Fig. 3. SEM images of the electro-polished recrystallized W after exposure to 500 heat loads shots (a) absorbed power density 24 MW/m2, (b) absorbed power density 36 MW/m2.
(SEM) using a field emission microscope. The subsurface cross-section information of the cyclic heat loaded area was obtained by focused ion beam (FIB) technology and imaging (SEM). In addition, electron backscatter diffraction (EBSD) was used to correlate the crystallographic orientation of grains to the surface morphology induced via cyclic heat loads. 3. Results and discussion 3.1. Microstructural analysis Fig. 1 shows the maximum temperature evolution of the samples simulated by FEM during one cyclic heat loads. The samples loaded at RT with 24, 36 and 48 MW/m2, respectively. The temperature of loaded area increases rapidly during 1 s pulsed loading and rapidly cool down to the initial temperature since the subjacent tungsten remains thermally unaffected. When the power density was increased from 24 to 48 MW/m2, a higher peak temperature and a faster heating rate are induced, which would result a more severe stress state at the loaded areas. Fig. 2 illustrates the surface morphology of the electro-polished recrystallized W after exposure to cyclic heat loads (absorbed power
density 48 MW/m2) in different cycle indexes, (a) 100 pulses, (b) 300 pulses, (c) 500 pulses and (d) 1000 pulses. It can be found that the surface thermal fatigue damage is gradually aggravated with the increase of the number of cycles. The microcracks (shear bands) are generally parallel to different directions for varying grains and are appeared in partial grains under fewer cycle loads, showing strong crystallographic orientation dependence. However, when the number of cycles increases to 1000 pulses, the sample was severely damaged (see Fig. 2d). As the number of cycles increases from 100 to 1000 times, the density of the intragranular shear bands in each grain becomes higher and the swelling of grain boundaries becomes more pronounced. Additionally, the transgranular shear bands and extruded flake structures are observed in these damaged areas (see Fig. 2e and f). The formation of extruded flake structures would cause pronounced surface roughening. Thermal fatigue and accumulated dislocations produced at elevated pulse numbers would create a highly stressed and deformed surface [8]. The surface morphology of the electro-polished recrystallized W exposed to 500 heat loading shots with two different absorbed power densities is shown in Fig. 3. As seen in Figs. 2c and 3, surface damage became more significant since a more severe stress state is induced at the loaded areas when the power density is increased from 24 to 48 MW/
Fig. 4. EBSD maps (a and b) and 70°-tilted oblique view SEM image of extruded flake structure morphology correlating to the orientation maps. The lattices of A, B and C grains embed in orientation map (b).
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Table 1 The detailed crystal orientation information of the different positions in A, B and C grains indicated in Fig. 2. Euler angle
A B
C
Miller indexes
φ1
Ф
φ2
(h k l)
[uvw]
Crystal orientation
a1
269.8
113.1
3.3
f021gh012i
89.9
63.8
357.8
ð021Þ (021)
[012]
a2
½012
Grain No.
b1
284.9
117.4
4.1
ð021Þ
½124
b2
285.1
117.4
4.0
ð021Þ
b3
277.0
27.2
187.2
b4
194.0
92.9
62.2
ð012Þ (210)
½124 [142]
b5
103.8
64.5
265.8
ð201Þ
c1
281.6
101.4
110.7
c2
188.0
109.4
255.9
ð311Þ (311)
m2 for 500 pulses. Therefore, it can be found that the number of cycles had little effect on the surface damage when the power density is relatively small (see Fig. 3a). The reason for the surface modification is that the thermal stress state of the samples changes alternatively between compressive and tensile stresses. The surface damages are ascribed to the swelling of grain boundaries and the irreversible plastic deformation of the heated grains due to compressive stresses during heat loading [9]. When the sample is heated by the transient load, the temperature of the loaded area increases rapidly, crossing the material's ductile to brittle transition temperature (DBTT), see Fig. 1. Thereby, the compressive stresses are induced during loading since the loaded area is subjected to thermal expansion and the surrounding area is colder and expands less. The induced compressive stresses can lead to the swelling of grain boundaries and the irreversible plastic deformation, depending on stress amplitude and temperature [8,11]. The tensile stresses are induced due to the volumetric shrinkage in the following cool down phase during the interval of loading. Due to this mechanism, the cycle alternately of compressive and tensile stresses is induced during cyclic heat loads. 3.2. Mechanism analysis The extruded flake structures on shear bands formed via cyclic heat loads are parallel to different directions for varying grains, strongly depending on crystallographic orientation of the grains. The characteristics of the surface morphology are similar with the results obtained in Ref [12]. The author conjectured that the repetitive anisotropic deformation causes these surface modifications. However, the detailed formation mechanism of this morphology is not concerned. To ascertain the formation of the extruded flake structures and get a precise explanation, EBSD was used to correlate the crystallographic orientation of grains to the surface morphology. Fig. 4 shows EBSD maps (a and
f021gh124i
½241 ½214 [103]
f311gh103i
½103
b) and 70°-tilted oblique view SEM image of extruded flake structure morphology correlating to the orientation maps. The extruded flake structures are induced under 500 cyclic heat loads with 48 MW/m2. The rotation angle of boundary between A and B grains obtained from the EBSD maps is b15°. Obviously, the extrusions (shear bands) can be easily extended to the adjacent grains, when the crystallographic orientation of two adjacent grains is close. However, a large deflection between the extrusions on either side of a grain boundary would be caused, if the misorientation between adjacent is large, see Fig. 4a and c. Moreover, the crystallographic and macroscopic orientations of the shear bands were calculated based on the method [9,13]. Euler angle information of the different positions in A, B and C grains is obtained from the EBSD image (see Fig. 4a), and the corresponding Miller indexes is calculated through the Euler angle. The detailed grain orientation information is illustrated in Table 1. After analysis, it found that the extrusion zone and the substrate of grains have the same crystal orientation although the Euler angle is different. Accordingly, we conjecture that slip deformation is the predominant mechanism for the formation of extrusion surface morphology under cyclic heat loads. Based on body-centered cubic structure, W commonly shear on three different crystallographic slip systems 〈111〉 {110}, 〈 111 〉 {112} and 〈111〉 {123} [14]. Table 2 shows the detailed information of slip systems occurring readily for A, B and C grains in 48 slip systems, Schmidt factor and the inclination angle of the slip bands to the xdirection (ψ1 the calculated angle of the intersection of slip plane and surface to the x-direction and ψ2 the measured angle between the slip bands and the x-direction from the micrograph in Fig. 4a). The slip system listed in Table 2 is most likely to occur (Schmidt factor is relatively large). The Schmidt factor can be calculated using m ¼ cosα cosβ− cosγ cosδ; where α and β are the angles of the slip direction (b)/ the normal
Table 2 The detailed information of slip systems occurring readily for A, B and C grains, respectively, Schmidt factor and the inclination angle of the slip bands to the x-direction.
Grain No.
Miller indexes (hkl) [uvw]
Slip systems n⋅b
A
ð021Þ½012
ð121Þ½111=ð121Þ½111 ð112Þ½111=ð112Þ½111
B
C
ð021Þ½124
ð311Þ½103
ψ1 (°)
ψ2 (°)
0.613
60.8
50–73
0.613
53.3
Schmidt factor (|m|)
ð110Þ½111=ð110Þ½111
0.572
24.1
ð101Þ½111=ð101Þ½111
0.572
41.8
ð231Þ½111=ð231Þ½111=ð213Þ½111 ð213Þ½111
0.617
48.2
ð121Þ½111
0.727
73.4
ð110Þ½111
0.665
36.7
ð132Þ½111
0.701
84.9
ð231Þ½111
0.727
60.8
ð112Þ½111
0.857
73.2
ð101Þ½111
0.772
58.9
ð123Þ½111
0.830
117.8
ð213Þ½111
0.853
92.5
73
73
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65
Fig. 5. FIB cross-sectional SEM images of the extruded flake structures induced via 1000 heat loads of absorbed power density 48 MW/m2. (a) Original, (b) after etching.
direction of slip surface (n) to the RD-direction. Accordingly, γ and δ are the angles of the slip direction (b)/ the normal direction of slip surface (n) to the ND-direction. Compared with ψ1 and ψ2, it was found that the slip bands are parallel to the traces of {112} slip planes with the surface. The A grain slip occurred on (121)[111], (121)[111], (112)[111] and (112)[111] slip systems, B grain on (121)[111] slip system and C grain on (112)[111] slip system. Four slip systems simultaneously occurred and then produced cross-slips in A grain, leading to the formation of a curved slip band morphology, see Fig. 4a. However, parallel slip bands are formed in B and C grains because there is only one slip system that occurred, see Fig. 4a. In order to further investigate thermal fatigue mechanism of W under cyclic heat loads, the FIB was implemented for the detailed observation of the cross-section of the extrusions. Fig. 5 shows cross-sectional SEM images of the extruded flake structures within the loaded surface before and after etching. Similar cross section information of extrusions is also presented in Ref [15]. Unfortunately, no useful information obtained from the cross section due to the effect of FIB processing and the formation of the extruded flake structures is not clearly explained. Therefore, the cross-section surface was treated with the etching solution (0.5 g K3[Fe(CN)6]: 0.5 g NaOH: 100 ml H2O) for 20 min in this study, aiming to get meaningful information, see Fig. 6b. After etching, parallel lines arranged in a certain direction appear on the crosssection surface. The parallel lines correspond to the extrusions on surface, and the lines of the two surfaces are connected to form a surface, which is a slip plane. Accordingly, it can be stated that the extruded
flake structures are pressed out material/grains along the crystallographic slip plane during cyclic slip caused by cyclic shear stress. In the case of tests, the surface temperature rapidly increases induced by pulsed loading, and gradually decreases to base temperature when the loading ends. Therefore, the surface temperature of the loaded area periodically altered following with the cyclic heat loads and then the cycle alternate of compressive and tensile stresses is induced on the loaded area. The shear stress induced during cyclic heat loads on crystallographic slip planes is different for varying grains, strongly depending on crystallographic orientation. When the cyclic shear stress arrived critical value on a slip system in some grains at the material surface, cyclic slip would occur. With the shear stress switching-over, irreversible slip will occur in the same slip band, but occur on adjacent parallel slip planes. As a consequence, the extrusion/intrusion structures are pressed out/in grains along the crystallographic slip planes during cyclic plastic deformation, see Fig. 6b and c. Additionally, the coexistent structure morphology of extrusion and intrusion could be created due to this mechanism, see Fig. 6d. 4. Conclusions Thermal fatigue resistance of the recrystallized W under cyclic heat loads has been studied in the electron beam facility with different loading parameters such as absorbed power density and cycle index. As the power density and the number of cycles increase, the density of the intragranular shear bands in each grain becomes higher and the
Fig. 6. The formation schematic diagrams of thermal fatigue damage morphology (extrusion and intrusion structures).
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swelling of grain boundaries becomes more pronounced. However, the number of cycles have little effect on the surface damage when the power density is relatively small. Based on the results, it can be stated that the influence of power density is greater than cycle index because the thermal stress is determined by loading power. Additionally, extruded flake structures on shear bands are observed in these damaged areas, which would cause serious surface roughening. The directions of shear bands are generally parallel to different directions for varying grains, showing strong grain orientation dependence. It found that the extrusion zone and the substrate have the same crystal orientation after calculating the crystallographic indexes of the extrusions and the substrate of grains. Accordingly, it can be stated that slip plastic deformation represents the predominant mechanism for thermal fatigue damage. The directions of the shear bands are found to be generally parallel to the traces of {112} slip planes with the surface, which implies that the extruded flake structures are pressed out material/grains along the crystallographic slip plane during cyclic slip. Acknowledgement This work was supported by the National Magnetic Confinement Fusion Science Program of China under Grant 2013GB109003, and the National Nature Science Foundation of China under contract no. 51571003. References [1] V. Philipps, Tungsten as material for plasma-facing components in fusion devices, J. Nucl. Mater. 415 (1) (2011) S2–S9.
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