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Effect of diluent on the synthesis of molybdenum disilicide by mechanically-induced self-propagating reaction
Journal of Alloys and Compounds 494 (2010) 301–304
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of diluent on the synthesis of molybdenum disilicide by mechanically-induced self-propagating reaction Peizhong Feng a,∗ , Akhtar Farid b , Xiaohong Wang a , Weisheng Liu a , Jie Wu a , Shuai Zhang a , Yinghuai Qiang 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 2 November 2009 Received in revised form 6 January 2010 Accepted 7 January 2010 Available online 15 January 2010 Keywords: Intermetallics Mechanical alloying Self-propagating reaction Diluent
a b s t r a c t The effect of the addition of diluent to Mo–Si system on the formation of MoSi2 by mechanically-induced self-propagating reaction was investigated in a high-energy ball mill. MoSi2 intermetallic was added as diluent to Mo–Si powder mixture. The structure and morphology of the reaction product was analyzed by X-ray diffraction and scanning electron microscope. The diluent decreased the adiabatic temperature, increased the ignition temperature and modified the Mo/Si reactants interface. Therefore, the incubation period of mechanically-induced self-propagating reaction was extended from 90 min to 140 min with the addition of diluent from 0 wt% to 10 wt% to Mo–Si powder mixture. The extended milling time reduced the size of agglomerated particles and fine MoSi2 product was obtained. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum disilicide (MoSi2 ) is currently of great interest as potential high-temperature structural materials because of its excellent stability in oxidation and corrosive environments. It is difficult to synthesize pure MoSi2 by conventional casting and powder pressing/sintering methods due to its high melting point (2030 ◦ C) and narrow compositional range [1]. The synthesis of MoSi2 by self-propagating high-temperature synthesis (SHS) and mechanical alloying (MA) have been shown in numerous investigations [2–6]. Self-propagating high-temperature synthesis (SHS) is a chemical reaction in a certain atmosphere to ignite powder compacts. The process is highly exothermic. The exothermic reactions is initiated at an ignition temperature Tig , and generates heat which is manifested in a maximum or combustion temperature, Tc (e.g. 1000–6500 K), which can volatilize low boiling point impurities, and results in purer products than those produced by conventional techniques [7]. SHS has engaged considerable attention as an affordable process to prepare a variety of refractory and high-temperature materials. These have been demonstrated in numerous investigations [7,8]. However, it is difficult to control the processing parameters of combustion synthesis reaction, and the end-product is largely porous powder compact.
∗ Corresponding author. Tel.: +86 516 83591870; fax: +86 516 83591870. E-mail address: [email protected] (P. Feng). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.01.016
Mechanical alloying (MA) is a solid-state powder processing technique. It involves repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill [9]. MA is capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or pre-alloyed powders [6,9]. The synthesis of a variety of alloy phases including solid solutions, quasicrystalline and crystalline intermetallic phases, and amorphous phases has spurred lots of research investigations in recent years [9,10]. However, the MA time is large to obtain the end products, sometime hundreds of hours [6,10] and is not suitable for industrial production. It is well known that when a highly exothermic chemical system is mechanically treated by ball milling, a combustion-like reaction can take place. This process is generally referred as mechanicallyinduced self-propagating reaction (MSR) [11–13]. This process combines the advantages of SHS and MA techniques. The endproduct is pulverized into fine particles. The milling time of MSR is short [11–14] and reduces the contamination from the balls and vials. Several researchers have reported the (explosive) formation of MoSi2 during mechanical alloying by mechanically-induced selfpropagating reaction [11,14]. The addition of diluents to the reactants will affect the milling process and the properties of the end-product will change. Jo et al. have reported the effect of the diluent content on MoSi2 synthesized by self-propagating high-temperature synthesis [2]. However, it is difficult to find any research report on the effect of the diluent on the formation of MoSi2 by mechanical alloying. The objective of this study is to investigate the effect of the
302
P. Feng et al. / Journal of Alloys and Compounds 494 (2010) 301–304 Table 1 Grain size, strain and lattice parameters of Mo. Sample no.
Grain size (nm)
Strain (%)
0 1 2 3 4
26.4 25.2 22.1 20.8 19.2
0.3803 0.3957 0.4500 0.4805 0.5251
Lattice parameters (nm) a
Fig. 1. The X-ray diffraction patterns taken at the induction time of mechanicallyinduced self-propagating reaction with various MoSi2 diluent content (a) as-milled for 90 min powder without diluent; (b) as-milled for 100 min powder with 2.5 wt% diluent; (c) as-milled for 110 min powder with 5.0 wt% diluent; (d) as-milled for 120 min powder with 7.5 wt% diluent; (e) as-milled for 140 min powder with 10.0 wt% diluent.
diluent content on the behavior of mechanically-induced selfpropagating reaction of Mo–Si system during mechanical alloying process. Based on the experimental results, a new method to alter the incubation time of MSR is proposed. Modification of incubation time will control the particle size of the final product. The structure and morphologies of MSR powders are discussed by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively, according to the proposed method. A fine grain structured product is obtained by the addition of diluent. 2. Experimental procedures The starting materials used in the present study were 99.9% pure elemental molybdenum powder with an average particle size of 3–5 m and 99.9% pure elemental silicon powder with an average particle size of 44 m. The mixture of elemental powders was prepared with the molar ratio of molybdenum to silicon being equal to 1:2. And then the diluent, MoSi2 intermetallic powder, was added into the mixture of molybdenum and silicon. The content of the diluent was 0 wt% (0# ), 2.5 wt% (1# ), 5.0 wt% (2# ), 7.5 wt% (3# ), 10.0 wt% (4# ), respectively. The mechanical alloying was performed using a high-energy vibratory ball mill. Three stainless steel vials (60 cm3 in volume) with bearing steel balls (10 mm and 6 mm in diameter) were used for milling. The frequency of vibration of the machine was 1000 rev min−1 . For each milling run, 5.0 g of the powder mixture was canned into the stainless steel vials containing bearing steel balls in a glove box container under an argon gas atmosphere to avoid contamination from 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 argon 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 intervals. When an obvious abrupt temperature increase was observed [15], the milling time was suggested to be the critical ball milling time (incubation period) of self-propagating reaction during mechanical alloying. The structure of milled powders was analyzed by a Rigaku D/max-RB X-ray diffractometer using Cu target (K␣, = 0.15406 nm) operating at 40 kV and 200 mA settings. The morphologies of milled powders were investigated using a Kevex LEO1450 scanning electron microscope (SEM).
Fig. 1 shows the X-ray diffraction patterns of samples with different diluent content after different milling times. The milling time represents the time for the occurrence of mechanically-induced
c
self-propagating reaction (MSR) during mechanical alloying. At this particular time, the surface temperature of the vial shows an acute variation. The strong peaks are of MoSi2 in all samples (Fig. 1). MoSi2 is synthesized by mechanically-induced self-propagating reaction after incubation period. In the present study, not all the Mo and Si powder mixture took part in the mechanically-induced selfpropagating reaction (MSR). And some Mo(Si) solid solution was residual (the site of Mo peaks in Fig. 1). In other words, the MSR in the present process was not uniform or integral. This is due to relatively low adiabatic temperature of MoSi2 (1942 K), which only is 143 K higher than the experiential suggestion of SHS (1800 K) [7]. The adiabatic temperature (Tad ) is the maximum combustion temperature of self-propagating high-temperature synthesis reaction under adiabatic conditions. It is calculated from the enthalpies of formation and specific heat of the product. On the basis of experimental observations, it has been suggested that systems with Tad ≤ 1800 K will not react in a self-propagating manner [7,8]. The powder is loose during mechanical alloying process, and the loose powder exhibits the state of the poor particle-particle contacts in the vial [16]. Therefore, the exothermic reaction ignites locally at one point. And, it is difficult that the heat evolved during the exothermic reaction propagates and ignites the reaction in other regions of reactants. Additionally, a part of heat is consumed in increasing the temperature of vial and balls. When the heat is not sufficient for SHS, the self-propagating reaction will extinguish. Simultaneously, the combustion wave is propagating irregular, in some instances, the wave propagates down at an angle. This is supported by the presence of strong Mo diffraction peaks (Fig. 1) and confirms the presence of residual reactants. These residual reactants transform into compound during subsequent heat treatment at relatively low temperature, such as at 700 ◦ C in vacuum [17]. It can also be seen from Fig. 1 that the incubation period of mechanically-induced self-propagating reaction (MSR) is prolonged from 90 min to 140 min with the increase of diluent. This indicates that the energy inducing the MSR is increased with the diluent content. This is well in accord with the previous findings where excessive Si acted as diluent and extended the incubation time and increased the ignition temperature of the powder mixture [18,19]. The grain size, strain and lattice parameters of Mo and MoSi2 are calculated from X-ray diffraction measurements, and they are shown in Tables 1 and 2, respectively. Table 1 shows that the grain size of Mo is decreasing with milling time. The stress induced due to mechanical alloying is conventionally described in terms of strain.
Table 2 Grain size, strain and lattice parameters of MoSi2 . Sample no.
Grain size (nm)
Strain (%)
Lattice parameters (nm) a
3. Results and discussions
b
0.31524 0.31416 0.31437 0.31463 0.31409
0 1 2 3 4
47.7 41.1 39.0 38.7 36.4
0.2222 0.2089 0.2458 0.2541 0.2357
0.32088 0.31999 0.31989 0.31990 0.32012
b
c 0.78574 0.78367 0.78374 0.78192 0.78273
P. Feng et al. / Journal of Alloys and Compounds 494 (2010) 301–304
Fig. 2. Scanning electron micrograph of the products of mechanically-induced selfpropagating reaction (a) as-milled for 90 min powder without diluent; (b and c) as-milled for 140 min powder with 10.0 wt% diluent.
It shows that the strain of Mo in increasing with milling time and hence the stress. Table 2 shows that the grain size of MoSi2 is also decreasing with milling time. Fig. 2 shows the SEM photograph of the products of mechanically-induced self-propagating reaction (MSR). The powder mixed without diluent shows large agglomerated particles with a diameter of 10 m (Fig. 2a). The agglomerated particles are consisted of submicron (300–500 nm) and large (2–3 m) particulates. The size of agglomerated particles is obviously decreased with the addition of diluent (Fig. 2b), and the agglomerated particles are mainly consisted of submicron particulates (200–300 nm) (Fig. 2c). During MSR, the diluent (MoSi2 ) will be thermally stable due to its high melting point (2303 K) and its particles will act as the heterogeneous nucleation sites and enhance the nucleation rate [2]. The increase in the nucleation rate
303
and milling time with the addition of diluents refine the powder product. During mechanical alloying, the interfacial energy because of refining, plus the deformation energy of cold-work, made the mechanically-induced self-propagating reaction (MSR) possible [16]. This indicates that the MSR can occur when the system energy is high enough. Thereby, the incubation time of MSR is the accumulating period of system energy. In incubation time, the powder particles are repeatedly fragmented, cold-welded, fractured and rewelded, and these processes promote the atomic diffusion and uniformity of reactants. The initial milling process also leads to a reduction in the crystallite size and an accumulation of defects in powder particles; this event introduces additional energy to the reactant system and, thus, effectively lowers the activation barrier for the reactions [9]. However, when the diluent is added into the reactants, the original material is increased from two kinds (Mo and Si) to three kinds (Mo, Si and MoSi2 ). Due to the addition of diluent in Mo and Si system, additional interfaces will appear in addition to simple Mo|Si interfaces. The new kind of interfaces will be of complex nature due to three phases. Diluent will find its place in existing simple Mo|Si interfaces. Hence, there is a good probability that the complex sandwich Mo|MoSi2 |Si||Mo|MoSi2 |Si interfaces will appear. These interfaces indicate that there is MoSi2 thin layer between some Mo and Si interfaces. This interface formation is further supported by the fact that the time for MSR is increased with diluent content due to reduced simple Mo|Si interfaces. In Mo–Si system, it is suggested that the brittle Si particles can be easily crushed into much small fragments and embedded into relatively plastic Mo lattices by diffusion. Thus, before the mechanically-induced self-propagating reaction (MSR), the Mo peaks gradually become wider and wider, while the Si peaks become weaker and weaker with the further milling. This indicates that the supersaturated Mo(Si) solid solution is formed [20]. Thereby, the formation of supersaturated Mo(Si) solid solution is an important foundation of the mechanically-induced selfpropagating reaction (MSR) [14]. In the present work, the diluent acts as a MoSi2 thin layer between the interface of Mo and Si. This increases the diffusion distance of Si into Mo lattices. It is difficult to form the supersaturated Mo(Si) solid solution. At the same time, some Si embeds into MoSi2 lattices, and forms the supersaturated MoSi2 (Si) solid solution [20]. These can further decrease the interface of Mo/Si reactants and delays the mechanically-induced self-propagating reaction (MSR). In SHS of MoSi2 , the heat conducted from the reaction front heats up the nearby Si and Mo mixture to the melting point of Si (1685 K) and the molten Si wets the solid Mo powders. Instead of forming MoSi2 at the liquid Si/solid Mo interface, Mo dissolved first into Si, the liquid Si phase becomes supersaturated with Mo and MoSi2 phase precipitates. This mechanism is characterized by Mo dissolution and MoSi2 precipitation, hence the dissolution and precipitation mechanism [2]. However, the complex sandwiches Mo|MoSi2 |Si||Mo|MoSi2 |Si interface result that the molten Si wets the solid Mo powders through the MoSi2 thin layer. And then the continuous dissolution and precipitation occur at the heterogeneous nucleation site of the diluent. With the increase of the diluent, the thickness of the diluent thin layer increases and makes wetting of the solid Mo by molten Si difficult and extends the incubation time of mechanically-induced self-propagating reaction (MSR). The adiabatic temperature (Tad ) is an important thermodynamic factor of SHS. With the increase of diluent content, the calculated adiabatic and measured combustion temperature of samples gradually decreases [2]. When the diluent content is 10 wt%, the calculated adiabatic temperature is only 1748 K. For the formation of compounds, it has been demonstrated empirically that the reaction will not be self-sustaining unless Tad > 1800 K. Thus the sample with
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P. Feng et al. / Journal of Alloys and Compounds 494 (2010) 301–304
amorphous phase, and then the amorphous phase is formed. For example, with the extending of milling, the stable phase T-MoSi2 may be translated into amorphous phase and metastable phase HMoSi2 . The present work indicates that the behavior of MSR can be controlled by the variation of diluent content, and polycrystalline MoSi2 powder can be obtained. The amorphous phase and metastable phase are restrained. 4. Conclusions
Fig. 3. Schematic diagram of the variation of the ignition temperature Tig and the temperature generated for ball milling collision Tc with milling time in mechanically-induced self-propagating reaction.
10% diluent cannot be synthesized by SHS model. When the sample with 10% diluent is pretreated to 400 K, stable wave propagation was obtained [2]. With the decrease of adiabatic temperature, the difficulty of SHS is increased. Unless the system is provided with enough high energy, e.g. preheat treatment [2] and pre-mechanical activation [4] the SHS will not be ignited. In present work, with further milling to 140 min, the system with diluent stored enough energy to induce MSR by mechanical impact. Fig. 3 shows the schematic diagram of the variation of the ignition temperature of MSR, Tig , the temperature generated for ball milling collision (ball-to-ball and ball-to-powder collisions) Tc and ball milling time. If Tc is higher than Tig , then the mechanicallyinduced self-propagating reaction can occur. Since the MA process refines the particle size and crystal size, Tig decreases with milling time. But, with increasing milling time Tc increases and reaches a steady state. The time at which the Tig and Tc intersect is the critical milling time, tig at which self-propagating reaction would occur [9]. However, the curve of Tig increases when the diluents is added in Mo–Si system. Thereby, when the milling time is extended , the T from tig to tig ig and Tc can intersect, and then the MSR is induced. MoSi2 compound acts as the diluent of Mo–Si system. The process of mechanically-induced self-propagating reaction (MSR) changes (induction time prolongs) and results in fine powder product. Thus, it is possible to design and control the milling time, structure and granularity of product. This is very important to the mechanical alloying (MA) process. MA is a solid-state powder processing technique. As long as the compound is formed, further milling accumulates defects in powder particles. In particular, the grain boundary defects are abundant. The additional energy to powder may overpass the barrier energy of the formation of
(1) Molybdenum disilicide was synthesized by mechanicallyinduced self-propagating reaction. The induction period of mechanically-induced self-propagating reaction was extended from 90 min to 140 min, with the fraction of diluent content from 0 wt% to 10 wt%. The diluent resulted in the decrease of adiabatic temperature, modified the Mo/Si reactants interface and extended the milling time. MoSi2 compound can acted as the diluent of Mo–Si system. (2) The extending milling time lead to fine grain structured product. The submicron particulates (200–300 nm) were formed. Acknowledgements The project was supported by the Natural Science Foundation of Jiangsu Province No. BK2009096 and Youth Foundation of China University of Mining and Technology No. 2008A046 and Jiangsu Postdoctoral Science Foundation No. 0902007B. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
N.S. Stoloff, Mater. Sci. Eng. A261 (1999) 169–180. S.W. Jo, G.W. Lee, J.T. Moon, Y.S. Kim, Acta Mater. 44 (1996) 4317–4326. H. Zhang, P. Chen, X. Liu, Rare Metal. Mater. Eng. 32 (2003) 70–72 (in Chinese). C. Gras, D. Vrel, E. Gaffet, F. Bernard, J. Alloys Compd. 314 (2001) 240–250. A.J. Heron, G.B. Schaffer, Mater. Sci. Eng. A352 (2003) 105–110. M. Zakeri, R. Yazdani-Rad, M.H. Enayati, M.R. Rahimipour, J. Alloys Compd. 403 (2005) 258–261. J.J. Moore, H.J. Feng, Prog. Mater. Sci. 39 (1995) 243–273. J.J. Moore, H.J. Feng, Prog. Mater. Sci. 39 (1995) 275–316. C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1–184. G.T. Fei, L. Liu, X.Z. Liu, L.D. Zhang, Q.Q. Zheng, J. Alloys Compd. 229 (1995) 280–282. B.K. Yen, T. Aizawa, J. Kihara, J. Am. Ceram. Soc. 79 (1996) 2221–2223. C. Deidda, F. Delogu, G. Cocco, J. Mater. Sci. 39 (2004) 5315–5318. B.K. Yen, T. Aizawa, J. Kihara, J. Am. Ceram. Soc. 81 (1998) 1953–1956. S.N. Patankar, S.Q. Xiao, J.J. Lewandowski, A.H. Heuer, J. Mater. Res. 8 (1993) 1311–1316. M. Atzmon, Phys. Rev. Lett. 64 (1990) 487–490. E. Ma, J. Pagan, G. Cranford, M. Atzmon, J. Mater. Res. 8 (1993) 1836–1844. P. Feng, F. Akhtar, X. Wang, H.S. Islam, X. Qu, Int. J. Refract. Met. Hard Mater. 26 (2008) 173–178. P. Feng, X. Qu, F. Akhtar, X. Du, H.S. Islam, C. Jia, J. Alloys Compd. 456 (2008) 304–307. M. Sannia, R. Orru, J.E. Garay, G. Cao, Z.A. Munir, Mater. Sci. Eng. A345 (2003) 270–277. R.B. Schwarz, S.R. Srinivasan, J.J. Petrovic, C.J. Maggiore, Mater. Sci. Eng. A155 (1992) 75–83.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of diluent on the synthesis of molybdenum disilicide by mechanically-induced self-propagating reaction Peizhong Feng a,∗ , Akhtar Farid b , Xiaohong Wang a , Weisheng Liu a , Jie Wu a , Shuai Zhang a , Yinghuai Qiang 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 2 November 2009 Received in revised form 6 January 2010 Accepted 7 January 2010 Available online 15 January 2010 Keywords: Intermetallics Mechanical alloying Self-propagating reaction Diluent
a b s t r a c t The effect of the addition of diluent to Mo–Si system on the formation of MoSi2 by mechanically-induced self-propagating reaction was investigated in a high-energy ball mill. MoSi2 intermetallic was added as diluent to Mo–Si powder mixture. The structure and morphology of the reaction product was analyzed by X-ray diffraction and scanning electron microscope. The diluent decreased the adiabatic temperature, increased the ignition temperature and modified the Mo/Si reactants interface. Therefore, the incubation period of mechanically-induced self-propagating reaction was extended from 90 min to 140 min with the addition of diluent from 0 wt% to 10 wt% to Mo–Si powder mixture. The extended milling time reduced the size of agglomerated particles and fine MoSi2 product was obtained. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum disilicide (MoSi2 ) is currently of great interest as potential high-temperature structural materials because of its excellent stability in oxidation and corrosive environments. It is difficult to synthesize pure MoSi2 by conventional casting and powder pressing/sintering methods due to its high melting point (2030 ◦ C) and narrow compositional range [1]. The synthesis of MoSi2 by self-propagating high-temperature synthesis (SHS) and mechanical alloying (MA) have been shown in numerous investigations [2–6]. Self-propagating high-temperature synthesis (SHS) is a chemical reaction in a certain atmosphere to ignite powder compacts. The process is highly exothermic. The exothermic reactions is initiated at an ignition temperature Tig , and generates heat which is manifested in a maximum or combustion temperature, Tc (e.g. 1000–6500 K), which can volatilize low boiling point impurities, and results in purer products than those produced by conventional techniques [7]. SHS has engaged considerable attention as an affordable process to prepare a variety of refractory and high-temperature materials. These have been demonstrated in numerous investigations [7,8]. However, it is difficult to control the processing parameters of combustion synthesis reaction, and the end-product is largely porous powder compact.
∗ Corresponding author. Tel.: +86 516 83591870; fax: +86 516 83591870. E-mail address: [email protected] (P. Feng). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.01.016
Mechanical alloying (MA) is a solid-state powder processing technique. It involves repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill [9]. MA is capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or pre-alloyed powders [6,9]. The synthesis of a variety of alloy phases including solid solutions, quasicrystalline and crystalline intermetallic phases, and amorphous phases has spurred lots of research investigations in recent years [9,10]. However, the MA time is large to obtain the end products, sometime hundreds of hours [6,10] and is not suitable for industrial production. It is well known that when a highly exothermic chemical system is mechanically treated by ball milling, a combustion-like reaction can take place. This process is generally referred as mechanicallyinduced self-propagating reaction (MSR) [11–13]. This process combines the advantages of SHS and MA techniques. The endproduct is pulverized into fine particles. The milling time of MSR is short [11–14] and reduces the contamination from the balls and vials. Several researchers have reported the (explosive) formation of MoSi2 during mechanical alloying by mechanically-induced selfpropagating reaction [11,14]. The addition of diluents to the reactants will affect the milling process and the properties of the end-product will change. Jo et al. have reported the effect of the diluent content on MoSi2 synthesized by self-propagating high-temperature synthesis [2]. However, it is difficult to find any research report on the effect of the diluent on the formation of MoSi2 by mechanical alloying. The objective of this study is to investigate the effect of the
302
P. Feng et al. / Journal of Alloys and Compounds 494 (2010) 301–304 Table 1 Grain size, strain and lattice parameters of Mo. Sample no.
Grain size (nm)
Strain (%)
0 1 2 3 4
26.4 25.2 22.1 20.8 19.2
0.3803 0.3957 0.4500 0.4805 0.5251
Lattice parameters (nm) a
Fig. 1. The X-ray diffraction patterns taken at the induction time of mechanicallyinduced self-propagating reaction with various MoSi2 diluent content (a) as-milled for 90 min powder without diluent; (b) as-milled for 100 min powder with 2.5 wt% diluent; (c) as-milled for 110 min powder with 5.0 wt% diluent; (d) as-milled for 120 min powder with 7.5 wt% diluent; (e) as-milled for 140 min powder with 10.0 wt% diluent.
diluent content on the behavior of mechanically-induced selfpropagating reaction of Mo–Si system during mechanical alloying process. Based on the experimental results, a new method to alter the incubation time of MSR is proposed. Modification of incubation time will control the particle size of the final product. The structure and morphologies of MSR powders are discussed by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively, according to the proposed method. A fine grain structured product is obtained by the addition of diluent. 2. Experimental procedures The starting materials used in the present study were 99.9% pure elemental molybdenum powder with an average particle size of 3–5 m and 99.9% pure elemental silicon powder with an average particle size of 44 m. The mixture of elemental powders was prepared with the molar ratio of molybdenum to silicon being equal to 1:2. And then the diluent, MoSi2 intermetallic powder, was added into the mixture of molybdenum and silicon. The content of the diluent was 0 wt% (0# ), 2.5 wt% (1# ), 5.0 wt% (2# ), 7.5 wt% (3# ), 10.0 wt% (4# ), respectively. The mechanical alloying was performed using a high-energy vibratory ball mill. Three stainless steel vials (60 cm3 in volume) with bearing steel balls (10 mm and 6 mm in diameter) were used for milling. The frequency of vibration of the machine was 1000 rev min−1 . For each milling run, 5.0 g of the powder mixture was canned into the stainless steel vials containing bearing steel balls in a glove box container under an argon gas atmosphere to avoid contamination from 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 argon 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 intervals. When an obvious abrupt temperature increase was observed [15], the milling time was suggested to be the critical ball milling time (incubation period) of self-propagating reaction during mechanical alloying. The structure of milled powders was analyzed by a Rigaku D/max-RB X-ray diffractometer using Cu target (K␣, = 0.15406 nm) operating at 40 kV and 200 mA settings. The morphologies of milled powders were investigated using a Kevex LEO1450 scanning electron microscope (SEM).
Fig. 1 shows the X-ray diffraction patterns of samples with different diluent content after different milling times. The milling time represents the time for the occurrence of mechanically-induced
c
self-propagating reaction (MSR) during mechanical alloying. At this particular time, the surface temperature of the vial shows an acute variation. The strong peaks are of MoSi2 in all samples (Fig. 1). MoSi2 is synthesized by mechanically-induced self-propagating reaction after incubation period. In the present study, not all the Mo and Si powder mixture took part in the mechanically-induced selfpropagating reaction (MSR). And some Mo(Si) solid solution was residual (the site of Mo peaks in Fig. 1). In other words, the MSR in the present process was not uniform or integral. This is due to relatively low adiabatic temperature of MoSi2 (1942 K), which only is 143 K higher than the experiential suggestion of SHS (1800 K) [7]. The adiabatic temperature (Tad ) is the maximum combustion temperature of self-propagating high-temperature synthesis reaction under adiabatic conditions. It is calculated from the enthalpies of formation and specific heat of the product. On the basis of experimental observations, it has been suggested that systems with Tad ≤ 1800 K will not react in a self-propagating manner [7,8]. The powder is loose during mechanical alloying process, and the loose powder exhibits the state of the poor particle-particle contacts in the vial [16]. Therefore, the exothermic reaction ignites locally at one point. And, it is difficult that the heat evolved during the exothermic reaction propagates and ignites the reaction in other regions of reactants. Additionally, a part of heat is consumed in increasing the temperature of vial and balls. When the heat is not sufficient for SHS, the self-propagating reaction will extinguish. Simultaneously, the combustion wave is propagating irregular, in some instances, the wave propagates down at an angle. This is supported by the presence of strong Mo diffraction peaks (Fig. 1) and confirms the presence of residual reactants. These residual reactants transform into compound during subsequent heat treatment at relatively low temperature, such as at 700 ◦ C in vacuum [17]. It can also be seen from Fig. 1 that the incubation period of mechanically-induced self-propagating reaction (MSR) is prolonged from 90 min to 140 min with the increase of diluent. This indicates that the energy inducing the MSR is increased with the diluent content. This is well in accord with the previous findings where excessive Si acted as diluent and extended the incubation time and increased the ignition temperature of the powder mixture [18,19]. The grain size, strain and lattice parameters of Mo and MoSi2 are calculated from X-ray diffraction measurements, and they are shown in Tables 1 and 2, respectively. Table 1 shows that the grain size of Mo is decreasing with milling time. The stress induced due to mechanical alloying is conventionally described in terms of strain.
Table 2 Grain size, strain and lattice parameters of MoSi2 . Sample no.
Grain size (nm)
Strain (%)
Lattice parameters (nm) a
3. Results and discussions
b
0.31524 0.31416 0.31437 0.31463 0.31409
0 1 2 3 4
47.7 41.1 39.0 38.7 36.4
0.2222 0.2089 0.2458 0.2541 0.2357
0.32088 0.31999 0.31989 0.31990 0.32012
b
c 0.78574 0.78367 0.78374 0.78192 0.78273
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Fig. 2. Scanning electron micrograph of the products of mechanically-induced selfpropagating reaction (a) as-milled for 90 min powder without diluent; (b and c) as-milled for 140 min powder with 10.0 wt% diluent.
It shows that the strain of Mo in increasing with milling time and hence the stress. Table 2 shows that the grain size of MoSi2 is also decreasing with milling time. Fig. 2 shows the SEM photograph of the products of mechanically-induced self-propagating reaction (MSR). The powder mixed without diluent shows large agglomerated particles with a diameter of 10 m (Fig. 2a). The agglomerated particles are consisted of submicron (300–500 nm) and large (2–3 m) particulates. The size of agglomerated particles is obviously decreased with the addition of diluent (Fig. 2b), and the agglomerated particles are mainly consisted of submicron particulates (200–300 nm) (Fig. 2c). During MSR, the diluent (MoSi2 ) will be thermally stable due to its high melting point (2303 K) and its particles will act as the heterogeneous nucleation sites and enhance the nucleation rate [2]. The increase in the nucleation rate
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and milling time with the addition of diluents refine the powder product. During mechanical alloying, the interfacial energy because of refining, plus the deformation energy of cold-work, made the mechanically-induced self-propagating reaction (MSR) possible [16]. This indicates that the MSR can occur when the system energy is high enough. Thereby, the incubation time of MSR is the accumulating period of system energy. In incubation time, the powder particles are repeatedly fragmented, cold-welded, fractured and rewelded, and these processes promote the atomic diffusion and uniformity of reactants. The initial milling process also leads to a reduction in the crystallite size and an accumulation of defects in powder particles; this event introduces additional energy to the reactant system and, thus, effectively lowers the activation barrier for the reactions [9]. However, when the diluent is added into the reactants, the original material is increased from two kinds (Mo and Si) to three kinds (Mo, Si and MoSi2 ). Due to the addition of diluent in Mo and Si system, additional interfaces will appear in addition to simple Mo|Si interfaces. The new kind of interfaces will be of complex nature due to three phases. Diluent will find its place in existing simple Mo|Si interfaces. Hence, there is a good probability that the complex sandwich Mo|MoSi2 |Si||Mo|MoSi2 |Si interfaces will appear. These interfaces indicate that there is MoSi2 thin layer between some Mo and Si interfaces. This interface formation is further supported by the fact that the time for MSR is increased with diluent content due to reduced simple Mo|Si interfaces. In Mo–Si system, it is suggested that the brittle Si particles can be easily crushed into much small fragments and embedded into relatively plastic Mo lattices by diffusion. Thus, before the mechanically-induced self-propagating reaction (MSR), the Mo peaks gradually become wider and wider, while the Si peaks become weaker and weaker with the further milling. This indicates that the supersaturated Mo(Si) solid solution is formed [20]. Thereby, the formation of supersaturated Mo(Si) solid solution is an important foundation of the mechanically-induced selfpropagating reaction (MSR) [14]. In the present work, the diluent acts as a MoSi2 thin layer between the interface of Mo and Si. This increases the diffusion distance of Si into Mo lattices. It is difficult to form the supersaturated Mo(Si) solid solution. At the same time, some Si embeds into MoSi2 lattices, and forms the supersaturated MoSi2 (Si) solid solution [20]. These can further decrease the interface of Mo/Si reactants and delays the mechanically-induced self-propagating reaction (MSR). In SHS of MoSi2 , the heat conducted from the reaction front heats up the nearby Si and Mo mixture to the melting point of Si (1685 K) and the molten Si wets the solid Mo powders. Instead of forming MoSi2 at the liquid Si/solid Mo interface, Mo dissolved first into Si, the liquid Si phase becomes supersaturated with Mo and MoSi2 phase precipitates. This mechanism is characterized by Mo dissolution and MoSi2 precipitation, hence the dissolution and precipitation mechanism [2]. However, the complex sandwiches Mo|MoSi2 |Si||Mo|MoSi2 |Si interface result that the molten Si wets the solid Mo powders through the MoSi2 thin layer. And then the continuous dissolution and precipitation occur at the heterogeneous nucleation site of the diluent. With the increase of the diluent, the thickness of the diluent thin layer increases and makes wetting of the solid Mo by molten Si difficult and extends the incubation time of mechanically-induced self-propagating reaction (MSR). The adiabatic temperature (Tad ) is an important thermodynamic factor of SHS. With the increase of diluent content, the calculated adiabatic and measured combustion temperature of samples gradually decreases [2]. When the diluent content is 10 wt%, the calculated adiabatic temperature is only 1748 K. For the formation of compounds, it has been demonstrated empirically that the reaction will not be self-sustaining unless Tad > 1800 K. Thus the sample with
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amorphous phase, and then the amorphous phase is formed. For example, with the extending of milling, the stable phase T-MoSi2 may be translated into amorphous phase and metastable phase HMoSi2 . The present work indicates that the behavior of MSR can be controlled by the variation of diluent content, and polycrystalline MoSi2 powder can be obtained. The amorphous phase and metastable phase are restrained. 4. Conclusions
Fig. 3. Schematic diagram of the variation of the ignition temperature Tig and the temperature generated for ball milling collision Tc with milling time in mechanically-induced self-propagating reaction.
10% diluent cannot be synthesized by SHS model. When the sample with 10% diluent is pretreated to 400 K, stable wave propagation was obtained [2]. With the decrease of adiabatic temperature, the difficulty of SHS is increased. Unless the system is provided with enough high energy, e.g. preheat treatment [2] and pre-mechanical activation [4] the SHS will not be ignited. In present work, with further milling to 140 min, the system with diluent stored enough energy to induce MSR by mechanical impact. Fig. 3 shows the schematic diagram of the variation of the ignition temperature of MSR, Tig , the temperature generated for ball milling collision (ball-to-ball and ball-to-powder collisions) Tc and ball milling time. If Tc is higher than Tig , then the mechanicallyinduced self-propagating reaction can occur. Since the MA process refines the particle size and crystal size, Tig decreases with milling time. But, with increasing milling time Tc increases and reaches a steady state. The time at which the Tig and Tc intersect is the critical milling time, tig at which self-propagating reaction would occur [9]. However, the curve of Tig increases when the diluents is added in Mo–Si system. Thereby, when the milling time is extended , the T from tig to tig ig and Tc can intersect, and then the MSR is induced. MoSi2 compound acts as the diluent of Mo–Si system. The process of mechanically-induced self-propagating reaction (MSR) changes (induction time prolongs) and results in fine powder product. Thus, it is possible to design and control the milling time, structure and granularity of product. This is very important to the mechanical alloying (MA) process. MA is a solid-state powder processing technique. As long as the compound is formed, further milling accumulates defects in powder particles. In particular, the grain boundary defects are abundant. The additional energy to powder may overpass the barrier energy of the formation of
(1) Molybdenum disilicide was synthesized by mechanicallyinduced self-propagating reaction. The induction period of mechanically-induced self-propagating reaction was extended from 90 min to 140 min, with the fraction of diluent content from 0 wt% to 10 wt%. The diluent resulted in the decrease of adiabatic temperature, modified the Mo/Si reactants interface and extended the milling time. MoSi2 compound can acted as the diluent of Mo–Si system. (2) The extending milling time lead to fine grain structured product. The submicron particulates (200–300 nm) were formed. Acknowledgements The project was supported by the Natural Science Foundation of Jiangsu Province No. BK2009096 and Youth Foundation of China University of Mining and Technology No. 2008A046 and Jiangsu Postdoctoral Science Foundation No. 0902007B. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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