Download Thermal and Mechanical Characterizations of W-armoured

Mechanical Characterization of W-armoured Plasma Facing Components after
Thermal Fatigue
D. Serret*, M. Richou M. Missirlian, and T. Loarer
* [email protected]
CEA, IRFM, F-13108 Saint Paul-lez-Durance, France
PACS reference :
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Abstract:
The future fusion device ITER aims at demonstrating the scientific and technical
feasibility of fusion power. Tens of thousands of W-armoured Plasma Facing
Components (PFCs) will be installed in the vertical targets of the ITER divertor and
submitted to high heat flux. The purpose of this paper is to present results on
mechanical and microstructural characterizations of tungsten PFCs after thermal
fatigue tests. On each component Vickers hardness measurements are performed. In
parallel, the mean grain diameter in the corresponding zone of tungsten material is
determined. The empirical Hall-Petch relation was adapted to experimental data.
However, due to the plateau effect on recrystallization hardness, this relation does
not seem to be relevant once recrystallization is complete: a new approach is
proposed to predict the margin to the tungsten melting onset.
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1. Introduction
The future fusion device ITER aims at demonstrating the scientific and technical
feasibility of fusion power. In the domain of the ITER Plasma Facing Component
(PFC) design, monoblock geometry has been chosen to sustain high heat flux loads
on the vertical targets of the divertor component.
The current ITER Baseline foresees the use of carbon and tungsten in the strikepoint and baffle region of divertor, respectively, for the initial non-active phase of
operation with hydrogen and helium plasmas. Thereafter a full tungsten divertor is
considered for the nuclear phase with deuterium and deuterium-tritium (D-T)
plasmas.
High heat flux tests were performed on W components with a monoblock-type
geometry consistent with the ITER divertor one. The results show that 10 MW/m² is
currently the limit without any degradation observed on the heat load and removal
capabilities [1-2]. Beyond, embrittlement of the W armour near the loaded surfaces
occurs (roughening, cracking) at 15 MW/m². Finally, after several hundreds of cycles
at 20 MW/m², cyclic fatigue induced damages result in higher and higher surface
temperature up to the appearance of melted W droplets [3-4]. So far, current
numerical simulation tools cannot predict correctly these experimental results. A
better characterization of structural damage of W armour material, due to fatigue
experiments, is required for a better understanding and prediction of fatigue lifetime
related to W-armoured components. The purpose of this paper is to present the first
results on mechanical and microstructural characterizations of tungsten armoured
actively cooled PFCs after thermal fatigue tests.
Among several tested samples, a study is carried out focused on 3 samples that
endured 3000 cycles at 10 MW/m² followed by 500 cycles at 20 MW/m². On each
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component Vickers hardness measurements are performed. In parallel, the mean
diameter of the tungsten grains in the related area is determined. Furthermore a
comparison with an empirical relation correlating grain diameter and Vickers
hardness is discussed whilst an analysis of tungsten PFCs is proposed for its use in
future fusion devices.
2. Experimental and numerical descriptions
2.1.
Monoblock PFC geometry
The geometry of PFCs used in the present work is presented in figure 1. The design
is similar to the one used in the ITER Divertor. Each component is 36 mm high, 28
mm wide and 12 mm thick. The minimum tungsten thickness, corresponding to the
minimum distance from the loaded surface to the copper heat sink structure, is 5 mm.
In order to assess the maximal acceptable defect size (i.e allowing steady state
operations without tungsten melting at the surface under heat flux exposure), some
calibrated defects were artificially implemented inside the monoblocks. The individual
tungsten blocks are made from tungsten sheet material. Pure copper casting
technology and hot radial pressing were used to assemble the CuCrZr cooling tube to
the Cu compliance layer and the Cu to the tungsten monoblock, respectively.
2.2.
Fatigue test campaign
A fatigue test campaign was performed on W monoblock PFCs using High Heat Flux
(HHF) loads in the electron beam facility FE200 [5]. The purpose of this campaign
was to study the detection and the evolution of artificially implemented and calibrated
defects under thermal fatigue [1]. Each component was actively cooled with a
pressurized (P=33 bar) and thermal controlled (Tinlet =120°C) water flow. During the
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HHF test campaign, the surface temperature of each component was monitored via
an infrared camera.
The testing protocol included two phases: a first cycling step of 3000 cycles at 10
MW/m², and a second one of around 500 cycles at 20 MW/m². At 20 MW/m², several
phenomena appeared: erosion and vertical cracks at the tungsten surface, initiation
of new defects or propagation of artificial defects at the W/Cu interface. These last
defects lead to a drop in the cooling efficiency resulting in a surface overheating, and
eventually to tungsten melting. Three W-monoblocks were selected on the basis of
the maximal surface temperature reached during the tests in order to estimate the
fatigue influence on their mechanical properties. It is worth noting that these samples
have sustained successfully 500 cycles at 20 MW/m². These monoblocks are
considered at a safe state since no apparent damage occurred during fatigue tests.
Due to different interface defect size, each component reached various maximal
surface temperature monitored by the infrared camera taking into account the low
emissivity value (0.28) of tungsten material [9]:

Sample 1: Tmax surf = 2500°C

Sample 2: Tmax surf = 2900°C

Sample 3: Tmax surf = 3200°C
2.3.
Mechanical and microstructural measurement set-ups
In order to estimate the fatigue influence on the mechanical properties of tungsten
monoblock, Vickers hardness measurement is carried out. The set-up is an AVK-CO
model of Mitutoyo. This test consists in applying during 30 s a controlled mechanical
load (294 N) with a diamond indenter and in correlating the diagonal size of the
indentation mark to the Vickers Hardness HV30/30.
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In parallel to this mechanical characterization, the local microstructure is analyzed
based on the NF ISO 643 norm [6]. Among several possibilities for determining the
mean tungsten grain diameter, counting the number of grain joint intersections with a
750 µm circle diameter is used. This method slightly overestimates the grain
diameter measurement [6]. However, since the average grain diameter defines the
recrystallization level that is correlated to the hardness value, this method is relevant
for the characterisation of the recrystallization.
Vickers hardness measurement and tungsten grain diameter can be linked by the
Hall-Petch relation [7-8] which is written in the form:
HV  a 
b
(kg.mm-2)
D grain
(1)
In the case of the tungsten material at the delivery (from single crystalline material
to nanometer polycrystalline grain made material), a and b equal 350 kg.mm-2 and 10
kg.mm-3/2 [9], respectively, and Dgrain represents the diameter of the grain.
2.4.
Thermal numerical simulation
Because of different evolution of artificial calibrated defects, monoblock overheating
induced different surface temperature exceeding sometimes the tungsten melting
point (≈ 3500°C). A correlation between number of cycles and the temperature
reached by tungsten is used as a reference for defining the fatigue status of the
monoblock.
In order to estimate the temperature reached by tungsten at each Vickers Hardness
and grain diameter measurements, a thermal numerical simulation of the realistic
geometry has been performed taking machined defects at W/Cu or Cu/CuCrZr
interfaces into account. Uniform heat flux (20 MW/m²) is applied on the top of the
monoblock and the Sieder-Tate correlation [10] is used to estimate the convective
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heat transfer coefficient in the CuCrZr tube. All these simulations are performed with
the ANSYS code whilst the thermo mechanical properties are taken from [11].
3. Results
For sample 2 (Tmax = 2900°C), several hardness profiles at different depth in the
direction parallel to the surface exposed to the heat flux are plotted in figure 2. These
point out the evolution of the tungsten hardness from around 365 HV30/30 in the
vicinity of the heated surface to 465 HV30/30 at a depth of 12 mm. At a depth of 6
mm from the heated surface, the hardness value of the non-recrystallized material is
similar to the delivered one (HVnon-recrist = 460 HV30/30 [12]). It is worth noting that
the experimental hardness measurement on the recrystallized material (365
HV30/30) is comparable to the theoretical value (HVrecrist = 360 HV30/30 [12]) and
that the hardness profile does not depend on the direction perpendicular to the heat
flux.
Steady-state temperature field inside sample 2 is shown in figure 3-a for 20 MW/m² of
heat flux deposition. During the HHF tests, once stationary conditions are reached, a
constant thermal gradient is established within the monoblock from the heated
surface to the cooling tube. The thermal gradient is mainly vertical which is in
agreement with hardness profile measurements. This numerical simulation allows
defining the recrystallization zone limit (Trecryst = 1350°C [12]). By comparing the
numerical results to the tungsten material state in terms of recrystallization, a
qualitative agreement is found as shown on figure 3-b.
As the tungsten grain size is linked to the cyclic thermal load undergone during HHF
tests, we aim at determining a quantitative correlation between grain size and Vickers
hardness for the maximal surface temperature of a component. The original HallPetch relation (1) and the experimental data of the sample 2 are presented on the
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figure 4-a, and it can be observed that the adapted theoretical relation does not fit at
all to the experimental results. As a consequence, an adaption of the Hall-Petch
relation by fitting the experimental data has been carried out, leading to a reassessment of parameters ‘a’ and ‘b’ at 65.5 and 70.4, respectively. Based on the
original Hall-Petch relation, the grain diameter in the non-recrystallized area is
estimated at 8 µm whereas the average experimental one is equal to 30 µm. The
discrepancy may be explained by the overestimation of the grain diameter
measurement technique, and/or the effect of the manufacturing process changing the
tungsten grade.
In figure 4-b the Vickers hardness is displayed as a function of the grain diameter and
the corresponding Hall-Petch fitting curves for the three samples which endured
different thermal gradients. Sample 1 reached a lower and sample 3 a higher
maximal surface temperature than sample 2. These fitted curves are defined in the
hardness range of 360-460 HV 30/30. These correlations are strongly dependent on
the maximal surface temperature. The higher is the tungsten maximal surface
temperature, the higher will be the maximal grain diameter. Whatever the thermal
gradient and the maximal surface temperature of the component: tungsten grains
close to the cooling tube will never or slightly be recrystallized. Otherwise copper
fusion temperature (1080°C) may be exceeded. Once Vickers hardness has reached
the recrystallization one (360 HV30/30), no evolution of the tungsten microstructure
may be identified by this single measurement. Here is the highlighted limitation of this
non-destructive examination method applied on a recrystallizing metal.
In order to predict the effect of the surface temperature of the tungsten PFC on the
margin to the melting onset, a new correlation is proposed using adimensional
hardness ( HV ) and adimensional grain diameter ( Dgrain ). Using adimensional
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variables defined in equation (2) and (3) allows adapting and testing the relation to
other tungsten grade materials.
HV 
HV  HV recryst
HV non recryst  HV recryst
D grain 
Where Dgrain
0
(2)
D grain  D grain0
(3)
D grainLimit  D grain0
is the non-recrystallized tungsten grain size and Dgrain
Limit
is the
maximal grain size before tungsten melting measured during post mortem analysis
(0.12 mm). Dgrain and HV are plotted on figure 5. Results for each component are
fitted by an exponential function which is written in the form:
HV  c exp( d Dgrain ) e
(4)
Interceptions of the Dgrain fitted curves with HV  0 are estimated ( Dgrain ( HV  0) and
plotted in figure 6 as a function of the maximal surface temperature. The margin to
the tungsten melting onset is defined by the gap to the value Dgrain ( HV  0)  1 and is
reduced by increasing the surface temperature. Because of the cyclic recrystallization
process occurrence, tungsten grains keep growing. Therefore, in the case of
additional fatigue cycles realized with the same maximal surface temperature, grains
will be increasingly large implying that the margin to the onset of melting is reduced
by the vertical displacement in the figure 6. This reasoning can be applied in the case
of coupled additional cycles and increasing surface temperature: slanting
displacement from curve to curve with increasing cycle number.
To complete the knowledge for the endured thermal cycling, more monoblocks with
several maximal surface temperatures are required. This data base will be
progressively filled in by tungsten PFCs that endured other fatigue cycle numbers.
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4. Conclusion
Tungsten Plasma Facing Components will be installed in the ITER divertor and
exposed to high heat flux. Their design has also to integrate the induced thermal
fatigue resulting from cycling. Vickers hardness and microstructure measurements
show the fatigue influence through the spatial grain diameter modification. This
evolution is well correlated to the thermal gradient in the monoblock. A relation
between Vickers hardness and grain diameter via the Hall-Petch one was also tested
and adapted to experimental data. However, due to the non evolution of
recrystallization hardness in function of the grain diameter, this relation does not
seem to be relevant once recrystallization is complete. A new approach is proposed
to predict the margin to the tungsten melting onset in the case of tungsten PFC.
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References
[1] Escourbiac F. et al. 2009 Fusion Eng. Des. 84 747
[2] Missirlian M. et al. 2010 Consequences of Fatigue on Heat Flux Removal
Capabilities of W Actively Cooled Plasma-Facing Components 23th International
Atomic Energy Agency Conference, Daejon, South Korea
[3] Missirlian M. et al. 2011 Fatigue Lifetime and Power Handling Capability of
Actively Cooled Plasma Facing Components for ITER Divertor 13th International
Workshop on Plasma-Facing Materials and Components for Fusion Applications,
Rosenheim, Germany
[4] Gavila P. et al. 2010 High Heat Flux Testing of Mock-ups for full Tungsten Divertor
26th Symposium on Fusion Technology, Porto, Portugal
[5] Bobin Vastra I. et al., Fusion Eng. Des., 2005, 75-79, 357
[6] NF ISO 643 Détermination micrographique de la grosseur de grain apparente,
2003
[7] Hall H.O. 1951 Proc. Phys. Soc. B 64 747
[8] Petch N.J. 1953 J. Iron Steel Inst. 174 25
[9] Lassner E. and Schubert W.-D. 1999 Tungsten: Properties, Chemistry, Technology
of the Element, Alloys, and Chemical Compounds (Springer)
[10] Sieder E.N. and Tate G.E. 1936 Ind. Eng. Chem. 28 1429
[11] ITER Material Properties Handbook, ITER Doc. No. 574 MA 2
[12] Tungsten Material Properties and Applications, Plansee Brochure 2000.
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Figure 1: Geometry sketch of tungsten monoblock
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Figure 2: Dependence of the Vickers hardness on the position in sample 2 (x=0 is on
the outer edge, see figures 1 and 3b).
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Figure 3: Numerical-experimental qualitative comparison of the recrystallization area
on sample 2
a) Results of numerical simulation for sample 2 at 20 MW/m²
b) Qualitative representation of recrystallized area based on Vickers hardness
measurements
a)
b)
Recrystallisation limit
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Figure 4: Effect of the fatigue test on the relation between tungsten grain diameter
and Vickers hardness measurement
a) Comparison of the original Hall-Petch relation to experimental data obtained after
HHF test (Sample 2)
b) Effect of the maximal surface temperature on the relation between grain diameter
and tungsten Vickers Hardness
a)
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b)
Figure 5: Plot between adimensional Vickers hardness and grain diameter
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Figure 6: Estimation of the margin to melting onset as function of the surface
temperature after 3000 cycles at 10 MW/m² and 500 cycles at 20 MW/m²
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