Effects of tantalum concentration on the microstructures and mechanical properties of tungsten-tantalum alloys

Effects of tantalum concentration on the microstructures and mechanical properties of tungsten-tantalum alloys

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ARTICLE IN PRESS

FUSION-9434; No. of Pages 7

Fusion Engineering and Design xxx (2017) xxx–xxx

Contents lists available at ScienceDirect

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Effects of tantalum concentration on the microstructures and mechanical properties of tungsten-tantalum alloys Zheng Wang a,b , Yue Yuan b,c,∗ , Kameel Arshad b,c , Jun Wang b,c , Zhangjian Zhou d , Jun Tang e , Guang-Hong Lu b,c a

Sino-French Engineer School, Beihang University, Beijing 100191, China Beijing Key Laboratory of Advanced Nuclear Materials and Physics, Beihang University, Beijing 100191, China c School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China d School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China e Institute of Nuclear Science and Technology, Sichuan University, Chengdu, Sichuan, 610064, China b

h i g h l i g h t s • • • • •

Tungsten-tantalum alloys with fine grain microstructure and high mechanical properties were fabricated by spark plasma sintering. Effects of tantalum concentration on the basic characterization of tungsten-tantalum alloys were studied. 10 wt.% of tantalum was considered to be the best option compared to 5, 15 and 20 wt.% by analyzing various testing results. Tantalum tends to gather together if its concentration is higher than 10 wt.%, and Ta precipitate has a negative effect on mechanical property. Pure tungsten fractured intergranularly, while the failure mode in tungsten-tantalum alloys was predominantly transgranular.

a r t i c l e

i n f o

Article history: Received 30 October 2016 Received in revised form 20 April 2017 Accepted 21 April 2017 Available online xxx Keywords: W-Ta alloys Ta concentration Microstructures Mechanical properties

a b s t r a c t Pure tungsten and tungsten-tantalum alloys with tantalum concentrations of 5, 10, 15 and 20 wt.% were fabricated by spark plasma sintering. Effects of tantalum concentration on the basic characterization of tungsten-tantalum alloys were studied. The results show that the addition of tantalum contributes to a decrease of grain size and an improvement of relative density and mechanical properties. Adding a small amount of tantalum (about 10 wt.%) increases the relative density significantly, while if the concentration of tantalum is higher than 15 wt.%, the relative density decreases apparently. The hardness of the alloy with 10 wt.% tantalum is highest, which is 508.65 HV, 42% higher than pure tungsten. The bending strength of the alloy with 5 wt.% tantalum reaches up to 742 MPa, which is 27.1% higher than pure tungsten. Pure tungsten fractures intergranularly, while the failure mode in the alloy is predominantly transgranular. SEM images show that if the concentration of tantalum is higher than 10 wt.%, tantalum tends to gather together, and this phenomenon has a negative effect on the mechanical property of materials. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Plasma-material interaction is an important but very complex area in the field of fusion technology for the success operation of future fusion reactors. The hot plasma of several millions Kelvin temperature is tried to contain in a tokamak by large magnetic fields but still the plasma facing components have to face high heat flux and intense beam of different radiations such as neutron,

∗ Corresponding author at: School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China. E-mail address: [email protected].com (Y. Yuan).

helium and hydrogen isotopes [1–3]. Thus, plasma-facing materials for future fusion reactors have to meet many requirements such as structural integrity needs to be kept even under severe thermal loads, thermally induced mechanical stresses and cyclic loading conditions [4]. They need to be resistant against chemical, physical sputtering and erosion and must have good properties such as thermal conductivity, high temperature strength, low ductile-to-brittle transition temperature (DBTT), high recrystallization temperature even after neutron and other irradiation damage. Tungsten (W) is chosen as the leading candidate for plasma facing material (PFM) in fusion devices due to its durability and favorable physical properties at elevated temperatures [5–8]. However, W has several disadvantages, such as high DBTT and

http://dx.doi.org/10.1016/j.fusengdes.2017.04.082 0920-3796/© 2017 Elsevier B.V. All rights reserved.

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recrystallization brittleness [6,9]. Adding a certain amount of tantalum (Ta) into W is expected to increase ductility and reduce DBTT [10–12]. Besides, it has been reported that the addition of Ta in W can suppress surface blistering under high fluence deuterium plasma irradiation [13]. Moreover, several studies related to the thermal shock performance of W-based materials indicate that the surface cracking threshold of W-Ta alloy is higher than pure W, because the existence of Ta changes the microstructure and grain size of materials [14–16]. The different concentrations of Ta into W and their comparative analysis for the evaluation of optimized combination of Ta and W with fine microstructures and good mechanical properties has not been studied well. The aim of this work is to fabricate high quality W-Ta alloy which is expected to resist the damage caused by irradiation of fusion reactors. In this work, effects of different Ta concentrations on the microstructures and mechanical properties of W-Ta alloys are studied.

Table 1 Characteristics of pure W and W-Ta alloy samples. Material

W

W-5Ta

W-10Ta

W-15Ta

W-20Ta

Average grain size (␮m) Bending strength (MPa) Vickers hardness (HV) Density (g/cm3 ) Theoretical density (g/cm3 ) Relative density (%)

6.04 584.31 358.22 18.26 19.25 94.84

3.15 741.62 414.20 18.10 19.10 94.73

2.48 706.27 508.65 18.26 18.96 96.29

2.42 647.59 466.58 17.93 18.82 95.27

2.49 586.81 489.68 17.72 18.68 94.89

2. Experimental Commercial powders of W (with an average particle size of 2 ␮m and a purity of 99.9%) and Ta (with an average particle size of 48 ␮m and a purity of 99.9%) were loaded in tungsten carbide mill pots in argon atmosphere. The alloy powders were mixed according to the Ta concentrations of 0, 5, 10, 15 and 20 wt.% (named W, W-5Ta, W-10Ta, W-15Ta, W-20Ta respectively). High energy ball milling (HEBM) process was operated at a speed of 380 rpm for 30 h and the weight ratio of ball to powder was 5:1. Then the powders were transferred from mill pots to a graphite die in a glove box which was filled with pure argon gas. The samples were sintered by spark plasma sintering (SPS) in vacuum under a pressure of 80 MPa, and the maximum temperature was 1800 ◦ C with heating rate of 100 ◦ C/min. The holding time was chosen to be 0 min in order to limit grain growth. Afterwards, the surface of all samples was mechanically polished to mirror-like finish. X-ray diffraction (XRD) was employed to investigate the alloying behavior due to milling and sintering. The morphology and grain size distribution of sintered samples were examined by a field emission scanning electron microscope (FE-SEM) equipped with electron backscatter diffraction (EBSD) system. Element distribution was detected by Energy Dispersive Spectrometer (EDS). In order to evaluate impurity content, oxygen and carbon content in various powders were tested by infrared absorption method. The relative density of each sample was obtained by the ratio of bulk density to theoretical density. Bulk density was measured by Archimedes method in water, and theoretical density was calculated by formula (1) [17]. =

w Ta x%w + (1 − x%)Ta

(1)

Vickers hardness was measured at room temperature with a load of 2.94 N for 15 s, and six different positions on polished surface were tested to obtain an average value. Three point bending tests were conducted on specimens with dimensions of 2 mm × 3 mm × 13 mm with a span of 10 mm and a crosshead speed of 0.5 mm/min. In order to get a representative value, each sample were measured three times for bending strength. 3. Results and discussion In order to study the effect of Ta concentration (0–20 wt.%) on the basic characteristics and microstructures of W-Ta alloys, average grain size (comes from W grains), bending strength, Vickers hardness and relative density of various sintered compacts were measured. Detailed data are summarized in Table 1, and the information will be discussed in the following sections.

Fig. 1. XRD patterns of (a) milled powders and (b) sintered compacts.

3.1. Alloying behavior Fig. 1 shows the XRD patterns of milled powder and sintered compacts. Small peaks of Ta can be found besides W peaks in all of the milled powders. However, for sintered compacts, small peaks of Ta only appear in W-15Ta/W-20Ta and cannot be observed in W5Ta/W-10Ta anymore. This means that HEBM cannot effectively implement alloying when Ta concentration is more than 5 wt.%, and SPS can promote alloying behavior to some extent. When the concentration of Ta reaches 15 wt.%, a portion of Ta may aggregate and become detectable. All the processes including powder mixing, extraction from ball miller and loading to furnace have been done in the inert environment of pure argon, thus oxygen and carbon contamination cannot be detected in XRD patterns. However, the chemical analysis shows that oxygen contents in all of these samples range from 0.13 wt.% to

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Fig. 2. Grain size distributions of various sintered compacts.

0.29 wt.% and carbon contents in all of these samples are no more than 210 ppm. 3.2. Grain size The average grain size of different samples is listed in Table 1, while Fig. 2 presents the analysis results of EBSD, showing the grain size distribution of various sintered compacts. Software “Channel5” was used to analyze EBSD data, which can distinguish each grain in the EBSD map, and calculate the grain size and the grain size distribution. The results indicate that average grain size of W-Ta alloys (∼2.5 ␮m) is much smaller than pure W (∼6 ␮m). For pure W, the grain size ranges from 0 to 15 ␮m, and takes 6–9 ␮m as the primary size. The grain size of W-5Ta is mainly 2–3 ␮m, and the largest grain size is 12 ␮m. With regard to W-10Ta, the largest grain size is no more than 7 ␮m, while the most frequent size is 1–2 ␮m. For W-15Ta and W-20Ta samples, the maximum of grain size is 9 ␮m and 8 ␮m, respectively, and take 1–3 ␮m as the primary size. It is obvious that, with the adding of Ta, the primary grain size reduces from 9 ␮m to about 2 ␮m. This means Ta effectively inhibits the grain growth. Grain refinement is inconspicuous when Ta content is relatively low, but when the concentration of Ta is equal or greater than 10 wt.%, it plays an important role in grain refinement. In Section 3.1, it can be seen that a portion of Ta may aggregate when the concentration of Ta reaches 15 wt.%. This result agrees well with former works, which claim that the second phase can act as obstacles only in solid phase sintering [18,19]. 3.3. Microstructure and relative density The microstructure morphology of polished surface on various samples is presented in Fig. 3. It can be seen that the grain size of pure W is bigger than W-Ta alloys. Uniform-distribution holes appear on the surface of pure W and W-5Ta samples. Density of holes is relatively low on the surface of W-10Ta. Ta particles formed by clustered Ta element can be observed in W-10Ta, W-15Ta and W-20Ta. Yellow circles marked out the Ta particles detected by EDS. Relatively bigger holes appear on the surface of W-15Ta and W-20Ta samples, and these bigger holes are concentrated in Ta particles. This phenomenon means that if the content of Ta is more than 10 wt.%, Ta would tend to aggregate. Stefan Wurster et al. observed similar aggregate phenomenon [20]. Moreover, the aggregation of

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Ta is bad to increase the relative density as there are many bigger holes in Ta area. Density and theoretical density of alloy compacts are listed in Table 1. The value of density is an average value of 5 measurements, and the standard error is no more than 1.5%. The relative density of all samples ranges from 94.73%–96.29%. Adding an amount of Ta (about 10 wt.%) increases the relative density. However, if the concentration of Ta is higher than 15 wt.%, the relative density decreases apparently. This result is consistent with the microstructure observations. The porosity observed in the polished surface (Fig. 3) is responsible for the lower density value. Relative density is affected by both grain size and alloying behavior. The smaller the grain size, the easier to upgrade the density. For W-15Ta and W-20Ta, although their grain sizes are similar to W-10Ta, their relative densities are not as high as W-10Ta. This is probably due to the alloying behavior. Just as XRD patterns (Fig. 1) illustrate, the alloying extent of W-15Ta and W-20Ta is weaker than W-10Ta. Fig. 3 exhibits SEM images of polished surface of various W-Ta alloys. Obviously, there are many Ta-gathering regions in samples of W-15Ta and W-20Ta (inside the yellow oval). Furthermore, a large number of holes exist in Ta-gathering regions, while holes in W area are relatively smaller and less. This result indicates that aggregation of Ta has much to do with relative density. The reason of more holes appear in Ta gathering region might be the particle size of origin Ta powder (48 ␮m) is much bigger than W powder (2 ␮m). The comparatively large size of Ta powder was selected because it has better stability against oxidation.

3.4. Fracture pattern and bending strength Fig. 4 shows the fracture morphology of various samples after three point bending test, which presents significant difference between pure W and W-Ta alloys. Pure W fractures along crystal, while the failure mode in the alloy is predominantly transgranular. In order to clearly display the transgranular fracture, the images of W-Ta alloys were magnified 10000 times. However, for pure W, a smaller magnification was chosen so as to obtain larger visual field. All fracture surfaces displayed here exhibit brittle fracture, no signs of macroscopic plastic deformation are visible. These images also display the changing behavior of the average grain sizes vs. Ta content. Grain size of pure W is much bigger than W-Ta alloys, and this result agrees well with the results in Section 3.2. The bending strength values of different samples are listed in Table 1, which are average values of three tests. And the standard error of each sample ranges from 9.5% to 17.4%. The present results indicate that that the bending strength firstly increases and then decreases with the Ta content increasing. The bending strength of W-5Ta reaches up to 742 MPa, which is 27.1% higher than that of pure W. It is worth mentioning that the relative density of W-5Ta is the lowest. The fracture morphologies observed by SEM which are in Fig. 4 may provide an explanation for this contrast. Fracture appearance of pure W is totally intergranular fracture, while fracture morphology of W-Ta alloys is predominantly transgranular fracture. That means the addition of Ta increased toughness of the material. W-5Ta reveals the highest bending strength, and this phenomenon suggests that adding a small amount of Ta plays an important role in dispersion strengthening. From the fracture morphology of W-15Ta and W-20Ta, it can be seen that the fracture mode round the holes is intergranular fracture. As shown in Fig. 3, there are multihole Ta precipitates within W-15Ta and W20Ta. It can be speculated that, the fracture mode in W-Ta alloyed matrix is transgranular fracture, and the crack may extend along the phase interface when it meets phase interface, and then intergranular fracture in a small area. That is why bending strength of alloys decreases with the Ta content increasing.

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Fig. 3. SEM and EDS images of polished surface on sintered samples.

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Fig. 4. Fracture morphology of sintered samples.

3.5. Vickers hardness Fig. 5 shows Vickers hardness of pure W and W-Ta alloys. Six different positions on polished surface were tested in order to get an average value. Obviously, the addition of Ta greatly improves Vickers hardness of W. The hardness of W-10Ta is the highest, which is 508.65 HV, 42% higher than that of pure W. Meanwhile, the higher the Ta content, the greater the error bar.

Results indicate that Vickers hardness is closely related to grain size. With decreasing of grain size, Vickers hardness becomes higher and higher. The curve in Fig. 5 increases at first, and smoothout when the concentration of Ta reaches 10%. This trend coincides well with grain sizes demonstrated in Section 3.2. Error bars of W-15Ta and W-20Ta are remarkable, this may root in weaker alloying behavior. There are plenty of holes in Ta gathering areas, and the value of Vickers hardness must be lower if the sampling point lies just in these areas. Some literatures consider that, hardness

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performances under plasma irradiation and heat loads are needed in the future. Acknowledgments This work was supported by the National Magnetic Confinement Fusion Science Program of China with Grant No. 2013GB109003, and the National Nature Science Foundation of China with Grant No. 51401012. G. H. Lu acknowledges the support from the China National Funds for Distinguished Young Scientists with Grant No. 51325103. References

Fig. 5. Variation of the Vickers hardness of W compacts as a function of the Ta doping content.

Table 2 Vickers hardness of pure W and W-Ta alloys. Material

W

W-5Ta

W-10Ta

W-15Ta

W-20Ta

Theoretical hardness (HV) Average hardness (HV) Difference (%)

478.67 358.22 25.16

528.17 414.20 21.58

550.80 508.65 7.65

553.28 466.58 15.67

550.40 489.68 11.03

increases with the increasing of density. But it seems that grain size is the more essential reason. According to Hall–Petch relationship, the hardness-grain size relation for W is H = H0 + KH d−1/2 , where H0 is 350 kg/mm2 , KH is 10 kg/mm2 , and d is grain size [21,22]. We calculated the hardness of pure W and W-Ta alloys according to their average grain size, and compared the calculated values with measured values in Table 2. For all samples, average measured hardness is lower than calculated values, and this may attributed to the low relative density. W-10Ta has a higher relative density, so its measured value is much closer to theoretical value. 4. Conclusions Pure W and W-Ta alloys with Ta concentrations of 5, 10, 15 and 20 wt.% were fabricated by spark plasma sintering. The results show that the addition of Ta (5–20 wt.%) reduces the average grain size by a factor of 3 compared to pure W (∼6 ␮m). Adding 10 wt.% of Ta increases the relative density from 94.8% to 96.3%. However, if the concentration of Ta is higher than 15 wt.%, the relative density decreases apparently to 94.9%. Vickers hardness is increased from 358.22 HV to 508.65 HV by adding 10 wt.% Ta and keeps constant with further increasing Ta concentration. The bending strength of the alloy with 5 wt.% Ta reaches up to 742 MPa, which is 27.1% higher than that of pure W, but gradually drops to 624 MPa when Ta concentration increases to 20 wt.%. The degraded properties for the alloys with >10 wt.% Ta are probably ascribed to the noticeable aggregation of Ta particles in the bulk. In conclusion, the addition of Ta in W contributes to refinement of microstructures and improving mechanical properties. SPS-sintered W–10 wt.% Ta exhibits superior properties compared to other alloys. Nevertheless, further investigations regarding the

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