ISSN 1063780X, Plasma Physics Reports, 2013, Vol. 39, No. 11, pp. 910–924. © Pleiades Publishing, Ltd., 2013. Original Russian Text © V.P. Budaev, Yu.V. Martynenko, L.N. Khimchenko, A.M. Zhitlukhin, N.S. Klimov, R.A. Pitts, J. Linke, B. Bazylev, N.E. Belova, A.V. Karpov, D.V. Kovalenko, V.L. Podkovyrov, A.D. Yaroshevskaya, 2013, published in Fizika Plazmy, 2013, Vol. 39, No. 11, pp. 1017–1032.
PLASMA–SURFACE INTERACTION
Recrystallization and Modification of the StainlessSteel Surface Relief under Photonic Heat Load in Powerful Plasma Discharges V. P. Budaeva, Yu. V. Martynenkoa, L. N. Khimchenkoc, A. M. Zhitlukhinb, N. S. Klimovb, R. A. Pittsd, J. Linkee, B. Bazylev f, N. E. Belovaa, A. V. Karpova, D. V. Kovalenkob, V. L. Podkovyrovb, and A. D. Yaroshevskayab a
National Research Centre Kurchatov Institute, pl. Akademika Kurchatova 1, Moscow, 123182 Russia b Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow, 142190 Russia c Project Center ITER, pl. Akademika Kurchatova 1, Moscow, 123182 Russia d ITER Organization, Route de VinonsurVerdon 13115, Saint Paul lezDurance, France e Forschungszentrum Jülich GmbH, EURATOM Association, D52425 Jülich, Germany f Karlsruhe Institute of Technology, IHM, P.O.Box 3640, D76021 Karlsruhe, Germany email:
[email protected] Received December 27, 2012; in final form, March 21, 2013
Abstract—Targets made of ITERgrade 316L(N)IG stainless steel and Russiangrade 12Cr18Ni10Ti stain less steel with a close composition were exposed at the QSPAT plasma gun to plasma photonic radiation pulses simulating conditions of disruption mitigation in ITER. After a large number of pulses, modification of the stainlesssteel surface was observed, such as the formation of a wavy structure, irregular roughness, and cracks on the target surface. Xray and optic microscopic analyses of targets revealed changes in the orienta tion and dimensions of crystallites (grains) over a depth of up to 20 µm for 316L(N)IG stainless steel after 200 pulses and up to 40 µm for 12Cr18Ni10Ti stainless steel after 50 pulses, which is significantly larger than the depth of the layer melted in one pulse (~10 µm). In a series of 200 tests of ITERgrade 316L(N)IG ITER stainless steel, a linear increase in the height of irregularity (roughness) with increasing number of pulses at a rate of up to ~1 µm per pulse was observed. No alteration in the chemical composition of the stainlesssteel surface in the series of tests was revealed. A model is developed that describes the formation of wavy irregu larities on the melted metal surface with allowance for the nonlinear stage of instability of the melted layer with a vapor/plasma flow above it. A decisive factor in this case is the viscous flow of the melted metal from the troughs to tops of the wavy structure. The model predicts saturation of the growth of the wavy structure when its amplitude becomes comparable with its wavelength. Approaches to describing the observed stochas tic relief and roughness of the stainlesssteel surface formed in the series of tests are considered. The recur rence of the melting–solidification process in which mechanisms of the hill growth compete with the spread ing of the material from the hills can result in the formation of a stochastic relief. DOI: 10.1134/S1063780X13110032
1. INTRODUCTION It is expected that, in tokamaktype fusion reac tors, including ITER, the plasmafacing materials of the vacuum chamber will be intensely affected by plasma and photonic radiation both in the steadystate regime and during transient plasma processes, such as disruption mitigated by massive injection of a noble gas (argon or neon). During transient plasma pro cesses, the thermal load on the plasmafacing compo nents will reach 0.2–5 MJ/m2 over a time interval from 0.1 to 1 ms [1]. Under the conditions of disrup tion mitigation by injection of noble gases, photonic radiation loads with an energy density of up to 1.3 MJ/m2 and a pulse duration of 5–10 ms are expected. Such radiation can penetrate into the gaps
and ports of the vacuum chamber and extremely affect the stainlesssteel surfaces of the vacuum chamber. This can lead to the melting of the surfaces. The dan ger comes from both macroscopic erosion and the modification of the material microstructure upon the recrystallization of the melted surface layer. These processes reduce the operation lifetime of the vacuum chamber and can deteriorate the strength properties of the constructional elements, which creates a danger of water leakage to the vacuum volume through the mod ified surface of the watercooled chamber elements. In recent years, experiments carried out on fusion devices have shown that the interaction of plasma with metal surfaces leads to the modification of their microstructure. For example, significant clusteriza tion of the surface structure of tungsten was found
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Table 1. Stainless steels used in the ITER design [4] Steel grade
Application of the material
304L 304B7 430 316L(N)IGI 316L(N)IGII
Elements of the vacuum vessel (ports) Elements of the neutron shield Ferromagnetic inserts of the vacuum vessel Firstwall modules produced by gas isostatic pressing Elements of the vacuum vessel and intravessel systems in which remelting of the exposed stainlesssteel sur faces should be provided 316L(N)IGIV Thinwall tubes for the first wall of the blanket modules 316L(N)IGV Tubes of collectors and pipelines
after tests on a plasma focus facility [2] and the GOL linear fusion device [3]. Mechanisms of solidification (recrystallization) of metal surfaces melted under the action of intense plasma loads are still unclear. Studies of such mechanisms and accumulation of experimen tal data are required to predict the behavior of metal elements of the ITER first wall. Table 1 [4] presents the list of stainless steels from which invessel components of ITER are produced. The components of ITER port plugs affected by high power photonic radiation during disruption mitigation will be produced from 316L stainless steel with a reduced carbon content (0.015–0.030%). The index N means a limited nitrogen content (0.06–0.08%), and the index IG (ITER Grade) reflects special requirements to the material intended for the use in ITER. In this work, we present results investigation of tar gets made of ITERgrade 316L(N)IG stainless steel, as well of Russiangrade 12Cr18Ni10Ti stainless steel with a close composition, exposed to plasma photonic radiation pulses with an energy density as high as 0.5 MJ/m2 and a duration of up to 0.5–1 ms, which simulate the load conditions of disruption mitigation in ITER. Such loads exceed the melting threshold almost twice. The microstructure of the tested surfaces was examined using microscopy, Xray spectroscopy, and structural analysis on spatial scales from a few nanometers to several hundreds of micrometers. The results are analyzed, a model describing the formation of a wavy structure on the melted surface layer with a vapor/plasma flow above it is proposed, and mecha nisms of the formation of a stochastic relief on the tar get surface under the plasma load are considered. 2. EXPERIMENTAL TECHNIQUE Stainlesssteel targets were tested at the QSPAT plasma gun (Troitsk Institute for Innovation and Fusion Research, Troitsk, Moscow, Russia), which is a onestage coaxial highcurrent plasma accelerator with a selfinduced magnetic field. The plasma is accelerated between two profiled coaxial electrodes to which an electric voltage from a power source is PLASMA PHYSICS REPORTS
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applied. The working gas is continuously supplied into the discharge gap. After gas breakdown, a bulk electric current with a density decreasing toward the accelera tor output arises in the discharge gap. The interaction of the radial component jr of this current with the azi muthal magnetic selffield Bϕ results in the generation of the longitudinal force Fz ~ jrBϕ, accelerating the plasma. The field configuration in the accelerator channel makes it possible to continuously accelerate plasma up to supersonic velocities. The duration of the flow is not limited by the time of flight, but is deter mined by the time during which the voltage is applied to the electrodes [5]. The system of the working gas (hydrogen) supply ensures the gas flow rate of up to 100 g/s. The accelerator is supplied from a capacitor bank consisting of six identical 8mF sections, which can be independently connected to the load. The con nection circuit of the sections makes it possible to vary the plasma flow duration from 0.2 to 0.6 ms. The energy stored in the capacitor bank is 600 kJ, the max imum working voltage being 5 kV. The discharge cur rent can be varied from 50 to 250 kA, and the working voltage can be varied in the range of 2–4 kV. The facil ity is used to test materials for ITER under the action of hydrogen and deuterium plasma flows with param eters of current disruptions and ELMy modes in ITER. Figure 1a shows the scheme of the experiment car ried out on QSPAT. A stainlesssteel target was placed at a distance of 100 mm from a massive graphite target, which was installed in a plasma flow. Powerful photo nic radiation is generated by braking argondoped (1– 2% of argon) hydrogen plasma flow on a solid (graph ite) target for normal plasma incidence on the target (Fig. 1). A significant fraction of the plasma energy is converted into photonic energy as a result of this pro cess, with a maximum efficiency of 40% being achieved. In this case, approximately 50 kJ is radiated into a halfsphere over ~0.5 ms. Photonic energy den sities up to 1.0 MJ/m2 can be achieved on targets placed outside the plasma flow, but facing the braking target. Such a load corresponds to the peak loads in ITER during disruption mitigation.
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0°
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3
1.5
5 4
2.5 3.0 Voltage U, kV
(c)
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Fig. 1. (a) Scheme of testing of stainlesssteel targets at the QSPAT facility. The discharge parameters in QSPAT: (b) dynamic pressure of the plasma flow, (c) energy density absorbed on the flow axis for different angles of incidence of the plasma flow, (d) plasma flow velocity, and (e) distribution of the thermal load over the target in the longitudinal (diamonds) and transverse (circles) directions for β = 60°.
Pressure p(t), atm
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Table 2. Chemical compositions of 316L(N)IG and 12Cr18Ni10Ti stainless steels (from [6]), % Steel 316L(N)IG 12X18H10T
C
Mn
Si
P
S
Cr
Ni
Mo
N
Cu
Ti
0.0181 0.0487
1.58 0.26
0.32 0.44
0.018 0.035
0, the profile λ–Hh(λy) has the same height distribution function as the profile h(y). The value Н = 0.5 corresponds to the trivial stochasticity (a Brownian relief), while the values 0.5 < H < 1 corre spond to structured reliefs with longrange correla tions. For the relief profiles shown in Figs. 3a and 4a, the Hurst exponent lies in the range from 0.76 to 0.79 (the procedure for estimating the Hurst exponent was described in [8]). For 316 L(N)IGII stainless steel, the height of irregularities calculated as a statistically averaged dis tance between the maxima and minima of the relief height increases linearly with the number of pulses (up to 250 pulses, see Fig. 6). 3.2. Analysis of the Target Microstructure Figure 7 shows scanning electron microscope (SEM) photographs of different exposed stainless steel targets. It is seen from Figs. 7a and 7b that cracks have formed on the surfaces of 316 L(N)IG and
12Cr18Ni10Ti stainlesssteel targets. The widths of cracks on the surface of 316 L(N)IG targets after 200 pulses reach 5 μm. For 12Cr18Ni10Ti targets, the crack width after 50 pulses is about 2 μm. On the sur faces of both the 316 L(N)IG and 12Cr18Ni10Ti tar gets, crystallites with a size of less than ~1 μm form after the solidification of the melted layer. The crystal lites formed on the 316 L(N)IG stainlesssteel sur face have distinct boundaries, whereas on the 12Cr18Ni10Ti stainlesssteel surface, it is impossible to distinguish individual grains. The metallographic analysis of the surface (Fig. 8) and transverse cross section (Fig. 9) of targets shows the cracking of the target surface after exposure. In 316 L(N)IG targets, the width and total number of cracks exceed those in 12Cr18Ni10Ti targets. On the surfaces of 12Cr18Ni10Ti targets, zones in the form of dark spots with a size of 5–20 μm are observed, which are identified by Xray microanalysis as regions with an enhanced titanium content. Possibly, this is because the Ti content in 12Cr18Ni10Ti stainless steel reaches ~0.8% (in 316 L(N)IG stainless steel, it is as low as 0.15%) and, after the action of plasma radiation, Ti is agglomerated in such zones on the target surface. It is worth noting some specific features of the modified (recrystallized) layer: (i) the depth of such a modified layer is smaller in the troughs and larger on the tops of the wavy relief; PLASMA PHYSICS REPORTS
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μm 300
μm 300
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21
mm h, nm (b)
h, μm 300
(c) 100
200 100
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20 x, mm 10–5 10–1
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101 f, 1/mm
Fig. 3. Properties of the surface relief of a 316 L(N)IG stainlesssteel target after the action of 100 plasma radiation pulses with a power density of 0.5 MJ/m2: (a) surface relief measured by a profilometer, (b) relief height profile, and (c) Fourier spectrum of the height profile in different cross sections of the relief.
(ii) the modified layer has a dendrite structure (a nonuniform arrangement of elongated treelike struc tures). The compositions of the original and exposed tar gets were studied by the energydispersion Xray microanalysis (Fig. 10). The spectra were recorded from a fairly large surface area, i.e., they are integral characteristics. The spectra presented in Fig. 10 show the elemental composition of targets and confirm its compliance with the industrial standards (see Table 2). For both 316 L(N)IG and 12Cr18Ni10Ti stainless steels, the spectra of the initial and exposed surfaces are absolutely identical, which indicates that the ele mental composition of the exposed surface does not change. The Xray structural analysis was performed using a Bruker Xray diffractometer with a copper tube (the wavelength of 1.5418A0). Diffraction images of the original materials are shown in Fig. 11. Analysis of these images has shown that the original 316 L(N)IG targets have an fcc structure (the austenite structure) and are initially textured (the dominant orientation of PLASMA PHYSICS REPORTS
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crystallite grains is 〈111〉). In addition, the size of per fect crystallites (estimated from the dimensions of coherent zones) along the axis 〈111〉 is nearly twice as large as that in the direction 〈100〉. The initial 12Cr18Ni10Ti stainlesssteel targets (before plasma exposure) have a twophase structure: in addition to the main austenite phase (73%), there is also the αphase of iron with a bcc structure (27%). No dominant orientation of grains is observed (i.e., the texture is absent). Metallography of the exposed targets has shown that their structure varies with depth (see Fig. 9). Therefore, we performed Xray structural analysis of targets at different depths. For this purpose, a surface layer of a given depth was removed by electrochemical polishing. Targets for such analysis were prepared in two stages. In the first stage, the targets were cut into pieces with dimensions of ~5 × 5 × 5 mm by the method of electrical erosion in water. Unlike mechan ical cutting, this method does not produce stresses in the target; therefore, it causes no spatial (structural) changes. Moreover, due to the presence of water and
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PDF, arb. units 0.18 (a) 0.16
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Film height Gaussian Cauchy–Lorentz
0.14 2
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Fig. 4. Properties of the surface relief of a 12Cr18Ni10Ti stainlesssteel target after the action of 50 plasma radiation pulses with a power density of 0.5 MJ/m2: (a) surface relief measured by a profilometer and (b) Fourier spectrum of the height profile in different cross sections of the relief.
the specific features of this method of cutting, the tar get is not heated above 100°С. The next stage was elec trochemical polishing of the exposed target surface in an electrolyte. Under the action of electric current in the electrolyte, the anode material is dissolved. An electrolyte consisting of 25% perchloric acid (HClO4) and 75% ethylene glycol monobutyl ether (C6H14O2/CH3(CH2)2CH2OCH2CH2OH) was used for electrochemical polishing. In the process of etch ing, a dc voltage of 20–22 V was applied to the target. In order to minimize the heating of the target and electrolyte, the magnitude of the current was main tained at a level of less than 0.1 A. The process of removing the material from the target surface was monitored by direct observation through an optical microscope. The thickness of the surface layer removed in the course of etching was measured using a micrometer. After each etching procedure, the surface was ana lyzed, which made it possible to study the target struc ture at different depths from the exposed surface. The results of analysis are presented in Fig. 12. The obtained results indicate the modification of the target structure after exposure. Although the target
0.15 0.10 0.05 0 –4 –3 –2 –1
0 1 2 3 (δh – 〈δh〉)/σδ
4
5
6
Fig. 5. Statistical properties (of the surface relief of (a) a 316 L(N)IG stainlesssteel target after the action of 100 QSPAT pulses (PDF of the increments of the relief height h(y) in Fig. 3a) and (b) a 12Cr18Ni10Ti stainlesssteel tar get after the action of 50 QSPAT pulses (PDF of the increments of the relief height h(y) in Fig. 4a). On the abscissa, the increment δh normalized to its standard devi ation σδ is plotted. For comparison, the normal distribu tion (dashed line) and the Cauchy–Lorentz law (solid line) are shown.
structure was initially different, the dominant orienta tion of grains on the exposed surfaces of both types of targets was 〈100〉 (see Figs. 12a, 12b). This texture per sists up to the depth of 40 μm for both stainless steels (Figs. 12c, 12d). As the depth increases, the texture 〈100〉 disappears and both types of targets become untextured. At the given depths, no αphase of iron in 12Cr18Ni10Ti targets is also observed. The fact that, for both untextured targets and tar gets highly textured in the direction 〈111〉, the new texture 〈100〉 appears indicates that the considered mechanism of action has much in common with the PLASMA PHYSICS REPORTS
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process of welding. In this process, the observed tex ture is related to the metal structure and the dominant orientation of grains for fcc and bcc structures is always 〈100〉. From the distribution over scales of the coherence regions before and after tests (Figs. 12e, 12f), it follows that, in both types of targets, the coherence regions (the smallest crystallites undistorted by dislocations, twins, and other defects) increase in size. For 316L(N)IG targets before the tests, the characteristic size of coherence regions ranges from 50 to 200 Å, whereas after the exposure tests, it increases to 200– 800 Å; moreover, the coherence regions enlarge with depth. 4. DISCUSSION OF THE RESULTS The main changes in the surfaces of targets made of ITERgrade 316 L(N)IG stainless steel and Russian grade 12Cr18Ni10Ti stainless steel after exposure by photonic heat load in powerful plasma discharges on QSPAT are as follows: (a) the formation of a melted layer with a thickness of up to 10 μm after each pulse; (b) the solidification (recrystallization) of this layer between two successive pulses; (c) the formation of crack and wavy irregularities (modulation of the relief height) on the surface; (d) an increase in the surface roughness with the number of pulses (the characteristic height of irregu larities increases by ~1 μm after each pulse and reaches 200 μm after 200 pulses; see Fig. 6); (e) the modification of the microstructure and tex ture of the surface layer as compared to the original material up to a depth of 200 μm, which is significantly deeper than the layer melted during one pulse (~10 μm). Changes (a)–(c) caused by the action of plasma flows on metals were observed earlier (see [9–13]). A new result is observation of changes (d) and (e). These changes can be explained by using models that take into account both the motion of the melted metal along the surface (which can result in a wavy relief) and the repetitive melting–solidification process accompanied by the plasma–surface interaction. The reported experiment has the following specific features. (i) The melting and solidification of the target sur face proceed under the conditions of intense plasma– surface interaction (rather than as isolated processes) and may involve processes of erosion and redeposition of the target material on the surface from the plasma. (ii) Significant modification of the surface struc ture takes place after a large number of tests. In this case, the discrete character of the process (with repet itive acts of melting and solidification) can play a more PLASMA PHYSICS REPORTS
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Δh, μm 250 200 150 100 50 0
50
100
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250 N
Fig. 6. Height of irregularities Δh on the 316 L(N)IG stainlesssteel surface vs. number N of plasma radiation pulses with a power density of up to 0.5 MJ/m 2 [6].
important role than the total energy density deposited in the target surface. Let us now consider the mechanism of the forma tion of a wavy perturbation on a melted metal surface, which results in a local growth of nonuniform struc tures after solidification. We will also consider the sta tistical approach that is used to describe the formation of a nonuniform relief on large spatial scales and allows one to apply methods employed in the physics of thin films [14]. 4.1. Model of the Formation of Wavy Irregularities on a Melted Metal Surface Let us consider the formation of waves on a melted metal surface caused by the development of tangential instability of the gas/vapor flow over the melted sur face. This process is similar to the excitation of waves on the water surface under the action of wind (see [15]). The formation of waves under the action of plasma flows on metals was observed for the first time in [16] and described in the linear approximation in [9–11]. Later on, this problem was also considered in [12, 13]. According to this model [9–11], the wavelength of exited perturbations of the melted layer is λmax = 3πα/ρ'U 2,
(1)
where α is the surface tension and ρ' and U are the mass density and velocity of vapor, respectively. In [9–11], a model of droplet erosion caused by the detachment of the upper part of the wave top by the gas/vapor flow was proposed. In this process, the wavelength is determined by the competition between the difference of the vapor pressure at the tops and troughs of a wave (described by the Bernoulli law) and
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Fig. 7. SEM photographs of target surfaces with different magnifications after the action of radiation pulses with a power density of up to 0.5 MJ/m2: (a) 316 L(N)IG stainless steel (200 pulses) and (b) 12Cr18Ni10Ti stainless steel (50 pulses) [6].
the surface tension force. Since the oscillation fre quency (see [9–11]) ω = (2πU/λ)(ρ'/(ρ + ρ') (2) is much lower than the instability growth rate (see [10–12]) γ = {2ρ'U3/3α}(ρ'/3ρ)1/2, (3) where ρ is the mass density of the liquid, the growth of waves passes into the nonlinear stage already in the first oscillation period. In this case, the liquid phase of the melted surface flows from troughs to tops with a velocity v determined by the viscous flow conditions: v ≈ (ρ'/ρ)U2h2/λχ,
(4)
where h is the thickness of the melted layer and χ is the kinematic viscosity. Then, the growth rate of the wave height H is dH/dt = vh/λ. (5) According to estimates, v ≈ 101–102 cm/s (see [5, 6]); therefore, the increment in the wave height per pulse is d = 1–10 μm. In the experiments carried out on QSPAT (see above), such a height of irregularities is reached only after 10 pulses (see Fig. 6). This estimate was made for a smooth substrate of the melted layer. The actual surface has a nonuniform profile with a typical roughness scale length of larger than 1 μm (Fig. 7). This roughness can decelerate the PLASMA PHYSICS REPORTS
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Fig. 8. Metallographs of target surfaces after the action of 50 radiation pulses with a power density of up to 0.5 MJ/m2: (a) 316 L(N)IG stainless steel and (b) 12Cr18Ni10Ti steel [6].
flow of the melted metal along the surface and slow down the growth of the wave. This slowing down pos sibly depends on the size of grains (crystallites) on the surface, i.e., the minimum roughness scale length of the original material. On the 12Cr18Ni10Ti surface, grains are smaller than those on the 316 L(N)IG sur face, which, probably, slows down the wave formation on the 12Cr18Ni10Ti stainless steel as compared to that on the 316 L(N)IG stainless steel. In the reported experiments, the values of ω and γ under the given load conditions on the tested surface are ω = 103 s–1 and γ = 106 s–1. In this case, the oscil lation period ω–1 is comparable with the duration of photonic pulses with parameters typical of mitigated disruptions in ITER, i.e., a steadystate wave process on spatial scales significantly exceeding the wave length has no time to be established. We also observed another example of how the tex ture influences the wave formation in the tests carried out on QSPAT. Two targets were cut from 316 L(N) IG stainless steel: one in parallel to the sheet surface and another perpendicular to it. After 50 QSPA pulses, the wave amplitude in the target cut perpendicular to the sheet surface was ~60 μm, while that in the target cut in parallel to the sheet surface was 100–110 μm. It should also be noted that droplet erosion arises as a result of blowing off the tops of the wave hills by the plasma/vapor flow [9]. Therefore, according to the above model, in the absence of a mechanism responsi ble for the formation of wavy irregularities, the droplet erosion is also absent. Since the force driving the growth of a wavy relief arises due to the difference in the pressures of the vapor/plasma flow above the hills and valleys, it may be expected that the growth of hills and deepening of valleys will either slow down or stop when the hill height becomes comparable with the wavelength. In this case, the vapor/plasma flow does not get into val leys and moves above hills and the difference in the pressures above the hills and valleys decreases. It should be noted that, for higher power plasma pulses, when both the density ρ' and the vapor/plasma flow PLASMA PHYSICS REPORTS
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velocity U are higher, the wavelength (λ = 3πα/ρ'U2) is shorter and the waves stop growing at smaller heights, although, in this case, the waves grow faster: dH/dt ~ ρ'U2h3/λ2. It should be noted that, as early as in [17], it was found that both the amplitude and the wave length increase with decreasing flow power. The difference between the elemental composi tions of the tested stainless steels is insignificant (Table 2). There is a difference in the carbon content, but both steels are classified as lowcarbon. In contrast to 12Cr18Ni10Ti steel, 316 L(N)IG steel contains Mo (2.5%), while the Ti content in 12Cr18Ni10Ti steel (~0.7%) is higher than in 316 L(N)IG steel (
