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International Journal of Refractory Metals & Hard Materials 26 (2008) 540–548 www.elsevier.com/locate/IJRMHM
Thermal–mechanical process in producing high dispersed tungsten–copper composite powder Yunping Li a,*, Shu Yu b b
a Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan State Key Laboratory for Powder Metallurgy, Central South University, Changsha, Hunan 410083, China
Received 25 May 2007; accepted 14 January 2008
Abstract In this study, the reduction mechanism of tungsten–copper oxide precursors WO3, CuWO4 as well as the mixtures of these two components were analyzed in detail by TGA–DTA, SEM as well as particle size distribution, etc. The results showed that after milling for the oxide precursors in alcohol the reduced tungsten–copper composite powder showed better particle size distributions than the other two processes of dry milling and water milling. Surface modification by nitrogen showed to be effective in the anti-oxidation of the newly reduced powder because the absorption of nitrogen in the particle surface inhibits further oxidation. The final sintered parts of the reduced powder at 1200 °C and 60 min showed to have an extremely high relative density than the traditional process. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Thermal–mechanical process; Tungsten–copper; Reduction; BET specific area; Particle size distribution
1. Introduction Recently, there has been a growing interest in tungsten– copper alloys owing to their superior thermal management properties and high microwave absorption capacity [1,2]. Traditionally, tungsten–copper alloy parts are fabricated by Cu infiltration of W preforms. Infiltration is a two-step process that wicks molten copper into the open pores of a previously sintered tungsten porous structure. Infiltration is a time-consuming process. Furthermore, the infiltration process does not result in a homogeneous microstructure and is not a net shape process, thus causing higher production costs [3]. In order to reduce costs and to produce net shaped components with a homogeneous microstructure, high relative density tungsten–copper parts can be produced by liquid-phase sintering of composite powders characterized by fine dispersions of both metals [1,2,4–9].
*
Corresponding author. Tel.: +81 22 215 2118; fax: +81 22 215 2116. E-mail address:
[email protected] (Y.P. Li).
0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.01.001
Such composite powders can be obtained by the reduction of copper and tungsten oxides in a hydrogen atmosphere. However, the final properties of tungsten–copper composites will be heavily affected if the precursor WO3, CuWO4x or its mixture cannot be completely reduced because the existence of WOx will greatly reduce the thermal conductivity and tensile strength of the final products. As reported previously [1,4], the milling process applied to the oxide precursor before the reduction process proved to be an effective method in improving the final property of reduced powder. After milling and reduction, the copper particles appear to be encapsulated by tungsten. The average particle size is very fine and this composite powder has an extremely high BET specific area. In order to improve this prospective process, the dynamic reduction behavior of precursors in hydrogen as well as the properties of the reduced composite powder has been analyzed. Different treatments to the oxide powder before and after reduction were applied and the effects to the precursors and the reduced powder have been analyzed in detail.
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2. Experimental The materials used in this study were commercial WO3 and CuO powder. A CuWO4 precursor was produced by oxidation in an air atmosphere at 1100 °C for about 60 min from the WO3 and CuO powder. Milling was carried out in an attrition mill (attritor, QM-1SP type) in order to disperse and smash the oxide powder. The ball to powder ratio of 3 (weight ratio) was maintained in the stainless vials. Milling was carried out in three different kinds of mediums: air, alcohol, as well as water in order to optimize the milling process. Water was used because of a lower cost than alcohol, and for its wide use in industry. To find an optimum reduction condition, TGA–DTA was conducted in hydrogen (dew point: 30 °C) with a heating rate of about 5 °C per min. On the basis of the results, reduction was carried out in a flowing hydrogen atmosphere in a tube furnace. Microstructure characterization was carried out by SEM. Phase analysis was conducted using XRD spectra. Medial particle size (Dev; 0:5T) analysis was conducted with a microplus type particle size analyzer. BET specific surface area was conducted in a monosorb type laser diffraction specific surface analyzer.
3.1. Reduction mechanism and dynamic behavior 3.1.1. Dynamic reduction behavior of WO3 powder It has been reported [10,11] that, there are four kinds of relatively stable tungsten oxides: yellow tungsten oxide (a phase)–WO3; blue tungsten oxide (b phase)–WO2.90; purple tungsten oxide (c phase)–WO2.72; and brown tungsten oxide (dphase)–WO2. The reduction process of WO3 into W can be generalized as follows: WO3 þ 3H2 ! W þ 3H2 O
ð1Þ
Because there are four kinds of tungsten oxides, the reaction above can be expressed by following four process steps [10,11]. WO3 þ 0:1H2 ! WO2:9 þ 0:1H2 O ðaÞ WO2:9 þ 0:18H2 ! WO2:72 þ 0:18H2 O ðbÞ WO2:72 þ 0:72H2 ! WO2 þ 0:72H2 O ðcÞ WO2 þ 2H2 ! W þ 2H2 O ðdÞ
All of the above are endothermic reactions. The higher the temperature, the higher the equilibrium constant KP, which is favorable for the reduction process. Although the reaction between WO3 and H2 is a solid– gas multiphase reaction, the vaporization of tungsten oxide should not be neglected. It has been found that [12] WO3 begins to vaporize at about 400 °C, and vaporizes at about 850 °C in H2 atmosphere at a relatively high rate of about 0.4–0.6% per hour, while the WO2 begins to vaporize at about 700 °C, and considerably vaporizes at about 1050 °C. In this study, the specific surface of the tungsten oxide is very high after milling (Fig. 1). The vaporizing rate should be much higher than this level. The vaporization of tungsten oxides are also closely related to the vapor contents. WO3 will form a kind of versatile compound (WOxHy) when changed into the gas phase in a state of homogeneous reaction. The pure tungsten deposition can be realized by the following reaction: WOx þ H2 ðgÞ ! WðsÞ þ H2 OðgÞ
WOx þ H2 O ! WOx H2 OðgÞ þ H2 ðgÞ
ð5Þ
The dynamic behavior of the tungsten oxide reduction process is obtained as shown in Fig. 2. It can be observed that, in the reduction of WO3 into a pure tungsten process in the present condition, there is a light reaction at about 360 °C. The reaction is completed at about 900 °C from the dynamic curve. However, if the particle size is fine enough as shown in the same figure for the reduction process after dry milling 8 h (the black solid line), the reaction can be completed at a lower temperature (about 800 °C). The milling does not have a great effect on the starting point. This may because in the initial stage of reduction the temperature is very low, and mainly proceeds as a solid–gas state reaction. While in a later stage, vaporization
ð2Þ
In the isobaric relation equilibrium constant KP and temperature T can be expressed by the following four equations [12], respectively 3266:9 þ 4:0667 T 4508:5 lg K pðbÞ ¼ þ 5:10866 T 904:83 lg K pðcÞ ¼ þ 0:90642 T 3255 lg K pðdÞ ¼ þ 1:650 T
ð4Þ
In another instance, tungsten particles can grow in vapor deposition though following two aspects [10]: WOx H2 OðgÞ þ H2 ðgÞ ! WðgÞ þ H2 OðgÞ
3. Results and discussion
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lg K pðaÞ ¼
ð3Þ
Fig. 1. BET specific area of WO3 powder in different mediums.
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Fig. 4. XRD of CuWO4 powder after reduction at 450 °C for 1 h.
mixture of pure copper and WO3. Therefore, the first reaction step is assumed to be CuWO4 þ H2 ! Cu þ WO3 þ H2 O Fig. 2. Dynamic behavior in reduction of WO3 powder in hydrogen atmosphere for the oxide powder without milling, and after milling in alcohol for about 8 h.
From reference [7], the reduction process of CuWO4 should begin with CuWO4 þ H2 ! Cu þ Cuð1xÞ WOð4xÞ þ H2 O
of tungsten oxides increase greatly and the homogeneous reaction begins to dominate gradually. After milling, the specific area increased one hundred times (from 0.2 m2/g to about 20 m2/g, Fig. 1), which has an extremely active effect on the reduction process. 3.1.2. The dynamic behavior of CuWO4 reduction process Fig. 3 shows the dynamic behavior in the reduction of CuWO4 powder produced from WO3 and CuO by oxidation at high temperature. There are two stages in the reduction process: decomposition of CuWO4 and reduction of tungsten oxides. For example for CuWO4 powder without the milling process, there is the first obvious reaction peak in the range of 300 °C to about 550 °C. Based on XRD analysis of the outcomes of the reaction at 550 °C for about 30 min (Fig. 4), the resultant is found to be composed of a
Fig. 3. Weight loss observing the reduction of CuWO4 in hydrogen atmosphere without milling, and after milling in alcohol for about 8 h.
ð6Þ
ð7Þ
and followed by Cuð1xÞ WOð4xÞ þ H2 ! Cu þ WO3 þ H2 O
ð8Þ
However, no obvious reaction was observed in TGA (see Fig. 3) or in XRD analysis (Fig. 4). This is because these two processes are progressing concurrently and the rate of the second reaction is extremely high. The interphase exists for a very short time. In the second reduction step, the reduction of WO3 was complete at about 750 °C, while the precursor without milling needed approximately 900 °C. That is to say that, in the decomposition–reduction process of CuWO4, the reduction of WO3 will readily proceed at a lower temperature. This is thought to be due to the appearance of pure copper in the first stage. Since only copper exists in the reaction system, and the freshly reduced copper has extremely high chemical activity and can act as an ideal base for the later reduction process as a result of the catalytic effect in providing an ideal location for the tungsten nucleation process and lowering the activation energy of single-phase tungsten. After milling, the dynamic reduction behavior of CuWO4 is very similar to that without milling. An obvious difference was observed, the reduction rate was improved, and to a certain extent, the reduction temperature was lowered. The first reaction stage lowered from about 500 °C to 450 °C, the second stage lowered from 750 °C to about 700 °C. The lowering of reduction temperatures is reasonably thought to be due to the increase in the specific area and the catalytic effect of newly reduced copper which improved the reaction rate greatly. 3.1.3. Dynamic behavior of CuWO4 + 0.382WO3 (wt%) oxide powder reduction process A compound with a mixture of CuWO4 + 0.382WO3 has the most commercial composition of W–20Cu after complete reduction. From Fig. 5, it can be observed that, due to the existence of WO3 before reduction, the dynamic
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Fig. 5. Dynamic behavior in reduction of CuWO4 + 0.382WO3 oxide powder in hydrogen atmosphere without milling, and after milling in alcohol for about 8 h.
reduction behavior is different from that of CuWO4: the starting temperature for the decomposition of CuWO4 was increased. For the oxide powder without milling, the starting point increased from 300 °C to about 380 °C, while powder after milling in alcohol for about 8 h, the starting point was further increased to about 550 °C. This reverse catalytic effect is thought to be closely related to the appearance of WO3. Because after milling the specific area of particles increased greatly, and the absorption ability between the particles increased accordingly. The conglomeration between the fine WO3 and CuWO4 particles changed the reduction dynamic behavior of the mixture. However, a detailed mechanism of reduction for CuWO4 with fine WO3 needs further research. Inhibiting the first stage reaction shortens the reduction temperature range between the CuWO4 and WO3, making the reduced copper not grow greatly as a result of composite powder with copper encapsulated by tungsten. Fig. 6 shows the back scatter in the two kinds of samples with and without milling, after the reduction process. The bright
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particles are tungsten because of the heavy atom number, and the grey ones are copper. The powder without milling shows a great number of aggregates and severe separation between the two phases. The powder, with milling before reduction, shows a uniform distribution between the two phases and the copper particles are encapsulated by the tungsten particles. The analysis of the components from the SEM shows that the apparent copper content decreases greatly. For example, after milling the powder for about 6 h in alcohol, the apparent copper content is about 6.3%, which is much lower than the true value. The apparent composition between the measured result and the theoretical are extremely different. It is reasonable to believe that in the composite powder, the copper powders are encapsulated by tungsten. Fig. 7 shows the copper composition of the reduced powders as a function of the milling time in different mediums. With an increase in milling time, a decrease in the apparent composition of copper is observed, showing an increase in the percent of composite powder for copper encapsulated by tungsten. The results in different milling medium demonstrate a similar behavior. The reduced powder after milling in alcohol shows better results than the other two processes, and has no obvious conglomeration with a separate particle size of 0.2–0.3 lm in a state of encapsulation by copper. Some researches [13] report that two-step reduction can avoid conglomeration. In the present research, one- and two-step reductions were applied according to the dynamic reduction behavior of the oxide powder. Before reduction, the powder was milled for about 8 h in air or alcohol medium. The one-step reduction process was applied at 750 °C for 60 min. For the two-step reduction process, the first step reduction was applied at 650 °C for about 60 min, and the second step at 750 °C for about 60 min. Fig. 8 shows the reduced powder profiles, after both dry and alcohol milling for about 8 h. It can be observed that two-step reduction can not reduce the number of aggregates in the present condition. On the other hand, it improves the conglomerating process, when compared to the one-step
Fig. 6. The SEM back scatter of the two kinds of samples after reduction at 750 °C for 60 min (a) without milling, and (b) after milling in alcohol for about 8 h.
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in the range of 0.2–0.3 lm and copper particles with larger sizes (Fig. 8d). In the case of tungsten–copper composite powder after milling in alcohol, the final distribution is better than that after dry milling and without larger size copper agglomeration. This is thought to be due the good dispersant in alcohol. Finally, in order to obtain a good encapsulated composite powder, a shortening in the reduction time and a lowering in the reduction temperature are basic principles. 3.2. Characteristics of the reduced powder
Fig. 7. The copper content of the composite powder after reduction as a function of milling time in different mediums.
reduction process. The reason is thought to be that the increase in time between the first step and the second results in the tungsten depositing on large size copper particles and for a growth in particle size as well as the conglomeration. The amplification of aggregate shows a loose structure and is composed of tungsten particle unit
Fig. 9 shows the particle size distribution of the reduced composite powder after dry and alcohol milling before reduction: the reduced powders demonstrate two peak distributions in the two kinds of powder. For example, for the one-step reduced powder after alcohol milling for 8 h before reduction, the particle size is mainly focused in the range of 0.1–1 lm. Although there are also some particles larger than 1.0 lm, the content of the large particles is negligible when compared with two-step reduction process. From Fig. 9b, it can be observed that after two-step reduction, a large amount of particles are distributed in a wider size range. In the case of the powder after dry milling, two-
Fig. 8. Profiles of the reduced powder in different processes.
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Fig. 9. Particle size distributions of the reduced powder after different processes.
step reduction and the distribution are much worse than the above two cases: the percent of the large particles increased greatly. So, from the particle size distributions of these four kinds of powders, a similar conclusion could be obtained: the two-step reduction process is not as suitable as that of the one-step and alcohol milling is the best treatment for the oxide precursors. Oxygen content influences the sintering ability of the tungsten–copper powder, especially with the appearance of WO4x. The WO4x residuals can not be eliminated completely in the sintering process, and will greatly influence the properties of the final products such as thermal conductivity and mechanical properties [13]. Therefore, controlling oxygen content of the reduced powder is a very important problem for tungsten–copper powder. Fig. 10
shows the oxygen content of the reduced powder decreases gradually with an increase of air and the alcohol milling time. With the increase of the reduction temperature, a decrease in oxygen content is observed. After alcohol milling for about 6 hours, and a reduction temperature of 850 °C, the oxygen content reaches about 100 ppm. The oxygen content is higher for the reduction at a lower temperature and a shorter milling time. In the case of the powder without milling at a reduction temperature of 750 °C, the oxygen content is about 2000 ppm. In this instance the powder is not completely reduced. However, with a milling time longer than 6 h, the oxygen contents are mostly lower than 700 ppm, and exhibit almost no obvious effect to the final properties of the sintered body.
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of the reduced powder. After the treatment of anti-oxidation and then holding powder in air for about 8 h, the oxygen content is only about 420 ppm. Although a longer nitrogen treatment was conducted in general, 3–4 h of nitrogen treatment is practicable in better effect on the anti-oxidization. It is thought that the successful surface modification on the tungsten–copper is most likely determined by the high specific area or the strong absorption ability of the reduced powder. The absorption of nitrogen on the particle surface should prevent the particles from contacting with oxygen. However, it was found that, surface modification in argon does not have an obvious effect on the anti-oxidation of the reduced powder. The surface treatment for anti-oxidation to the reduced powder as well as its mechanism needs a further research in future. Fig. 10. Oxygen contents of one-step reduced tungsten–copper powder after 750 °C and 850 °C for 60 min.
However, the oxygen content will increase gradually after reduction if the reduced powder is put in air. The reduced powder has an extremely high specific area and is easily oxidized. Fig. 11a shows the oxygen content of reduced powder (8 h alcohol milling, 750 °C for 60 min) as a function of the holding time in atmosphere. By increasing the holding time, a rapid increase in oxygen content is observed. In practice, the compression process and various treatments of the powder are conducted in air. Without any treatment to the reduced powder, the oxygen content will be increased greatly. So controlling the oxygen content after reduction, while keeping the particle size invariable, is also necessary in practical production. It was found that holding in nitrogen at room temperature can to some extent inhibit the further oxidation process. Fig. 11b shows the oxygen content of powder after different nitrogen treatments as a function of holding time in air. Even holding for just 2 h in nitrogen will greatly improve the anti-oxidation
3.3. Sintering behavior of one-step reduced powder After sintering at 1200 °C for 60 min of the one-step reduced powder with a nominal composition of W– 20 wt% Cu after alcohol milling for 8 h, the relative density of the sintered parts can reach to about 99.5% (Fig. 10). Table 1 is the properties comparison of the W–20 wt% Cu composite by different processes. Tungsten–copper composite can attain comparatively highest relative density by present thermo-mechanical process. The relative density of W–Cu materials sintered at 1200 °C is 99.5% and the thermal conductivity is 210 W m1 K1, respectively, about 89% of the predicted value (231 W m1 K1) by German [14]. The microstructure of sintered tungsten–copper composite is homogenized (Fig. 12). Every tungsten particle is capsulated by network like copper structure, and the overall microstructure is very near to that of the model devised by Germen [14]. High energy milling of oxide powder is effective to increase the density of sintered sample and the microstruc-
Fig. 11. Oxygen content of the tungsten–copper powder after alcohol milling (8 h) and one-step reduction process as a function of holding time in air atmosphere. Reduced powder (a) without surface modification, and (b) after surface modifications in nitrogen for different times.
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Table 1 The properties of tungsten–20 wt% Cu composite produced by thermo-mechanical process compared with other methods (1200 °C 60 min) Number
1# 2# 3# 4#
Powder process (W–20Cu)
Alcohol milling 8 h + One-step reduction 750 °C Activated sintering (Co 3.5%wt) Mechanical alloying (MA) + sintering Infiltration sintering
ture of sintered sample produced by reduced tungsten–copper powder with milling is more homogeneous than that by the reduced powder without milling. Sintered sample produced by reduced tungsten–copper powder without milling has micro-pore due to insufficient copper filling and copper pockets along spherical powder boundary due to copper filling in large pore. The relative density of sintered body is also affected by reduction process. The density of tungsten–copper composite after sintering of reduced powder with lower temperature is higher than that of with higher temperature. It is suggested that combination of thermal and mechanical processes could obtain the high density of tungsten– copper composite after liquid phase sintering. Surprisingly, the sintered sample produced by thermo-mechanical process showed nearly full density as a result of sintering at 1200 °C for 1 h without addition of the sintering activator. Microstructure of the W–20 wt% Cu composite sintered at 1200 °C for 60 min showed those tungsten particles of about 1 lm in size formed a homogenous network. The improved density after sintering in the thermo-mechanical process is due to the maintenance of homogeneous mixing state of superfine tungsten and copper particles without agglomeration until formation of liquid phase, and homogeneous redistribution of solid tungsten particles in the presence of liquid phase. Ozer and Kim etc [2,10,11] reported similar methods to produce superfine tungsten–copper composite powder by co-reduction of oxidized powder in hydrogen atmosphere.
Properties of final products Relative density (%)
Thermal conductivity (W m1 K1)
99.5 ± 0.2 <98 <99.5 <97
205 ± 10 <180 [15] <200 [16] <200 [17]
Ozer [2] used the W–CuO powder, and by applying a coreduction process to the composite powder after milling process for about 20 h, the relative density of sintered parts at 1300 °C in hydrogen reached to only 92–95%. A further densification process such as HIP has to be applied in order to further increase the relative density. Kim [10,11] used WO3 + CuO powder, after milling also for about 20 h, the grain sizes of both oxidized powder and the reduced powder were in the nano-scales by TEM observation. In this study, we did not conduced TEM observation to the oxidized or reduced powder. However, we obtained the final sintered parts with extremely high relative density of about 99.5% when sintered at 1200 °C for about 30 min. The main advantage of present research is thought to be as follows: we used the oxide precursor of CuWO4 + WO3. In the very initial stage of the oxides state, the Cu atoms and W atoms have been mixed into each other in atom scale. In the reduction process, due to existence of WO3, an inhibition of CuO reduction into pure copper was occurred. Pure tungsten and pure copper can be reduced in a very short temperature range, which then resulted in not so much of the agglomeration of copper particles or tungsten particles finally. Besides, present reduced powder has a very special characteristic: the composite powder has microstructure with copper particles encapsulated by the tungsten copper, which can inhibit the evaporation of copper during sintering process at high temperature. 4. Conclusions
Fig. 12. Microstructure of tungsten–copper composite produced by mechanical–thermal process after sintering at 1200 °C for 60 min.
1. The dynamic behavior of the reduction process of tungsten–copper oxides precursors in WO3, CuWO4 and CuWO4 + 0.382WO3 were analyzed by TGA– DTA. The results showed that milling to the precursors can be helpful in producing fine particle size distributed composite powder compared to that without milling. 2. The milling of the oxide precursors in alcohol before reduction showed the ability to produce the composite powder with fewer conglomerations after reduction than by dry milling or water milling. 3. Due to the extremely fine particles produced in the reduction process, the composite powder is extremely prone to be oxidized when placed in air. The powder modification by a hydrogen treatment proved to be effective to inhibit the oxidation.
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4. As an improvement of co-reduction process, the method of thermo-mechanical process can produce near full dense of W–Cu composite at lower sintering temperature. References [1] Li YP, Qu XH, Zheng ZS, Lei CM, Zou ZQ, Yu S. Inter J Refract Met H 2003;21:259–64. [2] Ozer O, Missiaen M, Laya S, Mitteau R. Mater Sci Eng A 2007;460– 461:525–31. [3] Raghu T, Sundaresan R, Ramakrishnan P, Rama Mohan TR. Mater Sci Eng 2001;A304–306:438–41. [4] Li YP, Qu XH, Zheng ZS, Yu S. J Cent South Univ Technol 2003;10:168–72. [5] Ling Y, Bai X, Changchun G. Mater Sci Forum 2003;423–425:49–54. [6] Kim BK, Hong SH. In: Proceedings of the powder metallurgy world congress2000. Kyoto: Japan Society of Powder and Powder Metallurgy; 2000. p. 682–5.
[7] Houck DL, Dorfman LP, Paliwal M. Proceedings of 7th international tungsten symposium, 24–27 September. Germany: Goslar; 2000. p. 390–409. [8] Kim DG, Kim EP, Kim YD, Moon IH. In: 15th Plansee seminar, RM 10, 21–24 November. Australia: Reutte/Tirol; 2001. p. 1–15. [9] Basu AK, Sale FR. J Mater Sci 1978;13:2703–11. [10] Kim TH, Yu JH, Lee JS. Nanostruct Mater 1997;9:213–5. [11] Kim DG, Oh ST, Joen H, Lee CH, Kim YD. J Alloy Compd 2003;354:239–42. [12] Zhang QX, Zhao QS. Metallurgy of tungsten and molybdenum. China: Metallurgical Industry Express; 2005. p. 151–82. [13] Kornilova VI, Konchakovskaya LD, Panichkina VV, Radchenko PYa, Skorokhod VV. Powder Metall Metal Ceram 1985;24:197–9 [English translation of Poroshkovaya Metallurgiya]. [14] German RM. Metall Trans A 1993;24A(8):1745–52. [15] Johnson LJ, German RM. Int J Powder Metall 1994;30(1): 91–102. [16] Raghu T, Sundaresan R, Ramakrishnan P, Rama Mohan TR. Mater Sci Eng 2001;A304–306(2):438–41. [17] Yang B, German RM. Adv Powder Metall Particulate Mater 1993;5:105–19.