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Mechanism for orientation dependence of blisters on W surface exposed to D plasma at low temperature
Journal of Nuclear Materials 477 (2016) 165e171
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Mechanism for orientation dependence of blisters on W surface exposed to D plasma at low temperature Y.Z. Jia a, W. Liu a, *, B. Xu a, G.-N. Luo b, S.L. Qu a, T.W. Morgan c, G. De Temmerman d a
Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China c FOM Institute DIFFER-Dutch Institute for Fundamental Energy Research, 5612AJ Eindhoven, The Netherlands d ITER Organization, Route de Vinon-sur-Verdon-CS90 046, 13067 St Paul Lez Durance Cedex, France b
h i g h l i g h t s The blistering behavior was severe on the [111] surface, while the [001] surfaces are the most resistant to blister formation. The CVD samples with [001] texture showed good resistance to blister formation, so it is suggested that it may be effective to alleviate blisters by texturing of W. The blister formation model based on the plastic deformation of W can well explain the heterogeneity of blister formation and the different shapes of blisters on surfaces with different normal directions. The [111] surface is more prone to blister formation, because the surface layer is easily deformed by the D2 gas pressure beneath the surface. The blister edges and steps were speculated to be induced by the slipping of dislocations.
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Article history: Received 10 January 2016 Received in revised form 26 April 2016 Accepted 3 May 2016 Available online 7 May 2016
The orientation dependence of blister formation induced by D plasma exposure at low temperature (about 523 K) on rolled tungsten and chemical vapor deposition (CVD) W samples was studied by scanning electron microscopy and electron backscatter diffraction. Severe blistering was observed on grains with surface normal directions close to [111], while the [001] surfaces are the most resistant to blister formation. Cavities induced by D2 gas were observed beneath [111], [110] and [001] surfaces, independently on whether blisters were observed on the surface or not. The [111] surface is more prone to blister formation, because it is easily plastically deformed by the D2 gas pressure. Some blister edges and steps were perpendicular to [110] directions, which may be induced by the slipping of dislocations on {110} planes. The blister morphology induced by D plasma can be well explained by the blister model based on plastic deformation mechanism. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tungsten (W) will be used as plasma facing material (PFM) in the ITER divertor, due to its favorable properties, such as low sputtering yield, high melting points and high thermal conductivity [1]. During ITER operations, in the strike-point region, W will be exposed to high fluxes and fluences of hydrogen isotopes (D and T). Strong surface modifications such as blistering can occur on W surfaces exposed to hydrogen isotope particles, even when the incident energy is below the threshold energy for displacement damage [2e4]. Moreover, our previous studies showed that high
* Corresponding author. E-mail address: [email protected] (W. Liu). http://dx.doi.org/10.1016/j.jnucmat.2016.05.011 0022-3115/© 2016 Elsevier B.V. All rights reserved.
flux (~1024 m 2 s 1) D plasma can induce blisters on W surface at temperatures up to 1273 K [5]. It was recently showed that blisters can affect the thermal shock behavior of W under ELM-like transient heat loads [6], raising concerns about the effect of blistering on the thermo-mechanical properties of W after high fluence plasma exposure. Many studies reported that blisters did not uniformly form over the surface exposed to energetic particles, but instead presented a strong dependence on the target orientations [5,7e9]. Previously, the orientation dependence of blister formation was reported for a variety of metals (Mo, Al and Nb et al.) bombarded by high energy (1e100 keV) hydrogen or helium ions, and this was attributed to the different implantation range and implanted amounts through surfaces with different normal directions (NDs) [7]. It was reported recently that when W was exposed to low-energy (~40 eV) D
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plasma, the resulting blisters also exhibited dependence on the surface NDs [8,9]. In general, blisters on W surface formed preferentially on grains whose surface NDs were close to the [111] direction, and fewer blisters formed on other surfaces at 523 K [8] and 573 K [9]. Miyamoto et al. [9] attributed this phenomenon to the deeper penetration range of implanted D atoms in the most open direction e [111] direction. However, the incident energy of D plasma used in Refs. [8,9] was only about 40 eV, and the implantation depth is only several nm making this hypothesis questionable for low energy ion irradiation. In our previous study, we reported that blisters formed at elevated temperature (~943 K) showed a similar dependence on the surface NDs, and some features of blisters were also related to the orientation of grains [5]. Since the slipping system of dislocations in W is strongly related to the grain orientation, a blister formation model based on the plastic deformation mechanism was proposed to explain the orientation dependence of blister formation under those conditions [5]. Based on this mechanism, the heterogeneity of blister formation on different surfaces was attributed to the relative difficulty of plastic deformation on surfaces with different NDs. In addition, other studies also found a strong relationship between the blister morphology and the plastic deformation mechanism of W at 600 K [10,11]. Hence, it is very likely that the plastic deformation mechanism can also explain the orientation dependence of blister at low temperature (around 500e600 K). In order to investigate the mechanism for the orientation dependence of blister formation induced by D plasma on W surface at low temperature, we observed the blisters formed on grains with different surface NDs by scanning electron microscope, electron backscatter diffraction, and focused ion beam. According to the experimental results, the orientation dependence of blisters is discussed based on the above-mentioned plastic deformation mechanism of W. 2. Experimental W disc samples (F30 3 mm2) were cut from a rolled sheet supplied by Advanced Technology & Materials CO., Ltd. (China). In addition, W samples prepared by chemical vapor deposition (CVD W) (15 15 2 mm3), were supplied by Xiamen Honglu Tungsten & Molybdenum Industry Co. Ltd. (China). All the samples were stress relieved at 1273 K at a background pressure of 5 10 4 Pa for 1 h after polishing and before plasma exposure. The samples were exposed to a high flux D plasma beam in the Pilot-PSI linear plasma generator [12], which is uniquely capable of producing low temperature plasmas with a high flux ranging from 1023 to 1025 m 2s 1. The vessel pressure before the plasma exposure was about 2 10 3 Pa. The residual oxygen and nitrogen fluxes to the W surface were magnitudes lower than the D plasma flux, and the optical emission spectroscopy did not show any signals related to O or N in the plasma beam, so the impurities have little effect on the blistering behavior on W surface. During the plasma exposure, electron density and temperature of the plasma beam were determined using Thomson scattering (TS) [13]. The D flux to the surface was determined from the TS-measured Te and ne, assuming the ions are accelerated to the sound speed at the sheath entrance (Bohm criterion) [14]. The peak ion flux was about 1.5 1024 m 2s 1 and the full-width at half-maximum (FWHM) of the plasma beam was ~12 mm. The ion energy was controlled by negatively biasing the sample and was fixed to ~38 eV. For all samples we used identical discharge durations of 70 s. During such a discharge, the peak fluence was about 0.8e1.1 1026 m 2. In order to reach higher accumulated fluence, the same discharge was repeated 6 times to achieve a total fluence about 6 1026 m 2. The
plasma conditions were kept constant throughout this study. The surface temperature was controlled by the water cooling system from the back side of the samples and was a balance between the incoming power flux from the plasma and the cooling efficiency. Because of the difference in sample dimensions, the surface temperatures for the rolled and CVD W samples were slightly different. For the rolled W samples the peak surface temperature was about 523 K, while for CVD samples it was about 563 K. The 2D surface temperature profile was measured by an infrared camera (FLIR A645 sc). After exposure, surface morphology changes of the center area were observed using a TESCAN MIRA 3 LMH high-resolution scanning electron microscope (SEM). The orientations of the surface grains have been determined by Oxford instrument NordlysMax2 electron backscatter diffraction technique (EBSD). A 20 keV electron beam with an incident angle of 70 to the normal of the analyzed surface area was used for the EBSD analysis with channel 5 analysis software, HKL. Focused ion beam (FIB) was performed to observe the cross-section morphology of the near surface region of the samples. 3. Results 3.1. Rolled tungsten samples After the exposure to D plasma at 523 K, blisters were clearly observed on rolled W surface, as shown in Fig. 1(a). The blisters did not occur uniformly over the whole surface. On some areas severe blistering occurred, while on other areas blisters were rarely observed. The heterogeneity of blister formation on the surface was analyzed by EBSD. Fig. 1(b) shows the normal direction (ND) e projected inverse pole figure (IPF) map of the same area as in Fig. 1(a). The grains with different NDs were drawn in different colors, according to the IPF key. Comparing the blistering morphology of Fig. 1(a) and the IPF map of Fig. 1(b), it is evident that blisters formed preferentially on grains with NDs close to [111], which appear bluer in the IPF map. On the contrary, on the grains with NDs close to [001] and [110] (red and green, respectively, in the IPF map of Fig. 1(b)), very few blisters could be observed. In order to describe the orientation dependence of blister formation precisely, the blister coverage of grains with different surface NDs was analyzed by SEM and EBSD, and the result is shown in Fig. 2. Eight groups of grains with similar NDs were analyzed, and surface NDs of all groups are labeled in the inverse pole figure in the inset. The blister coverage is defined as the ratio of blistered surface area to the analyzed grain surface area. The blistered areas and grain areas were both evaluated by a MATLAB program processing the SEM image. For each ND group, the blister coverage was obtained by analyzing about 20 grains with a similar ND. As shown in this figure, the blister coverage is the highest on grains with [111] ND e about 20%. For the grains with NDs close to [111] direction ([112], [122], and [123] directions), the blister coverage was about 10%e12%, about half of that on [111] grains. Moreover, for those grains with normal directions away from [111] direction ([126], [110], [012] and [001]), the blister coverage were even much lower (<6%), with the lowest blister coverage (~1.8%) observed on [001] surface. The blister coverage result is consistent with the SEM results shown in Fig. 1. To investigate the mechanism for the orientation dependence of blisters on different grains, the cross-section of grains with [111], [110] and [001] (the orientation of which was determined by EBSD) was observed using FIB milling of the surface and SEM observations. Fig. 3(a) shows the typical surface morphology of blisters on a grain with ND close to [111] direction, and Fig. 3(b) shows the crosssection morphology of the same grain along the black line in
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Fig. 1. (a) SEM surface morphology, (b) surface normal direction IPF map of the same surface area exposed to D plasma at 523 K.
Fig. 2. Blister coverage on surfaces with different surface normal directions.
Fig. 3(a). There were two large cavities beneath the lids of the two blisters, which is consistent with the surface morphology. According to the results reported before, the cavities are bubbles formed due to the accumulation of D atoms and precipitation of D2 molecules [15,16]. The high D2 gas pressure inside the cavities deformed the surface layer, resulting in the blister structure on the surface
[17,18]. Fig. 3(c) and (d) show the surface and cross-section views of the same area with a surface ND close to [110]. Two blisters were observed on the surface, which were caused by the gas pressure inside the cavities beneath the surface indicated by the red arrows in Fig. 3(d). Blisters on [110] grains were much flatter than those on [111] grain in Fig. 3(a) and (b). Moreover, there was a large cavity along the grain boundary as pointed by the blue arrow, but no obvious surface blister corresponding with this cavity. Fig. 3(e) shows the surface morphology of a grain with ND close to [001] direction, with no obvious blisters on the surface of this grain. However, there were also cavities beneath the surface as indicated by blue arrows in Fig. 3(f), but these cavities did not induce any obvious blisters on the surface at all. The blister morphology is related not only to the NDs of the surface, but also to some directions parallel to the surface. Many blisters on the surface with NDs close to [111] exhibit a hexagonal shape, as shown in Fig. 4(a), (c) and (e). The NDs of the three grains were quite close to [111] orientation as shown in the inverse pole figure, with only slight deviations from the [111] direction by 1.55 , 6.13 and 7.14 , respectively, as calculated by Euler angles of the grains. In addition, a stepped structure can be observed on the six sides of the blisters, while the top surface of the blister appears quite smooth. The Fig. 4(b), (d) and (f) are the [110] pole figures of the three grains. The pole figures show the orientations of all [110] directions of the grains in the sample coordinate system. The [110]
Fig. 3. (a), (c) and (e) are the surface morphologies on surfaces with surface normal directions close to [111], [110] and [001] directions respectively; (b), (d) and (f) are the crosssection morphologies corresponding with (a), (c) and (e).
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Fig. 4. (a), (c) and (e) are the surface morphologies on surface with surface normal directions close to [111] directions; (b), (d) and (f) are the [110] pole figures of grains in (a), (c) and (e) respectively.
directions, whose poles (the red ones) are located close to the big circle, were almost parallel to the grain surfaces and perpendicular to the surface NDs. It is found that the steps and edges of the hexagon six sides were almost perpendicular to the [110] directions of the grains, as shown by the [111] directions projected on the surface in the inset of Fig. 4(a), (c) and (e). This kind of hexagonal blister shape only occurred on the surfaces with NDs close to [111] direction, and did not appear on other surfaces. 3.2. CVD tungsten samples Fig. 5(a) and (b) shows the surface ND-projected IPF map and ND inverse pole figure of the CVD samples used in this study. The CVD samples are of strong [001] texture as shown in Fig. 5(a) and (b), so the NDs of grains on the exposed surface were mostly close to [001] with only a little parts of the grains with NDs away from [001]. Fig. 5(c) shows the surface morphology of the CVD sample exposed to D plasma at 563 K. Blisters were absent on most of the surface, and were observed only on some localized areas (marked by the black arrows). Fig. 5(d) shows the magnified image a particular blistered area in Fig. 5(c), and the inverse pole figure shows that the ND of the blistered area is away from [001] direction, so the orientation dependence of blisters on CVD sample is similar to that of rolled W samples. However, since the CVD samples were of strong [001] texture, the CVD W sample was in general more resistant to blistering over the whole surface. Therefore, it proved that the W sample with [001] texture can be used to alleviate the blister formation under D plasma conditions. 4. Discussion In this study, the exposure to D plasma caused blistering on the surface of rolled W at 523 K. Blisters formed preferentially on grains with ND close to [111] direction. These results are in line with observations made at different temperatures reported previously in literature [5,8,9]. The reason for the orientation dependence of blister formation at low temperature (500 Ke600 K) is not clearly explained in previous studies. In Ref. [9], it was attributed to the
openness of {111} planes of W, which will result in a deeper implantation range and a larger quantity of implanted D atoms in the near surface region. Following this explanation, the [110] surface should be the most resistant surface to the blister formation, because {110} planes are the densest lattice planes of W. However, our results show that it is the [001] surface, and not the [110] surface, which exhibits the lowest blister coverage. Eriksson et al. [19,20] investigated the implantation ranges of K (Potassium) ions along [001], [110] and [111] directions in W. It was found that the implantation range was the lowest along [110] direction, while the implantation ranges along the [001] and [111] directions are similar. The channeling effect of implanted ions therefore cannot explain the different blistering behavior on [001] and [111] surfaces. Moreover, Fig. 3(e) and (f) show that although no blisters were observed on [001] surface, cavities were observed beneath the [001] surface, which on the other hand can result in blisters on [111] surface. This means that the D atoms can also penetrate deeply beneath [001] surface as they do beneath [111] surface, but they cannot induce blisters on [001] surface easily. Therefore, the implantation range and quantity of D atoms may be different on surfaces with different NDs, but it appears to not be the main reason for the observed orientation dependence of blisters. In our previous study [5], we analyzed blisters formed on W surface at 943 K and proposed a new blister formation model based on plastic deformation mechanism as illustrated in Fig. 6. The slipping direction of dislocations in W is the [111] direction, so when the exposed surface is vertical to [111] direction, the surface layer will be easily deformed by the pressurized D2 gas in cavities beneath the surface, which will cause blister structures on surface [5]. This explanation is also applicable for the orientation dependence of blisters formed at 523 K in this study. It is found that the blister coverage is inversely related to the angle between the surface ND and the [111] direction as illustrated by Table 1. The strength of W can differ by a factor 2e4 along different directions [21,22]. Therefore, when the surface ND is away from the [111] direction, it is more difficult for the gas pressure to deform the surface layer, resulting in fewer blisters on the surface. Indeed, the cavities observed beneath the [001] and [110] surfaces cannot deform the
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Fig. 5. (a) Surface normal direction IPF map of CVD W sample; (b) inverse pole figure of surface normal directions of grains of CVD sample; (c) and (d) surface morphology of CVD sample exposed to D plasma at 563 K.
surface layer much, and the blisters were flatter or even absent as shown in Fig. 3(c) and (e). Thus, the model of plastic deformationinduced blister formation can well explain the heterogeneity of blister formation on surfaces with different NDs at 523 K. In this study, besides the orientation dependence of the blister formation, it is also found that some blisters on [111] surfaces showed a hexagonal shape, with their edges and steps to [110] directions, and the hexagonal blister shape did not appear on surfaces with NDs away from [111] direction. When the {110} slipping plane is perpendicular to the surface, the deformation due to dislocation slipping will leave steps perpendicular to the [110] direction on the blister surface as illustrated in Fig. 6. Because six {110} planes are perpendicular to the [111] surface, the blisters showed a hexagonal shape with six sides perpendicular to [110] directions as shown in Fig. 4. In addition, as for the blisters formed on [111] surface at 943 K, the edges were perpendicular to [110] or [112] directions [5]. It is known that the temperature has a strong effect on the slipping planes of dislocations in body-centered cubic (BCC) metals [23], so at 943 K dislocations may also slip on {112} planes besides the {110} planes in W, leaving edges perpendicular to [110] and [112] directions on the surface.
Following this explanation, if the edges and steps of blisters were due to plastic deformation of the surface layer, then blisters formed on surfaces with NDs other than [111] would also show some features related to the grain orientation. Fig. 7(a) shows the [110] pole figure of a grain with a [111] surface ND, and six [110] poles are located on the circle, which are perpendicular to the ND [111]. If the grain is rotated around one [110] direction, e.g. the Y axis in Fig. 7(a), then the other [110] poles will move towards the direction indicated by the red arrows. At the same time, the surface ND will change from the [111] direction to the [110] direction as indicated by the red arrow in Fig. 7(b). For the new orientation of the grain, only two [110] directions (the two red poles in Fig. 7(a)) are perpendicular to the surface ND, which means only two {110} planes are perpendicular to the surface. Therefore, we speculate that the blisters on the surface with ND located on the red line of Fig. 7(b) will have a shape as depicted in Fig. 7(c) e the long edges are perpendicular to [110] direction and other edges are not clearly related to given directions. In order to test this hypothesis, we observed some blisters on grains with surface ND located on the red line in Fig. 7(b). The results are shown in Fig. 8. The ND of the surface was shown in
Table 1 Blister coverage on different surfaces, and angles between the surface normal directions and [111] direction.
Fig. 6. Blister model based on plastic deformation mechanism of W material.
Surface ND
Blister coverage
Angle ( )
[001] [012] [126] [110] [123] [112] [122] [111]
1.76% 5.41% 3.24% 3.84% 10.43% 9.81% 11.47% 19.51%
54.74 39.23 35.76 35.26 22.21 19.47 15.79 0.00
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Fig. 7. (a) The change of [110] pole figure and (b) the change of inverse pole figure of surface ND, when the grain rotates around Y axis in (a); (c) the expected blister shape on surfaces with surface ND located on the red line of (b). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
the inverse pole figure of Fig. 8(a), and the ND is 12.6 away from the [111] direction. As shown in the pole figure of this grain in Fig. 8(b), only the red [110] poles were located on the big circle, which were perpendicular to the surface ND. Fig. 8(a) shows that the long edge of this blister is perpendicular to this [110] direction as we speculated. The surface NDs of grains showed in Fig. 8(c) and (e) were 21.6 and 28.5 away from the [111] direction, respectively, and the blister shape is also as expected with their long edges perpendicular to the [110] direction. Therefore, the welldefined and straight edges of blisters on [111] surfaces (Fig. 6) and other surfaces (Fig. 8) can be well explained by our model of blister formation based on plastic deformation. Also, it also should be noted that not all blisters show straight edges on surface, which may be affected by other factors, such as blister size and depth of the cavity. Although the implantation range of 40 eV D ions is limited to several nm, the high diffusion rate leads the D atoms to agglomerate around defects in deeper area, such as vacancies, dislocations, and grain boundaries. When the D concentration around the defects is high enough, the D atoms will recombine to form gas molecules, leading to the bubble formation [16,24]. Bubbles may
grow by punching dislocation loops and fracture between small bubbles [25,26]. The dislocation loops will glide along [111] direction, and the fracture toughness of W also depends strongly on crystal orientation [27,28]. In previous studies, it was found that bubbles tend to form on specific crystal planes in different metals [29,30]. Therefore, bubble formation may be related to the crystal orientation, but how exactly it in turn contributes to the blisters orientation dependence remains unclear. Finally, we analyzed the blister densities on surfaces with common surface NDs, such as [001], [110] and [111], finding that the lowest blister density occurred on the surfaces with [001] ND. Therefore, it is likely that a [001] texture would prevent blistering on W surfaces, as was confirmed with the results from the CVD samples. Selective texturing of W could therefore represent an interesting approach to mitigate hydrogen plasma-induced morphology changes on tungsten. However, the CVD samples used in this study have a large grain size, so the CVD W easily cracks along the grain boundaries between columnar grains, resulting in the brittleness of the CVD W materials. The CVD W material may therefore need more improvement before application in the divertor of future fusion devices.
Fig. 8. (a), (c) and (e) are the blister morphologies on surface with NDs on the red line in Fig. 6 (b); (b), (d) and (f) are the [110] pole figures of the grains in (a), (c) and (e) respectively. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
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5. Conclusions Blister formation by D plasma exposure of rolled W surfaces at 523 K show a strong dependence on grain orientation. The blistering behavior was severe on the surface with surface normal directions close to [111] direction, while the [001] surfaces are the most resistant to blister formation. Cavities induced by D2 gas were observed beneath [111], [110] and [001] surfaces, independently on whether blisters were observed on the surface or not. In addition, it was found that some edges and step structures of blisters were perpendicular to [110] directions on surfaces with certain NDs. In this study, we found that the model based on the different implantation range in grains with different surface orientations cannot completely explain the present experimental results, such as the blister surface coverage and presence of gas-filled cavities beneath surfaces with different normal directions. On the contrary, the blister formation model based on the plastic deformation of W induced by pressurized gas-filled cavities can well explain the heterogeneity of blister formation and the different shapes of blisters on surfaces with different NDs. The [111] surface is more prone to blister formation, because the surface layer is easily deformed by the D2 gas pressure beneath the surface. Moreover, the blister edges and steps perpendicular to [110] directions were speculated to be induced by the slipping of dislocations on {110} planes. After exposure to D plasma at 563 K, the CVD samples with [001] texture showed good resistance to blister formation, so it is suggested that it may be effective to alleviate blisters by texturing of W. Acknowledgement This work was supported by National Magnetic Confinement Fusion Science Program of China under Grant 2013GB109004, 2014GB117000, and the Joint Sino-German research project GZ 763,
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and the National Nature Science Foundation of China under Contract No. 51471092. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
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Mechanism for orientation dependence of blisters on W surface exposed to D plasma at low temperature Y.Z. Jia a, W. Liu a, *, B. Xu a, G.-N. Luo b, S.L. Qu a, T.W. Morgan c, G. De Temmerman d a
Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China c FOM Institute DIFFER-Dutch Institute for Fundamental Energy Research, 5612AJ Eindhoven, The Netherlands d ITER Organization, Route de Vinon-sur-Verdon-CS90 046, 13067 St Paul Lez Durance Cedex, France b
h i g h l i g h t s The blistering behavior was severe on the [111] surface, while the [001] surfaces are the most resistant to blister formation. The CVD samples with [001] texture showed good resistance to blister formation, so it is suggested that it may be effective to alleviate blisters by texturing of W. The blister formation model based on the plastic deformation of W can well explain the heterogeneity of blister formation and the different shapes of blisters on surfaces with different normal directions. The [111] surface is more prone to blister formation, because the surface layer is easily deformed by the D2 gas pressure beneath the surface. The blister edges and steps were speculated to be induced by the slipping of dislocations.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 January 2016 Received in revised form 26 April 2016 Accepted 3 May 2016 Available online 7 May 2016
The orientation dependence of blister formation induced by D plasma exposure at low temperature (about 523 K) on rolled tungsten and chemical vapor deposition (CVD) W samples was studied by scanning electron microscopy and electron backscatter diffraction. Severe blistering was observed on grains with surface normal directions close to [111], while the [001] surfaces are the most resistant to blister formation. Cavities induced by D2 gas were observed beneath [111], [110] and [001] surfaces, independently on whether blisters were observed on the surface or not. The [111] surface is more prone to blister formation, because it is easily plastically deformed by the D2 gas pressure. Some blister edges and steps were perpendicular to [110] directions, which may be induced by the slipping of dislocations on {110} planes. The blister morphology induced by D plasma can be well explained by the blister model based on plastic deformation mechanism. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tungsten (W) will be used as plasma facing material (PFM) in the ITER divertor, due to its favorable properties, such as low sputtering yield, high melting points and high thermal conductivity [1]. During ITER operations, in the strike-point region, W will be exposed to high fluxes and fluences of hydrogen isotopes (D and T). Strong surface modifications such as blistering can occur on W surfaces exposed to hydrogen isotope particles, even when the incident energy is below the threshold energy for displacement damage [2e4]. Moreover, our previous studies showed that high
* Corresponding author. E-mail address: [email protected] (W. Liu). http://dx.doi.org/10.1016/j.jnucmat.2016.05.011 0022-3115/© 2016 Elsevier B.V. All rights reserved.
flux (~1024 m 2 s 1) D plasma can induce blisters on W surface at temperatures up to 1273 K [5]. It was recently showed that blisters can affect the thermal shock behavior of W under ELM-like transient heat loads [6], raising concerns about the effect of blistering on the thermo-mechanical properties of W after high fluence plasma exposure. Many studies reported that blisters did not uniformly form over the surface exposed to energetic particles, but instead presented a strong dependence on the target orientations [5,7e9]. Previously, the orientation dependence of blister formation was reported for a variety of metals (Mo, Al and Nb et al.) bombarded by high energy (1e100 keV) hydrogen or helium ions, and this was attributed to the different implantation range and implanted amounts through surfaces with different normal directions (NDs) [7]. It was reported recently that when W was exposed to low-energy (~40 eV) D
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plasma, the resulting blisters also exhibited dependence on the surface NDs [8,9]. In general, blisters on W surface formed preferentially on grains whose surface NDs were close to the [111] direction, and fewer blisters formed on other surfaces at 523 K [8] and 573 K [9]. Miyamoto et al. [9] attributed this phenomenon to the deeper penetration range of implanted D atoms in the most open direction e [111] direction. However, the incident energy of D plasma used in Refs. [8,9] was only about 40 eV, and the implantation depth is only several nm making this hypothesis questionable for low energy ion irradiation. In our previous study, we reported that blisters formed at elevated temperature (~943 K) showed a similar dependence on the surface NDs, and some features of blisters were also related to the orientation of grains [5]. Since the slipping system of dislocations in W is strongly related to the grain orientation, a blister formation model based on the plastic deformation mechanism was proposed to explain the orientation dependence of blister formation under those conditions [5]. Based on this mechanism, the heterogeneity of blister formation on different surfaces was attributed to the relative difficulty of plastic deformation on surfaces with different NDs. In addition, other studies also found a strong relationship between the blister morphology and the plastic deformation mechanism of W at 600 K [10,11]. Hence, it is very likely that the plastic deformation mechanism can also explain the orientation dependence of blister at low temperature (around 500e600 K). In order to investigate the mechanism for the orientation dependence of blister formation induced by D plasma on W surface at low temperature, we observed the blisters formed on grains with different surface NDs by scanning electron microscope, electron backscatter diffraction, and focused ion beam. According to the experimental results, the orientation dependence of blisters is discussed based on the above-mentioned plastic deformation mechanism of W. 2. Experimental W disc samples (F30 3 mm2) were cut from a rolled sheet supplied by Advanced Technology & Materials CO., Ltd. (China). In addition, W samples prepared by chemical vapor deposition (CVD W) (15 15 2 mm3), were supplied by Xiamen Honglu Tungsten & Molybdenum Industry Co. Ltd. (China). All the samples were stress relieved at 1273 K at a background pressure of 5 10 4 Pa for 1 h after polishing and before plasma exposure. The samples were exposed to a high flux D plasma beam in the Pilot-PSI linear plasma generator [12], which is uniquely capable of producing low temperature plasmas with a high flux ranging from 1023 to 1025 m 2s 1. The vessel pressure before the plasma exposure was about 2 10 3 Pa. The residual oxygen and nitrogen fluxes to the W surface were magnitudes lower than the D plasma flux, and the optical emission spectroscopy did not show any signals related to O or N in the plasma beam, so the impurities have little effect on the blistering behavior on W surface. During the plasma exposure, electron density and temperature of the plasma beam were determined using Thomson scattering (TS) [13]. The D flux to the surface was determined from the TS-measured Te and ne, assuming the ions are accelerated to the sound speed at the sheath entrance (Bohm criterion) [14]. The peak ion flux was about 1.5 1024 m 2s 1 and the full-width at half-maximum (FWHM) of the plasma beam was ~12 mm. The ion energy was controlled by negatively biasing the sample and was fixed to ~38 eV. For all samples we used identical discharge durations of 70 s. During such a discharge, the peak fluence was about 0.8e1.1 1026 m 2. In order to reach higher accumulated fluence, the same discharge was repeated 6 times to achieve a total fluence about 6 1026 m 2. The
plasma conditions were kept constant throughout this study. The surface temperature was controlled by the water cooling system from the back side of the samples and was a balance between the incoming power flux from the plasma and the cooling efficiency. Because of the difference in sample dimensions, the surface temperatures for the rolled and CVD W samples were slightly different. For the rolled W samples the peak surface temperature was about 523 K, while for CVD samples it was about 563 K. The 2D surface temperature profile was measured by an infrared camera (FLIR A645 sc). After exposure, surface morphology changes of the center area were observed using a TESCAN MIRA 3 LMH high-resolution scanning electron microscope (SEM). The orientations of the surface grains have been determined by Oxford instrument NordlysMax2 electron backscatter diffraction technique (EBSD). A 20 keV electron beam with an incident angle of 70 to the normal of the analyzed surface area was used for the EBSD analysis with channel 5 analysis software, HKL. Focused ion beam (FIB) was performed to observe the cross-section morphology of the near surface region of the samples. 3. Results 3.1. Rolled tungsten samples After the exposure to D plasma at 523 K, blisters were clearly observed on rolled W surface, as shown in Fig. 1(a). The blisters did not occur uniformly over the whole surface. On some areas severe blistering occurred, while on other areas blisters were rarely observed. The heterogeneity of blister formation on the surface was analyzed by EBSD. Fig. 1(b) shows the normal direction (ND) e projected inverse pole figure (IPF) map of the same area as in Fig. 1(a). The grains with different NDs were drawn in different colors, according to the IPF key. Comparing the blistering morphology of Fig. 1(a) and the IPF map of Fig. 1(b), it is evident that blisters formed preferentially on grains with NDs close to [111], which appear bluer in the IPF map. On the contrary, on the grains with NDs close to [001] and [110] (red and green, respectively, in the IPF map of Fig. 1(b)), very few blisters could be observed. In order to describe the orientation dependence of blister formation precisely, the blister coverage of grains with different surface NDs was analyzed by SEM and EBSD, and the result is shown in Fig. 2. Eight groups of grains with similar NDs were analyzed, and surface NDs of all groups are labeled in the inverse pole figure in the inset. The blister coverage is defined as the ratio of blistered surface area to the analyzed grain surface area. The blistered areas and grain areas were both evaluated by a MATLAB program processing the SEM image. For each ND group, the blister coverage was obtained by analyzing about 20 grains with a similar ND. As shown in this figure, the blister coverage is the highest on grains with [111] ND e about 20%. For the grains with NDs close to [111] direction ([112], [122], and [123] directions), the blister coverage was about 10%e12%, about half of that on [111] grains. Moreover, for those grains with normal directions away from [111] direction ([126], [110], [012] and [001]), the blister coverage were even much lower (<6%), with the lowest blister coverage (~1.8%) observed on [001] surface. The blister coverage result is consistent with the SEM results shown in Fig. 1. To investigate the mechanism for the orientation dependence of blisters on different grains, the cross-section of grains with [111], [110] and [001] (the orientation of which was determined by EBSD) was observed using FIB milling of the surface and SEM observations. Fig. 3(a) shows the typical surface morphology of blisters on a grain with ND close to [111] direction, and Fig. 3(b) shows the crosssection morphology of the same grain along the black line in
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Fig. 1. (a) SEM surface morphology, (b) surface normal direction IPF map of the same surface area exposed to D plasma at 523 K.
Fig. 2. Blister coverage on surfaces with different surface normal directions.
Fig. 3(a). There were two large cavities beneath the lids of the two blisters, which is consistent with the surface morphology. According to the results reported before, the cavities are bubbles formed due to the accumulation of D atoms and precipitation of D2 molecules [15,16]. The high D2 gas pressure inside the cavities deformed the surface layer, resulting in the blister structure on the surface
[17,18]. Fig. 3(c) and (d) show the surface and cross-section views of the same area with a surface ND close to [110]. Two blisters were observed on the surface, which were caused by the gas pressure inside the cavities beneath the surface indicated by the red arrows in Fig. 3(d). Blisters on [110] grains were much flatter than those on [111] grain in Fig. 3(a) and (b). Moreover, there was a large cavity along the grain boundary as pointed by the blue arrow, but no obvious surface blister corresponding with this cavity. Fig. 3(e) shows the surface morphology of a grain with ND close to [001] direction, with no obvious blisters on the surface of this grain. However, there were also cavities beneath the surface as indicated by blue arrows in Fig. 3(f), but these cavities did not induce any obvious blisters on the surface at all. The blister morphology is related not only to the NDs of the surface, but also to some directions parallel to the surface. Many blisters on the surface with NDs close to [111] exhibit a hexagonal shape, as shown in Fig. 4(a), (c) and (e). The NDs of the three grains were quite close to [111] orientation as shown in the inverse pole figure, with only slight deviations from the [111] direction by 1.55 , 6.13 and 7.14 , respectively, as calculated by Euler angles of the grains. In addition, a stepped structure can be observed on the six sides of the blisters, while the top surface of the blister appears quite smooth. The Fig. 4(b), (d) and (f) are the [110] pole figures of the three grains. The pole figures show the orientations of all [110] directions of the grains in the sample coordinate system. The [110]
Fig. 3. (a), (c) and (e) are the surface morphologies on surfaces with surface normal directions close to [111], [110] and [001] directions respectively; (b), (d) and (f) are the crosssection morphologies corresponding with (a), (c) and (e).
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Fig. 4. (a), (c) and (e) are the surface morphologies on surface with surface normal directions close to [111] directions; (b), (d) and (f) are the [110] pole figures of grains in (a), (c) and (e) respectively.
directions, whose poles (the red ones) are located close to the big circle, were almost parallel to the grain surfaces and perpendicular to the surface NDs. It is found that the steps and edges of the hexagon six sides were almost perpendicular to the [110] directions of the grains, as shown by the [111] directions projected on the surface in the inset of Fig. 4(a), (c) and (e). This kind of hexagonal blister shape only occurred on the surfaces with NDs close to [111] direction, and did not appear on other surfaces. 3.2. CVD tungsten samples Fig. 5(a) and (b) shows the surface ND-projected IPF map and ND inverse pole figure of the CVD samples used in this study. The CVD samples are of strong [001] texture as shown in Fig. 5(a) and (b), so the NDs of grains on the exposed surface were mostly close to [001] with only a little parts of the grains with NDs away from [001]. Fig. 5(c) shows the surface morphology of the CVD sample exposed to D plasma at 563 K. Blisters were absent on most of the surface, and were observed only on some localized areas (marked by the black arrows). Fig. 5(d) shows the magnified image a particular blistered area in Fig. 5(c), and the inverse pole figure shows that the ND of the blistered area is away from [001] direction, so the orientation dependence of blisters on CVD sample is similar to that of rolled W samples. However, since the CVD samples were of strong [001] texture, the CVD W sample was in general more resistant to blistering over the whole surface. Therefore, it proved that the W sample with [001] texture can be used to alleviate the blister formation under D plasma conditions. 4. Discussion In this study, the exposure to D plasma caused blistering on the surface of rolled W at 523 K. Blisters formed preferentially on grains with ND close to [111] direction. These results are in line with observations made at different temperatures reported previously in literature [5,8,9]. The reason for the orientation dependence of blister formation at low temperature (500 Ke600 K) is not clearly explained in previous studies. In Ref. [9], it was attributed to the
openness of {111} planes of W, which will result in a deeper implantation range and a larger quantity of implanted D atoms in the near surface region. Following this explanation, the [110] surface should be the most resistant surface to the blister formation, because {110} planes are the densest lattice planes of W. However, our results show that it is the [001] surface, and not the [110] surface, which exhibits the lowest blister coverage. Eriksson et al. [19,20] investigated the implantation ranges of K (Potassium) ions along [001], [110] and [111] directions in W. It was found that the implantation range was the lowest along [110] direction, while the implantation ranges along the [001] and [111] directions are similar. The channeling effect of implanted ions therefore cannot explain the different blistering behavior on [001] and [111] surfaces. Moreover, Fig. 3(e) and (f) show that although no blisters were observed on [001] surface, cavities were observed beneath the [001] surface, which on the other hand can result in blisters on [111] surface. This means that the D atoms can also penetrate deeply beneath [001] surface as they do beneath [111] surface, but they cannot induce blisters on [001] surface easily. Therefore, the implantation range and quantity of D atoms may be different on surfaces with different NDs, but it appears to not be the main reason for the observed orientation dependence of blisters. In our previous study [5], we analyzed blisters formed on W surface at 943 K and proposed a new blister formation model based on plastic deformation mechanism as illustrated in Fig. 6. The slipping direction of dislocations in W is the [111] direction, so when the exposed surface is vertical to [111] direction, the surface layer will be easily deformed by the pressurized D2 gas in cavities beneath the surface, which will cause blister structures on surface [5]. This explanation is also applicable for the orientation dependence of blisters formed at 523 K in this study. It is found that the blister coverage is inversely related to the angle between the surface ND and the [111] direction as illustrated by Table 1. The strength of W can differ by a factor 2e4 along different directions [21,22]. Therefore, when the surface ND is away from the [111] direction, it is more difficult for the gas pressure to deform the surface layer, resulting in fewer blisters on the surface. Indeed, the cavities observed beneath the [001] and [110] surfaces cannot deform the
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Fig. 5. (a) Surface normal direction IPF map of CVD W sample; (b) inverse pole figure of surface normal directions of grains of CVD sample; (c) and (d) surface morphology of CVD sample exposed to D plasma at 563 K.
surface layer much, and the blisters were flatter or even absent as shown in Fig. 3(c) and (e). Thus, the model of plastic deformationinduced blister formation can well explain the heterogeneity of blister formation on surfaces with different NDs at 523 K. In this study, besides the orientation dependence of the blister formation, it is also found that some blisters on [111] surfaces showed a hexagonal shape, with their edges and steps to [110] directions, and the hexagonal blister shape did not appear on surfaces with NDs away from [111] direction. When the {110} slipping plane is perpendicular to the surface, the deformation due to dislocation slipping will leave steps perpendicular to the [110] direction on the blister surface as illustrated in Fig. 6. Because six {110} planes are perpendicular to the [111] surface, the blisters showed a hexagonal shape with six sides perpendicular to [110] directions as shown in Fig. 4. In addition, as for the blisters formed on [111] surface at 943 K, the edges were perpendicular to [110] or [112] directions [5]. It is known that the temperature has a strong effect on the slipping planes of dislocations in body-centered cubic (BCC) metals [23], so at 943 K dislocations may also slip on {112} planes besides the {110} planes in W, leaving edges perpendicular to [110] and [112] directions on the surface.
Following this explanation, if the edges and steps of blisters were due to plastic deformation of the surface layer, then blisters formed on surfaces with NDs other than [111] would also show some features related to the grain orientation. Fig. 7(a) shows the [110] pole figure of a grain with a [111] surface ND, and six [110] poles are located on the circle, which are perpendicular to the ND [111]. If the grain is rotated around one [110] direction, e.g. the Y axis in Fig. 7(a), then the other [110] poles will move towards the direction indicated by the red arrows. At the same time, the surface ND will change from the [111] direction to the [110] direction as indicated by the red arrow in Fig. 7(b). For the new orientation of the grain, only two [110] directions (the two red poles in Fig. 7(a)) are perpendicular to the surface ND, which means only two {110} planes are perpendicular to the surface. Therefore, we speculate that the blisters on the surface with ND located on the red line of Fig. 7(b) will have a shape as depicted in Fig. 7(c) e the long edges are perpendicular to [110] direction and other edges are not clearly related to given directions. In order to test this hypothesis, we observed some blisters on grains with surface ND located on the red line in Fig. 7(b). The results are shown in Fig. 8. The ND of the surface was shown in
Table 1 Blister coverage on different surfaces, and angles between the surface normal directions and [111] direction.
Fig. 6. Blister model based on plastic deformation mechanism of W material.
Surface ND
Blister coverage
Angle ( )
[001] [012] [126] [110] [123] [112] [122] [111]
1.76% 5.41% 3.24% 3.84% 10.43% 9.81% 11.47% 19.51%
54.74 39.23 35.76 35.26 22.21 19.47 15.79 0.00
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Fig. 7. (a) The change of [110] pole figure and (b) the change of inverse pole figure of surface ND, when the grain rotates around Y axis in (a); (c) the expected blister shape on surfaces with surface ND located on the red line of (b). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
the inverse pole figure of Fig. 8(a), and the ND is 12.6 away from the [111] direction. As shown in the pole figure of this grain in Fig. 8(b), only the red [110] poles were located on the big circle, which were perpendicular to the surface ND. Fig. 8(a) shows that the long edge of this blister is perpendicular to this [110] direction as we speculated. The surface NDs of grains showed in Fig. 8(c) and (e) were 21.6 and 28.5 away from the [111] direction, respectively, and the blister shape is also as expected with their long edges perpendicular to the [110] direction. Therefore, the welldefined and straight edges of blisters on [111] surfaces (Fig. 6) and other surfaces (Fig. 8) can be well explained by our model of blister formation based on plastic deformation. Also, it also should be noted that not all blisters show straight edges on surface, which may be affected by other factors, such as blister size and depth of the cavity. Although the implantation range of 40 eV D ions is limited to several nm, the high diffusion rate leads the D atoms to agglomerate around defects in deeper area, such as vacancies, dislocations, and grain boundaries. When the D concentration around the defects is high enough, the D atoms will recombine to form gas molecules, leading to the bubble formation [16,24]. Bubbles may
grow by punching dislocation loops and fracture between small bubbles [25,26]. The dislocation loops will glide along [111] direction, and the fracture toughness of W also depends strongly on crystal orientation [27,28]. In previous studies, it was found that bubbles tend to form on specific crystal planes in different metals [29,30]. Therefore, bubble formation may be related to the crystal orientation, but how exactly it in turn contributes to the blisters orientation dependence remains unclear. Finally, we analyzed the blister densities on surfaces with common surface NDs, such as [001], [110] and [111], finding that the lowest blister density occurred on the surfaces with [001] ND. Therefore, it is likely that a [001] texture would prevent blistering on W surfaces, as was confirmed with the results from the CVD samples. Selective texturing of W could therefore represent an interesting approach to mitigate hydrogen plasma-induced morphology changes on tungsten. However, the CVD samples used in this study have a large grain size, so the CVD W easily cracks along the grain boundaries between columnar grains, resulting in the brittleness of the CVD W materials. The CVD W material may therefore need more improvement before application in the divertor of future fusion devices.
Fig. 8. (a), (c) and (e) are the blister morphologies on surface with NDs on the red line in Fig. 6 (b); (b), (d) and (f) are the [110] pole figures of the grains in (a), (c) and (e) respectively. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
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5. Conclusions Blister formation by D plasma exposure of rolled W surfaces at 523 K show a strong dependence on grain orientation. The blistering behavior was severe on the surface with surface normal directions close to [111] direction, while the [001] surfaces are the most resistant to blister formation. Cavities induced by D2 gas were observed beneath [111], [110] and [001] surfaces, independently on whether blisters were observed on the surface or not. In addition, it was found that some edges and step structures of blisters were perpendicular to [110] directions on surfaces with certain NDs. In this study, we found that the model based on the different implantation range in grains with different surface orientations cannot completely explain the present experimental results, such as the blister surface coverage and presence of gas-filled cavities beneath surfaces with different normal directions. On the contrary, the blister formation model based on the plastic deformation of W induced by pressurized gas-filled cavities can well explain the heterogeneity of blister formation and the different shapes of blisters on surfaces with different NDs. The [111] surface is more prone to blister formation, because the surface layer is easily deformed by the D2 gas pressure beneath the surface. Moreover, the blister edges and steps perpendicular to [110] directions were speculated to be induced by the slipping of dislocations on {110} planes. After exposure to D plasma at 563 K, the CVD samples with [001] texture showed good resistance to blister formation, so it is suggested that it may be effective to alleviate blisters by texturing of W. Acknowledgement This work was supported by National Magnetic Confinement Fusion Science Program of China under Grant 2013GB109004, 2014GB117000, and the Joint Sino-German research project GZ 763,
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and the National Nature Science Foundation of China under Contract No. 51471092. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.
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