Mechanical properties and microstructural characterizations of potassium doped tungsten
Highlights
► The potassium-doped tungsten grade “WVWM” was studied. ► Tensile testing was performed in the high temperature vacuum furnace at 5 different temperatures. ► Microstructural investigations were performed and the recrystallization temperature as well as related material modifications determined. ► The mechanical properties from the annealed material showed acceptable behaviour especially with respect to the ductility.
Introduction
Tungsten and tungsten alloys are presently considered for the helium cooled divertor and possibly for the protection of the helium cooled first wall in DEMO designs. Mainly because of their high temperature strength, good thermal conductivity and low sputter rates. The material has also to be stable under high neutron irradiation and helium and hydrogen production rates (Norajitra et al., 2010). None of the W & W alloys developed so-far has been fully optimized for structure or armour application in fusion reactors. In addition, all the present grades exhibit low fracture toughness and high ductile-brittle transition temperature (DBTT) in their initial metallurgical condition and are further degraded after neutron irradiation. From a material science point of view, the DBTT is not a real material property. It depends on the stress state (tensile, compressive or shear stresses), the strain state (uniaxial, biaxial or triaxial) and the strain rate (Hirai et al., 2007). Comparison between different grades is possible only if similar conditions are used. However, DBTT cannot be directly transferred to design specifications. As a conclusion, fracture toughness versus temperature is much better and more useful for design purposes. No full characterization has been performed up to now for a reference tungsten grade that can be used for comparison (Rieth et al., 2010).
The potassium-doped tungsten grade “WVWM”, is fabricated by the company PLANSEE AG. The purpose of introducing potassium into tungsten is to create bubbles in the material which pin the grain boundaries. Because potassium is volatile above 740 °C, different methods to keep potassium inside the material is used until there are no more open pores through which the potassium could leave. The growth of these bubbles due to induced stress by creep or others (potassium diffusion assisted) could be detrimental and is particularly seen for small wires in light bulbs. For bulk materials, it does not seem to be a critical issue. Through the addition of potassium to tungsten and the use of pressing, sintering and thermomechanical treatment, a fibrous elongated structure can be produced which combines high temperature creep resistance with a low temperature ductility and a better corrosion resistance. Hence this paper focuses on (1) understanding the mechanical properties of potassium doped tungsten in both as-received and annealed state; (2) studying the stress–strain relationships at 5 different temperatures up to 2000 °C by high temperature tensile testing; (3) investigating microstructure of this WVWM by SEM and TEM in order to obtain better understanding of the material's real nature.
Section snippets
Material
The potassium doped tungsten (WVWM) is delivered in the form of forged rods with a diameter of 15 mm and a deformation degree of 1.7 (Pintsuk and Uytdenhouwen, 2010). The purity of this tungsten is above 99.99%, only 20–30 ppm of potassium is introduced by creating bubbles in tungsten which pin the grain boundaries.
High temperature tensile testing machine
Tensile testing was performed in the high temperature vacuum furnace shown in Fig. 1. Actually the tensile testing is performed on a mechanical test bench and the heating is done
The influence of strain rate on the ductility and the strength of annealed potassium doped W
The stress–strain curves for increasing temperatures for the as-received material tested at low deformation speed are shown in Fig. 2A. A large deflection in the slope for temperature at and above 1000 °C imposes the contribution of creep. At 300 °C, a large scatter in the stress–strain curves was found because it lies in the DBTT range of the material. Some of them fail prematurely while others exhibit already a non-negligible amount of ductility (up to 20%). Compare to it, the stress–strain
Conclusions
To summarize, potassium doped tungsten is promising as one of the candidates in fusion environment. The mechanical properties from the annealed material showed acceptable behaviour especially with respect to the ductility. The decrease in strain rate improved the ductility even further. Furthermore, all the data obtained from mechanical testing have been strongly supported by their microstructure images as evidence.
Acknowledgments
The authors greatly appreciate the scholarship awarded to Hua Sheng and financial support from SCK•CEN (Belgium).
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