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Tungsten foil laminate for structural divertor applications – Tensile test properties of tungsten foil
Journal of Nuclear Materials 434 (2013) 357–366
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Tungsten foil laminate for structural divertor applications – Tensile test properties of tungsten foil Jens Reiser a,⇑, Michael Rieth a, Anton Möslang a, Bernhard Dafferner a, Andreas Hoffmann b, Xiaoou Yi c, D.E.J. Armstrong c a b c
Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM-AWP), Germany PLANSEE SE, Reutte, Austria University of Oxford, Department of Materials, United Kingdom
a r t i c l e
i n f o
Article history: Received 5 November 2012 Accepted 3 December 2012 Available online 8 December 2012
a b s t r a c t This paper is the third part of our series on tungsten foil laminates. Within the tungsten laminate project we have succeeded in ductilizing tungsten by synthesizing a tungsten laminate made of tungsten foil, which is ductile. By assembling and joining several layers of tungsten foil, the ductile properties of the foil can be extended to the bulk. The aim of this paper is to present the results of tensile tests on 100-lm-thick tungsten foil in asreceived and recrystallized conditions (1 h at 2000 °C). The results show that the mechanical properties of tungsten foil are anisotropic and can be explained by considering (i) the texture of the tungsten foil of {1 0 0}h0 1 1i, (ii) the anisotropic grain shape (0.5 lm 3 lm 15 lm), (iii) the preferred slip direction of body-centered cubic (bcc) metals, the h1 1 1i direction, as well as the preferred cleavage plane of tungsten at room temperature, the {1 0 0} plane. Furthermore, the results give further hints for the sources and mechanism of the extraordinary ductility of tungsten foil such as the ‘foil effect’, which is the dislocation annihilation at the free surface. In particular, the ductile material behavior of tungsten foil in the recrystallized condition seems to benefit from the foil effect and this is how a plastic strain of about 30% in a tensile test at 600 °C can be explained. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction A divertor is a plasma facing, high heat flux component in a fusion reactor. The expected heat load for a divertor in a future fusion power reactor (DEMO) in a short-term scenario is expected to be about 10 MW/m2 with peaks of up to 20 MW/m2 due to plasma instabilities. As a divertor has to deal with such extreme heat loads it is quite obvious that tungsten, the material with the highest melting point of all metals (Tmelting = 3420 °C [1]), is considered as a candidate for several divertor applications. One application asks for a tungsten material that can be used for structural cooling pipes [2–4]. As these pipes are meant to be pressurized pipes (e.g. helium coolant, 600 °C, 100 bar) the tungsten material must fit the needs of a structural material. In this case the tungsten material must have a low brittle-to-ductile transition temperature (BDTT) as well as high fracture toughness (e.g. 20 MPa m1/2). Unfortunately tungsten is very brittle and until now has only been used as a functional and not as a structural material. So in this context ⇑ Corresponding author. Address. Karlsruhe Institute of Technology (KIT) – Campus Nord Institute for Applied Materials (IAM), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Tel.: +49 (0)721 608 23894. E-mail address: [email protected] (J. Reiser). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.12.003
the question of how to make tungsten ductile and how to shift the brittle-to-ductile transition (BDT) to lower temperatures arises. The attempts to ductilize tungsten can be categorized into three approaches: (i) the synthesis of a tungsten solid solution [5–8], (ii) the synthesis of a material with an ultra-fine-grained (UFG) microstructure realized by either severe plastic deformation (SPD) [9,10] or mechanical alloying [11,12], and (iii) the synthesis of a tungsten composite. Among the tungsten composites it is possible to distinguish between composites reinforced by (i) particles [13], (ii) short fibers [14], (iii) uniaxial long fibers [15,16], or (iv) a tungsten laminate. The approach that we assess in our work is the ductilization of tungsten by synthesizing a tungsten laminate. The idea is as follows. Tungsten foil is ductile; it can be bent plastically even at room temperature (RT) (see Fig. 1). Through the assembly and appropriate joining of several layers of tungsten foil it is possible to extend the ductile properties of the foil to the bulk [17,18]. The reason for and sources of the extraordinary ductility of tungsten foil are widely discussed. One reason might be the foil effect, according to which the dislocations that move to the free surface are annihilated. In our first paper within the tungsten foil project we showed that it is possible to extend the ductile properties of a tungsten foil to the bulk [17]. Compared to pure tungsten plate material we
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Fig. 2. Optical micrograph of the cross-section of 100-lm-thick tungsten foil in the as-received condition (top) and in the recrystallized condition (bottom). The rolling direction is from left to right.
Fig. 1. Preliminary bending tests with tungsten foil with a thickness of 0.1 mm. The tests were performed at room temperature. The rolling direction (0°) is from bottom to top. It can be clearly seen that tungsten foil is ductile at room temperature.
succeed in shifting the BDTT of about 300 °C (semi-finished products in the as-received condition) or 500 °C (semi-finished products in the recrystallized condition, 1 h at 2000 °C), respectively, to lower temperatures. Furthermore, by rolling up and joining a tungsten foil, a tungsten laminate pipe can be realized. These laminate pipes have extraordinary mechanical properties and might therefore be an interesting candidate for pressurized and safety relevant cooling pipes for a DEMO divertor. Our second paper focused on the microstructural analysis of tungsten foil in order to give a broad basis for the discussion of the mechanical properties as well as the extraordinary ductility of tungsten foil (e.g. the foil effect) [18]. In this paper the special mechanical properties of tungsten foil are assessed by tensile tests. Within this assessment we varied the thickness of the tungsten foil (0.1 mm, 0.2 mm, 0.3 mm, 0.5 mm, 1 mm), the orientation with respect to the rolling direction (0°, 45°, 90°), and the test temperature (RT, 600 °C) as well as the condition of the foil (as-received, recrystallized). The following chapters will address these questions: 1. How can we explain why the tensile test properties in the rolling direction (0°) and perpendicular to it (90°) are the same? 2. Is there a reason why the most ductile material behavior can be obtained on a tensile test sample tested in the 45° direction? 3. Can we find proof that the foil effect really takes place during a tensile test?
2. Material and microstructure The material chosen for the tensile tests is rolled 99.97% pure tungsten foil with a thickness of 0.1 mm. For the purpose of comparison, 99.97% pure tungsten foil/plate with thicknesses of 0.2 mm, 0.3 mm, 0.5 mm, and 1 mm as well as rolled 99.97% pure molybdenum foil with a thickness of 0.1 mm were also tested. These commercially available materials were produced by Plansee Metall GmbH, Reutte/Austria, in a powder metallurgical route. After sintering, the plate/foil was hot- and cold-rolled with a high degree of deformation. Details about the guaranteed purity of these materials can be found elsewhere [1]. The main focus in this work is the assessment of tungsten foil with a thickness of 100 lm. The microstructure of 100-lm-thick tungsten foil in the as-received condition has grains with dimensions of 0.5 lm 3 lm 15 lm (Fig. 2, top). This means that there are about 200 grains over the thickness of the foil. The texture of the foil is {1 0 0}h0 1 1i. This texture is established during cold work and is
the common texture of all cold-rolled body-centered cubic (bcc) metals [19] (see also molybdenum foil [20]). The microstructure of the tungsten foil consists of a subgrain structure whereas the grains/subgrains are in general nearly free of dislocations. However there are grains with a dislocation network but these are the exception. According to the hardness profile the recrystallization starts at 1100 °C (annealing for 1 h). The microstructure of 100-lm-thick tungsten foil in the recrystallized condition (e.g. 1 h at 2700 °C in hydrogen) consists of grains with dimensions of 100 lm 100 lm 100 lm (Fig. 2, bottom). This means that there is only one grain over the thickness of the foil. The texture of the foil is still {1 0 0}h0 1 1i. This means that there is no creation of a new nucleus and therefore no recrystallization. The same material behavior has also been observed in molybdenum foil [20]. The grains now have defined grain boundaries and the dislocation density is very low. More details about the microstructure of 100-lm tungsten foil can be found in our previous paper [18]. 3. Experimental: Setup of the tensile tests All tensile tests were performed in an electro-mechanical test device (Zwick100). This test device was modified and combined with a furnace (RT to 1400 °C) and a vacuum chamber (operation pressure: 106 mbar). The experiments were carried out using subsized specimens (gauge length: 7 mm; width: 2 mm). The test specimens were fabricated by electrical discharge machining (EDM) and all tests were displacement controlled with a strain rate of 0.1 mm/min. Tensile tests on 100-lm tungsten foil were performed at room temperature and 600 °C in three directions: in the rolling direction (0°), perpendicular to the rolling direction (90°), and at an angle of 45° to the rolling direction. The tungsten foil was either in the asreceived or in the recrystallized condition (1 h at 2000 °C). For each orientation, temperature, and foil condition, at least three samples were tested, and all samples failed between the gauge lengths. Fig. 3 shows what the tensile test setup looks like. 4. Results The first section of this chapter deals with tensile tests at RT on tungsten foil and plate material in the as-received condition. The following two sections will then show the results for 100-lm tungsten foil in the as-received and recrystallized conditions (1 h at 2000 °C) at 600 °C. Finally this chapter closes with a comparison of the results of tensile tests on tungsten foil and molybdenum foil. 4.1. Tensile tests at RT, foil/plate, as-received In a bending test at RT, 100-lm-thick tungsten foil shows ductile material behavior (see Fig. 1). However if a bending force is
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Fig. 3. Setup of the tensile tests performed in a vacuum furnace. The gauge length is 7 mm and the width of the test sample is 2 mm. All tests were displacement controlled with a strain rate of 0.1 mm/min.
applied on a tungsten laminate, this bending force can be considered as a sequence of tensile tests. So the question arises of whether tungsten foil shows ductile material behavior not only in a bending test but also in a tensile test at RT. Furthermore it would be interesting to see whether tungsten foil is anisotropic and how the anisotropy depends on the thickness of the foil/plate. To assess these questions tensile tests were performed on 100-lm tungsten foil in three directions: in the rolling direction (0°), perpendicular to the rolling direction (90°), and at an angle of 45°. Furthermore, tensile tests were performed on foil/plate with thicknesses of 0.2 mm, 0.3 mm, 0.5 mm, and 1 mm at 0° and 90° respectively. In the tensile test at RT, 100-lm tungsten foil in the as-received condition is ductile (see Fig. 4). As expected the Young’s modulus is isotropic [21] but the plastic material behavior is anisotropic. Plastic strain of about 1.5% can be measured when the test is performed in the rolling direction (0°) and about 4% at an angle of 45°. Perpendicular to the rolling direction (90°), almost no ductile material behavior was observed. The tensile strength is also anisotropic and rather high. In as well as perpendicular to the rolling direction the tensile strength is about 2000 MPa, while at an angle of 45° it is about 10% less. Fig. 5 shows the fractured surfaces of the tensile test samples. All samples that show plastic material behavior in a tensile test
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(0° and 45° orientations) show similar fracture surfaces (Fig. 5, left and middle) that can be described as a pronounced delamination. The tensile test samples tested in the 90° direction show the classical fracture surface of a brittle fracture (Fig. 5, right). The sample orientated at 45° shows the largest plastic strain of all tested orientations. This material behavior might be attributed to the texture of the foil in {1 0 0}h0 1 1i. This texture allows the main slip direction of bcc metals, the h1 1 1i direction, to appear at an angle of 45° to the direction of tension. It is well known from Mohr’s circle of stress that the maximum shear stress appears at an angle of 45°. This means that for a tensile test sample orientated in the 45° direction, the preferred slip direction of bcc metals and the direction where the maximum shear appears are congruent. This is why the plastic formability of tensile test samples orientated at 45° to the rolling direction of the foil is favoured. The texture of the 100-lm tungsten foil in {1 0 0}h0 1 1i is symmetric for a rotation of 90° around an axis that is perpendicular to the surface of the foil (e.g. x–y is the plane of the foil ? texture of {1 0 0}h0 1 1i is symmetrical for a 90° rotation around the z-axis). Accordingly the mechanical properties in the rolling direction (0°) and perpendicular to the rolling direction (90°) should be the same or at least in the same range. As this is not the case, beside the texture of {1 0 0}h0 1 1i the grain size of 0.5 lm 3 lm 15 lm also has to be included in the discussion about the mechanical properties of tungsten foil. According to the grain shape a tensile test sample orientated in the 90° direction has more grain boundaries that are perpendicular to the direction of tension than samples in the 0° orientation. It can be concluded that the anisotropic plastic material behavior of tungsten foil can be attributed on one hand to the texture of the foil and on the other to the grain shape. The classical picture of the influence of cold work like rolling on the tensile test properties can easily be explained for the example of steel (see Orowan–Taylor hardening). One way to harden steel is by cold working. The idea is that during cold-work hardening like rolling the dislocation density increases. This results in an increase of the tensile strength but also in a decrease of the ductility. The microstructural reason for the decrease in ductility can be found in the dislocation mobility, which is constrained due to the high dislocation density. The next paragraph will show that this classical picture of material behavior does not apply to tungsten foil and plate material. Fig. 6 shows the influence of cold work on tungsten foil and plate material. Tensile tests were performed at RT on tungsten foil/plate with thicknesses of 0.1 mm, 0.2 mm, 0.3 mm, 0.5 mm, and 1 mm in the rolling direction (0°) as well as perpendicular to the rolling direction (90°). As expected, the tensile strength increases with increasing degree of deformation and thus with decreasing thickness of the foil/plate. The tensile strength of 1mm tungsten plate material is about 1400 MPa whereas the tensile strength of 100-lm tungsten foil is about 2000 MPa. In contrast to the material behavior of cold worked steel, the ductility does not decrease with increasing degree of deformation or in other words with decreasing thickness of the foil/plate. The ductile material behavior can be measured independently of the thickness of the foil/plate. What all tensile tests on foil and plate material of different thicknesses have in common is that ductile material behavior can only be measured in the rolling direction (0°). Perpendicular to the rolling direction (90°) the material behavior is always brittle. 4.2. Tensile tests at 600 °C, foil, as-received
Fig. 4. This diagram shows the results of the tensile test on 100-lm-thick tungsten foil in the as-received condition at RT (see Fig. 3). Even at RT plastic material behavior can be observed. However the plasticity is anisotropic.
Tensile tests were performed not only at RT but also at 600 °C. Fig. 7 shows the stress–strain curve of 100-lm tungsten foil in the as-received condition tested at 600 °C. As expected, due to the 90° rotation symmetry of the texture of the foil, {1 0 0}h0 1 1i,
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Fig. 5. Fractured surfaces of tensile test samples of 100-lm-thick tungsten foil in the as-received condition tested at RT. Left to right: 0°, 45°, and 90° orientations.
Fig. 6. Stress–strain curves of tensile tests performed on tungsten foil/plate material in the as-received condition tested at RT. As expected the tensile strength increases with decreasing thickness of the foil. All tests have in common that ductile material behavior can only be measured in the rolling direction (0°).
4.3. Tensile tests at 600 °C, foil, recrystallized
Fig. 7. Stress–strain curves of tensile tests performed on 100-lm-thick tungsten foil in the as-received condition at 600 °C. As expected the plastic material behavior in the rolling direction (0°) and perpendicular to the rolling direction (90°) is congruent.
the plastic material behavior in the rolling direction (0°) and perpendicular to the rolling direction (90°) is congruent. The influence of the grain boundaries and the grain shape can no longer be observed at 600 °C. In common with the results for the tungsten foil tested at RT (see previous section) the most ductile material behavior, combined with a decrease of the tensile strength of about 10%, can be measured on a tensile test sample orientated in the 45° direction (for the argumentation, see previous section). It can be concluded that at 600 °C the anisotropic material behavior can easily be explained by the texture of the foil. At this test temperature the grain shape and the grain boundaries no longer influence the mechanical properties. Furthermore it can be determined that none of the stress–strain curves show any necking, which means that the uniform elongation (elongation without necking) is congruent to the total plastic strain.
Up to now the tensile tests have been performed only on tungsten foil in the as-received condition. In a new test series presented in this section, annealed tungsten foil will be assessed. The annealing conditions were 1 h at 2000 °C and 1 h at 2700 °C in hydrogen, respectively. As can be seen from Fig. 8, the tungsten foil in the recrystallized condition shows extreme plastic strain in a tensile test at 600 °C. Plastic strain of 25–45% can be measured. Comparing these results with the results obtained for tungsten foil in the as-received condition (also tested at 600 °C), the plastic strain increases by a factor of ten together with a decrease of the tensile strength by a factor of five. The decrease in the tensile strength of recrystallized tungsten was expected as it is well known that the tensile strength decreases when (i) the dislocation density decreases, (ii) the amount of grain boundaries decreases, and (iii) the grain size increases (see also Hall–Petch [22,23]). What is astonishing is the very large plastic strain of up to 45%. The reason for this incredible material behavior might be that the dislocations can move out of the foil surface: this is known as dislocation annihilation or the ‘foil effect’. Assessing the surface of the test sample after the test, it can be seen that the surface is now rough over the whole gauge length. This roughness is a hint that dislocation activity takes place at the surface and that the dislocation annihilation takes place over the whole gauge length. Fig. 9 shows a comparison of tensile test samples tested at 600 °C in the as-received (Fig. 9, left) and recrystallized condition (Fig. 9, right). The sample surfaces are now assessed in order to analyze the slip traces. Fig. 10 (top) shows an election microscopy assessment of the frontal surface of the foil after the tensile test. The surface shows slip traces that are independent of grain boundaries. It is well known from the work of Seeger [24] that the visible slip traces of bcc metals on the frontal surface are wavelike and cannot be explicitly attributed to the preferred slip planes of bcc metals, which are {1 1 0}, {1 1 2}, and {1 2 3} respectively. The reason for
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Fig. 10. Surfaces of 100-lm-thick tungsten foil tested at 600 °C in the recrystallized condition. Top: as expected the slip traces on the frontal surface are wavelike, which is a hint of frequent cross-slipping during plastic deformation. Bottom: the slip traces on the edge of the test sample can be identified as the h1 1 1i direction. Fig. 8. Results of tensile test on 100-lm-thick tungsten foil in the recrystallized condition at 600 °C. Top: foil was annealed for 1 h at 2000 °C. Bottom: foil was annealed for 1 h at 2700 °C. Extreme plastic strain of up to 40% combined with serrated flow can be observed.
Fig. 9. Picture of tensile test samples of 100-lm-thick tungsten foil tested at 600 °C. Left: foil in the as-received condition. Right: foil in the recrystallized condition (1 h/ 2000 °C). Even by the naked eye, the incredible plastic strain of the recrystallized foil as well as the rough surface can be seen. The rough surface is the evidence that dislocations moved out of the foil surface – the ‘foil effect’.
the wavelike slip traces is that the slip panes of bcc metals, {1 0 0}, {1 1 2}, and {1 2 3}, respectively, are nearly equally preferred, which leads to frequent cross-slipping during plastic deformation. Fig. 10
(bottom) shows the edge of a tested tensile test sample. Unfortunately the surface of the sample was damaged during sample manufacture, during EDM. Nevertheless slip traces are visible in the preferred slip direction of bcc metals, the h1 1 1i direction. Again this result also conforms with the results of Seeger, who found wavelike slip traces on the frontal surface as well as slip lines in the h1 1 1i direction on the edge of the sample [24]. Comparing the fracture surfaces of tensile test samples in the as-received and recrystallized conditions (see Fig. 11), it can be observed that delamination only occurs on samples in the as-received condition (Fig. 11, left). This goes for samples tested at RT as well as for those tested at 600 °C. Recrystallized tensile test samples show a fracture surface which is pointed and sharp (Fig. 11, right). Up to this point in this paper only tungsten foil has been investigated. In the next section the results of tensile tests on molybdenum foil will be presented and compared with the results for tungsten foil. The aim of this comparison is to see whether or not it is possible to transfer the results of mechanical tests (e.g. Charpy tests) on tungsten to molybdenum and vice versa.
4.4. Comparison with molybdenum foil In this section the results of tensile tests on 100-lm-thick molybdenum foil in the as-received condition carried out at RT and in the 0°, 45°, and 90° directions will be presented. It was shown elsewhere that the texture of molybdenum foil is congruent to that of tungsten foil, meaning that it is {1 0 0}h0 1 1i. As discussed previously this texture is symmetric for a rotation of 90°. According to this fact the mechanical properties in the
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Fig. 11. Fracture surfaces of tested tensile test samples of 100-lm-thick tungsten foil in 0° orientation. Left: foil in the as-received condition tested at RT. Right: foil in the recrystallized condition tested at 600 °C.
rolling direction (0°) and perpendicular to the rolling direction (90°) must be congruent. And this is exactly what can be measured in the tensile test on molybdenum foil at RT. In the rolling direction (0°) as well as perpendicular to the rolling direction (90°) there is about 4% plastic strain and a tensile strength of about 700 MPa (see Fig. 12). No influence of the grain shape or the different amount of grain boundaries perpendicular to the tensile direction can be observed. It can be concluded that the characteristics of the mechanical properties measured in a tensile test of molybdenum foil (as-received) at RT are congruent to those of tungsten foil (as-received) at 600 °C: (i) congruent curves are found in the rolling direction as well as perpendicular to it, (ii) the maximum plastic strain is measured on the sample at 45° orientation combined with a decrease in the tensile strength of about 10%, and (iii) there is no influence of the grain shape or the amount of grain boundaries that are perpendicular to the direction of tension. 5. Discussion This chapter comprises three parts. In the first, a definition of ‘structural tungsten material’ is sought. The second part deals with the difference in the material properties like ductility and fracture toughness. The chapter closes with a discussion on the plastic material behavior in the tensile tests performed in this work. 5.1. Attempt to define ‘structural tungsten material’ Pressurized components require a structural material, but the requirements for a ‘structural tungsten material’ are not defined
Fig. 12. Stress–strain curves of tensile tests performed on 100-lm-thick molybdenum foil in the as-received condition, tested at RT. As expected due to the 90° rotation symmetry of the {1 0 0}h0 1 1i texture, the plastic material behavior in the rolling direction (0°) and perpendicular to the rolling direction (90°) is congruent.
in any standard. So it is not that easy to define a ‘structural tungsten material’. However there are European standards that define the requirements for structural steels (see EN 10216-5, EN 12952-2, EN 13445-2, and EN 13480-2). The requirements mentioned in these standards are: (i) a Charpy energy of at least 27 J, measured with ISO-V test samples according to DIN EN ISO 1481 (10 10 55 mm3, 2 mm notch) at the lowest operation temperature, at least at room temperature and (ii) an ultimate strain of at least 14% measured in a tensile test according to EN ISO 6892-1. Further requirements are (iii) the capability to be formed (e.g. deep drawn) and (iv) an assessment of the feasibility of joining to other components, for example, welding pipe to pipe. What is not requested in these standards is the fracture toughness, KIC; however a fracture toughness of about 20 MPa m1/2 can be considered high. The standards mentioned above are only valid for steels. For other materials like nickel-based super-alloys or tungsten materials these standards are not relevant. In these cases the material can be certificated by either a VdTÜV material data sheet or a single case specification. 5.2. Ductility versus fracture toughness Ductility and fracture toughness must be rigidly separated as the two properties cannot be compared. Ductility is defined as the property of a material to irreversible plastic deformation whereas fracture toughness is the property of a material to withstand crack propagation. The ductility can be measured by mechanical tests such as tensile tests and is expressed as plastic strain [–]. Sources of plastic deformation of a material are either (i) the movement of edge and/or screw dislocations, (ii) twinning, or (iii) nano-crystalline effects like grain rotation, grain boundary sliding, grain boundary dislocation interaction, or grain rotation and alignment [25]. The fracture toughness like KIC has to be measured by fracture mechanical tests according to ASTM E399 and is expressed in MPa m1/2 [26]. As measuring of the fracture toughness is sometimes challenging and expensive it is also possible to measure the dynamic fracture toughness. This can be done by Charpy impact tests according to the EU standards DIN EN ISO 148-1 and 14556:2006-10. Using these tests, the dissipated energy in [J] is measured. Charpy impact properties are often measured on ISO-V samples with dimensions of 10 10 55 mm3 and a 2 mm notch. In some cases smaller KLST samples with dimensions of 3 4 27 mm3 and a 1 mm notch are used. Unfortunately it is not possible to transfer the results for the amount of dissipated energy from ISO-V samples to KLST samples easily. This means that it is not possible to predict how much energy will be dissipated by a KLST sample if an ISO-V sample made of the same material dissipates 27 J. Furthermore it is also not possible to translate the amount of dissipated energy measured by Charpy to a fracture toughness value. What
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the Charpy impact test and the fracture toughness tests have in common is that with both test methods the BDTT can be determined as the BDT occurs together with either an increase in the fracture toughness or an increase in the dissipated energy. In addition it must be mentioned that a material that shows ductile behavior in a tensile test must not have high fracture toughness. Considering, for example, the results of the tensile test at RT performed on 0.1 mm, 0.2 mm, 0.3 mm, 0.5 mm, and 1 mm foil/ plate material, it can be seen that for each sample the plastic strain measured in the rolling direction (0°) is in the same range (Fig. 6). However this does not mean that the fracture toughness of these materials is in the same range. For example the fracture toughness of 0.1 mm tungsten foil is expected to be much higher than the fracture toughness of 1 mm plate material even if the plastic strain measured in the tensile test is nearly the same. This is for example the reason why tungsten materials produced by powder injection molding (PIM) show excellent ductile behavior in a tensile test whereas Charpy impact properties are disappointing. 5.3. Discussion on tensile test properties Due to the symmetry of the {1 0 0}h0 1 1i texture of the 100-lm tungsten foil, the mechanical properties in the rolling direction (0°) and perpendicular to it (90°) must be in the same range. In samples oriented in the 45° direction, the main slip direction of bcc metals, the h1 1 1i direction, appears at an angle of 45° (see Fig. 13). According to Mohr’s circle of stress the maximum shear stress appears at an angle of 45°. So in a tensile test sample oriented in the 45° direction, (i) the direction with the maximum shear stress and (ii) the preferred slip direction are congruent. This is why the highest plastic strain can be obtained in samples oriented in the 45° direction. Tensile tests on 100-lm tungsten foil in the as-received condition tested at 600 °C (Fig. 7) and tensile tests on 100-lm molybdenum foil in the as-received condition tested at RT (Fig. 12) confirm this idea: in the rolling direction (0°) and perpendicular to the rolling direction (90°) the mechanical properties are nearly the same, and the highest ductile deformation can be measured on samples oriented in the 45° direction. The situation appears different for 100lm tungsten foil in the as-received condition tested at RT. Again, the highest plastic strain can be measured on a sample oriented in the 45° direction. But now the stress–strain curves of samples oriented at 0° and 90° are different. In the rolling direction (0°), plastic strain can be measured, but perpendicular to the rolling direction (90°) no plastic strain can be measured and the fracture surface
Fig. 13. This figure shows a top view of a tungsten foil (the x-axis is the rolling direction; the x–y plane is the flat surface of the foil). The texture of the tungsten foil, {1 0 0}h0 1 1i, is symmetric for a rotation of 90° around the z-axis, which is the out-of-plane axis. The mechanical properties of tungsten foil can be explained considering (i) the {1 0 0}h0 1 1i texture of the tungsten foil, (ii) the anisotropic grain shape (0.5 lm 3 lm 15 lm), and (iii) the preferred slip direction of bcc metals, the h1 1 1i direction, as well as the (iv) preferred cleavage plane of tungsten at room temperature, the {1 0 0} plane.
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shows a classic brittle fracture (see Figs. 4 and 5). This material behavior can be explained by the influence of the grain shape. Due to the grain shape, a 90° oriented sample has more grain boundaries perpendicular to the direction of tension than a sample oriented in the rolling direction (0°). So the weak grain boundaries of tungsten seem to affect this behavior. It can be concluded that beside the {1 0 0}h0 1 1i texture at low test temperatures the grain shape, 0.5 lm 3 lm 15 lm, has to be considered. A comparison of the results of tensile tests at 600 °C can be seen in Fig. 14. The top part of the figure shows as-received 100-lm tungsten foil; the middle, recrystallized 100-lm tungsten foil; and the bottom, as-received 1-mm tungsten plate. The stress-stain curves are profoundly different. The plate material is isotropic and shows a pronounced necking, whereas the foil does not. Why? Furthermore the curve of recrystallized tungsten foil shows a zigzag shape. Can this zigzag shape be attributed to an error in measurement or can this shape be explained by the foil effect? Let us start with the question of why plate material shows necking whereas foil material does not. A first attempt to explain this behavior by continuum mechanics does not work. Assuming that in both cases the clamping of the sample is proper, until necking starts there is a uniaxial stress state in both samples. Also, discussion about the state of plain stress or the state of plain strain does not help any further in this case. Another attempt to answer this question may be made using fracture mechanics. According to this idea the ratio of sample surface to volume of the sample is different for the foil and plate test samples. For a foil this ratio is larger, which means that surface effects like defects caused by sample preparation (EDM) have a stronger impact. I prefer another explanation: Necking, or in other words localization of deformation, as can be seen at the bottom of Fig. 14, is always attributed to local inhomogeneities like grain orientation or dislocation density gradients. Tungsten foil shows a plateau and no necking (Fig. 14, top). This is an indication that the material is homogeneous and sharp textured. It is an indication that the material has been put to its extremes and that its potential is completely utilized. The homogeneous distribution of the plastic strain might also be an indication of a high strain rate dependence of the material (see super plastic material behavior: deformation is localized, stress increases, strain rate increases, tensile strength increases, localization is stopped). However this mechanism requires pronounced dislocation diffusion. In my opinion the homogeneous distribution of the plastic strain is more likely to be attributed to a homogeneous distribution of the material properties (texture, dislocation density) than to high strain rate dependence. Furthermore at the discussed test temperatures dislocation diffusion (e.g. grain boundary sliding) is not very likely to be the dominating deformation process. Discussion on the results of tensile tests performed on recrystallized tungsten foil is very interesting (see Fig. 14, middle). What is curious at first sight is the extreme plastic strain of about 30%. Comparing these results with the results of tensile tests performed on tungsten single crystals it can be determined that the tensile strength and the plastic strain are in the same range [21]. This is not surprising as it was shown that a quasi-single crystal can be synthesized by annealing a foil [18,27]. A further question addresses the zigzag shape of the stress– strain curve. Here it is worth looking at the results of stress–strain curves of pre-deformed molybdenum single crystals. In these experiments a zigzag curve was also measured and was explained by the formation of dislocation cell structures [28]. The zigzag curve can be attributed to serrated flow. Serrated flow is well known in precipitation hardened (PH) aluminum as well as in
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condition has only one grain over the thickness of the foil, and size effects have to be considered [29–32]. If the movement of a dislocation is released it can penetrate through the whole foil without being arrested or restrained. The dislocation will continue moving until it finally reaches the surface of the foil, where it is annihilated: this is the foil effect. In my opinion, the extremely plastic and homogeneous deformation, combined with the rough surface of the foil after the test, is proof that dislocations have exited the foil, which annihilates them; this is the proof of the foil effect. By measuring the distribution range of the curve of serrated flow, r (20 MPa) (Fig. 14, middle), and using the cross-section area, A, of the tensile test sample (0.2 mm2), it is possible to calculate a force, F, for the dislocation activity according to
F ¼ r A ¼ 20
N 0:2 mm2 ¼ 4 N: mm2
ð1Þ
So a force of 4 N plays an important role in the discussion about the serrated flow of tungsten foil. Furthermore Fig. 14 gives hints on the forming capability of tungsten foil. It was shown in our previous work that 1-mm tungsten plate material can be deep drawn into a cap [33]. Looking at the stress–strain curves of the tested tungsten foil in Fig. 14 (top and middle) it is likely that a tungsten laminate made of recrystallized foil could be deep drawn more easily than a tungsten laminate made of foil in the as-received condition. Looking at the fracture surface of the tested 100-lm tungsten foil in the as-received condition tested at 600 °C (see Fig. 9 left) it can be observed that the fracture looks quite straight, as though the fracture follows a preferred cleavage plane. The same characteristic fracture surfaces were obtained on three-point-bending test samples [18,20]. This special kind of fracture that seems to penetrate along preferred cleavage planes might be explained by the preferred cleavage plane of tungsten at RT, which is the {1 0 0} plane [34]. This means that the texture of tungsten foil, {1 0 0}h0 1 1i, and the preferred cleavage plane of tungsten, {1 0 0}, form an angle of 45° with the rolling direction (see Fig. 13). After the analyses and the mechanical assessment of 100-lm tungsten foil in different conditions I have come to the preliminary conclusion that the ductility of tungsten foil is caused by (i) the high amount of mobile edge dislocation, (ii) the small grain size, which may lead to (ii-a) multiple slip at the grain boundaries [35] or (ii-b) nano-crystalline effects such as grain boundary rotation [25,35], and (iii) the foil effect – the dislocation annihilation on the free surface. In particular, the extreme plastic deformability of tungsten foil in the recrystallized condition seems to benefit from the foil effect. 6. Summary
Fig. 14. Comparison of stress–strain curves of tensile tests on tungsten at 600 °C. Top: results for 100-lm-thick tungsten foil in the as-received condition. Middle: results for 100-lm-thick tungsten foil in the recrystallized condition (1 h at 2000 °C). Bottom: results for 1-mm-thick tungsten plate material in the as-received condition.
steels in the context of strain ageing. During strain ageing a dislocation line is arrested on a carbon cloud. With increasing stress the dislocation line breaks away from the cloud. As a result of the arresting and breaking away from carbon clouds, serrated flow can be measured (for more details on serrated flow see also the Portevin-Le Chatelier effect). As the serrated flow measured on the tungsten foil cannot be attributed to precipitates or to carbon clouds one has to find another explanation. In my opinion the situation appears to be as follows. A tungsten foil in the recrystallized
The main question of the tungsten laminate project is to assess whether the ductile properties of a tungsten foil can be extended to the bulk. By synthesizing a tungsten laminate using the right combination of interlayer and joining technology it is possible to produce a tungsten laminate which shows improved properties in a Charpy impact test compared to pure tungsten plate material. So the main question can be answered with ‘Yes’. The microstructure of 100-lm tungsten foil in the as-received condition is as follows: Grain size and grain shape: 0.5 lm 3 lm 15 lm. This means that there are about 200 grains over the thickness of the foil. Texture: {1 0 0}h0 1 1i. This texture is established during cold work and is the saturation condition. This texture does not change on further cold working (see 25-lm tungsten foil). The same texture can also be found in molybdenum foil [20].
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In the grain: In general the grains are nearly free of dislocations. However there are grains with a dislocation network; these grains are the exception. Recrystallization: According to the distribution of the hardness, recrystallization starts at 1100 °C (annealing for 1 h). The same goes for tungsten foils with thicknesses of 25 lm and 200 lm.
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In the tensile test with recrystallized tungsten foil, the foil effect can be observed. The wavelike slip traces on the flat surface as well as the slip traces on the edges in the h1 1 1i direction correspond exactly with the results obtained by Seeger [24]. On the ductility of tungsten foil:
When 100-lm tungsten foil was exposed to extreme annealing (1 h at 2700 °C) the microstructure appeared to be as follows: Grain size and grain shape: 100 lm 100 lm 100 lm. This means that there is only one grain over the thickness of the foil. Texture: {1 0 0}h0 1 1i. There is no real recrystallization, as neither the creation of a nucleus nor the formation of a new and individually textured grain occurs. Moreover, the texture established during cold work stays the same and becomes even more pronounced. The same material behavior can also be observed for molybdenum foil [20]. In the grain: The grain boundaries are sharp and the dislocation density is low. The anisotropic mechanical properties of 100-lm tungsten foil can be explained by: The texture ({1 0 0}h0 1 1i), the anisotropic grain shape (0.5 lm 3 lm 15 lm), the preferred slip direction of bcc metals (h1 1 1i), and the preferred cleavage plane of tungsten ({1 0 0}). The results of three-point-bending tests on 100-lm tungsten foil can be summarized as follows: Tungsten foil in the as-received condition can be bent plastically even at RT. The ductility of tungsten foil is a thermally activated process. The BDTT is between 196 °C and RT (for foil in the as-received condition). In a three-point bending test at RT a foil annealed for 1 h at 900 °C is ductile while a foil annealed for 1 h at 1000 °C is brittle. Tungsten foil tested at RT fractures either along the preferred cleavage plane, {1 0 0}, or along the weak grain boundaries. Samples annealed up to 1 h at 1300 °C cleave along the {1 0 0} plane and the type of fracture is transgranular. At higher annealing temperatures the intercrystalline fracture becomes dominant. The results of tensile tests on 100-lm tungsten foil can be summarized as follows: Tungsten foil in the as-received condition is ductile not only in a bending test at RT but also in a tensile test. Tungsten foil in the as-received condition, tested at 600 °C. Due to the texture of the foil the mechanical properties in the rolling direction (0°) and perpendicular to it (90°) are equal and the most ductile material behavior is measured on a sample oriented in the 45° direction. Tungsten foil in the as-received condition, tested at RT. The greatest plastic strain can be obtained from a sample oriented in the 45° direction, as expected. But now the stress–strain curves of samples tested in the rolling direction (0°) and perpendicular to it (90°) are different. This can be explained by the anisotropic grain shape. The tensile tests show that tungsten foil is homogeneous and its properties are put to the extremes.
Assumption: The ductility of tungsten foil is caused by (i) the high amount of mobile edge dislocation, (ii) the small grain size, leading to possible multiple slip at the grain boundaries [35] or nano-crystalline effects such as grain boundary rotation [25,35], and (iii) the foil effect – the dislocation annihilation on the free surface. 7. Conclusion Tungsten foil is an extraordinary semi-finished product with curious ductile material behavior. Discussing about the ductility of 100-lm-thick tungsten foil in the as-received condition as well as in the recrystallized condition size effects have to be considered. In the as-received condition the 100-lm-thick tungsten foil has very small grains, so nano-crystalline effects have to be considered, discussing about the sources and mechanism of the ductile material behavior. In the recrystallized condition (e.g. annealing for 1 h at 2000 °C) the 100-lm tungsten foil has only one grain over the thickness of the foil. In this case the foil effect becomes relevant. In any case, tungsten foil is an excellent semi-finished product for the synthesis of tungsten laminates, especially tungsten laminate pipes for structural applications. Acknowledgements This work, supported by the European Communities, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. The authors are grateful to our colleagues from Plansee Metall GmbH, the University of Oxford, Department of Materials, and the Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM), for their support and valuable contributions. Special thanks go to Dr. W. Knabl (Plansee SE) and his team for their support and valuable discussion as well as to Mrs. D. Exner (KIT, IAM) for her help with the electron microscopy. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15] [16] [17]
www.plansee.com, 2012. M.S. Tillack et al., Fusion Eng. Des. 86 (2011) 71. P. Norajitra et al., Fusion Eng. Des. 82 (2007) 2740. J. Reiser, M. Rieth, Fusion Eng. Des. 87 (2012) 718. S. Wurster, B. Gludovatz, R. Pippan, J. Refract. Met. Hard Mater. 28 (2010) 692. L. Romaner, C. Ambrosch-Draxl, R. Pippan, Phys. Rev. Lett. 104 (2010) 195503. M. Rieth et al., Adv. Sci. Technol. 73 (2010) 11. M. Rieth et al., J. Nucl. Mater. 417 (2011) 463. M. Faleschini, H. Kreuzer, D. Kiener, R. Pippan, J. Nucl. Mater. 367–370 (2007) 800. M. Faleschini, W. Knabl, R. Pippan, Nano Eng. Mater. Struct. B 2T (15) (2006) 445. H. Kurishita et al., Mater. Sci. Eng. A 477 (2008) 162. H. Kurishita et al., J. Nucl. Mater. 398 (2010) 87. J. Hohe, P. Gumbsch, J. Nucl. Mater. 400 (2010) 218. V. Livramento, D. Nunes, J.B. Correia, P.A. Carvalho, R. Mateus, K. Hanada, N. Shohoji, H. Fernandes, C. Silva, E. Alves, Tungsten-tantalum composites for plasma facing components, in: Materials for Energy 2010, ENMAT2010, 4–8 July 2010, Karlsruhe, Germany. J. Du, T. Höschen, M. Rasinski, S. Wurster, W. Grosinger, J.-H. You, Compos. Sci. Technol. 70 (2010) 1482. J. Du, T. Höschen, M. Rasinski, J.-H. You, Mater. Sci. Eng. A 527 (2010) 1623. J. Reiser, M. Rieth, B. Dafferner, A. Hoffmann, J. Nucl. Mater. 423 (2012) 1.
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[18] J. Reiser, M. Rieth, B. Dafferner, A. Hoffmann, J. Nucl. Mater. 424 (2012) 197. [19] B. Hutchinson, Phil. Trans. R. Soc. Lond. A 1999 357 (2009) 1471. [20] J. Neges, B. Ortner, G. Leichtfried, H.P. Strüwe, Mater. Sci. Eng. A 196 (1995) 129. [21] E. Lassner, W.-D. Schubert, Tungsten: Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds, Kluwer Academic/Plenum Publishers, New York, 1999. [22] E.O. Hall, in: Proc. Phys. Soc. B 64 (1951) 747. [23] N.J. Petch, J. Iron Steel Inst. 174 (1953) 25. [24] B. Sestak, A. Seeger, Z. Metallkd 69 (1978) 195. [25] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427.
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ASTM E399-90 (1997), ASTM International, West Conshohocken PA, 2004. G.D. Rieck, Tungsten and Its Compounds, Pergamon Press, Oxford, 1967. p. 58. Ch. Ritches, A. Luft, D. Schulze, Kristall Technik 13 (1978) 791. J.S. Stölken, A.G. Evans, Acta Mater. 46 (14) (1998) 5109. A.G. Evans, J.W. Hutchinson, Acta Mater. 57 (2009) 1675. A.J. Bushby, D.J. Dunstan, Philos. Mag. (2010) 1. C.A. Colkert, E.T. Lilleodden, Philos. Mag. 86 (2006) 5567. J. Reiser, M. Rieth, B. Dafferner, S. Baumgärtner, R. Ziegler, A. Hoffmann, Fusion Eng. Des. 86 (2012) 2949. [34] P. Gumbsch, J. Nucl. Mater. 323 (2003) 304. [35] J. Koike, Mater. Sci. Forum 449–452 (2004) 665.
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Tungsten foil laminate for structural divertor applications – Tensile test properties of tungsten foil Jens Reiser a,⇑, Michael Rieth a, Anton Möslang a, Bernhard Dafferner a, Andreas Hoffmann b, Xiaoou Yi c, D.E.J. Armstrong c a b c
Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM-AWP), Germany PLANSEE SE, Reutte, Austria University of Oxford, Department of Materials, United Kingdom
a r t i c l e
i n f o
Article history: Received 5 November 2012 Accepted 3 December 2012 Available online 8 December 2012
a b s t r a c t This paper is the third part of our series on tungsten foil laminates. Within the tungsten laminate project we have succeeded in ductilizing tungsten by synthesizing a tungsten laminate made of tungsten foil, which is ductile. By assembling and joining several layers of tungsten foil, the ductile properties of the foil can be extended to the bulk. The aim of this paper is to present the results of tensile tests on 100-lm-thick tungsten foil in asreceived and recrystallized conditions (1 h at 2000 °C). The results show that the mechanical properties of tungsten foil are anisotropic and can be explained by considering (i) the texture of the tungsten foil of {1 0 0}h0 1 1i, (ii) the anisotropic grain shape (0.5 lm 3 lm 15 lm), (iii) the preferred slip direction of body-centered cubic (bcc) metals, the h1 1 1i direction, as well as the preferred cleavage plane of tungsten at room temperature, the {1 0 0} plane. Furthermore, the results give further hints for the sources and mechanism of the extraordinary ductility of tungsten foil such as the ‘foil effect’, which is the dislocation annihilation at the free surface. In particular, the ductile material behavior of tungsten foil in the recrystallized condition seems to benefit from the foil effect and this is how a plastic strain of about 30% in a tensile test at 600 °C can be explained. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction A divertor is a plasma facing, high heat flux component in a fusion reactor. The expected heat load for a divertor in a future fusion power reactor (DEMO) in a short-term scenario is expected to be about 10 MW/m2 with peaks of up to 20 MW/m2 due to plasma instabilities. As a divertor has to deal with such extreme heat loads it is quite obvious that tungsten, the material with the highest melting point of all metals (Tmelting = 3420 °C [1]), is considered as a candidate for several divertor applications. One application asks for a tungsten material that can be used for structural cooling pipes [2–4]. As these pipes are meant to be pressurized pipes (e.g. helium coolant, 600 °C, 100 bar) the tungsten material must fit the needs of a structural material. In this case the tungsten material must have a low brittle-to-ductile transition temperature (BDTT) as well as high fracture toughness (e.g. 20 MPa m1/2). Unfortunately tungsten is very brittle and until now has only been used as a functional and not as a structural material. So in this context ⇑ Corresponding author. Address. Karlsruhe Institute of Technology (KIT) – Campus Nord Institute for Applied Materials (IAM), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Tel.: +49 (0)721 608 23894. E-mail address: [email protected] (J. Reiser). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.12.003
the question of how to make tungsten ductile and how to shift the brittle-to-ductile transition (BDT) to lower temperatures arises. The attempts to ductilize tungsten can be categorized into three approaches: (i) the synthesis of a tungsten solid solution [5–8], (ii) the synthesis of a material with an ultra-fine-grained (UFG) microstructure realized by either severe plastic deformation (SPD) [9,10] or mechanical alloying [11,12], and (iii) the synthesis of a tungsten composite. Among the tungsten composites it is possible to distinguish between composites reinforced by (i) particles [13], (ii) short fibers [14], (iii) uniaxial long fibers [15,16], or (iv) a tungsten laminate. The approach that we assess in our work is the ductilization of tungsten by synthesizing a tungsten laminate. The idea is as follows. Tungsten foil is ductile; it can be bent plastically even at room temperature (RT) (see Fig. 1). Through the assembly and appropriate joining of several layers of tungsten foil it is possible to extend the ductile properties of the foil to the bulk [17,18]. The reason for and sources of the extraordinary ductility of tungsten foil are widely discussed. One reason might be the foil effect, according to which the dislocations that move to the free surface are annihilated. In our first paper within the tungsten foil project we showed that it is possible to extend the ductile properties of a tungsten foil to the bulk [17]. Compared to pure tungsten plate material we
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Fig. 2. Optical micrograph of the cross-section of 100-lm-thick tungsten foil in the as-received condition (top) and in the recrystallized condition (bottom). The rolling direction is from left to right.
Fig. 1. Preliminary bending tests with tungsten foil with a thickness of 0.1 mm. The tests were performed at room temperature. The rolling direction (0°) is from bottom to top. It can be clearly seen that tungsten foil is ductile at room temperature.
succeed in shifting the BDTT of about 300 °C (semi-finished products in the as-received condition) or 500 °C (semi-finished products in the recrystallized condition, 1 h at 2000 °C), respectively, to lower temperatures. Furthermore, by rolling up and joining a tungsten foil, a tungsten laminate pipe can be realized. These laminate pipes have extraordinary mechanical properties and might therefore be an interesting candidate for pressurized and safety relevant cooling pipes for a DEMO divertor. Our second paper focused on the microstructural analysis of tungsten foil in order to give a broad basis for the discussion of the mechanical properties as well as the extraordinary ductility of tungsten foil (e.g. the foil effect) [18]. In this paper the special mechanical properties of tungsten foil are assessed by tensile tests. Within this assessment we varied the thickness of the tungsten foil (0.1 mm, 0.2 mm, 0.3 mm, 0.5 mm, 1 mm), the orientation with respect to the rolling direction (0°, 45°, 90°), and the test temperature (RT, 600 °C) as well as the condition of the foil (as-received, recrystallized). The following chapters will address these questions: 1. How can we explain why the tensile test properties in the rolling direction (0°) and perpendicular to it (90°) are the same? 2. Is there a reason why the most ductile material behavior can be obtained on a tensile test sample tested in the 45° direction? 3. Can we find proof that the foil effect really takes place during a tensile test?
2. Material and microstructure The material chosen for the tensile tests is rolled 99.97% pure tungsten foil with a thickness of 0.1 mm. For the purpose of comparison, 99.97% pure tungsten foil/plate with thicknesses of 0.2 mm, 0.3 mm, 0.5 mm, and 1 mm as well as rolled 99.97% pure molybdenum foil with a thickness of 0.1 mm were also tested. These commercially available materials were produced by Plansee Metall GmbH, Reutte/Austria, in a powder metallurgical route. After sintering, the plate/foil was hot- and cold-rolled with a high degree of deformation. Details about the guaranteed purity of these materials can be found elsewhere [1]. The main focus in this work is the assessment of tungsten foil with a thickness of 100 lm. The microstructure of 100-lm-thick tungsten foil in the as-received condition has grains with dimensions of 0.5 lm 3 lm 15 lm (Fig. 2, top). This means that there are about 200 grains over the thickness of the foil. The texture of the foil is {1 0 0}h0 1 1i. This texture is established during cold work and is
the common texture of all cold-rolled body-centered cubic (bcc) metals [19] (see also molybdenum foil [20]). The microstructure of the tungsten foil consists of a subgrain structure whereas the grains/subgrains are in general nearly free of dislocations. However there are grains with a dislocation network but these are the exception. According to the hardness profile the recrystallization starts at 1100 °C (annealing for 1 h). The microstructure of 100-lm-thick tungsten foil in the recrystallized condition (e.g. 1 h at 2700 °C in hydrogen) consists of grains with dimensions of 100 lm 100 lm 100 lm (Fig. 2, bottom). This means that there is only one grain over the thickness of the foil. The texture of the foil is still {1 0 0}h0 1 1i. This means that there is no creation of a new nucleus and therefore no recrystallization. The same material behavior has also been observed in molybdenum foil [20]. The grains now have defined grain boundaries and the dislocation density is very low. More details about the microstructure of 100-lm tungsten foil can be found in our previous paper [18]. 3. Experimental: Setup of the tensile tests All tensile tests were performed in an electro-mechanical test device (Zwick100). This test device was modified and combined with a furnace (RT to 1400 °C) and a vacuum chamber (operation pressure: 106 mbar). The experiments were carried out using subsized specimens (gauge length: 7 mm; width: 2 mm). The test specimens were fabricated by electrical discharge machining (EDM) and all tests were displacement controlled with a strain rate of 0.1 mm/min. Tensile tests on 100-lm tungsten foil were performed at room temperature and 600 °C in three directions: in the rolling direction (0°), perpendicular to the rolling direction (90°), and at an angle of 45° to the rolling direction. The tungsten foil was either in the asreceived or in the recrystallized condition (1 h at 2000 °C). For each orientation, temperature, and foil condition, at least three samples were tested, and all samples failed between the gauge lengths. Fig. 3 shows what the tensile test setup looks like. 4. Results The first section of this chapter deals with tensile tests at RT on tungsten foil and plate material in the as-received condition. The following two sections will then show the results for 100-lm tungsten foil in the as-received and recrystallized conditions (1 h at 2000 °C) at 600 °C. Finally this chapter closes with a comparison of the results of tensile tests on tungsten foil and molybdenum foil. 4.1. Tensile tests at RT, foil/plate, as-received In a bending test at RT, 100-lm-thick tungsten foil shows ductile material behavior (see Fig. 1). However if a bending force is
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Fig. 3. Setup of the tensile tests performed in a vacuum furnace. The gauge length is 7 mm and the width of the test sample is 2 mm. All tests were displacement controlled with a strain rate of 0.1 mm/min.
applied on a tungsten laminate, this bending force can be considered as a sequence of tensile tests. So the question arises of whether tungsten foil shows ductile material behavior not only in a bending test but also in a tensile test at RT. Furthermore it would be interesting to see whether tungsten foil is anisotropic and how the anisotropy depends on the thickness of the foil/plate. To assess these questions tensile tests were performed on 100-lm tungsten foil in three directions: in the rolling direction (0°), perpendicular to the rolling direction (90°), and at an angle of 45°. Furthermore, tensile tests were performed on foil/plate with thicknesses of 0.2 mm, 0.3 mm, 0.5 mm, and 1 mm at 0° and 90° respectively. In the tensile test at RT, 100-lm tungsten foil in the as-received condition is ductile (see Fig. 4). As expected the Young’s modulus is isotropic [21] but the plastic material behavior is anisotropic. Plastic strain of about 1.5% can be measured when the test is performed in the rolling direction (0°) and about 4% at an angle of 45°. Perpendicular to the rolling direction (90°), almost no ductile material behavior was observed. The tensile strength is also anisotropic and rather high. In as well as perpendicular to the rolling direction the tensile strength is about 2000 MPa, while at an angle of 45° it is about 10% less. Fig. 5 shows the fractured surfaces of the tensile test samples. All samples that show plastic material behavior in a tensile test
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(0° and 45° orientations) show similar fracture surfaces (Fig. 5, left and middle) that can be described as a pronounced delamination. The tensile test samples tested in the 90° direction show the classical fracture surface of a brittle fracture (Fig. 5, right). The sample orientated at 45° shows the largest plastic strain of all tested orientations. This material behavior might be attributed to the texture of the foil in {1 0 0}h0 1 1i. This texture allows the main slip direction of bcc metals, the h1 1 1i direction, to appear at an angle of 45° to the direction of tension. It is well known from Mohr’s circle of stress that the maximum shear stress appears at an angle of 45°. This means that for a tensile test sample orientated in the 45° direction, the preferred slip direction of bcc metals and the direction where the maximum shear appears are congruent. This is why the plastic formability of tensile test samples orientated at 45° to the rolling direction of the foil is favoured. The texture of the 100-lm tungsten foil in {1 0 0}h0 1 1i is symmetric for a rotation of 90° around an axis that is perpendicular to the surface of the foil (e.g. x–y is the plane of the foil ? texture of {1 0 0}h0 1 1i is symmetrical for a 90° rotation around the z-axis). Accordingly the mechanical properties in the rolling direction (0°) and perpendicular to the rolling direction (90°) should be the same or at least in the same range. As this is not the case, beside the texture of {1 0 0}h0 1 1i the grain size of 0.5 lm 3 lm 15 lm also has to be included in the discussion about the mechanical properties of tungsten foil. According to the grain shape a tensile test sample orientated in the 90° direction has more grain boundaries that are perpendicular to the direction of tension than samples in the 0° orientation. It can be concluded that the anisotropic plastic material behavior of tungsten foil can be attributed on one hand to the texture of the foil and on the other to the grain shape. The classical picture of the influence of cold work like rolling on the tensile test properties can easily be explained for the example of steel (see Orowan–Taylor hardening). One way to harden steel is by cold working. The idea is that during cold-work hardening like rolling the dislocation density increases. This results in an increase of the tensile strength but also in a decrease of the ductility. The microstructural reason for the decrease in ductility can be found in the dislocation mobility, which is constrained due to the high dislocation density. The next paragraph will show that this classical picture of material behavior does not apply to tungsten foil and plate material. Fig. 6 shows the influence of cold work on tungsten foil and plate material. Tensile tests were performed at RT on tungsten foil/plate with thicknesses of 0.1 mm, 0.2 mm, 0.3 mm, 0.5 mm, and 1 mm in the rolling direction (0°) as well as perpendicular to the rolling direction (90°). As expected, the tensile strength increases with increasing degree of deformation and thus with decreasing thickness of the foil/plate. The tensile strength of 1mm tungsten plate material is about 1400 MPa whereas the tensile strength of 100-lm tungsten foil is about 2000 MPa. In contrast to the material behavior of cold worked steel, the ductility does not decrease with increasing degree of deformation or in other words with decreasing thickness of the foil/plate. The ductile material behavior can be measured independently of the thickness of the foil/plate. What all tensile tests on foil and plate material of different thicknesses have in common is that ductile material behavior can only be measured in the rolling direction (0°). Perpendicular to the rolling direction (90°) the material behavior is always brittle. 4.2. Tensile tests at 600 °C, foil, as-received
Fig. 4. This diagram shows the results of the tensile test on 100-lm-thick tungsten foil in the as-received condition at RT (see Fig. 3). Even at RT plastic material behavior can be observed. However the plasticity is anisotropic.
Tensile tests were performed not only at RT but also at 600 °C. Fig. 7 shows the stress–strain curve of 100-lm tungsten foil in the as-received condition tested at 600 °C. As expected, due to the 90° rotation symmetry of the texture of the foil, {1 0 0}h0 1 1i,
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Fig. 5. Fractured surfaces of tensile test samples of 100-lm-thick tungsten foil in the as-received condition tested at RT. Left to right: 0°, 45°, and 90° orientations.
Fig. 6. Stress–strain curves of tensile tests performed on tungsten foil/plate material in the as-received condition tested at RT. As expected the tensile strength increases with decreasing thickness of the foil. All tests have in common that ductile material behavior can only be measured in the rolling direction (0°).
4.3. Tensile tests at 600 °C, foil, recrystallized
Fig. 7. Stress–strain curves of tensile tests performed on 100-lm-thick tungsten foil in the as-received condition at 600 °C. As expected the plastic material behavior in the rolling direction (0°) and perpendicular to the rolling direction (90°) is congruent.
the plastic material behavior in the rolling direction (0°) and perpendicular to the rolling direction (90°) is congruent. The influence of the grain boundaries and the grain shape can no longer be observed at 600 °C. In common with the results for the tungsten foil tested at RT (see previous section) the most ductile material behavior, combined with a decrease of the tensile strength of about 10%, can be measured on a tensile test sample orientated in the 45° direction (for the argumentation, see previous section). It can be concluded that at 600 °C the anisotropic material behavior can easily be explained by the texture of the foil. At this test temperature the grain shape and the grain boundaries no longer influence the mechanical properties. Furthermore it can be determined that none of the stress–strain curves show any necking, which means that the uniform elongation (elongation without necking) is congruent to the total plastic strain.
Up to now the tensile tests have been performed only on tungsten foil in the as-received condition. In a new test series presented in this section, annealed tungsten foil will be assessed. The annealing conditions were 1 h at 2000 °C and 1 h at 2700 °C in hydrogen, respectively. As can be seen from Fig. 8, the tungsten foil in the recrystallized condition shows extreme plastic strain in a tensile test at 600 °C. Plastic strain of 25–45% can be measured. Comparing these results with the results obtained for tungsten foil in the as-received condition (also tested at 600 °C), the plastic strain increases by a factor of ten together with a decrease of the tensile strength by a factor of five. The decrease in the tensile strength of recrystallized tungsten was expected as it is well known that the tensile strength decreases when (i) the dislocation density decreases, (ii) the amount of grain boundaries decreases, and (iii) the grain size increases (see also Hall–Petch [22,23]). What is astonishing is the very large plastic strain of up to 45%. The reason for this incredible material behavior might be that the dislocations can move out of the foil surface: this is known as dislocation annihilation or the ‘foil effect’. Assessing the surface of the test sample after the test, it can be seen that the surface is now rough over the whole gauge length. This roughness is a hint that dislocation activity takes place at the surface and that the dislocation annihilation takes place over the whole gauge length. Fig. 9 shows a comparison of tensile test samples tested at 600 °C in the as-received (Fig. 9, left) and recrystallized condition (Fig. 9, right). The sample surfaces are now assessed in order to analyze the slip traces. Fig. 10 (top) shows an election microscopy assessment of the frontal surface of the foil after the tensile test. The surface shows slip traces that are independent of grain boundaries. It is well known from the work of Seeger [24] that the visible slip traces of bcc metals on the frontal surface are wavelike and cannot be explicitly attributed to the preferred slip planes of bcc metals, which are {1 1 0}, {1 1 2}, and {1 2 3} respectively. The reason for
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Fig. 10. Surfaces of 100-lm-thick tungsten foil tested at 600 °C in the recrystallized condition. Top: as expected the slip traces on the frontal surface are wavelike, which is a hint of frequent cross-slipping during plastic deformation. Bottom: the slip traces on the edge of the test sample can be identified as the h1 1 1i direction. Fig. 8. Results of tensile test on 100-lm-thick tungsten foil in the recrystallized condition at 600 °C. Top: foil was annealed for 1 h at 2000 °C. Bottom: foil was annealed for 1 h at 2700 °C. Extreme plastic strain of up to 40% combined with serrated flow can be observed.
Fig. 9. Picture of tensile test samples of 100-lm-thick tungsten foil tested at 600 °C. Left: foil in the as-received condition. Right: foil in the recrystallized condition (1 h/ 2000 °C). Even by the naked eye, the incredible plastic strain of the recrystallized foil as well as the rough surface can be seen. The rough surface is the evidence that dislocations moved out of the foil surface – the ‘foil effect’.
the wavelike slip traces is that the slip panes of bcc metals, {1 0 0}, {1 1 2}, and {1 2 3}, respectively, are nearly equally preferred, which leads to frequent cross-slipping during plastic deformation. Fig. 10
(bottom) shows the edge of a tested tensile test sample. Unfortunately the surface of the sample was damaged during sample manufacture, during EDM. Nevertheless slip traces are visible in the preferred slip direction of bcc metals, the h1 1 1i direction. Again this result also conforms with the results of Seeger, who found wavelike slip traces on the frontal surface as well as slip lines in the h1 1 1i direction on the edge of the sample [24]. Comparing the fracture surfaces of tensile test samples in the as-received and recrystallized conditions (see Fig. 11), it can be observed that delamination only occurs on samples in the as-received condition (Fig. 11, left). This goes for samples tested at RT as well as for those tested at 600 °C. Recrystallized tensile test samples show a fracture surface which is pointed and sharp (Fig. 11, right). Up to this point in this paper only tungsten foil has been investigated. In the next section the results of tensile tests on molybdenum foil will be presented and compared with the results for tungsten foil. The aim of this comparison is to see whether or not it is possible to transfer the results of mechanical tests (e.g. Charpy tests) on tungsten to molybdenum and vice versa.
4.4. Comparison with molybdenum foil In this section the results of tensile tests on 100-lm-thick molybdenum foil in the as-received condition carried out at RT and in the 0°, 45°, and 90° directions will be presented. It was shown elsewhere that the texture of molybdenum foil is congruent to that of tungsten foil, meaning that it is {1 0 0}h0 1 1i. As discussed previously this texture is symmetric for a rotation of 90°. According to this fact the mechanical properties in the
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Fig. 11. Fracture surfaces of tested tensile test samples of 100-lm-thick tungsten foil in 0° orientation. Left: foil in the as-received condition tested at RT. Right: foil in the recrystallized condition tested at 600 °C.
rolling direction (0°) and perpendicular to the rolling direction (90°) must be congruent. And this is exactly what can be measured in the tensile test on molybdenum foil at RT. In the rolling direction (0°) as well as perpendicular to the rolling direction (90°) there is about 4% plastic strain and a tensile strength of about 700 MPa (see Fig. 12). No influence of the grain shape or the different amount of grain boundaries perpendicular to the tensile direction can be observed. It can be concluded that the characteristics of the mechanical properties measured in a tensile test of molybdenum foil (as-received) at RT are congruent to those of tungsten foil (as-received) at 600 °C: (i) congruent curves are found in the rolling direction as well as perpendicular to it, (ii) the maximum plastic strain is measured on the sample at 45° orientation combined with a decrease in the tensile strength of about 10%, and (iii) there is no influence of the grain shape or the amount of grain boundaries that are perpendicular to the direction of tension. 5. Discussion This chapter comprises three parts. In the first, a definition of ‘structural tungsten material’ is sought. The second part deals with the difference in the material properties like ductility and fracture toughness. The chapter closes with a discussion on the plastic material behavior in the tensile tests performed in this work. 5.1. Attempt to define ‘structural tungsten material’ Pressurized components require a structural material, but the requirements for a ‘structural tungsten material’ are not defined
Fig. 12. Stress–strain curves of tensile tests performed on 100-lm-thick molybdenum foil in the as-received condition, tested at RT. As expected due to the 90° rotation symmetry of the {1 0 0}h0 1 1i texture, the plastic material behavior in the rolling direction (0°) and perpendicular to the rolling direction (90°) is congruent.
in any standard. So it is not that easy to define a ‘structural tungsten material’. However there are European standards that define the requirements for structural steels (see EN 10216-5, EN 12952-2, EN 13445-2, and EN 13480-2). The requirements mentioned in these standards are: (i) a Charpy energy of at least 27 J, measured with ISO-V test samples according to DIN EN ISO 1481 (10 10 55 mm3, 2 mm notch) at the lowest operation temperature, at least at room temperature and (ii) an ultimate strain of at least 14% measured in a tensile test according to EN ISO 6892-1. Further requirements are (iii) the capability to be formed (e.g. deep drawn) and (iv) an assessment of the feasibility of joining to other components, for example, welding pipe to pipe. What is not requested in these standards is the fracture toughness, KIC; however a fracture toughness of about 20 MPa m1/2 can be considered high. The standards mentioned above are only valid for steels. For other materials like nickel-based super-alloys or tungsten materials these standards are not relevant. In these cases the material can be certificated by either a VdTÜV material data sheet or a single case specification. 5.2. Ductility versus fracture toughness Ductility and fracture toughness must be rigidly separated as the two properties cannot be compared. Ductility is defined as the property of a material to irreversible plastic deformation whereas fracture toughness is the property of a material to withstand crack propagation. The ductility can be measured by mechanical tests such as tensile tests and is expressed as plastic strain [–]. Sources of plastic deformation of a material are either (i) the movement of edge and/or screw dislocations, (ii) twinning, or (iii) nano-crystalline effects like grain rotation, grain boundary sliding, grain boundary dislocation interaction, or grain rotation and alignment [25]. The fracture toughness like KIC has to be measured by fracture mechanical tests according to ASTM E399 and is expressed in MPa m1/2 [26]. As measuring of the fracture toughness is sometimes challenging and expensive it is also possible to measure the dynamic fracture toughness. This can be done by Charpy impact tests according to the EU standards DIN EN ISO 148-1 and 14556:2006-10. Using these tests, the dissipated energy in [J] is measured. Charpy impact properties are often measured on ISO-V samples with dimensions of 10 10 55 mm3 and a 2 mm notch. In some cases smaller KLST samples with dimensions of 3 4 27 mm3 and a 1 mm notch are used. Unfortunately it is not possible to transfer the results for the amount of dissipated energy from ISO-V samples to KLST samples easily. This means that it is not possible to predict how much energy will be dissipated by a KLST sample if an ISO-V sample made of the same material dissipates 27 J. Furthermore it is also not possible to translate the amount of dissipated energy measured by Charpy to a fracture toughness value. What
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the Charpy impact test and the fracture toughness tests have in common is that with both test methods the BDTT can be determined as the BDT occurs together with either an increase in the fracture toughness or an increase in the dissipated energy. In addition it must be mentioned that a material that shows ductile behavior in a tensile test must not have high fracture toughness. Considering, for example, the results of the tensile test at RT performed on 0.1 mm, 0.2 mm, 0.3 mm, 0.5 mm, and 1 mm foil/ plate material, it can be seen that for each sample the plastic strain measured in the rolling direction (0°) is in the same range (Fig. 6). However this does not mean that the fracture toughness of these materials is in the same range. For example the fracture toughness of 0.1 mm tungsten foil is expected to be much higher than the fracture toughness of 1 mm plate material even if the plastic strain measured in the tensile test is nearly the same. This is for example the reason why tungsten materials produced by powder injection molding (PIM) show excellent ductile behavior in a tensile test whereas Charpy impact properties are disappointing. 5.3. Discussion on tensile test properties Due to the symmetry of the {1 0 0}h0 1 1i texture of the 100-lm tungsten foil, the mechanical properties in the rolling direction (0°) and perpendicular to it (90°) must be in the same range. In samples oriented in the 45° direction, the main slip direction of bcc metals, the h1 1 1i direction, appears at an angle of 45° (see Fig. 13). According to Mohr’s circle of stress the maximum shear stress appears at an angle of 45°. So in a tensile test sample oriented in the 45° direction, (i) the direction with the maximum shear stress and (ii) the preferred slip direction are congruent. This is why the highest plastic strain can be obtained in samples oriented in the 45° direction. Tensile tests on 100-lm tungsten foil in the as-received condition tested at 600 °C (Fig. 7) and tensile tests on 100-lm molybdenum foil in the as-received condition tested at RT (Fig. 12) confirm this idea: in the rolling direction (0°) and perpendicular to the rolling direction (90°) the mechanical properties are nearly the same, and the highest ductile deformation can be measured on samples oriented in the 45° direction. The situation appears different for 100lm tungsten foil in the as-received condition tested at RT. Again, the highest plastic strain can be measured on a sample oriented in the 45° direction. But now the stress–strain curves of samples oriented at 0° and 90° are different. In the rolling direction (0°), plastic strain can be measured, but perpendicular to the rolling direction (90°) no plastic strain can be measured and the fracture surface
Fig. 13. This figure shows a top view of a tungsten foil (the x-axis is the rolling direction; the x–y plane is the flat surface of the foil). The texture of the tungsten foil, {1 0 0}h0 1 1i, is symmetric for a rotation of 90° around the z-axis, which is the out-of-plane axis. The mechanical properties of tungsten foil can be explained considering (i) the {1 0 0}h0 1 1i texture of the tungsten foil, (ii) the anisotropic grain shape (0.5 lm 3 lm 15 lm), and (iii) the preferred slip direction of bcc metals, the h1 1 1i direction, as well as the (iv) preferred cleavage plane of tungsten at room temperature, the {1 0 0} plane.
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shows a classic brittle fracture (see Figs. 4 and 5). This material behavior can be explained by the influence of the grain shape. Due to the grain shape, a 90° oriented sample has more grain boundaries perpendicular to the direction of tension than a sample oriented in the rolling direction (0°). So the weak grain boundaries of tungsten seem to affect this behavior. It can be concluded that beside the {1 0 0}h0 1 1i texture at low test temperatures the grain shape, 0.5 lm 3 lm 15 lm, has to be considered. A comparison of the results of tensile tests at 600 °C can be seen in Fig. 14. The top part of the figure shows as-received 100-lm tungsten foil; the middle, recrystallized 100-lm tungsten foil; and the bottom, as-received 1-mm tungsten plate. The stress-stain curves are profoundly different. The plate material is isotropic and shows a pronounced necking, whereas the foil does not. Why? Furthermore the curve of recrystallized tungsten foil shows a zigzag shape. Can this zigzag shape be attributed to an error in measurement or can this shape be explained by the foil effect? Let us start with the question of why plate material shows necking whereas foil material does not. A first attempt to explain this behavior by continuum mechanics does not work. Assuming that in both cases the clamping of the sample is proper, until necking starts there is a uniaxial stress state in both samples. Also, discussion about the state of plain stress or the state of plain strain does not help any further in this case. Another attempt to answer this question may be made using fracture mechanics. According to this idea the ratio of sample surface to volume of the sample is different for the foil and plate test samples. For a foil this ratio is larger, which means that surface effects like defects caused by sample preparation (EDM) have a stronger impact. I prefer another explanation: Necking, or in other words localization of deformation, as can be seen at the bottom of Fig. 14, is always attributed to local inhomogeneities like grain orientation or dislocation density gradients. Tungsten foil shows a plateau and no necking (Fig. 14, top). This is an indication that the material is homogeneous and sharp textured. It is an indication that the material has been put to its extremes and that its potential is completely utilized. The homogeneous distribution of the plastic strain might also be an indication of a high strain rate dependence of the material (see super plastic material behavior: deformation is localized, stress increases, strain rate increases, tensile strength increases, localization is stopped). However this mechanism requires pronounced dislocation diffusion. In my opinion the homogeneous distribution of the plastic strain is more likely to be attributed to a homogeneous distribution of the material properties (texture, dislocation density) than to high strain rate dependence. Furthermore at the discussed test temperatures dislocation diffusion (e.g. grain boundary sliding) is not very likely to be the dominating deformation process. Discussion on the results of tensile tests performed on recrystallized tungsten foil is very interesting (see Fig. 14, middle). What is curious at first sight is the extreme plastic strain of about 30%. Comparing these results with the results of tensile tests performed on tungsten single crystals it can be determined that the tensile strength and the plastic strain are in the same range [21]. This is not surprising as it was shown that a quasi-single crystal can be synthesized by annealing a foil [18,27]. A further question addresses the zigzag shape of the stress– strain curve. Here it is worth looking at the results of stress–strain curves of pre-deformed molybdenum single crystals. In these experiments a zigzag curve was also measured and was explained by the formation of dislocation cell structures [28]. The zigzag curve can be attributed to serrated flow. Serrated flow is well known in precipitation hardened (PH) aluminum as well as in
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condition has only one grain over the thickness of the foil, and size effects have to be considered [29–32]. If the movement of a dislocation is released it can penetrate through the whole foil without being arrested or restrained. The dislocation will continue moving until it finally reaches the surface of the foil, where it is annihilated: this is the foil effect. In my opinion, the extremely plastic and homogeneous deformation, combined with the rough surface of the foil after the test, is proof that dislocations have exited the foil, which annihilates them; this is the proof of the foil effect. By measuring the distribution range of the curve of serrated flow, r (20 MPa) (Fig. 14, middle), and using the cross-section area, A, of the tensile test sample (0.2 mm2), it is possible to calculate a force, F, for the dislocation activity according to
F ¼ r A ¼ 20
N 0:2 mm2 ¼ 4 N: mm2
ð1Þ
So a force of 4 N plays an important role in the discussion about the serrated flow of tungsten foil. Furthermore Fig. 14 gives hints on the forming capability of tungsten foil. It was shown in our previous work that 1-mm tungsten plate material can be deep drawn into a cap [33]. Looking at the stress–strain curves of the tested tungsten foil in Fig. 14 (top and middle) it is likely that a tungsten laminate made of recrystallized foil could be deep drawn more easily than a tungsten laminate made of foil in the as-received condition. Looking at the fracture surface of the tested 100-lm tungsten foil in the as-received condition tested at 600 °C (see Fig. 9 left) it can be observed that the fracture looks quite straight, as though the fracture follows a preferred cleavage plane. The same characteristic fracture surfaces were obtained on three-point-bending test samples [18,20]. This special kind of fracture that seems to penetrate along preferred cleavage planes might be explained by the preferred cleavage plane of tungsten at RT, which is the {1 0 0} plane [34]. This means that the texture of tungsten foil, {1 0 0}h0 1 1i, and the preferred cleavage plane of tungsten, {1 0 0}, form an angle of 45° with the rolling direction (see Fig. 13). After the analyses and the mechanical assessment of 100-lm tungsten foil in different conditions I have come to the preliminary conclusion that the ductility of tungsten foil is caused by (i) the high amount of mobile edge dislocation, (ii) the small grain size, which may lead to (ii-a) multiple slip at the grain boundaries [35] or (ii-b) nano-crystalline effects such as grain boundary rotation [25,35], and (iii) the foil effect – the dislocation annihilation on the free surface. In particular, the extreme plastic deformability of tungsten foil in the recrystallized condition seems to benefit from the foil effect. 6. Summary
Fig. 14. Comparison of stress–strain curves of tensile tests on tungsten at 600 °C. Top: results for 100-lm-thick tungsten foil in the as-received condition. Middle: results for 100-lm-thick tungsten foil in the recrystallized condition (1 h at 2000 °C). Bottom: results for 1-mm-thick tungsten plate material in the as-received condition.
steels in the context of strain ageing. During strain ageing a dislocation line is arrested on a carbon cloud. With increasing stress the dislocation line breaks away from the cloud. As a result of the arresting and breaking away from carbon clouds, serrated flow can be measured (for more details on serrated flow see also the Portevin-Le Chatelier effect). As the serrated flow measured on the tungsten foil cannot be attributed to precipitates or to carbon clouds one has to find another explanation. In my opinion the situation appears to be as follows. A tungsten foil in the recrystallized
The main question of the tungsten laminate project is to assess whether the ductile properties of a tungsten foil can be extended to the bulk. By synthesizing a tungsten laminate using the right combination of interlayer and joining technology it is possible to produce a tungsten laminate which shows improved properties in a Charpy impact test compared to pure tungsten plate material. So the main question can be answered with ‘Yes’. The microstructure of 100-lm tungsten foil in the as-received condition is as follows: Grain size and grain shape: 0.5 lm 3 lm 15 lm. This means that there are about 200 grains over the thickness of the foil. Texture: {1 0 0}h0 1 1i. This texture is established during cold work and is the saturation condition. This texture does not change on further cold working (see 25-lm tungsten foil). The same texture can also be found in molybdenum foil [20].
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In the grain: In general the grains are nearly free of dislocations. However there are grains with a dislocation network; these grains are the exception. Recrystallization: According to the distribution of the hardness, recrystallization starts at 1100 °C (annealing for 1 h). The same goes for tungsten foils with thicknesses of 25 lm and 200 lm.
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In the tensile test with recrystallized tungsten foil, the foil effect can be observed. The wavelike slip traces on the flat surface as well as the slip traces on the edges in the h1 1 1i direction correspond exactly with the results obtained by Seeger [24]. On the ductility of tungsten foil:
When 100-lm tungsten foil was exposed to extreme annealing (1 h at 2700 °C) the microstructure appeared to be as follows: Grain size and grain shape: 100 lm 100 lm 100 lm. This means that there is only one grain over the thickness of the foil. Texture: {1 0 0}h0 1 1i. There is no real recrystallization, as neither the creation of a nucleus nor the formation of a new and individually textured grain occurs. Moreover, the texture established during cold work stays the same and becomes even more pronounced. The same material behavior can also be observed for molybdenum foil [20]. In the grain: The grain boundaries are sharp and the dislocation density is low. The anisotropic mechanical properties of 100-lm tungsten foil can be explained by: The texture ({1 0 0}h0 1 1i), the anisotropic grain shape (0.5 lm 3 lm 15 lm), the preferred slip direction of bcc metals (h1 1 1i), and the preferred cleavage plane of tungsten ({1 0 0}). The results of three-point-bending tests on 100-lm tungsten foil can be summarized as follows: Tungsten foil in the as-received condition can be bent plastically even at RT. The ductility of tungsten foil is a thermally activated process. The BDTT is between 196 °C and RT (for foil in the as-received condition). In a three-point bending test at RT a foil annealed for 1 h at 900 °C is ductile while a foil annealed for 1 h at 1000 °C is brittle. Tungsten foil tested at RT fractures either along the preferred cleavage plane, {1 0 0}, or along the weak grain boundaries. Samples annealed up to 1 h at 1300 °C cleave along the {1 0 0} plane and the type of fracture is transgranular. At higher annealing temperatures the intercrystalline fracture becomes dominant. The results of tensile tests on 100-lm tungsten foil can be summarized as follows: Tungsten foil in the as-received condition is ductile not only in a bending test at RT but also in a tensile test. Tungsten foil in the as-received condition, tested at 600 °C. Due to the texture of the foil the mechanical properties in the rolling direction (0°) and perpendicular to it (90°) are equal and the most ductile material behavior is measured on a sample oriented in the 45° direction. Tungsten foil in the as-received condition, tested at RT. The greatest plastic strain can be obtained from a sample oriented in the 45° direction, as expected. But now the stress–strain curves of samples tested in the rolling direction (0°) and perpendicular to it (90°) are different. This can be explained by the anisotropic grain shape. The tensile tests show that tungsten foil is homogeneous and its properties are put to the extremes.
Assumption: The ductility of tungsten foil is caused by (i) the high amount of mobile edge dislocation, (ii) the small grain size, leading to possible multiple slip at the grain boundaries [35] or nano-crystalline effects such as grain boundary rotation [25,35], and (iii) the foil effect – the dislocation annihilation on the free surface. 7. Conclusion Tungsten foil is an extraordinary semi-finished product with curious ductile material behavior. Discussing about the ductility of 100-lm-thick tungsten foil in the as-received condition as well as in the recrystallized condition size effects have to be considered. In the as-received condition the 100-lm-thick tungsten foil has very small grains, so nano-crystalline effects have to be considered, discussing about the sources and mechanism of the ductile material behavior. In the recrystallized condition (e.g. annealing for 1 h at 2000 °C) the 100-lm tungsten foil has only one grain over the thickness of the foil. In this case the foil effect becomes relevant. In any case, tungsten foil is an excellent semi-finished product for the synthesis of tungsten laminates, especially tungsten laminate pipes for structural applications. Acknowledgements This work, supported by the European Communities, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. The authors are grateful to our colleagues from Plansee Metall GmbH, the University of Oxford, Department of Materials, and the Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM), for their support and valuable contributions. Special thanks go to Dr. W. Knabl (Plansee SE) and his team for their support and valuable discussion as well as to Mrs. D. Exner (KIT, IAM) for her help with the electron microscopy. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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