Microstructure and High-Temperature Performance of High K-Doped Tungsten Fibers Used as Reinforcement of Tungsten Matrix

Abstract

Tungsten (W) fiber-reinforced tungsten (W_f/W) composite with ultra-high strength and high-temperature resistance is considered an attractive candidate material for plasma-facing materials (PFM) in future fusion reactors. The main component of W_f/W composite is tungsten wire, which is obtained through powder metallurgy and the drawing process. In this paper, high potassium (K)-doped tungsten wires with 98 ppm of K and 61 ppm of impurities are prepared using traditional and optimized processing technologies, respectively, and a comparative study with conventional K-doped tungsten wires with 83 ppm of K and 80 ppm of impurities is conducted. The high-temperature mechanical properties as well as the microstructure’s evolution of the prepared tungsten wires are investigated. The results show that the high-temperature performance of K-doped tungsten wires is improved by increasing the K content and by simultaneously reducing the impurities. By adopting small compression deformation and low-temperature processing technology, the high-temperature performance of high K-doped tungsten wires can be further improved. A microstructure analysis indicates that the excellent high-temperature performance is attributed to a combination of the small K bubble size, high K bubble number density, and long K bubble string, which are produced through optimization of the processing technology. A study on the processing technology and the performance of tungsten wires with a high K content and a high purity can provide important information regarding W_f/W composites.

1. Introduction

Tungsten, with its excellent high-temperature performance, high thermal conductivity, and good resistance to corrosion and sputtering, has been widely researched and used for several decades [1,2,3,4,5,6,7]. Compared with refractory carbides or oxides, potassium (K) bubbles form the strongest high-temperature barriers in tungsten materials [5]. K-doped bulk tungsten is deemed an attractive potential plasma-facing material (PFM) for future fusion reactors and is considered one of the two most promising W-based alloys for the diverter in demonstration reactors (DEMOs) [8]. Different from bulk tungsten’s intrinsic toughening mechanism, the W_f/W composite relies on an extrinsic toughening mechanism, which has better ductility and a higher strength in long-term high temperature applications [9]. Although the preparation of W_f/W composite is not as mature as bulk tungsten material, it is still considered an attractive candidate. W_f/W composites can be prepared using two methods: by depositing W on K-doped tungsten wires and by mixing tungsten powder and K-doped tungsten wires and then sintering them using a powder metallurgical process. In these two methods, K-doped tungsten wire plays an important role as a reinforcing phase for the whole composite; K-doped tungsten wire with excellent properties is the premise for preparing W_f/W composite with high performance. Thus, improving the properties of tungsten wire is one way to improve the properties of W_f/W composite.

The high-temperature properties of K-doped tungsten include recrystallization temperature, recrystallized grain length-to-width ratio, creep property, and high-temperature strength. According to the mature experience of K-doped tungsten wire, there are several ways to improve high-temperature performance: (1) increasing the effective concentration of potassium to obtain high K bubble number density; (2) improving the purity of the tungsten material to reduce the influence of impurities on high-temperature performance; and (3) designing reasonable processing technology to obtain K bubbles with a long string and a small size. Different from the equiaxed recrystallization structure of pure tungsten, the recrystallization structure of K-doped tungsten wire is swallowtail-like, and this type of structure usually exhibits better high-temperature performance due to the blocking effect of K bubbles on the sliding of grain boundaries [10,11]. This phenomenon is identified as the strengthening effect of K bubbles [12,13]. Enough K content must be doped to confirm the effect. The K bubble number density increases almost linearly with K content [14]; however, more K content is not better, as an increase in K content causes tungsten to tend to harden and crack. Meanwhile, excess potassium can become an impurity phase, which is harmful to high-temperature performance. In terms of purity, the mechanical, physical, and metallurgical properties of tungsten are very sensitive to the presence of impurities [15], especially the Al, Si, Fe, O, S, and N elements. In terms of processing technology, tungsten processing is hot processing, and temperature is one of the key processing parameters [16]. Too high a processing temperature will lead to an increase in wire brittleness, the shortening of the K bubble string, and the growth of K bubbles, resulting in a decline in high-temperature performance. On the contrary, too low a processing temperature will lead to an increase in wire cracking due to the stress concentration being too large. Selecting the appropriate processing temperature can improve the high-temperature performance of the wire and reduce cracks. Pass compression deformation is another important processing parameter during the tungsten wire drawing process. Excessive pass compression deformation will lead to excessive deformation stress and cracks. If the pass compression deformation is too small it can improve the process safety factor and make the process easy to carry out, though the deformation is uneven [17], and deformation is easy to concentrate on the surface of tungsten material. Therefore, it is very important and meaningful to study the relationships among potassium content, impurities, the processing technology of tungsten wire, and its high-temperature performance.

Some works have been done in this field. Tang et al. prepared a series of K-doped bulk tungsten with K contents from 46 to 144 parts per million (ppm) in original powders and found that the K-doped W samples with 82 ppm of K presented the highest Vickers hardness, tensile strength, and thermal shock resistance. The explanation for this could be that the strengthening effect of the K bubble was weaker for the low-K-doped W sample with only 46 ppm of K, while much higher impurities for the high K-doped W sample with K over 108 ppm led to the degradation of mechanical performance [18]. Tanoue et al. studied the effect of potassium content on the formation and growth of recrystallized grains of tungsten wire when the potassium content was 42 ppm; the recrystallized grain was small and the grain’s length-to-width ratio was low; with an increase in potassium content to 70–94 ppm, the recrystallized grain became longitudinal and elongated [19]. Schade mentioned that K bubbles are distributed in tungsten wire as longitudinal rows for low K-doped W with a concentration in the range of a few tens in terms of ppm; K bubbles interact with all lattice defects and act as pinning points for dislocations and dislocation networks [1,5]. He believes that drawing deformation and the annealing process can increase the dispersion of potassium bubbles. Namely, the deformation process can reduce the average size of potassium bubbles while annealing can make the potassium tube split and increase the number of potassium bubbles and the length of potassium bubble strings [20]. Nikolić et al. investigated the influence of heating treatments on pure and K-doped drawn tungsten wires and found that the K-doped wire only experiences milder structural changes, annealing below 1400 °C, in sharp contrast with the pure wire, which experiences severe modifications upon heat treatments from 900 °C [21]. According to Brilliant’s report, the recrystallization behavior of two wires with almost the same potassium contents (76 and 77 ppm) and ingot densities can be completely different when different processing methods were used [22]. Too high a K content can easily lead to processing difficulties and large K bubbles; hence, the K content in K-doped tungsten wire is usually less than 90 ppm; only a few reports exist on ultra-high K-doped tungsten with concentration above 95 ppm. There are still a number of issues that should be further studied on the process and performance of tungsten wires with ultra-high potassium content above 95 ppm.

In this paper, two conventional K-doped tungsten wires with 83 ppm of K and 80 ppm of impurities as well as two high K doped ones with 98 ppm of K and 61 ppm of impurities are obtained using a traditional and an optimizing processing technology, respectively. The high-temperature performance, tensile property, internal structure, and the K bubble evolution of the tungsten wires are measured and discussed. Both K-doped tungsten wire and K-doped bulk tungsten are made using the powder metallurgy process based on the mechanism of K-bubble strengthening; thus, the studying processing technology and the performance of K-doped tungsten wire with a high potassium content and purity can provide some important options for W_f/W composites.

2. Materials and Methods

2.1. Materials

There are many specifications for tungsten wire, among which Φ 0.39 mm tungsten wire is an important semi-finished product. In this experiment, two kinds of tungsten rods with different potassium contents and impurities were selected to draw from Φ 3.1 mm to Φ 0.39 mm using two different processing technologies. One was the traditional process with 13 passes of compression deformation, and the average pass deformation was about 28%. The other was an optimized process by reducing the maximum heating temperature of the furnace and increasing the mold temperature to effectively increase the deformation temperature to ease work-hardening, and there were 23 compression passes with an average pass deformation about 20%. The details of the four obtained tungsten wires with Φ 0.39 mm are listed in

Table 1. Details of the prepared Φ 0.39 mm tungsten wires.

Wire	W, %	K, ppm	Main Impurity, ppm							Process (from Φ 3.1 mm to Φ 0.39 mm)
			Al	Si	Fe	Na	Mo	Cr	O
1#-0	≥99.95	83	11	8	9	5	20	4	14	traditional process 1
1#-1	≥99.95	83	11	8	9	5	20	4	14	optimized process 2
2#-0	≥99.95	98	6	5	9	5	20	2	5	traditional process
2#-1	≥99.95	98	6	5	9	5	20	2	5	optimized process

¹ Compression passes of 13 with average pass deformation of ~28%. ² Compression passes of 23 with average pass deformation of ~20% by reducing the maximum heating temperature and increasing the mold temperature.

. The chemical composition of the drawn tungsten wires was similar to that of the raw tungsten rods. The contents of K, Na, and Fe were determined using an atomic adsorption spectroscopy (AAS, CAAM-2001, Hanshi, Beijing, China), the content of O was determined using an oxygen and nitrogen analyzer (EMGA-620W, Horiba, Kyoto, Japan), and the other impurities, such as Si, Al, Mo, and Cr, were determined using an atomic emission spectroscopy (AES, ULTIMA 2000, Jobin Yvon, Paris, France). The K and impurity contents of tungsten wire 1# were 83 ppm and 80 ppm, respectively. For tungsten wire 2#, K content was 98 ppm, and the main impurity content was only 61 ppm. It can be seen that the K content of tungsten wire 2# was higher than that of tungsten wire 1#; meanwhile, the impurity content was nearly 20 ppm lower, especially for the contents of the Al, Si, and O elements.

2.2. Test Methods

2.2.1. High-Temperature Performance Test and Microstructure Observation

Due to the dead-weight of the tungsten wire at a high temperature, its strength decreases rapidly and the grain boundary slips, which makes the tungsten wire sag. Sag value, recrystallization temperature, and recrystallization grain length-to-width ratio can reflect the high-temperature performance of tungsten wire.

The sag values of the four prepared tungsten wires were tested using a V-type high-temperature testing machine (GV-IA, Institute of electric light source and material science, Nanjing, China) and the test samples are shown in Figure 1. The sag values of the tungsten wires were obtained according to the Japanese Industrial Standard H 4460-2002 [23]. The tungsten wires with a 230-mm length were bent into a “V” shape and the fusing current (FC) of the four kinds of tungsten wires were measured. The recrystallization behavior of the four prepared tungsten wires in the as-worked fiber structures were tested under different heat-treatment temperatures. The microstructures of the tungsten wires were observed by a metallographic microscope. The longitudinal sections of the as-worked and heat-treated tungsten wires were prepared by grinding, polishing, and etching (30% K₃Fe(CN)₆ and 10% NaOH aqueous solution in a volume ratio of 1:1). The temperature when a coarse crystal structure is generated is defined as the recrystallization start temperature. When the microstructure is completely recrystallized, the corresponding minimum temperature is taken as the complete recrystallization temperature of the tungsten wire. The grain length-to-width ratio is expressed by the ratio of longitudinal grain boundary length to radial grain boundary length after completing recrystallization. The evolution of K tubes and bubbles of the tungsten wires were also investigated using a scanning electron microscope (SEM, S-4800ⅡHitach, Tokyo, Japan). The observation surfaces were prepared by truncating wire along the longitudinal direction at room temperature.

2.2.2. Tensile Test

In addition to the sag value and recrystallization behavior, we also measured the tensile strength of the prepared tungsten wires. The tensile tests were conducted at room temperature (RT) and at a high temperature of 1800 °C using a Shimadzu tensile testing machine (AGS-H, Shimadzu, Kyoto, Japan) equipped with a homemade heating furnace. The length of the test samples was 200 mm. It should be noted that the tungsten wires were heated and tested in a hydrogen atmosphere at 1800 °C.

3. Results and Discussion

3.1. High-Temperature Performance

Table 2. High-temperature performance test results of the samples.

Wire	Sag Value	Grain Length-to-Width Ratio	Recrystallization Start Temperature	Recrystallization End Temperature
1#-0	2.3	9	1850 °C	2180 °C
1#-1	1.27	12	1910 °C	2230 °C
2#-0	1.28	18	1960 °C	2450 °C
2#-1	0.8	18	2020 °C	2450 °C

shows the results of the V-type high-temperature tests. For sample 1#-0, the sag value was 2.3, the grain length-to-width ratio was only 9, and the recrystallization start temperature was 1850 °C; however, the sag value of sample 1#-1, of which the composition was the same as sample 1#-0 but that underwent different processing, was reduced to 1.27, the grain length-to-width ratio was increased to 12, and the recrystallization start temperature was increased to 1910 °C. Samples 2#-0 and 2#-1 showed similar change trends. The sag value of 2#-1 could be further reduced to 0.8 and the recrystallization start temperature could be further increased to 2020 °C. The result of sample 2#-1 was the best of the four samples. These data show that the high-temperature properties of K-doped tungsten wires were improved by matching proper processing, increasing the potassium content, and reducing the impurities.

3.2. Tensile Properties

The tensile test results of the tungsten wires are shown in Figure 2. The tensile strength at room temperature was above 2000 MPa with maxima of 2276 MPa, which is much higher than Tang’s spark plasma sintering K-doped tungsten (200~300 MPa) [18] and higher than Lied’s rolled K-doped bulk tungsten (1800~1900 MPa for samples with a thickness of 0.35 mm) [24]. The tensile strength could be maintained around 400 MPa with a maxima of 445 MPa at 1800 °C. High-temperature tensile data are very scarce in the literature, and we could only find data on K-doped W alloy at 1300 °C with a high strength of 240 MPa [25], which is much lower than our measured tensile strength, even at 1800 °C. From the perspective of the elemental composition, the tensile strengths of 2#-0 and 2#-1 were higher than those of 1#-0 and 1#-1. From the perspective of the processing technology, the tensile strengths of 2#-1 and 1#-1 were higher than those of 2#-0 and 2#-1. However, tensile strength shows a more significant increase with the K content and impurities than that with the processing technology; hence, the effect of the K content and impurities is more obvious.

3.3. Microstructure Evolution

The microstructures of these as-worked tungsten wires are shown in Figure 3. It can be seen that the microstructure is composed of long and narrow fibers along the longitudinal direction, and there are a few transverse grain boundaries. Since the total deformation of tungsten wire is more than 98% in the drawing process from Φ 3.1 mm to Φ 0.39 mm, which is very large, the result in the distance between the transverse grain boundaries is lengthened, and it is difficult to observe the transverse interface. In addition, with continuous drawing, some transverse grain boundaries will gradually turn to the direction parallel to the fibers and become longitudinal fiber boundaries [26]. The fiber of sample 1#-0 is coarse and curved, while the fibers of sample 2#-0 and 2#-1 are longer, straighter, and narrower. The coarsening of the fiber structure is caused by uneven deformation. When a grain orientation has a “hard orientation” relative to the drawing stress, the deformation is smaller than other grains; thus, the final fiber is coarser and wider [27].

The recrystallized microstructure evolutions of these four heat-treated tungsten wires are shown in Figure 4. The number and the length-to-width of recrystallized grains are different in these four samples. There are more recrystallized grains in sample 1#-0, and the grain length-to-width ratio is small. Using the optimized process, the recrystallized microstructure of sample 1#-1 is a long and interlocked grain, and the number of small grains is obviously reduced. The results show that the recrystallized microstructure of sample 2# is a swallowtail-like, long interlocked grain, whether using the traditional or the optimized process, and the number of recrystallized grains is further lessened by the optimization process, and only two grains are found on the observation surface of sample 2#-1.

3.4. Potassium Tube and Bubble Evolution

The above mentioned high-temperature performance, tensile properties, and microstructure of tungsten wire are closely related to dopants and impurities. After sintering, most K compounds, which are doped in the powder mixing stage, are volatilized away and only a small amount of potassium is left in the final drawn tungsten wire, which is usually several ppm. However, trace residual potassium has a great impact on the recrystallization characteristics of doped tungsten wire [28]; therefore, it is necessary to investigate the evolution of potassium in the four kinds of tungsten wires.

Potassium has a very low solubility at the grain boundary of tungsten [29]. From the sintering ingot stage, potassium can only adhere to the inner wall of the sintering holes. Those sintered holes containing potassium are flattened and stretched during the pressure process to form K tubes or bubbles, arranged along the wire’s longitudinal direction [30]. Figure 5 shows that only sample 1#-0 can occasionally show a few split K tubes in the as-worked fiber structure along the longitudinal direction of the wire. K tubes or bubbles in the other three tungsten wires are almost invisible. According to the microstructure of the as-worked tungsten wires in Figure 3, it shows that the other three tungsten wires have a relatively developed fiber structure along the longitudinal direction, and the high density of the dislocation entanglements hide the K tubes [31]. The fiber boundary of sample 1#-0 is coarse and curved, while the K tubes or bubbles become visible.

With heat treatment proceeding, K tubes will break and become small, round bubbles due to surface tension, forming a series of K bubbles arranged along the wire longitudinal direction [32]. As shown in Figure 5, at the start of recrystallization, a small number of potassium bubbles can be observed at the boundary or inside the grain. The K bubble size of samples 1#-0 and 1#-1 are generally 10–170 nm, sample 2#-0 occasionally has several large-sized potassium bubbles (diameter about 350 nm), the K bubble size of sample 2#-1 is significantly smaller than that of sample 2#-0, which is generally 10–90 nm. Combined with the results in Table 2 and Figure 4, sample 1#-0 has the lowest recrystallization start temperature and the most recrystallized start grains, corresponding to the biggest K bubble size and is easily observed, while sample 2#-1 had the highest recrystallization start temperature and the fewest recrystallized start grains (only one grain on the observation surface) corresponding to the smallest K bubble size and was most difficult to be observed. It seems that the size and the active degree of the K tube or bubble are related to the recrystallization behavior of tungsten wire. The K tube and bubble begin to change early, which lead to a lower recrystallized start temperature, and small size K bubbles have a good pinning effect, which lead to difficulty in the grain boundary migration, thus delaying the recrystallization start and forming a few number of recrystallized grains; similar opinions are mentioned in Ref. [17].

The recrystallization will end at a higher heat treatment temperature, and a large number of potassium bubbles can obviously be observed at this time, as shown in Figure 5 x-3(x = a, b, c, and d). The K bubble arrangement of sample 1#-0 is a little disordered, while those of the other three tungsten wires were linear along the longitudinal direction of the wire besides several single randomly distributed K bubbles.

Table 3. K tube and bubble of Φ 0.39 mm tungsten wires at the stage of recrystallization.

Wire	K Bubble Size, nm	K Bubble Number Density, m−2	Length of K Bubble String, μm
1#-0	20–410	3–4 × 1012	1–3
1#-1	20–290	3–4 × 1012	3–6
2#-0	20–830	3–4 × 1012	1–5
2#-1	20–285	4–5 × 1012	3–8

lists some information about potassium bubbles at the stage of recrystallization. The size of the K bubble and the length of the K bubble string are measured by the measurement software attached to a SEM, and the K bubble number density is obtained by counting the number of potassium bubbles per unit area in the SEM photos. Assuming the fracture surface is almost flat, the maximal K bubble size of sample 1#-0 reached 410 nm and sample 2#-0 reached 830 nm. Sample 2#-1 showed the smallest average K bubble size, the highest K bubble number density, and the longest K bubble string length. Combined with the results in Table 2 and Figure 4, sample 2#-1 has excellent high-temperature performance and a long recrystallized grain structure; it shows that a small K bubble size, long K bubble string along the longitudinal direction, and high K bubble’s number density are conductive to strong resistance of grain migration along the radial direction; thus, the grains tend to grow along the longitudinal direction, and the grain’s length-to-width ratio become larger, forming a swallowtail-like, long interlocked recrystallized grain structure, which is an important precondition for excellent high-temperature performance [33].

From Table 3, it can be concluded that both the processing technology and the composition content affect the evolution of the K bubble and the microstructure. Compared to the optimized process, the traditional process has a larger pass deformation, and the deformation heat is greater. Meanwhile, the traditional process sets a higher processing temperature so that the tungsten wire is processed in a higher temperature condition and leads to K tubes and bubbles that are easy to break up and grow; thus, the fiber structure of the as-worked tungsten wire is easily coarsened. As we know, the degree of difficulty that the grain boundary can overcome at the pinning sites will depend on the amount of stored energy produced due to the deformation [22]. The amount of energy stored in tungsten wire by the traditional process with a larger pass deformation is more than that by the optimized process; hence, the grain boundaries overcome the K bubble pinning obstacles more easily with the traditional process and, thus, form recrystallized grains, which leads to a lower recrystallization temperature. On the other hand, tungsten wire with a high K content is easily formed into large-sized potassium bubbles at high temperatures [34] (refer to K bubble size of sample 2#-0). Large-sized K bubbles tend to merge and grow continuously and finally become a kind of fracture source, which is a potential failure for high K-doped tungsten wire. Usually, the K content in K-doped tungsten is less than 90 ppm; in this paper, the K content in tungsten wires 2#-0 and 2#-1 were as high as 98 ppm, and the size of some K bubbles reached 830 nm when the processing technology is not optimized, such as in sample 2#-0. By optimizing the processing technology, the premature formation of large-sized K bubbles was reduced, as in sample 2#-1. Namely, it is usable to obtain tungsten materials with better high-temperature performance from tungsten with a higher K content through the combination with an optimizing processing technology.

4. Conclusions

In this paper, we studied the microstructure and high-temperature performance of tungsten wires with 98 ppm of K and 61 ppm of impurities using two different processing technologies, comparing them to tungsten wires with 83 ppm of K content and 80 ppm of impurities. It is found that a proper pass deformation and processing temperature are helpful for tungsten wires with a high K content and that this method can be applied to the preparation of tungsten materials with better high-temperature performance. The main conclusions are as follows:

(1)

Both optimizing the processing technology and increasing the potassium content can promote the high-temperature performance of a K-doped tungsten wire, and the effect of increasing the potassium content and improving the purity is more obvious. However, it should be noted that an ultra-high potassium content tends to form large potassium bubbles, which need to be avoided using the optimizing processing technology.

(2)

Tungsten wire with 98 ppm of K and 61 ppm of impurities prepared using the optimizing processing technology presented the highest tensile strength both at RT and 1800 °C and the lowest sag value and the highest recrystallization start temperature.