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Effects of tungsten and aluminum additions on the formation of molybdenum disilicide by mechanically-induced self-propagating reaction
Journal of Alloys and Compounds 490 (2010) 388–392
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effects of tungsten and aluminum additions on the formation of molybdenum disilicide by mechanically-induced self-propagating reaction Xiaohong Wang a , Peizhong Feng a,∗ , Farid Akhtar b , Jie Wu a , Weisheng Liu a , Yinghuai Qiang a , Zhenzhong Wang a a b
School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, PR China Department of Metallurgical and Materials Engineering, University of Engineering and Technology, Lahore 54890, Pakistan
a r t i c l e
i n f o
Article history: Received 7 September 2009 Received in revised form 29 September 2009 Accepted 30 September 2009 Available online 9 October 2009 Keywords: Intermetallics Powder metallurgy Mechanical alloying X-ray diffraction
a b s t r a c t The effects of tungsten and aluminum additions to Mo–Si system on the formation of MoSi2 by mechanically-induced self-propagating reaction in a high-energy ball mill were investigated by Xray diffraction. The incubation time for mechanically-induced self-propagating reaction to form MoSi2 was 90 min in Mo–Si system. With the addition of tungsten to Mo–Si system, the incubation time of mechanically-induced self-propagating reaction of (Wx ,Mo1−x )Si2 system was prolonged. It was due to the decrease of adiabatic temperature with increase in x, and the mechanically-induced self-propagating reaction was not observed until 150 min of high-energy milling in (Wx ,Mo1−x )Si2 powder sample with x equals to 0.4. Conversely, the minor aluminum (y < 0.2 in Mo(Si1−y ,Aly )2 system) substituting for silicon had shorten the incubation period and accelerated the reaction. When tungsten and aluminium were added simultaneously to Mo–Si system, the mechanically-induced self-propagating reaction was observed in (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 sample, but it was not observed in (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 and (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction MoSi2 has been regarded as a promising material for hightemperature structural applications. Due to its high melting point of 2030 ◦ C, relatively low density of 6.24 g/cm3 , excellent resistance to oxidation at elevated temperature, good thermal conductivity and electrical conductivity, MoSi2 may tolerate high service temperatures in the field of energy machines, gas turbine parts, heat shields and tiles [1–3]. MoSi2 has been extensively studied during the past 20 years. Significant progress has been made in both scientific and technological developments [2,3]. Today the main application of MoSi2 is as heating elements for electric furnaces [1]. Unfortunately, applications of MoSi2 are limited by its low fracture toughness at room temperature and poor creep resistance at high temperature. These can lead to the problems of component reliability. Thus the key issue is to improve fracture toughness and creep resistance at elevated temperature of MoSi2 [1–5]. Approaches to improve low temperature fracture toughness and high-temperature creep properties of MoSi2 have been focused on introducing secondary reinforcement and alloying. Alloying strategies include the addition of a third element, such as W, Al, Mg, Ge,
∗ Corresponding author. Tel.: +86 516 83591870; fax: +86 516 83591870. E-mail address: [email protected] (P. Feng). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.09.189
Re, Nb, Ta and V, etc. [2,5–9]. W, Re, Nb, Ta and V atoms substitute Mo atoms in MoSi2 . Al, Mg and Ge atoms substitute Si atoms in MoSi2 [7]. Especially, W and Al are considered as the important toughening and strengthening elements [10–14]. Mechanical alloying (MA) which is well known as a nonequilibrium processing technique has been applied to many systems, including molybdenum silicides [15–17]. The MA of MoSi2 has been extensively studied [15–19]. One of the formation mechanisms of MoSi2 by mechanical alloying is similar to the self-propagating high-temperature synthesis (SHS) [16–18]. The characteristic of this mechanism is a spontaneous rapid high-temperature synthesis reaction induced by a few hours of high-energy milling. This behavior is so far reported in MoSi2 , Ta2 Si3 , Nb5 Si3 and Al50 Ni50 systems by many researchers [18,20]. And this mechanism is named as mechanically-induced self-propagating reaction (MSR) [16]. MSR is analogous to that of the thermally ignited SHS [16]. The SHS process is highly exothermic. A highly exothermic reaction evolves enough heat to heat up the adjacent layer of reactants to react and by this the reaction becomes self-sustained. The high combustion temperature (e.g. 1000–6500 K) volatilizes low boiling point impurities and gives purer products. However, it is difficult to control the process of SHS, and the end product is largely porous powder compact. The process of MSR combines the advantages of SHS and MA techniques. The end-product is pulverized into fine particles. The milling time of MSR is short [16,18] and hence reduces the contamination from the balls and vials.
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Though the effects of alloying on the preparation of MoSi2 by mechanical alloying and self-propagating high-temperature synthesis are reported in many studies [1,19], it is difficult to find the effects of alloying on the synthesis of MoSi2 by mechanicallyinduced self-propagating reaction. Based on the mechanism of mechanically-induced selfpropagating reaction (MSR), the objective of this study is to investigate the effects of tungsten and aluminum additions on the formation of MoSi2 by self-propagating reaction during the mechanical alloying. The structure of as-milled powders and heat treated powders is analyzed by X-ray diffraction (XRD). 2. Experimental procedures The starting materials were 99.9% pure elemental molybdenum powder with an average particle size of 3–5 m, 99.9% pure elemental silicon powder with an average particle size of 44 m, 99.5% pure elemental tungsten powder with an average particle size of 3.3 m and 99.5% pure elemental aluminum powder with an average particle size of 44 m. The elemental powders were carefully weighed and mixed in the stoichiometry of MoSi2 , (W0.1 ,Mo0.9 )Si2 , (W0.2 ,Mo0.8 )Si2 , (W0.3 ,Mo0.7 )Si2 and (W0.4 ,Mo0.6 )Si2 (W–Mo–Si system); Mo(Si0.95 ,Al0.05 )2 , Mo(Si0.9 ,Al0.1 )2 , Mo(Si0.8 ,Al0.2 )2 and Mo(Si0.7 ,Al0.3 )2 (Mo–Si–Al system); (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 , (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 and (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 (W–Mo–Si–Al system), respectively. A high-energy vibrating mill with three vials was used for the mechanical alloying experiments. The stainless steel vials (60 cm3 in volume) with bearing steel balls (6 and 10 mm in diameter) were used for milling. The frequency of vibration of the machine was 1000 revolution/min. For each milling run, 5.0 g of the powder mixture was canned into stainless steel vials containing bearing steel balls in a glove box under an argon gas atmosphere to avoid contamination from the air. The weight ratio of the balls to the powder mixture was 15:1. The vials were sealed with a rubber O ring and the milling thus proceeded in a stationary atmosphere. The milling program was set to pause for 6 min for every 10 min of milling to prevent excessive heating during the milling process. The surface temperature on the top of the vial was measured with a digital thermometer at fixed intervals. When an obvious and abrupt temperature increase was observed, as reported by Atzmon [20], it was considered to be the critical ball milling time (incubation period) of self-propagating reaction during mechanical alloying. The heat treatment of the as-milled powders was carried out in vacuum at 1000 ◦ C for 1 h. The structure of the as-milled powders and heat treated powders was analyzed by a Rigaku Dmax-RB X-ray diffraction (XRD) using a Cu target (K␣, = 0.15406 nm), operating at 40 kV and 200 mA settings. The compositional analyses were conducted through energy dispersive spectroscopy (EDS, Kevex-Sigma).
Fig. 1. X-ray diffraction patterns of W–Mo–Si powder mixture high-energy ball milled for different times (a) MoSi2 stoichiometry; (b) (W0.1 ,Mo0.9 )Si2 stoichiometry; (c) (W0.2 ,Mo0.8 )Si2 stoichiometry; (d) (W0.3 ,Mo0.7 )Si2 stoichiometry; (e) (W0.4 ,Mo0.6 )Si2 stoichiometry.
3. Results and discussions Fig. 2. A temperature trace of the vial during milling of Mo:2Si powders.
3.1. Tungsten addition Fig. 1 shows the X-ray diffraction (XRD) patterns of W–Mo–Si powder mixtures after different milling times. The milling time is that particular time when the surface temperature of the vial shows an abrupt change. Fig. 2 shows a temperature trace of the vial during milling of Mo:2Si powders (MoSi2 stoichiometry). The surface temperature increases from 27.8 ◦ C to 38.9 ◦ C when the powder is milled for 90 min (incubation period) (Fig. 2). Strong peaks of XRD pattern in Fig. 1a corresponds to MoSi2 . So, the high-energy milling of Mo:2Si powders synthesizes MoSi2 by mechanically-induced self-propagating reaction (MSR) after 90 min of incubation period. The MSR is neither uniform nor integral, and hence much reactant is residual. This is the reason for the relatively low adiabatic temperature of MoSi2 (1943 K) in MA process (loose powders), which only is 143 K higher than the experiential criterion of SHS (1800 K). The heat transfer from the reaction front is difficult. The combustion wave propagation is irregular, and in some instances, the wave propagates down at an angle. This behavior is observed in Mo–Si [17–18], Al–Ni [20], CuO–Fe [21], PbO2 –TiO [15], etc. The acute temperature variation is also observed after 90 min milling in the x = 0.1 and 0.2 samples in (Wx ,Mo1−x )Si2 system. The X-ray diffraction patterns in Fig. 1b and c show that a great deal of MoSi2 and WSi2 are formed by the MSR. For the sample
with x = 0.3 in (Wx ,Mo1−x )Si2 system, the acute temperature change variation is observed after 100 min of milling. The MoSi2 is synthesized (Fig. 1d) with slightly prolonged incubation time for MSR. With further increase of tungsten content in (Wx ,Mo1−x )Si2 system to x = 0.4, mechanically-induced self-propagating reaction is not observed up to 150 min (1.5 times for the x = 0.3 sample) of milling. The acute temperature variation is not observed in the (W0.4 ,Mo0.6 )Si2 sample, and the absence of MSR is corroborated by the XRD peaks in Fig. 1e. Fig. 1(a–e) shows that the product mainly consists of C11b /A2, C11b /A2, C11b /A2, C11b /A2 and A2, respectively. The MSR cannot occur in the (W0.4 ,Mo0.6 )Si2 sample. It has a close relationship with the addition of tungsten to Mo–Si system. It is calculated that the adiabatic temperature (Tad ) of MoSi2 is 1943 K, and that of WSi2 is 1512 K (Table 1). On the basis of experiential observations, it has been suggested that systems with Table 1 Thermodynamic properties and adiabatic temperatures of MoSi2 and WSi2 . ◦
−1
Compound H298 (J mol MoSi2 WSi2
−131710 −92751
) Cp (J mol−1 K−1 ) 67.831 + 11.970 × 10−3 T − 6.569 × 105 T−2 67.831 + 11.042 × 10−3 T − 6.092 × 105 T−2
Tad (K) 1943 1512
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Table 2 Crystal structure, lattice parameters, space group and formula units per cell of compounds of Mo, W, Si and Ala . Phase
Crystal structure
Si Mo W Al MoSi2 WSi2 Mo(Si,Al)2 W(Si,Al)2
Diamond cubic bcc bcc fcc Tetragonal Tetragonal Hexagonal Hexagonal
Lattice parameters (nm) a
a
b
0.5431 0.3417 0.3165 0.4049 0.3205 0.3211 0.4674 0.4726
Space group
Formula units per cell
Fd3m Im3m Im3m Fm3m I4/mmm I4/mmm P6222 P6222
8 4 2 2 2 2 3 3
c
0.7845 0.7829 0.6558 0.6581
The data from the MDI Jade 5.0.
Tad ≤ 1800 K will not react in a self-propagating manner [22]. MoSi2 can be synthesized by SHS, but WSi2 cannot be. The adiabatic temperature of (Wx ,Mo1−x )Si2 system decreases with the increase of tungsten. The adiabatic temperature of the (W0.3 ,Mo0.7 )Si2 sample (x = 0.3) is 1813.7 K, which is still higher than the experiential criterion, thus the new compound can be formed by the SHS in x = 0.3 sample. The adiabatic temperature decreases to 1770.6 K for samples with tungsten content x = 0.4, which is less than the experiential criterion of SHS (Fig. 3), and hence new compound will not form by SHS. Fig. 3 shows variation of adiabatic temperature with x value in (Wx, Mo1−x )Si2 system. The adiabatic temperature is calculated from the reaction of (1−x)Mo + xW + 2Si → (Wx, Mo1−x )Si2 . Thus, it is linear with increase of tungsten. In the present work, the results of MSR are consistent with the calculation of the adiabatic temperature. The XRD patterns of the W–Mo–Si powder mixtures milled and heat treated at 1000 ◦ C for 1 h are shown in Fig. 4. It can be seen from Fig. 4 that the Mo, W and Si have reacted during heat treatment. The phases formed are MoSi2 , WSi2 and trace amounts of Mo5 Si3 and W5 Si3 with minor amounts of residual reactants. As the crystal structures and lattice parameters of Mo, W and their silicides are similar. The crystal structures, lattice parameters, space group and formula units per cell of compounds of Mo, W, Si and Al are present in Table 2. And the atoms of Mo and W can replace each other during synthesis and form a continuous solid solution (Wx ,Mo1−x )Si2 [10,11]. The EDS analyses show that Mo and W elements are well distributed. Thus (W0.1 ,Mo0.9 )Si2 , (W0.2 ,Mo0.8 )Si2 , (W0.3 ,Mo0.7 )Si2 and (W0.4 ,Mo0.6 )Si2 compounds/solid solutions are formed by MSR and heat treatment in the present work.
Fig. 3. Variation of adiabatic temperature with x value in (Wx ,Mo1−x )Si2 system.
Fig. 4. X-ray diffraction patterns of W–Mo–Si powder mixture high-energy ball milled and heat treated at 1000 ◦ C for 1 h (a) MoSi2 stoichiometry; (b) (W0.1 ,Mo0.9 )Si2 stoichiometry; (c) (W0.2 ,Mo0.8 )Si2 stoichiometry; (d) (W0.3 ,Mo0.7 )Si2 stoichiometry; (e) (W0.4 ,Mo0.6 )Si2 stoichiometry.
3.2. Aluminum addition Fig. 5 shows the X-ray diffraction patterns of Mo–Si–Al powder mixtures after different milling times. The acute temperature variation is observed after 80, 70 and 90 min of milling in Mo(Si1−y ,Aly )2 system for y = 0.05, 0.1 and 0.2, respectively. The X-ray diffraction confirms the formation of MoSi2 and Mo(Si,Al)2 compounds by MSR
Fig. 5. X-ray diffraction patterns of Mo–Si–Al powder mixture high-energy ball milled for different times (a) Mo(Si0.95 ,Al0.05 )2 stoichiometry; (b) Mo(Si0.9 ,Al0.1 )2 stoichiometry; (c) Mo(Si0.8 ,Al0.2 )2 stoichiometry; (d) Mo(Si0.7 ,Al0.3 )2 stoichiometry.
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Fig. 6. X-ray diffraction patterns of Mo–Si–Al powder mixture high-energy ball milled and heat treated at 1000 ◦ C for 1 h (a) Mo(Si0.95 ,Al0.05 )2 stoichiometry; (b) Mo(Si0.9 ,Al0.1 )2 stoichiometry; (c) Mo(Si0.8 ,Al0.2 )2 stoichiometry; (d) Mo(Si0.7 ,Al0.3 )2 stoichiometry.
Fig. 7. X-ray diffraction patterns of W–Mo–Si–Al powder mixture highenergy milled for different times (a) (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 stoichiometry; (b) (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 stoichiometry; (c) (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 stoichiometry.
in Mo(Si1−y ,Aly )2 system. However, the milling time is prolonged to 140 min (∼1.5 times for the y = 0.2 sample) and the acute temperature variation is not observed in the y = 0.3 sample (Fig. 5d). Though the diffraction peaks of Si are weaken, the diffraction peaks of Mo are very strong and peaks for any new compound phase are absent. Thus the y = 0.3 sample has not gone through MSR. Fig. 5(a–d) show that the product mainly consists of C11b /A2, C11b /C40/A2, C40/A2 and A2/A4, respectively. The melting point of aluminum is 660 ◦ C, and that of silicon and Al-12.2at%Si eutectic are 1410 ◦ C and 577 ◦ C, respectively. The mechanism of MoSi2 by SHS is characterized by the melting of Si, the wetting of Mo by molten Si, the dissolution of Mo in the molten Si and the precipitation of MoSi2 from the liquid phase. Severe collisions and friction take place amongst vial, balls and powders during ball milling. These collisions lead to energy concentration in small volumes between the vial and a ball, resulting in higher local temperature upon impact, and the self-sustaining reaction is then triggered at these “hot spots” [17,20]. On the other hand, the temperature of the powders during high-energy milling will increase due to the kinetic energy of the grinding medium, and it can increase to >1000 ◦ C [15]. The self-sustaining reaction can be ignited at this temperature. Because the melting point of Al and Al-12.2at%Si eutectic is less than that of Si, the temperature of “hot spots” and the powders may exceed prior to the melting point of Si. The molten Al and Al–Si eutectic will wet the solid Mo powders. This is beneficial to the contact of the reactants and the reduction of the incubation time of MSR. The X-ray diffraction patterns of the Mo–Si–Al powder mixture milled and heat treated at 1000 ◦ C for 1 h are shown in Fig. 6. It can be seen from Fig. 6d that the structure of y = 0.3 sample is C40 Mo(Si,Al)2 . The structure of MoSi2 is Cllb . The structure of product is changed from Cllb to C40. The standard enthalpy of formation of hypothetical compounds C11b –MoAl2 , C40–MoAl2 and C11b –MoSi2 are 19.0, 21.0, 45.156 kJ/mol, respectively [23]. The decrease of standard enthalpy increases the difficulty of formation of compounds. This variation of structure leads to the result that the self-sustaining reaction cannot be induced in Mo(Si1−y ,Aly )2 system for y = 0.3 (Fig. 5d). Fig. 6 shows the structure change from Cllb to C40 in Mo(Si1−y ,Aly )2 system with increase in y. It can be seen from Fig. 6 that the structure of y = 0.05 sample is only Cllb MoSi2 (Fig. 6a). The reflection line of C40 Mo(Si,Al)2 is observed in the y = 0.1 sample, and the product is a mixture of Cllb and C40 (Fig. 6b). The
MoSi2 peaks are weakened and the Mo(Si,Al)2 peaks are strengthened in the y = 0.2 sample (Fig. 6c). Only Mo(Si,Al)2 can be indexed in the y = 0.3 sample (Fig. 6d). This change in structure can be explained by an equilibrium phase diagram of Mo–Si–Al system [24]. On the diagram, the material lies in the monophasic Cllb , biphasic Cllb and C40, and monophasic C40 region with the increase of the aluminum, respectively [14,24,25]. Thus Mo(Si0.95 ,Al0.05 )2 , Mo(Si0.9 ,Al0.1 )2 , Mo(Si0.8 ,Al0.2 )2 and Mo(Si0.7 ,Al0.3 )2 compounds are formed in the present work.
3.3. Tungsten and aluminum additions Fig. 7 shows the X-ray diffraction patterns of W–Mo–Si–Al powder mixture system after different milling times. The acute temperature variation is observed after 100 min milling in the x = 0.1, y = 0.1 sample of (Wx ,Mo1−x )(Si1−y ,Aly )2 system. The X-ray diffraction shows that the silicide and alumino-silicide of Mo and W are formed. However, the milling time is prolonged to 170 min for the x = 0.2, y = 0.2 sample and 190 min for the x = 0.3, y = 0.3 sample, respectively, the acute temperature variation is not observed. The XRD patterns confirm that the MSR does not occur and there is not any new phase. Fig. 7(a–c) shows that the product mainly consists of C11b /C40/A2, A2/A4 and A2/A4, respectively. The addition of W leads to the decrease of the adiabatic temperature (as discussed above). And the incubation time is also rapidly increased, when y = 0.2 in Mo–Si–Al system (Fig. 5). Thus, the MSR cannot be induced in the x ≥ 0.2, y ≥ 0.2 sample of W–Mo–Si–Al system. The X-ray diffraction patterns of the W–Mo–Si–Al powder mixture milled and heat treated at 1000 ◦ C for 1 h are shown in Fig. 8. The structure of the x = 0.1, y = 0.1 sample mainly consists of Cllb MoSi2 and WSi2 . Small amount of C40 Mo(Al,Si)2 and W(Al,Si)2 and trace amounts Mo5 Si3 and W5 Si3 are also present. When x = 0.2, y = 0.2, the primary product becomes C40 phase, with the Cllb phase formed in minor amount. When x = 0.3, y = 0.3, the intensities of peaks from C40 phase is further increased, and that of Cllb phase is decreased. As the crystal structure and lattice parameters of silicide and alumino–silicide of Mo and W are also similar (Table 2), it is believed that the W and Al atoms substitute for Mo and Si atoms, respectively. The EDS analyses show that Mo and W elements are well distributed. The compounds/solid solution (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 , (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 and (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 are formed.
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formed by the MSR. And the MSR is not induced in the x ≥ 0.2, y ≥ 0.2 sample. The solid solution of (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 , (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 and (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 are formed. Acknowledgements The project was supported by Doctoral Fund of Ministry of Education of China No. 20070290523, Natural Science Foundation of Jiangsu Province No. BK2009096 and Youth Foundation of China University of Mining and Technology No. 2008A046. Xiaohong Wang would like to acknowledge Research and Innovation Project for College Graduate of Jiangsu Province CX09B 122Z. References
Fig. 8. X-ray diffraction patterns of W–Mo–Si–Al powder mixture high-energy milled and heat treated at 1000 ◦ C for 1 h (a) (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 stoichiometry; (b) (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 stoichiometry; (c) (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 stoichiometry.
4. Conclusions 1. With the increase of W in (Wx ,Mo1−x )Si2 powder mixture system, the incubation time of mechanically-induced selfpropagating reaction is prolonged because of the decrease of adiabatic temperature. And self-sustaining reaction cannot be induced in the (W0.4 ,Mo0.6 )Si2 sample (i.e. x = 0.4 sample). 2. With the increase of Al in Mo(Si1−y ,Aly )2 powder mixture system, the incubation time of mechanically-induced self-propagating reaction is shortened from 90 min (the y = 0 sample) to 70 min (the y = 0.1 sample), and then, the incubation time is reverted to 90 min in the y = 0.2 sample. Self-sustaining reaction cannot be induced in the y = 0.3 sample, even after increasing milling time to 140 min. 3. The acute temperature variation is observed after 100 min milling in the x = 0.1, y = 0.1 sample of (Wx ,Mo1−x )(Si1−y ,Aly )2 system. The silicide and alumino–silicide of Mo and W are
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effects of tungsten and aluminum additions on the formation of molybdenum disilicide by mechanically-induced self-propagating reaction Xiaohong Wang a , Peizhong Feng a,∗ , Farid Akhtar b , Jie Wu a , Weisheng Liu a , Yinghuai Qiang a , Zhenzhong Wang a a b
School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, PR China Department of Metallurgical and Materials Engineering, University of Engineering and Technology, Lahore 54890, Pakistan
a r t i c l e
i n f o
Article history: Received 7 September 2009 Received in revised form 29 September 2009 Accepted 30 September 2009 Available online 9 October 2009 Keywords: Intermetallics Powder metallurgy Mechanical alloying X-ray diffraction
a b s t r a c t The effects of tungsten and aluminum additions to Mo–Si system on the formation of MoSi2 by mechanically-induced self-propagating reaction in a high-energy ball mill were investigated by Xray diffraction. The incubation time for mechanically-induced self-propagating reaction to form MoSi2 was 90 min in Mo–Si system. With the addition of tungsten to Mo–Si system, the incubation time of mechanically-induced self-propagating reaction of (Wx ,Mo1−x )Si2 system was prolonged. It was due to the decrease of adiabatic temperature with increase in x, and the mechanically-induced self-propagating reaction was not observed until 150 min of high-energy milling in (Wx ,Mo1−x )Si2 powder sample with x equals to 0.4. Conversely, the minor aluminum (y < 0.2 in Mo(Si1−y ,Aly )2 system) substituting for silicon had shorten the incubation period and accelerated the reaction. When tungsten and aluminium were added simultaneously to Mo–Si system, the mechanically-induced self-propagating reaction was observed in (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 sample, but it was not observed in (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 and (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction MoSi2 has been regarded as a promising material for hightemperature structural applications. Due to its high melting point of 2030 ◦ C, relatively low density of 6.24 g/cm3 , excellent resistance to oxidation at elevated temperature, good thermal conductivity and electrical conductivity, MoSi2 may tolerate high service temperatures in the field of energy machines, gas turbine parts, heat shields and tiles [1–3]. MoSi2 has been extensively studied during the past 20 years. Significant progress has been made in both scientific and technological developments [2,3]. Today the main application of MoSi2 is as heating elements for electric furnaces [1]. Unfortunately, applications of MoSi2 are limited by its low fracture toughness at room temperature and poor creep resistance at high temperature. These can lead to the problems of component reliability. Thus the key issue is to improve fracture toughness and creep resistance at elevated temperature of MoSi2 [1–5]. Approaches to improve low temperature fracture toughness and high-temperature creep properties of MoSi2 have been focused on introducing secondary reinforcement and alloying. Alloying strategies include the addition of a third element, such as W, Al, Mg, Ge,
∗ Corresponding author. Tel.: +86 516 83591870; fax: +86 516 83591870. E-mail address: [email protected] (P. Feng). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.09.189
Re, Nb, Ta and V, etc. [2,5–9]. W, Re, Nb, Ta and V atoms substitute Mo atoms in MoSi2 . Al, Mg and Ge atoms substitute Si atoms in MoSi2 [7]. Especially, W and Al are considered as the important toughening and strengthening elements [10–14]. Mechanical alloying (MA) which is well known as a nonequilibrium processing technique has been applied to many systems, including molybdenum silicides [15–17]. The MA of MoSi2 has been extensively studied [15–19]. One of the formation mechanisms of MoSi2 by mechanical alloying is similar to the self-propagating high-temperature synthesis (SHS) [16–18]. The characteristic of this mechanism is a spontaneous rapid high-temperature synthesis reaction induced by a few hours of high-energy milling. This behavior is so far reported in MoSi2 , Ta2 Si3 , Nb5 Si3 and Al50 Ni50 systems by many researchers [18,20]. And this mechanism is named as mechanically-induced self-propagating reaction (MSR) [16]. MSR is analogous to that of the thermally ignited SHS [16]. The SHS process is highly exothermic. A highly exothermic reaction evolves enough heat to heat up the adjacent layer of reactants to react and by this the reaction becomes self-sustained. The high combustion temperature (e.g. 1000–6500 K) volatilizes low boiling point impurities and gives purer products. However, it is difficult to control the process of SHS, and the end product is largely porous powder compact. The process of MSR combines the advantages of SHS and MA techniques. The end-product is pulverized into fine particles. The milling time of MSR is short [16,18] and hence reduces the contamination from the balls and vials.
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Though the effects of alloying on the preparation of MoSi2 by mechanical alloying and self-propagating high-temperature synthesis are reported in many studies [1,19], it is difficult to find the effects of alloying on the synthesis of MoSi2 by mechanicallyinduced self-propagating reaction. Based on the mechanism of mechanically-induced selfpropagating reaction (MSR), the objective of this study is to investigate the effects of tungsten and aluminum additions on the formation of MoSi2 by self-propagating reaction during the mechanical alloying. The structure of as-milled powders and heat treated powders is analyzed by X-ray diffraction (XRD). 2. Experimental procedures The starting materials were 99.9% pure elemental molybdenum powder with an average particle size of 3–5 m, 99.9% pure elemental silicon powder with an average particle size of 44 m, 99.5% pure elemental tungsten powder with an average particle size of 3.3 m and 99.5% pure elemental aluminum powder with an average particle size of 44 m. The elemental powders were carefully weighed and mixed in the stoichiometry of MoSi2 , (W0.1 ,Mo0.9 )Si2 , (W0.2 ,Mo0.8 )Si2 , (W0.3 ,Mo0.7 )Si2 and (W0.4 ,Mo0.6 )Si2 (W–Mo–Si system); Mo(Si0.95 ,Al0.05 )2 , Mo(Si0.9 ,Al0.1 )2 , Mo(Si0.8 ,Al0.2 )2 and Mo(Si0.7 ,Al0.3 )2 (Mo–Si–Al system); (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 , (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 and (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 (W–Mo–Si–Al system), respectively. A high-energy vibrating mill with three vials was used for the mechanical alloying experiments. The stainless steel vials (60 cm3 in volume) with bearing steel balls (6 and 10 mm in diameter) were used for milling. The frequency of vibration of the machine was 1000 revolution/min. For each milling run, 5.0 g of the powder mixture was canned into stainless steel vials containing bearing steel balls in a glove box under an argon gas atmosphere to avoid contamination from the air. The weight ratio of the balls to the powder mixture was 15:1. The vials were sealed with a rubber O ring and the milling thus proceeded in a stationary atmosphere. The milling program was set to pause for 6 min for every 10 min of milling to prevent excessive heating during the milling process. The surface temperature on the top of the vial was measured with a digital thermometer at fixed intervals. When an obvious and abrupt temperature increase was observed, as reported by Atzmon [20], it was considered to be the critical ball milling time (incubation period) of self-propagating reaction during mechanical alloying. The heat treatment of the as-milled powders was carried out in vacuum at 1000 ◦ C for 1 h. The structure of the as-milled powders and heat treated powders was analyzed by a Rigaku Dmax-RB X-ray diffraction (XRD) using a Cu target (K␣, = 0.15406 nm), operating at 40 kV and 200 mA settings. The compositional analyses were conducted through energy dispersive spectroscopy (EDS, Kevex-Sigma).
Fig. 1. X-ray diffraction patterns of W–Mo–Si powder mixture high-energy ball milled for different times (a) MoSi2 stoichiometry; (b) (W0.1 ,Mo0.9 )Si2 stoichiometry; (c) (W0.2 ,Mo0.8 )Si2 stoichiometry; (d) (W0.3 ,Mo0.7 )Si2 stoichiometry; (e) (W0.4 ,Mo0.6 )Si2 stoichiometry.
3. Results and discussions Fig. 2. A temperature trace of the vial during milling of Mo:2Si powders.
3.1. Tungsten addition Fig. 1 shows the X-ray diffraction (XRD) patterns of W–Mo–Si powder mixtures after different milling times. The milling time is that particular time when the surface temperature of the vial shows an abrupt change. Fig. 2 shows a temperature trace of the vial during milling of Mo:2Si powders (MoSi2 stoichiometry). The surface temperature increases from 27.8 ◦ C to 38.9 ◦ C when the powder is milled for 90 min (incubation period) (Fig. 2). Strong peaks of XRD pattern in Fig. 1a corresponds to MoSi2 . So, the high-energy milling of Mo:2Si powders synthesizes MoSi2 by mechanically-induced self-propagating reaction (MSR) after 90 min of incubation period. The MSR is neither uniform nor integral, and hence much reactant is residual. This is the reason for the relatively low adiabatic temperature of MoSi2 (1943 K) in MA process (loose powders), which only is 143 K higher than the experiential criterion of SHS (1800 K). The heat transfer from the reaction front is difficult. The combustion wave propagation is irregular, and in some instances, the wave propagates down at an angle. This behavior is observed in Mo–Si [17–18], Al–Ni [20], CuO–Fe [21], PbO2 –TiO [15], etc. The acute temperature variation is also observed after 90 min milling in the x = 0.1 and 0.2 samples in (Wx ,Mo1−x )Si2 system. The X-ray diffraction patterns in Fig. 1b and c show that a great deal of MoSi2 and WSi2 are formed by the MSR. For the sample
with x = 0.3 in (Wx ,Mo1−x )Si2 system, the acute temperature change variation is observed after 100 min of milling. The MoSi2 is synthesized (Fig. 1d) with slightly prolonged incubation time for MSR. With further increase of tungsten content in (Wx ,Mo1−x )Si2 system to x = 0.4, mechanically-induced self-propagating reaction is not observed up to 150 min (1.5 times for the x = 0.3 sample) of milling. The acute temperature variation is not observed in the (W0.4 ,Mo0.6 )Si2 sample, and the absence of MSR is corroborated by the XRD peaks in Fig. 1e. Fig. 1(a–e) shows that the product mainly consists of C11b /A2, C11b /A2, C11b /A2, C11b /A2 and A2, respectively. The MSR cannot occur in the (W0.4 ,Mo0.6 )Si2 sample. It has a close relationship with the addition of tungsten to Mo–Si system. It is calculated that the adiabatic temperature (Tad ) of MoSi2 is 1943 K, and that of WSi2 is 1512 K (Table 1). On the basis of experiential observations, it has been suggested that systems with Table 1 Thermodynamic properties and adiabatic temperatures of MoSi2 and WSi2 . ◦
−1
Compound H298 (J mol MoSi2 WSi2
−131710 −92751
) Cp (J mol−1 K−1 ) 67.831 + 11.970 × 10−3 T − 6.569 × 105 T−2 67.831 + 11.042 × 10−3 T − 6.092 × 105 T−2
Tad (K) 1943 1512
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Table 2 Crystal structure, lattice parameters, space group and formula units per cell of compounds of Mo, W, Si and Ala . Phase
Crystal structure
Si Mo W Al MoSi2 WSi2 Mo(Si,Al)2 W(Si,Al)2
Diamond cubic bcc bcc fcc Tetragonal Tetragonal Hexagonal Hexagonal
Lattice parameters (nm) a
a
b
0.5431 0.3417 0.3165 0.4049 0.3205 0.3211 0.4674 0.4726
Space group
Formula units per cell
Fd3m Im3m Im3m Fm3m I4/mmm I4/mmm P6222 P6222
8 4 2 2 2 2 3 3
c
0.7845 0.7829 0.6558 0.6581
The data from the MDI Jade 5.0.
Tad ≤ 1800 K will not react in a self-propagating manner [22]. MoSi2 can be synthesized by SHS, but WSi2 cannot be. The adiabatic temperature of (Wx ,Mo1−x )Si2 system decreases with the increase of tungsten. The adiabatic temperature of the (W0.3 ,Mo0.7 )Si2 sample (x = 0.3) is 1813.7 K, which is still higher than the experiential criterion, thus the new compound can be formed by the SHS in x = 0.3 sample. The adiabatic temperature decreases to 1770.6 K for samples with tungsten content x = 0.4, which is less than the experiential criterion of SHS (Fig. 3), and hence new compound will not form by SHS. Fig. 3 shows variation of adiabatic temperature with x value in (Wx, Mo1−x )Si2 system. The adiabatic temperature is calculated from the reaction of (1−x)Mo + xW + 2Si → (Wx, Mo1−x )Si2 . Thus, it is linear with increase of tungsten. In the present work, the results of MSR are consistent with the calculation of the adiabatic temperature. The XRD patterns of the W–Mo–Si powder mixtures milled and heat treated at 1000 ◦ C for 1 h are shown in Fig. 4. It can be seen from Fig. 4 that the Mo, W and Si have reacted during heat treatment. The phases formed are MoSi2 , WSi2 and trace amounts of Mo5 Si3 and W5 Si3 with minor amounts of residual reactants. As the crystal structures and lattice parameters of Mo, W and their silicides are similar. The crystal structures, lattice parameters, space group and formula units per cell of compounds of Mo, W, Si and Al are present in Table 2. And the atoms of Mo and W can replace each other during synthesis and form a continuous solid solution (Wx ,Mo1−x )Si2 [10,11]. The EDS analyses show that Mo and W elements are well distributed. Thus (W0.1 ,Mo0.9 )Si2 , (W0.2 ,Mo0.8 )Si2 , (W0.3 ,Mo0.7 )Si2 and (W0.4 ,Mo0.6 )Si2 compounds/solid solutions are formed by MSR and heat treatment in the present work.
Fig. 3. Variation of adiabatic temperature with x value in (Wx ,Mo1−x )Si2 system.
Fig. 4. X-ray diffraction patterns of W–Mo–Si powder mixture high-energy ball milled and heat treated at 1000 ◦ C for 1 h (a) MoSi2 stoichiometry; (b) (W0.1 ,Mo0.9 )Si2 stoichiometry; (c) (W0.2 ,Mo0.8 )Si2 stoichiometry; (d) (W0.3 ,Mo0.7 )Si2 stoichiometry; (e) (W0.4 ,Mo0.6 )Si2 stoichiometry.
3.2. Aluminum addition Fig. 5 shows the X-ray diffraction patterns of Mo–Si–Al powder mixtures after different milling times. The acute temperature variation is observed after 80, 70 and 90 min of milling in Mo(Si1−y ,Aly )2 system for y = 0.05, 0.1 and 0.2, respectively. The X-ray diffraction confirms the formation of MoSi2 and Mo(Si,Al)2 compounds by MSR
Fig. 5. X-ray diffraction patterns of Mo–Si–Al powder mixture high-energy ball milled for different times (a) Mo(Si0.95 ,Al0.05 )2 stoichiometry; (b) Mo(Si0.9 ,Al0.1 )2 stoichiometry; (c) Mo(Si0.8 ,Al0.2 )2 stoichiometry; (d) Mo(Si0.7 ,Al0.3 )2 stoichiometry.
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Fig. 6. X-ray diffraction patterns of Mo–Si–Al powder mixture high-energy ball milled and heat treated at 1000 ◦ C for 1 h (a) Mo(Si0.95 ,Al0.05 )2 stoichiometry; (b) Mo(Si0.9 ,Al0.1 )2 stoichiometry; (c) Mo(Si0.8 ,Al0.2 )2 stoichiometry; (d) Mo(Si0.7 ,Al0.3 )2 stoichiometry.
Fig. 7. X-ray diffraction patterns of W–Mo–Si–Al powder mixture highenergy milled for different times (a) (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 stoichiometry; (b) (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 stoichiometry; (c) (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 stoichiometry.
in Mo(Si1−y ,Aly )2 system. However, the milling time is prolonged to 140 min (∼1.5 times for the y = 0.2 sample) and the acute temperature variation is not observed in the y = 0.3 sample (Fig. 5d). Though the diffraction peaks of Si are weaken, the diffraction peaks of Mo are very strong and peaks for any new compound phase are absent. Thus the y = 0.3 sample has not gone through MSR. Fig. 5(a–d) show that the product mainly consists of C11b /A2, C11b /C40/A2, C40/A2 and A2/A4, respectively. The melting point of aluminum is 660 ◦ C, and that of silicon and Al-12.2at%Si eutectic are 1410 ◦ C and 577 ◦ C, respectively. The mechanism of MoSi2 by SHS is characterized by the melting of Si, the wetting of Mo by molten Si, the dissolution of Mo in the molten Si and the precipitation of MoSi2 from the liquid phase. Severe collisions and friction take place amongst vial, balls and powders during ball milling. These collisions lead to energy concentration in small volumes between the vial and a ball, resulting in higher local temperature upon impact, and the self-sustaining reaction is then triggered at these “hot spots” [17,20]. On the other hand, the temperature of the powders during high-energy milling will increase due to the kinetic energy of the grinding medium, and it can increase to >1000 ◦ C [15]. The self-sustaining reaction can be ignited at this temperature. Because the melting point of Al and Al-12.2at%Si eutectic is less than that of Si, the temperature of “hot spots” and the powders may exceed prior to the melting point of Si. The molten Al and Al–Si eutectic will wet the solid Mo powders. This is beneficial to the contact of the reactants and the reduction of the incubation time of MSR. The X-ray diffraction patterns of the Mo–Si–Al powder mixture milled and heat treated at 1000 ◦ C for 1 h are shown in Fig. 6. It can be seen from Fig. 6d that the structure of y = 0.3 sample is C40 Mo(Si,Al)2 . The structure of MoSi2 is Cllb . The structure of product is changed from Cllb to C40. The standard enthalpy of formation of hypothetical compounds C11b –MoAl2 , C40–MoAl2 and C11b –MoSi2 are 19.0, 21.0, 45.156 kJ/mol, respectively [23]. The decrease of standard enthalpy increases the difficulty of formation of compounds. This variation of structure leads to the result that the self-sustaining reaction cannot be induced in Mo(Si1−y ,Aly )2 system for y = 0.3 (Fig. 5d). Fig. 6 shows the structure change from Cllb to C40 in Mo(Si1−y ,Aly )2 system with increase in y. It can be seen from Fig. 6 that the structure of y = 0.05 sample is only Cllb MoSi2 (Fig. 6a). The reflection line of C40 Mo(Si,Al)2 is observed in the y = 0.1 sample, and the product is a mixture of Cllb and C40 (Fig. 6b). The
MoSi2 peaks are weakened and the Mo(Si,Al)2 peaks are strengthened in the y = 0.2 sample (Fig. 6c). Only Mo(Si,Al)2 can be indexed in the y = 0.3 sample (Fig. 6d). This change in structure can be explained by an equilibrium phase diagram of Mo–Si–Al system [24]. On the diagram, the material lies in the monophasic Cllb , biphasic Cllb and C40, and monophasic C40 region with the increase of the aluminum, respectively [14,24,25]. Thus Mo(Si0.95 ,Al0.05 )2 , Mo(Si0.9 ,Al0.1 )2 , Mo(Si0.8 ,Al0.2 )2 and Mo(Si0.7 ,Al0.3 )2 compounds are formed in the present work.
3.3. Tungsten and aluminum additions Fig. 7 shows the X-ray diffraction patterns of W–Mo–Si–Al powder mixture system after different milling times. The acute temperature variation is observed after 100 min milling in the x = 0.1, y = 0.1 sample of (Wx ,Mo1−x )(Si1−y ,Aly )2 system. The X-ray diffraction shows that the silicide and alumino-silicide of Mo and W are formed. However, the milling time is prolonged to 170 min for the x = 0.2, y = 0.2 sample and 190 min for the x = 0.3, y = 0.3 sample, respectively, the acute temperature variation is not observed. The XRD patterns confirm that the MSR does not occur and there is not any new phase. Fig. 7(a–c) shows that the product mainly consists of C11b /C40/A2, A2/A4 and A2/A4, respectively. The addition of W leads to the decrease of the adiabatic temperature (as discussed above). And the incubation time is also rapidly increased, when y = 0.2 in Mo–Si–Al system (Fig. 5). Thus, the MSR cannot be induced in the x ≥ 0.2, y ≥ 0.2 sample of W–Mo–Si–Al system. The X-ray diffraction patterns of the W–Mo–Si–Al powder mixture milled and heat treated at 1000 ◦ C for 1 h are shown in Fig. 8. The structure of the x = 0.1, y = 0.1 sample mainly consists of Cllb MoSi2 and WSi2 . Small amount of C40 Mo(Al,Si)2 and W(Al,Si)2 and trace amounts Mo5 Si3 and W5 Si3 are also present. When x = 0.2, y = 0.2, the primary product becomes C40 phase, with the Cllb phase formed in minor amount. When x = 0.3, y = 0.3, the intensities of peaks from C40 phase is further increased, and that of Cllb phase is decreased. As the crystal structure and lattice parameters of silicide and alumino–silicide of Mo and W are also similar (Table 2), it is believed that the W and Al atoms substitute for Mo and Si atoms, respectively. The EDS analyses show that Mo and W elements are well distributed. The compounds/solid solution (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 , (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 and (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 are formed.
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formed by the MSR. And the MSR is not induced in the x ≥ 0.2, y ≥ 0.2 sample. The solid solution of (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 , (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 and (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 are formed. Acknowledgements The project was supported by Doctoral Fund of Ministry of Education of China No. 20070290523, Natural Science Foundation of Jiangsu Province No. BK2009096 and Youth Foundation of China University of Mining and Technology No. 2008A046. Xiaohong Wang would like to acknowledge Research and Innovation Project for College Graduate of Jiangsu Province CX09B 122Z. References
Fig. 8. X-ray diffraction patterns of W–Mo–Si–Al powder mixture high-energy milled and heat treated at 1000 ◦ C for 1 h (a) (W0.1 ,Mo0.9 )(Si0.9 ,Al0.1 )2 stoichiometry; (b) (W0.2 ,Mo0.8 )(Si0.8 ,Al0.2 )2 stoichiometry; (c) (W0.3 ,Mo0.7 )(Si0.7 ,Al0.3 )2 stoichiometry.
4. Conclusions 1. With the increase of W in (Wx ,Mo1−x )Si2 powder mixture system, the incubation time of mechanically-induced selfpropagating reaction is prolonged because of the decrease of adiabatic temperature. And self-sustaining reaction cannot be induced in the (W0.4 ,Mo0.6 )Si2 sample (i.e. x = 0.4 sample). 2. With the increase of Al in Mo(Si1−y ,Aly )2 powder mixture system, the incubation time of mechanically-induced self-propagating reaction is shortened from 90 min (the y = 0 sample) to 70 min (the y = 0.1 sample), and then, the incubation time is reverted to 90 min in the y = 0.2 sample. Self-sustaining reaction cannot be induced in the y = 0.3 sample, even after increasing milling time to 140 min. 3. The acute temperature variation is observed after 100 min milling in the x = 0.1, y = 0.1 sample of (Wx ,Mo1−x )(Si1−y ,Aly )2 system. The silicide and alumino–silicide of Mo and W are
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