Comparison of K-doped and pure cold-rolled tungsten sheets: Tensile properties and brittle-to-ductile transition temperatures

Philipp Lied,Wolfgang Pantleon,Carsten Bonnekoh,Simon Bonk,Andreas Hoffmann,Jens Reiser,Michael Rieth

doi:10.1016/j.jnucmat.2020.152664

Philipp Lied, Wolfgang Pantleon + Show 5 more

Open Access

https://doi.org/10.1016/j.jnucmat.2020.152664

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Abstract

For high-temperature environments, as in future fusion reactors, the use of tungsten materials has been sincerely discussed in the last decade. Although severe cold-rolling of tungsten leads to significant improvements in mechanical properties, the fine-grained microstructure of such tungsten material has to be stabilized. For that, the use of potassium-doping (K-doping) in tungsten sheets is investigated in our ongoing study. In this work, we compare mechanical properties of warm- and cold-rolled sheets of pure tungsten and K-doped tungsten (for five different degree of deformation respectively) by means of fracture toughness tests and tensile tests.Fracture toughness and brittle-to-ductile transition temperatures (TBDT) are assessed, showing a slightly lower transition temperature for the cold-rolled K-doped sheets (lower than −100 °C for the 50 µm thick foil). The better performance of the K-doped sheet is related to its finer grain size. The thickest K-doped sheet shows a much higher TBDT than its pure tungsten counterpart. This is presumably caused by the presence of several tens of micrometre thick bands, containing only low angle boundaries, in the microstructure of the K-doped sheet.Tensile tests reveal an outstanding yield strength of 2860 MPa and an ultimate tensile strength of 2970 MPa for the thinnest K-doped sheet with similar, but slightly lower values for the pure tungsten counterpart. Both thinnest sheets show a drastic increase in ultimate tensile strength in correlation to their mean grain size, much higher than expected by a Hall-Petch relation. This deviation has been observed for the microhardness as well and is assumed to be caused by an extraordinary increase in the density of dislocations.Our results indicate that no disadvantages in tensile strength and brittle-to-ductile transition are to be expected compared to pure tungsten, when K-doped tungsten is used to inhibit recrystallization in high-temperature environments.

Highlights

The use of tungsten (W) and tungsten-based materials in future fusion power reactors has been investigated and discussed for several years [1]
The slightly lower TBDT of the cold-rolled K-doped sheets compared to the pure W sheets is related to a smaller grain size of the K-doped sheets
We suggest, based on observations from a previous study, that K-bubbles in the W matrix do not directly enhance grain refinement during rolling, but inhibit growth of grains during heating of the W sheets between rolling steps

Summary

Introduction

The use of tungsten (W) and tungsten-based materials in future fusion power reactors has been investigated and discussed for several years [1]. Tungsten is the element with the highest melting temperature among all metals (Tm = 3693 K [2]), it possesses a low vapour pressure in the plasma chamber of a fusion reactor It exhibits a high wear resistance, as well as good hightemperature strength and good thermal conductivity. In order to avoid this disadvantage, the influence of cold-working on tungsten has been assessed in several studies [3,4,5,6,7]; mechanical properties like tensile properties [8,9] and fracture toughness [7,10] have been investigated in-depth on such severely rolled sheets Such thin cold-rolled tungsten sheets could be joined to a laminate bulk material [11] or laminate pipes [12] for applications in fusion related environments

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