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Effect of molybdenum doping on the microstructure, micro-hardness and thermal shock behavior of WKMoTiY alloy
Journal of Alloys and Compounds 678 (2016) 533e540
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of molybdenum doping on the microstructure, micro-hardness and thermal shock behavior of WeKeMoeTieY alloy Ye Xiao a, Bo Huang a, **, Bo He a, Ke Shi a, Youyun Lian b, Xiang Liu b, Jun Tang a, * a
Key Laboratory of Radiation Physics and Technology of Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China b Southwest Institute of Physics, Chengdu 610064, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2016 Received in revised form 24 March 2016 Accepted 4 April 2016 Available online 6 April 2016
WeKeMoeTieY powder were mechanically alloyed (MA) and then consolidated through spark plasma sintering (SPS). Four different concentrations of molybdenum, i.e., (0, 2, 5, 10) wt%, were applied to investigate the behavior of the alloys. The microstructure, Vickers micro-hardness and thermal shock behavior were analyzed. It is found that the molybdenum doping can mechanically alloy with tungsten and occupy tungsten lattice, which results in a correspondingly decrease of Vickers micro-hardness and thermal conductivity at the room temperature. Among the molybdenum doped samples, WeK-2wt% MoeTieY alloy displays highly enhanced resistance against thermal shock. It is suggested that appropriate molybdenum doping in WeKeMoeTieY alloy is beneficial in consolidating and improving its thermal shock resistance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Tungsten Molybdenum Microstructure Micro-harness Thermal shock
1. Introduction Tungsten and tungsten based alloys have attracted wide interests for their various applications at high temperature circumstances due to its outstanding properties, such as highest melting temperature among metals, low thermal expansion, high thermal conductivity, etc [1e5]. For example, tungsten based alloys are regarded as promising plasma facing materials (PFMs) candidates in fusion reactors, which should sustain extremely severe working conditions, such as high flux plasma etching, high energy neutron irradiation and high transient heat loads up to 1 GW/m2, etc [6e10]. However, the properties of tungsten-based alloys still need to be further optimized to meet the needs of actual applications [7e17]. Especially, cracking caused by transient heat loads would seriously shorten the lifetime of materials and devices [17e25]. Therefore, the thermal shock resistance is one of the most important properties to accommodate the special working conditions. There are several ways to improve their thermal resistance.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Huang), [email protected] (J. Tang). http://dx.doi.org/10.1016/j.jallcom.2016.04.027 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Besides microstructure modulating, tuning the composition of tungsten alloys is another effective methods, which can achieve solid solution strengthening [26e29], oxide dispersion strengthening (ODS) [30e32], potassium bubbles strengthening, etc [33e36]. According to this strategy, researchers have developed novel WeMoeTieY tungsten alloy, named cast tungsten or We13I [6,37,38]. In this system, it is regarded that molybdenum can act as solid solution strengthening, while Ti and Y can absorb free oxygen in the alloy, since the free oxygen is thought to be the original damage source when encountering irradiation [39,40]. More importantly, Ti and molybdenum particle has an excellent thermal stability due to the decrease in the interfacial energy [41]. And the doped yttrium can also act as dispersion strengthening phase. Owing to these multiple strengthening effects, WeMoeTieY alloy displays excellent mechanical properties [4,42]. In addition, potassium bubble doping is another interesting strengthening technique, which has been widely used in tungsten and molybdenum alloys [33,43,44]. For example, tungsten-potassium alloy was applied in lamp filament industry since 1930s [34]. In the previous studies, tungsten-potassium alloy has shown remarkable improvement in mechanical and thermal shock properties compared with pure tungsten [18,33]. Based on these results, potassium bubbles may perform similar function as other dispersion phase. Inspired by these strengthening strategies mentioned above,
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a multi-element alloyed WeKeMoeTieY material is thought to have good performance owing to its mechanical properties and thermal shock resistance. However, how the microstructure can influence its properties? And what is the strengthening effect of each element? Therefore, this novel and interesting WeKeMoeTieY alloy system need to be systematically studied. In the previous work, we have studied the thermal shock behavior of potassium-doped tungsten [9,45]. Herein, dense WeKeMoeTieY ingots were prepared by spark plasma sintering (SPS) using AKS (AleKeSi)-tungsten and Mo, Ti and Y powder successfully. Furthermore, the molybdenum doping effect to the micro-hardness, microstructure and thermal shock behavior of WeKeMoeTieY were investigated. The comparison of different tungsten based alloys after thermal shock test helps to study the mechanical properties and thermal shock resistance.
Table 2 Mixing ratios of each component of W, K, Mo, Ti, and Y.
WeKeTieY WeK-2% MoeTieY WeK-5% MoeTieY WeK-10% MoeTieY
W
K
99.8% 97.8% 94.8% 89.8%
82 82 82 82
ppm ppm ppm ppm
Mo
Ti
Y
0% 2% 5% 10%
0.1% 0.1% 0.1% 0.1%
0.1% 0.1% 0.1% 0.1%
2. Experimental procedure 2.1. Ball-milling by planetary grinding mill AKS-W, Mo (purity>99.9%), Ti (purity>99.99%) and Y (purity>99.9%) powder were used as raw materials. Then, the typical impurities of AKS-W are listed in Table 1. And the Mixing ratios of each component of W, K, Mo, Ti, and Y are listed in Table 2. These mixed powders were alloyed mechanically in a planetary ball mill (QM-3SP04, China) in an argon gas atmosphere for 40 h. With ball-to-powder ratio of 5:1 (diameter of 10 mm and 5 mm tungsten carbide balls with a ratio of 2:1) and a rotation speed of 300 rpm, the powder was sufficiently mixed. Meanwhile, the grain size of powder is supposed to be refined [39]. We turned over the jars which contain the mixed powder every 2.5 h in order to prevent powder from agglomeration. To minimize contaminations in ball milling, balls and jars in this experiment are made of WC-8wt% Co.
Fig. 1. The SPS temperature and pressure curve for sintering process.
K3Fe(CN)6þ2.5 g NaOHþ25 ml H2O. The diameters of more than 100 grains were measured through the smooth surface after etching in each sample. 2.3. Characterizing procedure The thermal shock test was carried out in the electron beam test facility EMS-60 at Southwest Institute of Physics (SWIP, China). In the test, the heat flux we use can be defined as absorbed power Pabs, which can be expressed as the following formula [9]:
2.2. Sintering progress and polishing
Pabs ¼ UIa
The as-prepared powder after ball-milling was weighted and fed into a graphite die. And carbon paper was used to separate the powder from the graphite die. The consolidation of the samples was carried out through SPS (LABOX-325, Japan) equipment. The consolidation process of SPS was mainly affected by sintering parameters, including sintering temperature, applied pressure, holding time, heating rate and sintering atmosphere besides the factor of powder [46]. Optimization of the sintering parameters for the SPS sintering of AKS-doped tungsten has been carried out in our previous work [45]. Then the SPS temperature and pressure curve of sintering program in this study was illustrated in Fig. 1. With a uniaxial pressure from 0 MPa to 80 MPa applied, the preprepared powder were heated to 1750 C with the heating rate of 100 C/min and held for 3 min. And then, the powder was cooled down to the room temperature through about 30 min. All the sintering process was conducted in vacuum. The size of obtained sintered samples was about 15.0 mm in diameter and 2.0 mm in thickness. The consolidated samples were polished in order to characterize the samples as exactly as possible. Finally, all the samples were annealed in a vacuum furnace at 1273 K for 2 h. The grain boundaries were revealed by etching in a solution of 2.5 g
Where U, I and a donate the acceleration voltage, beam current, and absorption coefficient, respectively. Given an electron absorption coefficient of 0.55, the acceleration voltage of 120 kV, the beam current of 90 mA and the scanning area of 4 4 mm2, the absorbed power density of the heat flux was up to 0.37 GW/m2. Therefore, in this paper, a single pulse with 0.37 GW/m2 power density is concentrated on a scanning area of 4 4 mm2 for 5 ms at room temperature, in order to characterize the resistance against thermal shock. The powder after milling and sintered ingots were characterized by X-ray diffraction (XRD) and the density of the sintered samples was determined by Archimedes' principle. The polished samples were subjected to Vickers micro-hardness testing at room temperature using a Vickers diamond pyramid with a load of 1.96 N for 10 s. The crystallite size was estimated by XRD analysis using the Scherrer formula. Microstructure of fracture surface was also characterized a field-emission scanning electron microscopy (FESEM, Hitachi S4800) and elements analysis was conducted using the energy dispersion spectroscopy (EDS).
Table 1 Impurity composition of AKS-W (potassium-doped tungsten) powder.
3.1. Characterization of milling powder
Elements
Si
Al
K
Fe
Co
Ni
N
C
S
Contents (ppm)
185
30
82
28
4
4
13
<10
<10
3. Results and discussion
The crystallite size evolution corresponding to ball milling can be estimated by XRD measurements. Taking the WeK-2wt% MoeTieY powder as an example, the XRD spectra after different
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case, the higher dislocation and finer grain structure formed because of the lower recovery rate, which results from the lower stacking fault energy [52]. Therefore, the reduction in crystallite size of tungsten with Mo alloying may be attributed to decrease in stacking fault energy. In the XRD spectra, the peaks of W and Mo were seriously overlapped in 110 and 200 peaks which results from their similar lattice parameters. It is hard to assess the alloying as a function of milling time through the 110 and 200 peaks. In this case, the 211 peak was applied to confirm the alloying. As shown in Fig. 2(b), it is obviously shown that the peak intensity of Mo decreases and the peaks of W and Mo combine into one with addition of ball-milling time [4]. Based on the situation, it is deduced that the molybdenum was mechanically alloy with tungsten in this process. 3.2. Characterization of sintering samples
Fig. 2. XRD spectra of W-K-2wt% MoeTieY: (a) W-K-2wt% MoeTieY powder after different ball-milling times with the crystallite size in the illustration; (b) Effect of milling time on alloying behavior in WeMo showing 211 peak.
ball-milling time were shown in Fig. 2(a). It is displayed that the alloys consist of a single tungsten-based phase with BCC (bodycentered cubic) structure. And the tungsten peaks become widen obviously after the first 10 h. Then, the peaks become widen slightly in the following 30 h. Based on the Scherrer formula, we can conclude that the powder has been refined [47]. The crystallite size of the powder after different ball-milling time was calculated and shown in the illustration. It can be seen that the crystallite size of the ball-milled powder after 40 h decreases to 16.8 nm, which is quite different from that of the raw composites powder (about 149.1 nm). During the ball-milling progress, the ultimate crystallite size depends on the minimum grain size that can sustain dislocation pileup within a grain and the rate of recovery progress which is determined by dislocation cross-slip [48]. Owing to the recombination and annihilation of dislocation to form low angle boundaries, dislocation cross-slip, which is strongly influenced by the stacking fault energy, plays an important role in recovery progress [49,50]. Generally speaking, the stacking fault energy would decrease with alloying, such as W and Mo in this system, whose stacking fault energies are 500 and 430 MJ/m2, respectively [51]. During the ball-milling progress, the segregation of solute atoms to the stacking faults could be responsible for partial dislocation separating further to each other [51]. With the reducing stacking fault energy, it becomes more difficult to produce cross-slip. In this
3.2.1. The densification and microstructure analysis The measured results, including grain size, lattice parameter, density and micro-hardness of the spark plasma sintered WeK(0e10) wt% MoeTieY alloys samples are listed in Table 3. The density of the samples is trending downward with the increase in molybdenum content, and the relative density of these samples was well constant with the molybdenum addition. Among those ingots, the one of 2 wt % molybdenum doping had the highest relative density of 97.1% corresponding to a theoretical density of 18.509 g/cm3. Upon the relative density, it can be concluded that all samples are well consolidated after spark plasma sintering. Lattice parameters (BCC structure, a ¼ b ¼ c) were calculated from the XRD pattern of the corresponding samples. The XRD spectra of WeK-(0e10) wt% MoeTieY alloy and the lattice parameter are shown in Fig. 3. It is shown that the lattice constant decreases slightly with the molybdenum addition. Four main peaks were attributed to tungsten and no molybdenum peaks were observed, suggesting that the molybdenum was well mechanically alloyed with tungsten and occupy tungsten lattice. Ti and Y peaks were not detectable owing to its low content. To further confirm the alloying details, the elements analysis was also carried out. Taking the WeK-10 wt% MoeTieY alloy as an example, the heterogeneous distribution is illustrated by the chemical arrangement in the equilibrated alloy in Fig. 4. Fig. 4(a) shows the appearance under SEM and Fig. 3(bed) shows the local chemical map based on energy dispersive spectroscopy (EDS). From the ellipse area, it is found that the distribution of molybdenum is similar to that of tungsten, conforming that the molybdenum is mechanically alloyed with tungsten and occupy tungsten lattice to some extent. However, when concentrating on the square area, Ti assembled in the tungsten-less area. It is suggested that Ti should mainly distribute in the grain boundaries. Our other high resolution study demonstrated that the Y dispersed with Y particles covered with oxide layer and potassium mainly dispersed in the grain with a form of potassium bubbles. This should be the reason that they are not observed in this work. The fracture morphology of the sintered samples is also characterized by SEM, as shown in Fig. 5 (aed). It can be found that the molybdenum doping in WeKeMoeTieY alloys did not change the grain size obviously. Brittle fracture could be seen in the WeKeMoeTieY alloys, including inter-granular fracture and transgranular fracture. In the trans-granular area, potassium bubbles with the grain size from 20 to 100 nm were clearly observed inside the grains (the area pointed out by yellow arrow), and the dark gray phase is mainly distributed at the grain boundaries. EDS results revealed that the dark gray phase was Ti-rich region (the area pointed out by black arrow), while the light gray region was mainly the molybdenum doping on the tungsten matrix. It was also
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Table 3 Grain size, lattice parameter, theoretical density, density, relative density and Vickers micro-hardness of the WeKeMoeTieY alloys. Samples
Grain size (mm)
Lattice parameter (nm)
Density (g/cm3)
Relative density (%)
Vickers micro-hardness (MPa)
WeKeTieY WeK-2% MoeTieY WeK-5% MoeTieY WeK-10% MoeTieY
5.18 4.84 4.74 4.71
0.31863 0.31767 0.31725 0.31711
18.619 18.509 18.197 17.669
96.4 97.1 96.7 96.0
464.95 425.49 400.35 391.93
relationship [54]:
sy ¼ s0 þ ky d1=2
Fig. 3. The XRD spectra of W-K-(0, 2, 5, 10) wt% MoeTieY alloys and the relation curve between lattice parameter and molybdenum concentration in the inset.
Where sy is the yield stress, s0 is the friction stress, d is the average grain size, and ky is the stress concentration factor. Based on the formula, the larger inverse square of grain size would cause increased strength, as well as the micro-hardness with the same trend [55]. And the micro-hardness should be increased due to the refinement of grains. Paradoxically, the micro-hardness of WeK(0e10) wt.% MoeTieY alloys changed as 464.95, 425.49, 400.35, 391.93 HV, showing a decrease trend with the addition of molybdenum. Obviously, the anomalous trend indicates that the grain size was not the decisive factor to the hardness in this system. When comparing micro-hardness to relative density in Fig. 6, the HV values decrease sharply with the molybdenum content from 0 to 10 wt%, while the relative densities and grain size of all the samples remain relatively stable. It is suggested that such apparent
Fig. 4. Chemical analysis of the WeK-10 wt% MoeTieY alloy: (a) the morphology under SEM (bed) The Mo, Ti and W elemental map confirms a heterogeneous structure.
displayed that the average size of the Ti-rich phase is ranging from 100 nm to 1 mm. Because of the low content of potassium, it cannot be observed in the EDS spectra. 3.2.2. Vickers micro-hardness analysis Vickers micro-hardness of the polished samples was tested at room temperature. It is well known that the hardness of bulk samples can be influenced by three factors, i.e., grain size, relative density and doping content [53]. The relationship between grain size and micro-hardness is base on the Hall-Petch (HeP)
reduction in micro-hardness should be attributed to the distorting effect of doped atoms (Mo) on the base metal (W) crystal lattice, besides the lower hardness of Mo compared to W, rather than the grain size or relative density. Another explanation may be due to non-homogeneous microstructure in WeKeMoeTieY alloy [42]. 3.2.3. Thermal shock tests Transient heat load tests were carried out under 0.37 GW/m2 heat load. The surface morphology of the samples after thermal shock is shown in Fig. 7. Cracks occurred in all the samples except
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Fig. 5. The fracture surface morphology of the sintered samples: (a) WeKeTieY (b) WeK-2% MoeTieY (c) WeK-5% MoeTieY (d) WeK-10% MoeTieY and EDX spectra of the WeK2% MoeTieY alloy: (e) dark gray region (f) light gray region.
WeK-2% MoeTieY, which might be attributed to the increase of ductility. The crack number of WeK-10% MoeTieY is relatively less than WeKeTieY and WeK-5% MoeTieY while the crack distance is larger. The maximum crack width of the four investigated samples is shown in Fig. 8. It is displayed that the maximum crack width decreases before appropriate molybdenum doping, then increases with excessive molybdenum content. More detailed information can be obtained from the surface roughness of the thermal loaded sample, as shown in Fig. 8 (eeh). With the increased Mo doping, the surface roughness evolved as 0.3412, 0.1256, 0.2364 and 0.1504 mm, respectively, which reflected the plastic deformation. Because the
plastic deformation strongly relates to the yield strength, as well as the downtrend of micro-hardness, it can be concluded that the strength also decrease. Besides the mechanical strength, thermal conductivity is another factor to influence its thermal shock properties. Thus the thermal conductivity of WeKeMoeTieY alloys was measured, as shown in Fig. 9. It showed a sharply decrease with the addition of molybdenum. In this way, the increased crack width should be due to the decrease of strength and thermal conductivity. Because the strength, toughness and thermal conductivity strongly determine the thermal shock properties of materials, the overall performance of WeK-2% MoeTieY alloy might contribute to the toughness. Therefore, alloying elements could have significant
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influence on the intense transient loads resistance and the mechanical stability. It is suggested that the molybdenum doping has solid solution strengthening effects on the thermal shock resistance. In another word, the appropriate molybdenum addition (<2 wt%) have positive effects on tungsten to prevent cracking in transient high heat load, while excessive molybdenum addition do not have this effect. .
4. Conclusion
Fig. 6. The relative density and Inverse Square of grain size with the addition of molybdenum with the Vickers micro-hardness trend in the illustration.
Dense WeKeMoeTieY alloys were sintered through spark plasma sintering (SPS) technique. The relative density of all these samples was about 97%. It is found that the addition of molybdenum can lead to the decrease of Vickers micro-hardness in the room temperature. The sample with 2% molybdenum doping
Fig. 7. The surface morphology after thermal shock: (a) WeKeTieY (b) WeK-2% MoeTieY (c) WeK-5% MoeTieY (d) WeKe10% MoeTieY. The insets show the outline of cracks.
Fig. 8. The cracked surface of the samples after thermal shock: (a) WeKeTieY (b) WeK-2% MoeTieY (c) WeK-5% MoeTieY (d) WeK-10% MoeTieY and the surface roughness of the samples after thermal shock: (e) WeKeTieY (f) WeK-2% MoeTieY (g) WeK-5% MoeTieY (h) WeK-10% MoeTieY.
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[12]
[13]
[14]
[15]
[16]
[17]
Fig. 9. Thermal conductivity of WeKeMoeTieY alloys with the addition of molybdenum.
[18]
[19]
displays high thermal shock resistance than those with other doping contents. It was concluded that appropriate amount of molybdenum doping is beneficial in improving the thermal shock resistance, while excessive molybdenum doping brings negative effects. In a technological point of view, given the exceptionally thermal shock behavior of crystalline BCC metals studied in the test, the present results developed a new family of engineering tungsten-based alloys. Meanwhile, our experimental work on WeKeMoeTieY alloy is an example of improving the thermal shock resistance. These discoveries may provide helpful guideline to improve the properties of tungsten-based plasma facing materials (PFMs), as well as the approach which applied in WeKeMoeTieY alloy might be used again to a variety of different metals. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11475118), the International Thermonuclear Experimental Reactor (ITER) Program Special (No. 2011GB108005), and the National Fund of China for Fostering Talents in Basic Science (J1210004). References
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of molybdenum doping on the microstructure, micro-hardness and thermal shock behavior of WeKeMoeTieY alloy Ye Xiao a, Bo Huang a, **, Bo He a, Ke Shi a, Youyun Lian b, Xiang Liu b, Jun Tang a, * a
Key Laboratory of Radiation Physics and Technology of Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China b Southwest Institute of Physics, Chengdu 610064, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2016 Received in revised form 24 March 2016 Accepted 4 April 2016 Available online 6 April 2016
WeKeMoeTieY powder were mechanically alloyed (MA) and then consolidated through spark plasma sintering (SPS). Four different concentrations of molybdenum, i.e., (0, 2, 5, 10) wt%, were applied to investigate the behavior of the alloys. The microstructure, Vickers micro-hardness and thermal shock behavior were analyzed. It is found that the molybdenum doping can mechanically alloy with tungsten and occupy tungsten lattice, which results in a correspondingly decrease of Vickers micro-hardness and thermal conductivity at the room temperature. Among the molybdenum doped samples, WeK-2wt% MoeTieY alloy displays highly enhanced resistance against thermal shock. It is suggested that appropriate molybdenum doping in WeKeMoeTieY alloy is beneficial in consolidating and improving its thermal shock resistance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Tungsten Molybdenum Microstructure Micro-harness Thermal shock
1. Introduction Tungsten and tungsten based alloys have attracted wide interests for their various applications at high temperature circumstances due to its outstanding properties, such as highest melting temperature among metals, low thermal expansion, high thermal conductivity, etc [1e5]. For example, tungsten based alloys are regarded as promising plasma facing materials (PFMs) candidates in fusion reactors, which should sustain extremely severe working conditions, such as high flux plasma etching, high energy neutron irradiation and high transient heat loads up to 1 GW/m2, etc [6e10]. However, the properties of tungsten-based alloys still need to be further optimized to meet the needs of actual applications [7e17]. Especially, cracking caused by transient heat loads would seriously shorten the lifetime of materials and devices [17e25]. Therefore, the thermal shock resistance is one of the most important properties to accommodate the special working conditions. There are several ways to improve their thermal resistance.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Huang), [email protected] (J. Tang). http://dx.doi.org/10.1016/j.jallcom.2016.04.027 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Besides microstructure modulating, tuning the composition of tungsten alloys is another effective methods, which can achieve solid solution strengthening [26e29], oxide dispersion strengthening (ODS) [30e32], potassium bubbles strengthening, etc [33e36]. According to this strategy, researchers have developed novel WeMoeTieY tungsten alloy, named cast tungsten or We13I [6,37,38]. In this system, it is regarded that molybdenum can act as solid solution strengthening, while Ti and Y can absorb free oxygen in the alloy, since the free oxygen is thought to be the original damage source when encountering irradiation [39,40]. More importantly, Ti and molybdenum particle has an excellent thermal stability due to the decrease in the interfacial energy [41]. And the doped yttrium can also act as dispersion strengthening phase. Owing to these multiple strengthening effects, WeMoeTieY alloy displays excellent mechanical properties [4,42]. In addition, potassium bubble doping is another interesting strengthening technique, which has been widely used in tungsten and molybdenum alloys [33,43,44]. For example, tungsten-potassium alloy was applied in lamp filament industry since 1930s [34]. In the previous studies, tungsten-potassium alloy has shown remarkable improvement in mechanical and thermal shock properties compared with pure tungsten [18,33]. Based on these results, potassium bubbles may perform similar function as other dispersion phase. Inspired by these strengthening strategies mentioned above,
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a multi-element alloyed WeKeMoeTieY material is thought to have good performance owing to its mechanical properties and thermal shock resistance. However, how the microstructure can influence its properties? And what is the strengthening effect of each element? Therefore, this novel and interesting WeKeMoeTieY alloy system need to be systematically studied. In the previous work, we have studied the thermal shock behavior of potassium-doped tungsten [9,45]. Herein, dense WeKeMoeTieY ingots were prepared by spark plasma sintering (SPS) using AKS (AleKeSi)-tungsten and Mo, Ti and Y powder successfully. Furthermore, the molybdenum doping effect to the micro-hardness, microstructure and thermal shock behavior of WeKeMoeTieY were investigated. The comparison of different tungsten based alloys after thermal shock test helps to study the mechanical properties and thermal shock resistance.
Table 2 Mixing ratios of each component of W, K, Mo, Ti, and Y.
WeKeTieY WeK-2% MoeTieY WeK-5% MoeTieY WeK-10% MoeTieY
W
K
99.8% 97.8% 94.8% 89.8%
82 82 82 82
ppm ppm ppm ppm
Mo
Ti
Y
0% 2% 5% 10%
0.1% 0.1% 0.1% 0.1%
0.1% 0.1% 0.1% 0.1%
2. Experimental procedure 2.1. Ball-milling by planetary grinding mill AKS-W, Mo (purity>99.9%), Ti (purity>99.99%) and Y (purity>99.9%) powder were used as raw materials. Then, the typical impurities of AKS-W are listed in Table 1. And the Mixing ratios of each component of W, K, Mo, Ti, and Y are listed in Table 2. These mixed powders were alloyed mechanically in a planetary ball mill (QM-3SP04, China) in an argon gas atmosphere for 40 h. With ball-to-powder ratio of 5:1 (diameter of 10 mm and 5 mm tungsten carbide balls with a ratio of 2:1) and a rotation speed of 300 rpm, the powder was sufficiently mixed. Meanwhile, the grain size of powder is supposed to be refined [39]. We turned over the jars which contain the mixed powder every 2.5 h in order to prevent powder from agglomeration. To minimize contaminations in ball milling, balls and jars in this experiment are made of WC-8wt% Co.
Fig. 1. The SPS temperature and pressure curve for sintering process.
K3Fe(CN)6þ2.5 g NaOHþ25 ml H2O. The diameters of more than 100 grains were measured through the smooth surface after etching in each sample. 2.3. Characterizing procedure The thermal shock test was carried out in the electron beam test facility EMS-60 at Southwest Institute of Physics (SWIP, China). In the test, the heat flux we use can be defined as absorbed power Pabs, which can be expressed as the following formula [9]:
2.2. Sintering progress and polishing
Pabs ¼ UIa
The as-prepared powder after ball-milling was weighted and fed into a graphite die. And carbon paper was used to separate the powder from the graphite die. The consolidation of the samples was carried out through SPS (LABOX-325, Japan) equipment. The consolidation process of SPS was mainly affected by sintering parameters, including sintering temperature, applied pressure, holding time, heating rate and sintering atmosphere besides the factor of powder [46]. Optimization of the sintering parameters for the SPS sintering of AKS-doped tungsten has been carried out in our previous work [45]. Then the SPS temperature and pressure curve of sintering program in this study was illustrated in Fig. 1. With a uniaxial pressure from 0 MPa to 80 MPa applied, the preprepared powder were heated to 1750 C with the heating rate of 100 C/min and held for 3 min. And then, the powder was cooled down to the room temperature through about 30 min. All the sintering process was conducted in vacuum. The size of obtained sintered samples was about 15.0 mm in diameter and 2.0 mm in thickness. The consolidated samples were polished in order to characterize the samples as exactly as possible. Finally, all the samples were annealed in a vacuum furnace at 1273 K for 2 h. The grain boundaries were revealed by etching in a solution of 2.5 g
Where U, I and a donate the acceleration voltage, beam current, and absorption coefficient, respectively. Given an electron absorption coefficient of 0.55, the acceleration voltage of 120 kV, the beam current of 90 mA and the scanning area of 4 4 mm2, the absorbed power density of the heat flux was up to 0.37 GW/m2. Therefore, in this paper, a single pulse with 0.37 GW/m2 power density is concentrated on a scanning area of 4 4 mm2 for 5 ms at room temperature, in order to characterize the resistance against thermal shock. The powder after milling and sintered ingots were characterized by X-ray diffraction (XRD) and the density of the sintered samples was determined by Archimedes' principle. The polished samples were subjected to Vickers micro-hardness testing at room temperature using a Vickers diamond pyramid with a load of 1.96 N for 10 s. The crystallite size was estimated by XRD analysis using the Scherrer formula. Microstructure of fracture surface was also characterized a field-emission scanning electron microscopy (FESEM, Hitachi S4800) and elements analysis was conducted using the energy dispersion spectroscopy (EDS).
Table 1 Impurity composition of AKS-W (potassium-doped tungsten) powder.
3.1. Characterization of milling powder
Elements
Si
Al
K
Fe
Co
Ni
N
C
S
Contents (ppm)
185
30
82
28
4
4
13
<10
<10
3. Results and discussion
The crystallite size evolution corresponding to ball milling can be estimated by XRD measurements. Taking the WeK-2wt% MoeTieY powder as an example, the XRD spectra after different
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case, the higher dislocation and finer grain structure formed because of the lower recovery rate, which results from the lower stacking fault energy [52]. Therefore, the reduction in crystallite size of tungsten with Mo alloying may be attributed to decrease in stacking fault energy. In the XRD spectra, the peaks of W and Mo were seriously overlapped in 110 and 200 peaks which results from their similar lattice parameters. It is hard to assess the alloying as a function of milling time through the 110 and 200 peaks. In this case, the 211 peak was applied to confirm the alloying. As shown in Fig. 2(b), it is obviously shown that the peak intensity of Mo decreases and the peaks of W and Mo combine into one with addition of ball-milling time [4]. Based on the situation, it is deduced that the molybdenum was mechanically alloy with tungsten in this process. 3.2. Characterization of sintering samples
Fig. 2. XRD spectra of W-K-2wt% MoeTieY: (a) W-K-2wt% MoeTieY powder after different ball-milling times with the crystallite size in the illustration; (b) Effect of milling time on alloying behavior in WeMo showing 211 peak.
ball-milling time were shown in Fig. 2(a). It is displayed that the alloys consist of a single tungsten-based phase with BCC (bodycentered cubic) structure. And the tungsten peaks become widen obviously after the first 10 h. Then, the peaks become widen slightly in the following 30 h. Based on the Scherrer formula, we can conclude that the powder has been refined [47]. The crystallite size of the powder after different ball-milling time was calculated and shown in the illustration. It can be seen that the crystallite size of the ball-milled powder after 40 h decreases to 16.8 nm, which is quite different from that of the raw composites powder (about 149.1 nm). During the ball-milling progress, the ultimate crystallite size depends on the minimum grain size that can sustain dislocation pileup within a grain and the rate of recovery progress which is determined by dislocation cross-slip [48]. Owing to the recombination and annihilation of dislocation to form low angle boundaries, dislocation cross-slip, which is strongly influenced by the stacking fault energy, plays an important role in recovery progress [49,50]. Generally speaking, the stacking fault energy would decrease with alloying, such as W and Mo in this system, whose stacking fault energies are 500 and 430 MJ/m2, respectively [51]. During the ball-milling progress, the segregation of solute atoms to the stacking faults could be responsible for partial dislocation separating further to each other [51]. With the reducing stacking fault energy, it becomes more difficult to produce cross-slip. In this
3.2.1. The densification and microstructure analysis The measured results, including grain size, lattice parameter, density and micro-hardness of the spark plasma sintered WeK(0e10) wt% MoeTieY alloys samples are listed in Table 3. The density of the samples is trending downward with the increase in molybdenum content, and the relative density of these samples was well constant with the molybdenum addition. Among those ingots, the one of 2 wt % molybdenum doping had the highest relative density of 97.1% corresponding to a theoretical density of 18.509 g/cm3. Upon the relative density, it can be concluded that all samples are well consolidated after spark plasma sintering. Lattice parameters (BCC structure, a ¼ b ¼ c) were calculated from the XRD pattern of the corresponding samples. The XRD spectra of WeK-(0e10) wt% MoeTieY alloy and the lattice parameter are shown in Fig. 3. It is shown that the lattice constant decreases slightly with the molybdenum addition. Four main peaks were attributed to tungsten and no molybdenum peaks were observed, suggesting that the molybdenum was well mechanically alloyed with tungsten and occupy tungsten lattice. Ti and Y peaks were not detectable owing to its low content. To further confirm the alloying details, the elements analysis was also carried out. Taking the WeK-10 wt% MoeTieY alloy as an example, the heterogeneous distribution is illustrated by the chemical arrangement in the equilibrated alloy in Fig. 4. Fig. 4(a) shows the appearance under SEM and Fig. 3(bed) shows the local chemical map based on energy dispersive spectroscopy (EDS). From the ellipse area, it is found that the distribution of molybdenum is similar to that of tungsten, conforming that the molybdenum is mechanically alloyed with tungsten and occupy tungsten lattice to some extent. However, when concentrating on the square area, Ti assembled in the tungsten-less area. It is suggested that Ti should mainly distribute in the grain boundaries. Our other high resolution study demonstrated that the Y dispersed with Y particles covered with oxide layer and potassium mainly dispersed in the grain with a form of potassium bubbles. This should be the reason that they are not observed in this work. The fracture morphology of the sintered samples is also characterized by SEM, as shown in Fig. 5 (aed). It can be found that the molybdenum doping in WeKeMoeTieY alloys did not change the grain size obviously. Brittle fracture could be seen in the WeKeMoeTieY alloys, including inter-granular fracture and transgranular fracture. In the trans-granular area, potassium bubbles with the grain size from 20 to 100 nm were clearly observed inside the grains (the area pointed out by yellow arrow), and the dark gray phase is mainly distributed at the grain boundaries. EDS results revealed that the dark gray phase was Ti-rich region (the area pointed out by black arrow), while the light gray region was mainly the molybdenum doping on the tungsten matrix. It was also
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Table 3 Grain size, lattice parameter, theoretical density, density, relative density and Vickers micro-hardness of the WeKeMoeTieY alloys. Samples
Grain size (mm)
Lattice parameter (nm)
Density (g/cm3)
Relative density (%)
Vickers micro-hardness (MPa)
WeKeTieY WeK-2% MoeTieY WeK-5% MoeTieY WeK-10% MoeTieY
5.18 4.84 4.74 4.71
0.31863 0.31767 0.31725 0.31711
18.619 18.509 18.197 17.669
96.4 97.1 96.7 96.0
464.95 425.49 400.35 391.93
relationship [54]:
sy ¼ s0 þ ky d1=2
Fig. 3. The XRD spectra of W-K-(0, 2, 5, 10) wt% MoeTieY alloys and the relation curve between lattice parameter and molybdenum concentration in the inset.
Where sy is the yield stress, s0 is the friction stress, d is the average grain size, and ky is the stress concentration factor. Based on the formula, the larger inverse square of grain size would cause increased strength, as well as the micro-hardness with the same trend [55]. And the micro-hardness should be increased due to the refinement of grains. Paradoxically, the micro-hardness of WeK(0e10) wt.% MoeTieY alloys changed as 464.95, 425.49, 400.35, 391.93 HV, showing a decrease trend with the addition of molybdenum. Obviously, the anomalous trend indicates that the grain size was not the decisive factor to the hardness in this system. When comparing micro-hardness to relative density in Fig. 6, the HV values decrease sharply with the molybdenum content from 0 to 10 wt%, while the relative densities and grain size of all the samples remain relatively stable. It is suggested that such apparent
Fig. 4. Chemical analysis of the WeK-10 wt% MoeTieY alloy: (a) the morphology under SEM (bed) The Mo, Ti and W elemental map confirms a heterogeneous structure.
displayed that the average size of the Ti-rich phase is ranging from 100 nm to 1 mm. Because of the low content of potassium, it cannot be observed in the EDS spectra. 3.2.2. Vickers micro-hardness analysis Vickers micro-hardness of the polished samples was tested at room temperature. It is well known that the hardness of bulk samples can be influenced by three factors, i.e., grain size, relative density and doping content [53]. The relationship between grain size and micro-hardness is base on the Hall-Petch (HeP)
reduction in micro-hardness should be attributed to the distorting effect of doped atoms (Mo) on the base metal (W) crystal lattice, besides the lower hardness of Mo compared to W, rather than the grain size or relative density. Another explanation may be due to non-homogeneous microstructure in WeKeMoeTieY alloy [42]. 3.2.3. Thermal shock tests Transient heat load tests were carried out under 0.37 GW/m2 heat load. The surface morphology of the samples after thermal shock is shown in Fig. 7. Cracks occurred in all the samples except
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Fig. 5. The fracture surface morphology of the sintered samples: (a) WeKeTieY (b) WeK-2% MoeTieY (c) WeK-5% MoeTieY (d) WeK-10% MoeTieY and EDX spectra of the WeK2% MoeTieY alloy: (e) dark gray region (f) light gray region.
WeK-2% MoeTieY, which might be attributed to the increase of ductility. The crack number of WeK-10% MoeTieY is relatively less than WeKeTieY and WeK-5% MoeTieY while the crack distance is larger. The maximum crack width of the four investigated samples is shown in Fig. 8. It is displayed that the maximum crack width decreases before appropriate molybdenum doping, then increases with excessive molybdenum content. More detailed information can be obtained from the surface roughness of the thermal loaded sample, as shown in Fig. 8 (eeh). With the increased Mo doping, the surface roughness evolved as 0.3412, 0.1256, 0.2364 and 0.1504 mm, respectively, which reflected the plastic deformation. Because the
plastic deformation strongly relates to the yield strength, as well as the downtrend of micro-hardness, it can be concluded that the strength also decrease. Besides the mechanical strength, thermal conductivity is another factor to influence its thermal shock properties. Thus the thermal conductivity of WeKeMoeTieY alloys was measured, as shown in Fig. 9. It showed a sharply decrease with the addition of molybdenum. In this way, the increased crack width should be due to the decrease of strength and thermal conductivity. Because the strength, toughness and thermal conductivity strongly determine the thermal shock properties of materials, the overall performance of WeK-2% MoeTieY alloy might contribute to the toughness. Therefore, alloying elements could have significant
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influence on the intense transient loads resistance and the mechanical stability. It is suggested that the molybdenum doping has solid solution strengthening effects on the thermal shock resistance. In another word, the appropriate molybdenum addition (<2 wt%) have positive effects on tungsten to prevent cracking in transient high heat load, while excessive molybdenum addition do not have this effect. .
4. Conclusion
Fig. 6. The relative density and Inverse Square of grain size with the addition of molybdenum with the Vickers micro-hardness trend in the illustration.
Dense WeKeMoeTieY alloys were sintered through spark plasma sintering (SPS) technique. The relative density of all these samples was about 97%. It is found that the addition of molybdenum can lead to the decrease of Vickers micro-hardness in the room temperature. The sample with 2% molybdenum doping
Fig. 7. The surface morphology after thermal shock: (a) WeKeTieY (b) WeK-2% MoeTieY (c) WeK-5% MoeTieY (d) WeKe10% MoeTieY. The insets show the outline of cracks.
Fig. 8. The cracked surface of the samples after thermal shock: (a) WeKeTieY (b) WeK-2% MoeTieY (c) WeK-5% MoeTieY (d) WeK-10% MoeTieY and the surface roughness of the samples after thermal shock: (e) WeKeTieY (f) WeK-2% MoeTieY (g) WeK-5% MoeTieY (h) WeK-10% MoeTieY.
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[12]
[13]
[14]
[15]
[16]
[17]
Fig. 9. Thermal conductivity of WeKeMoeTieY alloys with the addition of molybdenum.
[18]
[19]
displays high thermal shock resistance than those with other doping contents. It was concluded that appropriate amount of molybdenum doping is beneficial in improving the thermal shock resistance, while excessive molybdenum doping brings negative effects. In a technological point of view, given the exceptionally thermal shock behavior of crystalline BCC metals studied in the test, the present results developed a new family of engineering tungsten-based alloys. Meanwhile, our experimental work on WeKeMoeTieY alloy is an example of improving the thermal shock resistance. These discoveries may provide helpful guideline to improve the properties of tungsten-based plasma facing materials (PFMs), as well as the approach which applied in WeKeMoeTieY alloy might be used again to a variety of different metals. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11475118), the International Thermonuclear Experimental Reactor (ITER) Program Special (No. 2011GB108005), and the National Fund of China for Fostering Talents in Basic Science (J1210004). References
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