Evolution of carbon and oxygen concentration in tungsten prepared by field assisted sintering and its effect on ductility

Highlights

• Tungsten powder was sintered using different process parameters (various dwell times, atmospheres and foils)

• Stress-strain analysis was performed at several temperatures

• Sintering parameters leading to the lowest impurity concentration and improved mechanical properties were described

• Mechanism of carbon intake and oxygen concentration reduction during the sintering at various conditions was explained

Abstract

Good thermal conductivity, high-temperature strength and low tritium retention favour the use of tungsten as a plasma facing material for fusion reactors. However, high ductile-to-brittle transition temperature (DBTT) of tungsten limits its application. The most viable way of industrial-scale tungsten production is powder metallurgy. Conventional methods use high temperatures and long dwell times. However, this can degrade the mechanical properties of tungsten due to excessive grain growth. Therefore, new sintering techniques and unconventional sintering parameters are explored. The aim is to achieve less demanding single step processing method with the potential to improve mechanical properties. In this study, pure tungsten samples were prepared by Field Assisted Sintering using different dwell times, atmospheres and foils. The effect of the parameters on the impurity concentration was analysed and the mechanical behaviour was evaluated using stress-strain analysis. Significant improvement in mechanical performance has been reached for short dwell times (2 min) and vacuum or Ar + H₂ atmosphere.

Introduction

Tungsten is a refractory metal with high density, high melting point, good thermal conductivity and high-temperature strength. Therefore, tungsten is suitable for a broad range of applications, e.g. for weights or counterbalances, heating elements, crucibles, electrodes etc. In the last decades, application of tungsten in nuclear fusion is pursued, not only for the thermal properties, but also for high sputtering resistance and low hydrogen retention, as an essential part of the so called “Plasma Facing Components” (PFCs). PFCs are parts of the nuclear fusion facilities which are closest to the burning plasma and as such are exposed to significant heat and particle fluxes, including neutron irradiation. While the melting point over 3400 °C is favourable regarding the high-temperature applications, the ductile-to-brittle transition temperature (DBTT) and recrystallization temperature set certain limits for operation. Below the DBTT, tungsten is brittle and is not suitable for accommodating thermomechanical stresses resulting from the preceding heat loading. The DBTT depends on the material purity, microstructure and post-processing. For tungsten the values usually lie between 300°C and 700 °C [[1], [2], [3], [4]]; however, after recrystallization DBTT can overcome 1000 °C [5]. DBTT can be shifted to lower temperatures by alloying with certain elements, e.g. Re [6], Ir [7] or Zr [8], or by applying plastic deformation. However, this further complicates the production process and other properties, such as recrystallization temperature, may be negatively affected. Recrystallization is the formation of new grain structure in deformed material [9] that is at high temperatures followed by further grain growth. Usually the recrystallization and excessive grain growth is accompanied by reduction in strength and hardness while increasing ductility. However, for tungsten recrystallization is undesirable, as decrease of hardness [10], fracture strength [11] or yield stress [12] and increase of DBTT [5] was observed.

The high melting point of tungsten on the other hand significantly increases the fabrication difficulty. Conventional metallurgical routes, i.e. melting and casting are not feasible in large quantities. Also, the machining is challenging due to the brittleness. Therefore, methods of powder metallurgy are mostly used. The process consists of two major steps, i.e. powder preparation and compaction. Powders can be either supplied from an external manufacturer or can be prepared by mixing, milling or mechanical alloying. Subsequent compaction, i.e. application of high temperature and pressure, results in the diffusion bonding of the powder particles and in the formation of a solid sample. The sintering temperature for tungsten is usually in the range of 1700–2000 °C [3,13,14]. It can be decreased by addition of a small amount of other elements, such as palladium or nickel [15]; however, this can negatively influence the mechanical properties. Multiple compaction methods are distinguished with either successive or simultaneous heating and pressing, e.g. hot or cold isostatic pressing, microwave sintering, field assisted sintering etc. Field assisted sintering technique (FAST, commonly designated also as Spark Plasma Sintering, SPS) offers several advantages, such as lower temperature and shorter time required for compaction, and faster heating/cooling rate achievable. Faster compaction, i.e. shorter exposure to high temperatures, also enables preventing the excessive grain growth. Principle of FAST and used equipment are described in Fig. 1.

The key characteristic of FAST is the use of electric current for heating. Important components are the die, punches and electrodes. The powder is placed into the die, which is put between the punches. Powder is separated from the punches and the die by a protective foil, usually graphite. The purpose of the foil is to prevent sticking of the compacted powder to the die and to ease the extrusion of the sample. It was also observed that non-graphite foil can suppress the carbon diffusion from the die and punches, which are commonly made of graphite [17,18]. The foil location is highlighted in Fig. 1. The punches are connected to the electrodes, enabling the electric current propagation. During the compaction, high pressure (usually several tens of MPa) and pulsed electric current are applied on the powder. Due to the electric current flowing through the powder, significant Joule heat is generated at the particle contact points, facilitating local deformation and diffusion. This is, along with the high pressure, the mechanism of compaction. As conductive powder is heated directly by the current and not by the heating of the die, as for the conventional metallurgy, very high heating and cooling rates are achievable, up to 2000 °C/min. This shortens the time required for the compaction, resulting in energy savings, in limited grain growth and in the possibility of non-equilibrium materials fabrication.

Several papers focusing on DBTT of tungsten prepared by FAST were published previously. For example, in the authors' previous study performed in collaboration with Tanure et al. [19], samples sintered at 2000 °C and 70 MPa in Ar + H₂ atmosphere for 2 min were tested at 400°C and 600 °C. At both temperatures, the material was already ductile, indicating that the DBTT is lower than 400 °C. In the work of Yin et al., tungsten samples prepared by FAST at 2000 °C and 70 MPa in Ar + H₂ atmosphere exhibited a fine-grained microstructure and the DBTT between 300 and 400 °C [20]. Novak et al. performed study on sintering W and W-WC samples by FAST [14]. Pure W samples, sintered at 1900 °C and 60 MPa in vacuum for 5 min, exhibited brittle behaviour up to 400 °C. Although the ductility improved at 600 °C, the total elongation was lower than 2%, and therefore the transition apparently occurs slightly over 600 °C. Miao et al. prepared pure W samples by two-step FAST process with 20 min holding at 1300 °C and 2 min holding at 1800 °C, with the pressure of 47.7 MPa [21]. Flowing argon was used during the sintering. The DBTT was between 600 °C and 700 °C. Same result was achieved by Liu et al. as well, after sintering at 1800 °C and 47.7 MPa in argon atmosphere for 2 min [8].

As mentioned above, the key parameters characterising FAST compaction are temperature and pressure, together with dwell time. However, there are numerous other parameters which were not studied thoroughly, e.g. the type of used atmosphere or material of the sintering foil etc. Reports on the impurity content resulting from various sintering conditions are also scarce. The main impurities are carbon and oxygen. Thus, in this paper, analysis of carbon and oxygen concentration in tungsten samples sintered using various dwell time, atmosphere and foil is presented. Furthermore, comparison of microstructure and stress-strain performance with the focus on the onset of ductility is provided. Discussion on the sintering conditions leading to lowest impurity levels is performed together with the explanation of mechanism behind carbon and oxygen concentration evolution. Comparison with already reported data is provided whenever the literature is available. In other cases, appropriate ceramic and metallic materials are discussed.