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Development and characterization of powder metallurgically produced discontinuous tungsten
fiber reinforced tungsten composites
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2017 Phys. Scr. 2017 014005
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| Royal Swedish Academy of Sciences Physica Scripta
Phys. Scr. T170 (2017) 014005 (7pp) https://doi.org/10.1088/0031-8949/2017/T170/014005
Development and characterization of powder
metallurgically produced discontinuous
tungsten fiber reinforced tungsten
composites
Y Mao1,7 , J W Coenen1 , J Riesch2 , S Sistla3, J Almanstötter4,
B Jasper1, A Terra1, T Höschen2 , H Gietl2,5, M Bram6, J Gonzalez-Julian6,
Ch Linsmeier1 and C Broeckmann3
1
Institut für Energie und Klimaforschung—Plasmaphysik, Forschungszentrum Jülich GmbH, D-52425
Jülich, Germany
2
Max-Planck-Institut für Plasmaphysik, D-85748 Garching b. München, Germany
3
Institut für Werkstoffanwendungen im Maschinenbau (IWM), RWTH Aachen University, D-52062
Aachen, Germany
4
OSRAM GmbH, SP PRE PLM DMET, Mittelstetter Weg 2, D-86830 Schwabmünchen, Germany
5
Technische Universität München, Boltzmannstrasse 15, D-85748 Garching, Germany
6
Institut für Energie und Klimaforschung—Materials Synthesis and Processing, Forschungszentrum Jülich
GmbH, D-52425 Jülich, Germany
E-mail: y.mao@fz-juelich.de
Received 24 May 2017, revised 26 July 2017
Accepted for publication 14 August 2017
Published 31 August 2017
Abstract
In future fusion reactors, tungsten is the prime candidate material for the plasma facing components.
Nevertheless, tungsten is prone to develop cracks due to its intrinsic brittleness—a major concern
under the extreme conditions of fusion environment. To overcome this drawback, tungsten fiber
reinforced tungsten (Wf/W) composites are being developed. These composite materials rely on an
extrinsic toughing principle, similar to those in ceramic matrix composite, using internal energy
dissipation mechanisms, such as crack bridging and fiber pull-out, during crack propagation. This
can help Wf/W to facilitate a pseudo-ductile behavior and allows an elevated damage resilience
compared to pure W. For pseudo-ductility mechanisms to occur, the interface between the fiber and
matrix is crucial. Recent developments in the area of powder-metallurgical Wf/W are presented.
Two consolidation methods are compared. Field assisted sintering technology and hot isostatic
pressing are chosen to manufacture the Wf/W composites. Initial mechanical tests and
microstructural analyses are performed on the Wf/W composites with a 30% fiber volume fraction.
The samples produced by both processes can give pseudo-ductile behavior at room temperature.
Keywords: tungsten, fiber reinforced composites, powder metallurgy, pseudo ductile behavior,
crack propagation resistance
(Some figures may appear in colour only in the online journal)
1. Introduction
Tungsten is currently the main candidate material for plasma resistance against erosion, high melting point, low tritium
facing component in future fusion reactors due to its retention and benign activation behavior by neutron irradia-
tion [1, 2]. However, its application is strongly restricted by
7
Author to whom any correspondence should be addressed. its inherent brittleness. Under the extreme conditions of
0031-8949/17/014005+07$33.00 1 © 2017 Forschungszentrum Jülich Printed in the UK
Phys. Scr. T170 (2017) 014005 Y Mao et al
fusion environment with high transient heat loads and neutron typical yttria thin film cross section prepared by focused ion
irradiation, a tungsten plasma facing material will face the beam cut.
possibility of crack formation during operation and sub- In a next step, the tungsten fibers were mixed with the
sequent catastrophic failure [3]. Tungsten fiber reinforced tungsten powders by manual shaking in a vessel with a fiber
tungsten (Wf/W) composites have been developed to over- volume fraction of 30%. The similar density of the tungsten
come this issue, relying on extrinsic toughing mechanisms by powders and the tungsten fibers leads to a random distribution
using commercially available tungsten wires as reinforcement of the tungsten fibers in the mixture.
[4–6]. By incorporation of the fibers, some local energy dis- Afterwards, the fiber-powder mixture was sintered
sipation mechanism during crack propagation can be realized, (consolidated) by a FAST or HIP process, respectively. The
such as fiber pull-out, ductile deformation of the fibers, crack FAST and HIP process parameters are shown in table 1.
deflection and interface de-bonding. These mechanisms Schematic diagrams of the processes are shown in
enable Wf/W to behave pseudo ductile [7], similar to ceramic figure 2. During the FAST process, the powder-fiber mixture
matrix composites [8–10]. The crucial factor to realize pseudo was consolidated in a graphite die with 20 mm inner diameter.
ductility is the existence of a relatively weak fiber-matrix The sintering was performed under vacuum below 0.1 mbar.
interface [11–13]. Tungsten powder and fiber mixture were then heated by
The two potential methods to manufacture Wf/W com- current Joule heating through the sample under high uniaxial
posites are chemical vapor deposition (CVD) [7, 14] and pressure (in z direction) [29]. As result, coin shape samples
powder metallurgy processes (PM) [15–18]. Both production (20 mm diameter and ∼5 mm height) was produced (figure 3).
routes have shown the possibility to achieve the expected For the HIP process, the powder/fiber mixture was firstly
pseudo ductility. Compared to the CVD production route, the uniaxial (in z direction in figure 2) pre-pressed at 200 MPa and
PM process, as a more standard industrial process, has several then put into a tantalum capsule. After vacuum seal welding, the
benefits, such as substantial experience with bulk production, tantalum capsule was then consolidated in the HIP chamber.
higher sample density and an easier realization of alloy pro- The isostatic pressure was applied by means of an Argon gas
duction. However, the potential issue of the PM process is inlet. The HIP sample after consolidation is cylindrical in shape
that, the high temperature during sintering will cause recrys- with ∼18 mm diameter and ∼14 mm height (figure 3).
tallization of the tungsten fiber and, hence, weaken the fibers The mass density of the sintered Wf/W samples after
ductility and strength [5, 19]. Additionally, the fiber-matrix consolidation was measured by the Archimedes principle.
interface will be damaged during the sintering process due to The results are given in table 1.
the high temperature and pressure [15, 20]. Figures 4 and 5 represent an overview of the typical
In this work, as introducing long fibers into the PM pro- microstructure of the as-fabricated Wf/W samples for both
cesses is technically difficult, discontinuous tungsten fibers fabrication process respectively. The cross sections shown are
(2.4 mm length) are chosen to reinforce the tungsten matrix. parallel and perpendicular to z axis, respectively. A relatively
The samples were prepared via different PM processes: field homogeneous fiber distribution can be observed in all cases.
assisted sintering technology (FAST) [21–25] and hot isostatic Potential fiber distribution inhomogeneity (areas marked by dash
pressing (HIP) [26–28]. The study focuses on the micro- lines in figures 4 and 5) could be caused by insufficient shaking
structure and load capability of the PM produced Wf/W. during mixing or punch/capsule compression during sintering.
The samples from both processes tend to have a
2-dimentional planar fiber orientation distribution. Most fibers
2. Wf/W manufacturing and microstructure tend to orient parallel to x–y plane. For the FAST process, it is
assumed to be due to the height reduction in the z-direction
The raw materials of the Wf/W composites fabrication are pure during sintering. For the HIP process, it is caused by the
tungsten powders (provided by OSRAM GmbH) with 5 μm uniaxial pre-pressing (in z direction) during green body
average particle size (fischer sub sieve size) and potassium preparation. Additionally, fiber deformation after consolida-
doped short tungsten fibers (provided by OSRAM GmbH) with tion is visible, especially when fibers are in contact with each
2.4 mm length and 0.24 mm diameter. The tungsten fibers were other, as marked by solid circles in figure 5.
produced by a drawing process and then cut into the required Figure 6 represents the detailed view of the fiber-matrix
length. Due to the drawn microstructure with elongated grains, interface region. An energy dispersive x-ray spectroscopy
the tungsten fibers show ductile behavior and extremely high (EDX) mapping analysis was performed to detect local
tensile strength (∼3000 MPa) [5, 19]. Since potassium is interface chemical composition. The location of elemental
insoluble in tungsten the doping (approx. 75 ppm) is present in yttrium signal are shown in green. The fiber-matrix interfaces
form of nano-disperse rows of bubbles along the elongated are visible after sintering for the samples from both processes.
grains pinning the grain boundaries and thus leading to a good In contrast to the dense and homogenous yttria layer in the as-
high temperature microstructure stability [5]. coated (see figure 1), the yttria interface show damage after
As a first step of sample manufacturing, the tungsten the consolidation process. Especially the outer shell of the
fibers were coated with a 2.5 μm yttria layer to form a dedi- interface got penetrated by the tungsten powders owing to the
cated fiber-matrix interface, by using a magnetron sputtering external pressure and the high temperature [15, 20, 30, 31].
process. Coating production details are shown in [20]. The interface damage during the FAST product looks, at a
Figure 1 presents a scanning electron microscope image of a first glance, more severe (figure 6). We can see from the EDX
2
Phys. Scr. T170 (2017) 014005 Y Mao et al
Figure 1. Scanning electron microscope (SEM) image of the as-deposited yttrium oxide thin film surface on single tungsten fiber (a); SEM
image of the thin film cross section prepared by focused ion beam (FIB) cut showing yttrium oxide thin film structure (b) (the visible
additional top layer of platinum was deposited during sample preparation for FIB cut).
Figure 2. Schematic diagram of FAST (a) and HIP (b) processes.
Table 1. FAST and HIP process parameters and relative density of the samples.
Temperature Pressure Holding time Heating rate Relative density
−1
FAST 190 °C 60 MPa 4 min 200 K min ∼94%
HIP 1600 °C 200 MPa 2h 10 K min−1 ∼98%
results, in the FAST Wf/W, more yttria particles diffused into 3. Fracture behavior
the matrix and the diffusion distance is larger. This is prob-
ably due to the higher process temperature. Also an effect To observe the crack propagation behavior of the Wf/W
called dielectric breakdown effect might play a role [30, 31], composites, a pre-notched 3-point bending test was per-
as yttria is electrically isolating. formed at room temperature (RT) on samples produced by the
3
Phys. Scr. T170 (2017) 014005 Y Mao et al
Figure 3. Representative Wf/W samples after powder metallurgical consolidation.
Figure 4. Light microscope image of the overview microstructure of the FAST and HIP produced Wf/W, the cross sections are parallel to z
axis. The dash lines mark the potential inhomogeneous fiber distribution areas.
Figure 5. Light microscope image of the overview microstructure of the FAST and HIP produced Wf/W, the cross sections are perpendicular
to z axis. The dash lines mark the potential inhomogeneous fiber distribution areas and the solid circles mark the fiber deformation positions.
above described processes. Sample dimensions were testing machine (Instron GmbH, Darmstadt, Germany) with a
18×2×4 mm3 (length×width×thickness) with a displacement rate of 5 μm s−1.
∼2 mm deep notch. The pre-notch was prepared by diamond Typical load-displacement curves of pure W and Wf/W
wire cutting followed by manually razor blade polishing. The composites produced by FAST, as well as Wf/W composites
bending tests were performed with an Instron 3342 universal produced by HIP are presented in figure 7. It is important to
4
Phys. Scr. T170 (2017) 014005 Y Mao et al
Figure 6. SEM images of the fiber-matrix interface region of the FAST and HIP produced Wf/W composites. An energy dispersive x-ray
spectroscopy mapping analysis was performed. The location of elemental yttrium signal are shown in green.
note that the absolute force values in these curves are not Additionally, with respect to the fracture behavior, some
directly comparable. Because the pre-notch was manually difference between FAST and HIP samples can be identified.
prepared, therefore, the sharpness and the depth of the notches The main difference is after maximum loading: for FAST
are not identical. The fracture surfaces of the samples after the sample, the load drop is more continuous (has more steps); for
pre-notched 3-point bending tests are shown in figure 8. HIP sample, there is a sudden load drop to a low value. We
As shown in figure 7(a), at RT, the pure tungsten sample expect this difference is caused by the different fiber-matrix
show pure linearly elastic loading until a severe load drop to interface strength. For FAST samples, in some regions, the
zero during failure without having any plastic deformation. fiber and matrix get sintered together, as we can see from
Also a clear brittle intergranular fracture surface is observed figure 6. In this case the bonding between fiber and matrix
as shown in figure 8(a). will be very strong and we can consider it like a tungsten/
Both Wf/W samples give different force-displacement yttria mixed interface, instead a clean yttria interface. So the
curves: after the first load-drop, the load still increases; a fiber-matrix interface strength is higher for FAST produced
massive load-drop occurs after reaching the maximum load; Wf/W. During the pull-out stage after multiple fiber failure, a
higher strength interface could dissipate more energy (fiber-
afterwards, the samples tend to have a stepwise or continuous
matrix friction is higher) so we end up with a more con-
load-drop instead of a completely failure.
tinuous load decreasing compared to HIP sample with a
The matrix of the composites exhibits also intergranular
relatively low interface strength. The interface strength
fracture, similar to pure W. Interface de-bonding, fiber pull-
influence on Wf/W fracture behavior will be reported in
out can be observed in both cases, as shown in figures 8(b)
future study.
and (c). Additionally, the overall fracture surface of the
Wf/W is more uneven compared to the pure tungsten.
The above results give a strong indication that the PM
4. Conclusion and outlook
produced Wf/W composites are able to exhibit typical pseudo
ductile behavior similar to Wf/W produced by CVD [4]: the Wf/W composites with ∼94% and ∼98% density are pro-
first load-drop signifies the matrix failure; after the matrix duced by FAST and HIP processes, respectively. Both pro-
failure, the load still increases due to the crack bridging by the cesses give similar microstructures after consolidation with a
fibers; the massive load-drop indicates the multiple fiber relatively homogeneous fiber distribution. Fibers tend to have
failure; then the stepwise load bearing capability is probably a 2-dimentional planar orientation. The fiber-matrix interface
supplied by frictional fiber pull-out from the matrix. The remains intact after PM processes. The PM produced Wf/W
uneven fracture surface gives a hint for observed crack samples are able to show a pseudo-ductility behavior at RT.
deflection. Here the fiber pull-out, fiber elastic bridging and This is in contrast to complete brittle behavior of pure tung-
crack deflection are likely the energy dissipation mechanisms sten. The improved resistance against fracture relies on
which contribute to the elevated fracture resistance compared energy dissipation mechanisms, such as fiber pull-out, crack
to pure tungsten [9, 32]. bridging by the fibers and crack deflection. For the first time,
5
Phys. Scr. T170 (2017) 014005 Y Mao et al
Figure 7. Typical load-displacement curves of FAST produced pure W, Wf/W and HIP produced Wf/W during pre-notched 3-point bending
tests.
Figure 8. SEM image of the fracture sections after the pre-notched 3-point bending tests of FAST produced pure W (a), Wf/W (b) and HIP
produced Wf/W (c).
pseudo ductile behavior of a HIP produced Wf/W composite T Höschen https://orcid.org/0000-0002-4966-1091
at RT was presented. Ch Linsmeier https://orcid.org/0000-0003-0404-7191
The fiber volume fraction influence on the composite
properties has also been studied and will be reported in the
future. In the next step, the influence of the fiber-matrix inter- References
face strength on the composite will be studied. The Wf/W
composites with different interface materials will be produced
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